Improving carbonate reservoir models with digital outcrop modeling: An example from the Cretaceous Natih Formation, Sultanate of Oman

Adams, E.W., M. Pal and A.E. Csoma

Commonly, interiors of Middle East carbonate platforms are stratigraphically correlated in a layer-cake fashion making the assumption that the platform interior was represented by flat, extensive and undifferentiated shallow-water environments creating wide, laterally continuous facies belts. However, high-resolution, 3-D seismic data from the subsurface of Oman of Cretaceous epeiric platform carbonates reveals a complicated internal stratigraphic architecture, comprising depositional geometries such as platform incisions and clinoforms. The internal spatial complexity, facies heterogeneities and reservoir property distribution of these architectures remains unknown. In order to ground-truth and quantify both the external and internal make up of these geobodies, outcrop analogues can be used. In Jabal Madmar in Oman, these geobodies were recognized in outcrops of the Natih Formation. The outcrops are exposed along several canyons that cut deeply into an anticline structure and as such provide pseudo 3-D exposures.

A reservoir model should be robust for predicting responses to IOR and EOR processes. For that reason it should adequately account for critical embedded geologic heterogeneities and geobodies. One of the most reliable sources of information on heterogeneities comes from outcrops because it allows examining geological variability at all scales. By deploying a digital outcrop modelling workflow, geobodies observed in outcrop can be quantified efficiently and effectively. In essence, outcrop-based geologic features are spatially positioned and recorded with digital mapping and surveying techniques – in our case high-resolution GPS and LiDAR – in 3-D and on scales ranging from centimetre (hand specimen observations) to kilometre (mountain range observations). The large amount of collected data is assimilated, visualized, and modelled to create a digital outcrop model (DOM), which delivers numerical data on matrix heterogeneity and structural overprint.

In Jabal Madmar, the first type of geobody is a seismic-scale incision (depth of 15 m and width of c. 600 m) that was formed during a relative fall in sea level. Dolomitized wackestone facies infilling the incision indicate that diagenesis played a role in defining the reservoir properties of the incision. The second type of geobody is recognised by hectometre-scale (100 m long) medium-angle to high-angle (1–5°) stratal surfaces bounding clinoform sets that are observed within grainstone shoal complexes. The clinoforms are characterised by complex internal architectures and lithological and diagenetic changes.

The main objective of the study was to develop recommendations and guidelines on ways to incorporate these type of geobodies into static and dynamic models. Special attention was given for modelling clinoforms in PetrelTM. Although upscaling will need considerable attention, complex and high-resolution clinoform models must be considered in reservoir modelling workflows in order to correctly derive static and dynamic rock properties for input into a full-field model. Besides direct subsurface applications, the digital outcrop models can be presented as virtual reality (VR) training datasets spanning resolutions ranging from sector-scale to thin section-scale.

High-resolution lithofacies analyses of predominantly organic-rich carbonates from the Cenomanian Natih-B intra-shelf basin, North Oman

Al Balushi, S.A.K., J.H.S. Macquaker and C. Hollis

During the Late Albian to Early Turonian, a major phase of carbonate platform growth took place in North Oman, which recorded the deposition of the Natih Formation over siliciclastic sediments of the Nahr Umr Formation. Seven lithostratigraphic units have been identified in the Natih Formation (members Natih-A to Natih-G downward, reflecting major changes in relative sea level that mainly controlled their deposition). Uplift and erosion in the Turonian ended the development of this carbonate platform, which enabled the influx of fine-grained terrigenous material of the Fiqa Formation from the Arabian craton.

Intra-shelf basin environments developed twice in the extensive carbonate platform of the Natih Formation (upper Natih-B and lower Natih-E) as a result of major transgressions, enhanced by differential sedimentation rates. This study is focused on the Middle – Late Cenomanian Natih-B Member because the intra-shelf basin within it includes more organic-carbon-rich sediments (up to 40-m-thick source-rock interval, average about 4% total organic carbon (TOC)) that have sourced hydrocarbons to the adjacent Natih Formation reservoirs. Examples of time-equivalent intra-shelf-basinal carbonates to those of the Natih-B Member in the Arabian Plate include the Shilaif/Khatiah Formation of United Arab Emirates, Rumaila Formation of eastern Saudi Arabia, Kuwait and southern Iraq, and upper Sarvak Formation of southwestern Iran.

Our research, utilizing high-resolution petrographic (optical and backscattered electron-optical) and geochemical (X-ray diffraction (XRD) and TOC) analyses on samples collected from North Oman (Adam Foothills and Natih field), has revealed that the predominantly fine-grained carbonates of the Natih-B Member contain a great deal of previously unrecognized lithofacies variability, developed at cm- to sub-mm scales. Most geologists have largely overlooked the significance of this small-scale variability, attributing changes observed to either influence of varying primary production and/or oxygen concentration in the water column, or differential effects of weathering once the sediment had been subaerially exposed.

Detailed analyses of the textures (e.g. relict thin beds, burrow mottles, organo-mineralic aggregates, lenticular laminae and scoured surfaces) and components (including both fossil and mineral assemblages) present at these small scales suggest that this lithofacies variability reflects bed-scale processes. Moreover, it indicates that many of the organic-carbon-rich intra-shelf-basinal sediments, rather than being deposited in predominantly low-energy settings as a result of a continuous rain of detritus to the sediment-water interface, were actually deposited, at least partially, by episodic and advective events and then were subsequently disrupted by diminutive burrowing organisms. Bioturbation, although subtle, is widespread in these sediments, which also contain evidence of in-situ fauna (including thick-shelled oysters, flattened pectens and benthic foraminifera) suggesting that during deposition, bottom-water conditions were most likely oxic to dysoxic rather than persistently anoxic. These sediments, following compaction, developed thin depositional beds that might have been mistaken for depositional laminae by some geologists.

Furthermore, analyses of successive samples within laterally-equivalent intervals suggest that there was considerable temporal and spatial variability in sediment supply and accommodation availability, in addition to local changes in primary production, sediment accumulation and burial rates that fundamentally controlled organic-matter enrichment in the Natih-B Member.

Stratigraphic trapping: The Natih Formation in North Oman

Al Busaidy, T.

With more than 50 years of exploration in Oman it is not surprising that the conventional structural plays have been creamed. Exploration in Oman has been focusing therefore more and more on the more difficult unconventional plays, including stratigraphic traps. The Natih was recognised early on as a potential candidate for stratigraphic trapping because of strong lateral facies variations in combination with intercalated source rocks that have been proven to contribute to charge in some areas.

The Natih Formation forms the interior part of an extensive carbonate platform that covered the eastern margin of the Arabian Shield during the mid Cretaceous. The formation consists of a number of repetitive sedimentary cycles ranging from several tens to 150 m in thickness. Each cycle starts from basal, generally thin, variably argillaceous mud-supported units (potential seals and source rocks) followed by thick deepening and shallowing carbonates ending with shell-rich or pelletal-skeletal grain-supported units (the reservoirs). Superimposed on these cycles, two to five third-order, generally shoaling-upward sequences have been distinguised by previous studies.

Extensive penetrations and supporting studies and seismic have provided the very detailed understanding that is required to assess its stratigraphic trapping potential and the definition of statigraphic traps conceptually. The testing of one of these traps confirmed the models, but the absence of hydrocarbons indicated a low sealing potential of the intra-Natih mudstones. A full assessment of the stratigraphic trapping potential of the Natih led to a consolidation of the play, essentially remaining in monitoring mode. Play geometries with trapping potential exist, but they remain very subtle and are difficult to distinguish from subtle structural traps or combinations of structural stratigraphic traps. Volume potential is therefore also difficult to assess and remains a challenge. Recent serendipitous Natih discoveries in 2008 and 2009 confirm this potential, but also confirm the difficulty in volume assessment.

Roles of predicting fracture patterns in fold and thrust belts: A case study from the Salakh Arch, Sultanate of Oman

Al-Kindy, M.

The Salakh Arch is an arcuate chain of six doubly plunging anticlines that collectively extend for about 75 km in the southern edge of the northern Oman Mountains. The anticlines are interpreted as buckles of more than 2 km carbonate thick sequence (from Natih to Khuff formations), underlain by less competent inter-bedded sandstone and shale sequence, detached on a regionally-extensive evaporite horizon. The outcrops of Natih and Upper Kahmah formations, along with the seismic data in the area, offer a good framework to assess the relation between the structural evolution of the Arch at a regional scale and the fracture development at a local scale. Results from this study indicate that fractures relate to the orientation of the local structure, with some sets parallel and some sets perpendicular to local hinge lines. The majority of fractures have orientations consistent with having formed by flexural slip/flexural flow on fold limbs. These structures can be predicted from finite stratal dip, simple curvature and 3-D folding restoration maps, whereas Gaussian curvature and 3-D faulting restoration maps can be used as proxies for fault-related fractures. Local hinge-related fractures may reflect local tangential longitudinal strain during large-scale fold-tightening. Those fold structures that have formed obliquely to the regional shortening direction show additional hinge-perpendicular fracture arrays indicating weak axial extension, presumably developed as the arcuate thrust belt amplified. Our analysis illustrates the importance of taking a 3-D approach, especially within non-cylindrical fold.

The effect of bioturbation on permeability and water-flood recovery in a Natih carbonate sequence

Al-Musallami, T.

A detailed evaluation of cores, image logs and outcrop analogues of a 40 year old field in North Oman indicates that some of the main reservoir intervals are heavily bioturbated. The intervals are part of the Cretaceous carbonate sequence of the Natih Formation. These reservoir units comprise of grainstones and packstones deposited primarily in inner to mid-ramp environments. Burrows are nodular on a cm scale with cemented burrow rims splitting the volume into three parts: (1) inter-burrow matrix, (2) cemented burrow linings and (3) intra-burrow fills.

The nodular texture is expressed in core, image logs and in outcrop respectively by differences in hydrocarbon stain, resistivity and weathering due to differential cementation. These features in turn define porosity, pore shape distribution, permeability and saturation characteristics, which are key parameters that affect water-flood performance.

In order to quantify the effect of bioturbation a detailed petrographic analyses in combination with a comparison of mini-permeameter measurements with plug data and production tests were conducted. The results of the analyses were then used to build dedicated small-scale simulation models to test fluid flow through the burrow network and helped assess the water-flood efficiency.

Sequence stratigraphy of the Albian to Cenomanian interval in the northwest Fars area (southwest Iran)

Asillian, H., Y. Lasemi, D. Morsalnejad, M. Jalali and A. Khosravi-Sereshki

The Albian to Turonian sediments are over 600 m thick in the study area consisting mainly of shallow-water carbonates. In total five surface sections were measured in the Coastal and sub-Coastal Fars, then correlated with eleven wells. The datasets are organised in three SE-NW and SW-NE oriented transects. Facies analysis resulted in the recognition of a range of tidal flat to open-marine sedimentary environments. The sedimentary facies consist of stromatolite, rudist debris, oligosteginid and other planktonic foraminifera in mudstone to boundstone sedimentary textures.

Six third-order sedimentary sequences were distinguished in this time interval. Lateral facies and thickness changes introduce local and regional controls on the sedimentation patterns. The regional factors could be related to the Cenomanian – Turonian uplift and erosion that created a patchy nature in these units, while the local factors are attributed to salt movements and the influence of the well known Kazerun and Nezam Abad faults. The overall thickness variations show that the depocentre was located between these two faults in the sub-Coastal Fars area.

The mid-Cretaceous unconformity in the southern Arabian Gulf

Burchette, T., A. Al Suwaidi and M.E.I. Elsaid

Obduction of the Oman Ophiolite during the Late Cenomanian and Early Turonian caused structural reworking of much of the northeastern Arabian craton. In the eastern part of the southern Arabian Gulf, this event resulted in the growth of a shallow peripheral bulge that exposed preceding carbonate sediments of the Wasia Group and the development of a foreland basin that reconfigured Late Cretaceous depositional systems. Halokinesis was accelerated in many diapirs rooted in the underlying Proterozoic Hormuz salt basin to form, substantially by the Early Turonian, many of the familiar offshore oil and gas-bearing structures of the region.

In north-eastern offshore Abu Dhabi and in adjacent Dubai and Iran, the Fateh, SW Fateh, Rashid, and Sirri fields form a cluster of such genetically-related, salt-cored structures in which the Mishrif is oil-bearing. In addition, other non-productive structures, for example the Sir Abu Nu’air Island and Mandous salt piercements, formed in the area at the same time. A map of the top Mishrif surface shows that these features surround a pronounced area of salt withdrawal and subsidence to the north of Sir Abu Nu’air Island. Subsidence here allowed the formation of a marine re-entrant onto the Mishrif shelf which contains an additional pre-Laffan (sensu stricto) Mishrif depositional cycle. The Sir Abu Nu’air trough is overlain by a modern sea-floor depression, implying that salt withdrawal has continued at a slower rate until quite recently.

The top of the Mishrif Formation in same area is characterised by regional exposure: the “mid-Cretaceous” or “pre-Aruma” unconformity. In places proximal to the Oman Mountains, over the location of the peripheral bulge, the Mishrif has been substantially eroded or entirely removed. More distally, over the crests of some salt domes, erosion is equally pronounced. Here too, the Mishrif has been locally stripped away, although it is preserved off-structure. Through a combination of non-deposition and erosion, the Mishrif succession on this eastern side of the “Shilaif basin” is significantly thinner than in other locations.

The existence of the top-Mishrif unconformity and its impact on Mishrif reservoir quality has been noted elsewhere, but its sedimentology has only been cursorily examined. In offshore NE Abu Dhabi and western Dubai, the Mishrif surface is characterised everywhere by exposure and erosion and is commonly fissured and overlain by up to several metres of coal and palaeo-soil rich in organic material. Cavities and karstic fissures in the top few metres of the formation are also commonly infilled by terrestrial plant-bearing material. In areas of more intense karstification, caves and valleys or dolines filled with breccias and soil-derived material are present.

Samples from above the unconformity contain boehmite, a kaolinitic clay typical of lateritic or bauxitic soils developed over carbonate terrains in pluvial tropical environments. This surface can therefore be characterised as a lowland coastal karst analogous to that seen in some modern exposed tropical and sub-tropical carbonate terrains. A picture of the top-Mishrif unconformity emerges as one of erosion and entrenchment in uplands over the mid-Cretaceous peripheral bulge and isolated salt domes with the development of a marginal, at least seasonally-humid, vegetated lowland over lower-lying western slopes where the water table was probably periodically close to surface, giving rise to swamps and mires.

Reservoir modelling from interpretation of 3-D virtual outcrops and field data: A case study from the Upper Sarvak Formation, Chenareh Gorge, Lurestan, Iran

Casini, G., S. Homke, J.-C. Embry, T. Lee Scarrott, I. Romaire, N. Fernández, N.A.H. Pickard, I. Sharp, M.R. Jamaledini, A. Jahanpanah, F. Livbjerg, P. Gillespie, C. Mehl, M. Jalali, L. Aghajari, J. Vergés and D.W. Hunt

Although very important for reservoir characterisation and modelling, matrix and fracture heterogeneities are generally poorly incorporated into reservoir models. Indeed, modelling of subsurface reservoirs is often exclusively based on well log and seismic data, which are not able to capture many of the heterogeneities characterizing matrix and fracture properties across the reservoir. The analysis of reservoir outcrop analogues plays a key role for the characterization and modelling of matrix and fracture heterogeneities, especially when well log and seismic data are limited or of poor quality. A case study of reservoir heterogeneity characterization and modelling based on integration of fieldwork and 3-D virtual outcrop interpretation is presented.

The case study is located in the Chenareh anticline, Simply Folded Belt of the Zagros Mountains, Lurestan, Iran. Along the Chenareh Gorge, the Upper Sarvak-Ilam stratigraphic section of the Bangestan Group is spectacularly exposed. The gorge cuts entirely across the Chenareh anticline allowing continuous fracture and matrix characterisation from backlimb to forelimb. The section, Cenomanian – Turonian in age, has been extensively studied both in the field and through interpretation of high resolution 3-D photorealistic models based on LiDAR technology (up to 0.05 m resolution) and QuickBird satellite imagery (0.70 m resolution).

Data from extensive fieldwork along the gorge and surrounding areas permit detailed characterisation of matrix and fracture heterogeneities across the anticline. Facies, pore type and sequence stratigraphy have been characterised in the field and through the analysis of samples collected systematically along the measured section. Data on fracture and fault network geometry have been also collected systematically through the whole stratigraphic interval. Fieldwork data have been integrated with data manually and automatically extracted from 3-D photorealistic models based on LiDAR technology and QuickBird satellite imagery. The use of high-resolution 3-D photorealistic models allows quick collection of high-quality data for quantitative and qualitative analysis of matrix and fracture heterogeneities. Integration of fieldwork and 3-D virtual outcrop interpretation was fundamental for modelling matrix and fracture heterogeneities of the Chenareh Gorge fractured reservoir analogue.

Fault and fracture development in foreland fold and thrust belts: Insight from the Lurestan Province, Zagros Mountains, Iran

Casini, G., J. Vergés, I. Romaire, N. Fernández, E. Casciello, E. Saura, S. Homke, J.-C. Embry, P. Gillespie, L. Aghajari, H. Noroozi, M. Sedigh, J. Bagheri and D.W. Hunt

The Simply Folded Belt of the Zagros Mountains, Iran, represents one of the best examples of foreland fold and thrust belt. A regional fault and fracture analysis of the Cenomanian – Coniacian Ilam and Sarvak formations, exposed in southern Lurestan Province, is presented as a case study for fault and fracture development in folded belts. The area is characterised by the occurrence of gentle to tight anticlines and synclines whose NW-SE axial traces are parallel to the general trend of the belt. Fold style is intimately related to both vertical and lateral facies distribution. The two formations belong to the Bangestan Group and, in this area, they represent the oldest strata exposed in the core of most anticlines outcropping at surface.

Distribution, kinematics and timing of faults and fractures have been characterised through extensive fieldwork and interpretation of orthorectified QuickBird imagery and 3-D virtual outcrop models based on LiDAR technology. Data have been collected from 10 anticlines covering an area of approximately 150 x 150 km. Key outcrops for fracture and fault kinematics interpretations are presented.

Field observations and interpretation of QuickBird and 3-D photorealistic models suggest a complex fault and fracture geometry and timing relationship. Both fractures and faults record pre-folding to uplift-related deformations. Pre-folding structures are typically represented by small-scale, flat-ramp-flat geometry thrusts, systematic veins and stylolites, which are superimposed on inherited syn-sedimentary normal faults. Folding-related structures generally reactivated pre-existing fracture and fault planes. Strike-slip faulting is typically recorded as the last faulting event and is probably related to late stage of fold tightening. All structures are geometrically and kinematically consistent with the trend of the Arabian passive margin and its subsequent tectonic inversion. Uplift and stress release induced opening and propagation of through-going fractures.

Faults and fracture orientations generally change accordingly with local fold trend. Symmetry between fracture and fold orientation, although commonly interpreted as evidence for folding-related fracture development, is here interpreted as evidence of syn-to post-folding local vertical axis passive rotation.

Reservoir characterisation of emersion surfaces and their associated diagenesis: Natih Formation, Cretaceous, Sultanate of Oman

Champagne, J., C. Durlet, C. Grélaud, P. Razin and S. Schroeder

The characterization of emersion surfaces in carbonates and their associated diagenesis could provide helpful tools for reservoir studies. Outcrops observations combinated with data from oil fields should lead to predictive models of reservoir properties associated with emersion surfaces. The Natih Formation constitutes the top of a thick succession of Cretaceous epeiric platform deposits. It is subdivided into three third-order sequences. These sequences are frequently capped by emersive horizons, which could be associated with incisions. Detailed studies of the lateral distribution and chronology of diagenetic phases associated with emersion surfaces form the basis for the sedimentological and diagenetic model. The results emphasize the heterogeneous character of these surfaces from a microscopic to a regional scale. The diagenetic sequence and the characterization of fluids associated with petrophysical data constrain the modifications of reservoir quality, in relation with the geological context. The complex interplays between the factors responsible for reservoirs properties could make difficult the recognition of exposure surfaces. That is why an integrated approach is needed to find predictable patterns for the detection of unconformities in carbonates.

Sequence stratigraphy and depositional systems in the Albian – Turonian of the Arabian Plate: Implications for regional exploration and reservoir description

Davies, R.B. and M. Simmons

The Albian to Turonian succession on the Arabian Plate shows marked spatial and temporal variation in depositional characteristics that have (a) led to a confusing lithostratigraphic nomenclature, (b) reflect its sequence stratigraphic organisation, and (c) have important ramifications for the exploitation of discovered hydrocarbons and the discovery of further reserves. It is bounded top and bottom by tectonically-enhanced, platform-wide unconformities (K90 SB and K150 SB of Sharland et al. 2001, 2004). Between these major unconformities, the succession shows important large-scale cyclicity that can be related to eustatic sea-level changes (see Simmons et al. abstract). Overall the proportion of siliciciclastic input decreased with time whilst carbonates became more widespread.

Immediately above the basal K90 SB unconformity, in proximal areas coarse-grained braid plain deposits characterise the Albian to Turonian succession. In complete contrast, the only record of siliciclastics in the most distal settings may be of slightly argillaceous and dense carbonates or clay infills to karst although both still exert significant influence over reservoir compartmentilisation and connectivity. Intermediate types of siliciclastics comprise major reservoir facies, and key regional and intra-formational seals. Carbonates range from low-energy platform-top mudstones to wackestones, rudist bioherms and storm-reworked rudstones forming variable and complex reservoir facies, to relatively deepwater carbonates. Intermediate mixed facies include areas of major source rock deposition in restricted intra-shelf basins. All depositional components can be fitted within a regional sequence stratigraphic framework that predicts further exploration potential as well as forecasting primary controls on reservoir layering and compartmentalisation. The regional asymmetry of depositional systems is a major control over prospectivity at the regional scale and, for the major fields at least, is predicted to impact reservoir architecture at the field scale.

Stratigraphic framework of the Natih Formation in the Sultanate of Oman

Droste, H.J.

Carbonates of the Albian to Turonian Natih Formation are important hydrocarbon reservoirs in the Sultanate of Oman. A regional sequence stratigraphic study integrating seismic and well data of Interior Oman led to a better understanding of the reservoir and seal distribution as well as the stratigraphic trapping potential.

Deposition took place on an epeiric shelf with carbonate platform development at the ocean ward margin, located in the North and East of Oman, while clastics predominated along the exposed Arabian Shield in the southwest. Lateral shifts in clastic and carbonate facies belts driven by changes in relative sea level and climate resulted in a hierarchical stacking of depositional cycles of several 10s up to some 150 m thick.

Two major flooding events, with widespread deposition of pelagic carbonates, occur in the Late Albian and Late Cenomanian. Both are associated with the creation of significant depositional topography (up to 100 m) as a result of aggradational carbonate growth along the margin. This was followed by a phase of platform progradation over more than 100 km towards the interior of the epeiric shelf. Variations in the type and amount of sediment input both in time and space caused major variations in reservoir geometry and properties within these prograding complexes.

During the sea-level rises there was widespread deposition of organic-rich pelagic carbonates linked to high organic production and/or restricted bottom-water circulation. Contrary to what has been suggested in previous models these organic-rich sediments were not deposited within intra-platform basins surrounded by carbonate platforms but within the deeper water belt located between the carbonate margin and the shallow marine clastics lining the Arabian Shield.

A major fall in sea level in the Middle Cenomanian led to exposure and channel incision of the platforms and a major influx of clastics. Fine-grained clastics also covered most of the lower Cenomanian platform during the initial stage of the following relative sea level rise. A second major drop in sea level occurred in the Turonian and is also associated with the influx of clastics. Quartz sands trapped between the exposed carbonate platforms may provide opportunities for stratigraphic traps.

Detection and distribution of Thief Perm in the Late Albian Mauddud Formation, Sabariyah and Raudhatain fields, Kuwait: A dual porosity dual permeability methodology for model implementation and history matching

El Din Ibrahim, H., H. Ashok, M. May, A. Quinn, L. Brinton, J. Keay, J. Kalbus and J. Dozzo

An integrated analysis of a new detailed core descriptions, image and open-hole logs, PLT and PTA data, and production history in the Late Albian (Early Cretaceous) Mauddud Formation of Sabariyah and Raudhatain fields, North Kuwait, reveal the presence of dual perm- dual porosity carbonate reservoirs. A methodology was developed to distribute this secondary pore system within the dynamic model and achieve an excellent history match. Early water breakthrough within months of water injection in this high porosity/low permeability reservoir indicated the likely presence of a secondary high perm low volume pore system or “Thief ”. This was further supported by the lack of identification of a water-swept zone in infill wells despite field-wide water cuts of 30%.

Earlier studies suggested that fractures could only play a minor role in this enhanced permeability reservoir. Even though the fields are highly faulted, image log analyses supported these earlier conclusions as the zone of disturbance due to tectonic fractures associated with any fault is generally limited laterally less than 3 ft. Numerous small fractures associated with hard nodular features were observed on the resistivity images and in core but these have very limited extent (<10 cm). These fractures do not appear to make a significant contribution to porosity or large permeability in the reservoir (not associated with thief zones). The Image logs were not successful in identifying of thief zones; however there were only three wells to analyze that had both PLT and image logs.

A basic open-hole well log suite was available. An attempt was made to create a secondary porosity indicator (SPI) from the well logs using the traditional Density Porosity minus Sonic Porosity. The vertical resolution of this technique is limited to the resolution of the tools – 2 or more ft thick. A comparison of the SPI with PLT logs showed a very poor correlation indicating the log-based secondary porosity indicator could not be used to reliably identify thief zones. This was not surprising as thief zones were also not identified by the conventional core analysis with few core points in excess of the average matrix permeability.

In general, porosity development is predominantly a function of grain and matrix dissolution in carbonate lithofacies, with fewer occurrences of preserved primary interparticle porosity in grain-dominated lithologies/lithofacies. Dissolution porosity in carbonates, particularly in highstand sequences of the Upper Mauddud, appears to be related to multi-generational subaerial exposure events that occurred at the tops of vertically stacked cycles/sequences. The result was numerous and compounded dissolution phases resulting in good to exceptional porosity development as indicated by dissolution breccia and rubble intervals in the core. The low volume high perm secondary pore system is attributed to thin stratigraphically conformable streaks of enhanced dissolution created during these periods of exposure.

A thief zone facies log was created and derived from core interpretation, foot by foot PLT spike and permeability logs, well test permeabilities, and high flow production zones. PLT data are considered the most reliable indicators of secondary perm and are present in 1/6 of the total well stock. PLT permeability logs were estimated in single phase producers and injectors. These logs were calibrated to well test data where possible and used to generate secondary perm logs. Median thief permeability thickness (kh) ranged between 50–3,700 mDft depending on stratigraphic flow unit. Values as high as 30,000 mDft were observed. Average matrix kh is generally less than 20 mDft.

Initial thief maps were created using geocellular modeling software. Thiefs were modeled at the resolution of the dynamic model as the observed data is of insufficient resolution to support a more refined model. Two methods were used to map the thiefs, object and pixel based. In object modeling, a thief is represented by discrete geometric bodies distributed in a 3-D grid. Because of the uncertainty in the thief connectivity and the dominance of thief zones in reservoir performance, the pixel based thief description was created as a more continuous case. In the pixel based approach, thief kh was distributed directly from the all kh observations including both thief and matrix. Thief perm was distributed in both cases using the Sequential Gaussian Simulation technique.

Both object and pixel-based descriptions were considered for the history match. It is likely that a high quality history match could be achieved from either description. However, model results indicated a preference for the pixel-based more continuous thief zone case. The thief description was iteratively modified during the history-matching process as the well performance data provides vital information about thief continuity. An excellent match was achieved and will likely lead to significant modification of the field development plans at considerable savings to Kuwait Oil Company.

Constraints on regional-scale stratigraphic forward modelling in the Upper Sarvak Formation, Lurestan, Iran

Embry, J.-C., D. Hunt, I. Sharp, G. Casini, S. Homke, T.L. Scarrott and M.R. Jamaledini

Forward numerical stratigraphic modelling is the simulation of sedimentary processes through time. The DIONISOSTM software, developed as a research consortium by the Institut Français du Pétrole, is used in this work to constrain a range of controls on the Upper Sarvak hydrocarbon-bearing carbonate platform-basin deposits at the regional scale (Lurestan Province). This technique is used to better predict stratigraphic geometries and facies distribution through time, with focus on understanding reservoir facies distribution, volume and connectivity. The model simulates Cenomanian – Turonian carbonate sedimentation (Upper Sarvak Formation) in an area of 70,500 km2. The simulation is based on three main parameters: accommodation (tectonic subsidence and eustasy), sediment supply (in situ production for carbonates) and transport (mainly gravity-driven diffusion).

To date a combination of 42 wells and outcrop sections have been correlated into a sequence stratigraphic framework, and 13 facies maps and 11 isochore maps have been prepared. The sequence stratigraphic framework is then used to produce a 4-D “static” DIONISOSTM geomodel that is matched to the wells, surface sections and stratigraphic architecture. The geomodel allows the geologist to verify the geologic database and interpretation. Once quality control of the “static” geomodel is complete, tectonic subsidence maps are computed for each interval, using (1) the interval thickness map, (2) its initial and final bathymetries, and (3) eustatic sea-level (i.e. Gradstein, 2004). This tectonic subsidence history is used as an input parameter for the forward stratigraphic modelling. The last step of the forward stratigraphic modelling workflow consists of constraining the the dynamics of a sedimentary system in response to different parameters, such as carbonate productivity, sediment transport, wave energy or clastic input. This is achieved through sensitivity testing. 4-D stratigraphic modelling that incorporates physical processes to forward model the stratigraphic architecture is revolutionizing the way we predict reservoir facies distribution and quantify uncertainties around this prediction. This technique allows:

  • A better understanding of the parameters controlling the stratigraphic architecture, such as tectonic or the external controls,

  • Improvements in prediction of platform-basin facies length scales,

  • Highlighting of the important changes in reservoir facies and architecture/stacking associated with ocean facing, intra-shelf and isolated platform margins.

  • A thorough understanding of reservoir facies distribution and connectivity that can be used as a template to aid model building of early phase fields where well data is sparse.

  • A depositional/facies template that can be further utilised for regional-scale diagenetic and seismic modelling studies.

Sedimentary and stratigraphic heterogeneities within third-order sequences in the Natih Formation (Late Albian – Early Turonian)

Grélaud, C., P. Razin, P.W. Homewood, V.C. Vahrenkamp and A.M. Schwab

The Natih Formation (Late Albian – Early Turonian) corresponds to a very broad, extensive inner carbonate platform that developed in Oman over a width of more than 1,000 km. Because it is overall an aggrading carbonate platform system, the third-order stratigraphic sequences appear very tabular and isopach, which often leads to “layer-cake” correlations, even at the scale of subsurface reservoirs. However, within these third-order sequences, a high level of sedimentary and stratigraphic heterogeneity can be found, and the heterogeneities are linked with:

(1) the development and fill of intra-shelf basins: different types of clinoforms (inclined time surfaces) with varying angle, lateral extent, direction of progradation and facies association; (2) phases of subaerial exposure: development of incisions (1 km wide, 12 to 20 m thick, up to 100 km long, with different types of fill) and forced regressive wedges (high angle oblique clinoforms of limited 3-D extent); and (3) phases of flooding of the platform: development of tidal channels and associated bioclastic sandwaves and shoals (mainly filled with coarse high-energy facies organized as complex sets of inclined sediment bodies).

These stratigraphic intervals have been studied on several outcrops of the Oman Mountains (Adam Foothills and Al Jabal Al Akhdar). The regional stratigraphic framework (van Buchem et al., 2002) has been reviewed and extended by regional 3-D seismic interpretation. A particular focus was made on specific reservoir-scale sediment bodies (channelized structures, sedimentary wedges) on both outcrops and seismic data and the technique of synthetic seismic modeling has been used to improve seismic interpretation.

The results of this study have been used to refine the static reservoir model of the Natih Formation in a giant oil field in northern Oman.

Diagenesis of the Albian – Turonian Formations of the Middle East

Hollis, C.

Diagenetic modification of the Albian – Turonian in the Middle East is complex and multiphase, and has provided a critical control on the distribution of an effective matrix pore system in reservoirs of this age. Data has been gathered from outcrop and subsurface case studies, which span a period of over 20 years, from across the region. During this time, there has been a significant evolution in our understanding of the depositional setting, tectonic evolution and burial history of the succession. Furthermore, conceptual diagenetic models have evolved, and interpretations presented in the literature in some way reflect this. Nevertheless, the published data (from Iraq, Iran, Kuwait, United Arab Emirates, Saudi Arabia and Oman) also reveal a surprising number of consistent trends.

In particular, three key paragenetic events appear to control the distribution of matrix porosity. (1) Marine cementation, along discontinuities formed during falls in relative sea level, and within Thalassionoides burrow fills. (2) Formation of incipient karst at intra-formational sequence boundaries and (locally) deep-cutting karst associated with late Cenomanian – Early Turonian exposure. (3) Leaching of micrite and early diagenetic cements to create large volumes of secondary micro- and macroporosity. Historically this leaching has been attributed to the ingress of meteoric pore waters, but more recent models point to dissolution from deep burial brines.

This paper will assess the relative importance of these key events across the region, and consider the influence of depositional setting, burial history and hydrocarbon charge on controlling the spatial distribution both diagenetic products and matrix porosity. This will provide the basis of a predictive regional diagenetic framework, a necessary first step to truly understanding the controls on reservoir quality in the Albian – Turonian of the Middle East.

Carbonate platform high-frequency cycles, Natih Sequence I (E Member), Sultanate of Oman: Contrasting transgressive- and regressive-constrained reservoir-scale heterogeneities

Homewood, P., M. Mettraux, C. Grélaud, P. Razin and V.Vahrenkamp

The Natih Formation carbonate system and depositional sequences of Oman are well established, and the heterogeneities at the reservoir scale, studied on outcrops of the Oman Mountains and Adam foothills, have already proven useful for static reservoir modelling. The carbonate depositional system of the Natih E has been shown to prograde from three nucleation areas in North Oman: West, North and East. Whereas attention has been focused on the margin to basin facies of the outcrops of the eastern and western platforms, since they tie directly into several oilfields through adjacent 3-D seismic surveys, the build up of the Northern platform provides insight on facies and reservoir heterogeneities from accumulations of more proximal, shallower carbonate platform environments where clay intervals are absent.

Stacks of metre-thick to several-metres-thick high-frequency cycles (genetic units) may form an apparent layer cake up to one hundred metres in thickness. Although these thick and laterally extensive carbonates do not show any clay intervals that would hinder fluid flow across thinner units, stylolite-rich seams and heavily dolomitised beds (or bed surfaces) do create potential flow barriers that should be taken into account for reservoir modelling of such apparently more homogeneous, thick layers.

The > 100 m thick proximal platform carbonates of the Natih sequence I (E member) in particular show two highly different facies associations, during transgressive («landward stepping») and during regressive («seaward stepping») stacking patterns. Landward stepping high-frequency cycles show a progradational half-cycle with cross bedded to homogeneous low-angle clinoform-set bounded packstones and wackestones, followed by a transgressive half-cycle of wackestones to mudstones with more or less pronounced horsetail stylolite seams. The cycle thickness diminishes from 10m or so to 2–3 m and cycles become more and more symmetrical up-section. A most pronounced stylolite-seamed bed is at the turn around from this first facies association to the second, and is interpreted to be the MFS of Sequence I. Seaward stepping high-frequency cycles become more and more asymmetrical up-section and are terminated by a well developed karst penetrating >10m below an emersion surface, the SB between sequences I and II. The seaward stepping high-frequency cycles show wackestones to packstones with a more or less intensely bioturbated upper part, covered by a thin stylolite-seamed wackestone to mudstone. The bioturbated bed tops are commonly dolomitised, and early cementation of the upper parts of the bed is shown to have preserved the burrow pattern shape.

The dolomitised, early cemented grainstones to packstones of the seaward stepping units, and the stylolite-seamed mudstone to wackestones of the landward stepping units create extensive, layer parallel permeability barriers at the 1–10 m scale and should be taken into when constructing models for fluid flow in such apparently homogeneous units.

Retreat of shelf margins in the Albian to Turonian of northeast Arabia

Horbury, A.D.

Albian – Turonian carbonates in northeast Arabia show a progressive retreat of their shelf margins towards palaeohighs. This retreat is controlled by localised tectonic collapse of the pre-existing platforms and re-location of platform margins onto structurally high areas.

By the end of the Albian, there had developed an extensive clastic-backed carbonate platform across most of the northern Middle East. By the end of the Turonian, shallow-water platform carbonates were all but absent, restricted to a few locations on major palaeohighs. The demise of this platform system took place in two phases. An initial retreat from its maximum progradation in the Late Albian, took place in the latest Albian, when the majority of the northern Iraq platform was drowned and the platform margin re-located itself to a NW-oriented axis running through Baghdad into the Bala Rud shoal area of Iran. Behind this developed an intrashelf basin, with shelf carbonates reappearing in the Western Desert area of Iraq. Following a major lowstand in the later part of the Early Turonian, the study area was covered with deeper-water carbonates, excepting the area of Massive Limestone and Mardin Limestone deposition in northeast Syria and southeast Turkey.

Each phase of retreat followed locally significant tectonism that marks the initiation of the foredeep and coeval transtensional basin system that characterise the later part of the Cretaceous. In particular, collapse of the Albian platform margin and relocation of the shallow-water carbonates to the Baghdad area, was controlled by initiation of a transtensional structure in the Tikrit-Amara area, whilst to the southeast in Iran the Cenomanian margin sits directly above the older Albian margin and no retreat is shown. Retreat is significant in that it fundamentally affects location of reservoir rocks, their diagenesis (e.g. tendency to be karstified) and sealing capacity of overlying deeper-water facies. In the middle of the stratigraphy is the Mishrif Formation, which is arguably the most important reservoir in Iraq. The Mishrif Formation only succeeds as a play where it is covered by deeper-water and marly Khasib or Kometan formation carbonates as a seal; in East Baghdad, the more shelfal facies of the Khasib Formation do not adequately seal the Mishrif Formation reservoir.

The Albian sedimentary record of southeast Arabia: Facies, sequence stratigraphy and depositional environments

Immenhauser, A.

This contribution aims at summarizing the presently published data set on facies, sequence stratigraphy and depositional environments of the Albian units in southeast Arabia. The latest Aptian to Albian argillaceous units in southeast Arabia, forming a regionally significant seal, have received comparably little attention in the past because they do not form a source or reservoir rock themselves. During the latest Aptian to Late Albian, a major influx of terrigenous material from the emerged Arabian-Nubian Shield to the northwest onto the Arabian carbonate platform led to the deposition of the argillaceous Nahr Umr Formation within the Bab Basin. Throughout the southern Gulf, the Nahr Umr is principally an impermeable unit of clayey facies acting as a seal for the Shu’aiba carbonate reservoir rocks. To the northeast, in the present-day Jabal Akhdar, this shaley facies gradually changes into rudist and coral dominated limestone facies termed Al Hassanat Formation. The Al Hassanat Formation was deposited on a wave-impinged oceanic swell facing the Neo-Tethys Ocean. The Al Hassanat probably formed a rather narrow ribbon (not more than a few kilometres in width) trending approximately parallel to the present-day oceanic margin of North Oman. During the Late Aptian and Early Albian, an uplifted and exposed high separated the Al Hassanat Platform from the coeval Nahr Umr deposits of the intra-cratonic basin. This high was submerged during the middle Albian, and the Nahr Umr records three pulses of Al Hassanat rudist floatstones prograding southward into the intracratonic basin (forming the marker units in the Nahr Umr). During the latest Albian, the Nahr Umr reflects a cleaning upward and during the Cenomanian is overlaid by calcareous units of the Natih Formation. At Jabal Akhdar, the Nahr Umr was overthrust during the Cenomanian by nappes of the Hawasina Complex and the Semail Ophiolite. The Nahr Umr is characterized by important orbitolinid foraminifera accumulations. Similar Orbitolina facies is found for example in the Albacete-Prebetic area of southeast Spain. Commonly, Orbitolina-dominated sediments are related to episodes of terrigenous influx, increase in water depth, and a tendency of platforms to evolve into more ramp-type depositional settings.

Eustasy and tectonic controls on the sedimentary patterns in the Albian – Cenomanian succession in the Lurestan (Zagros)

Jamali, A.M. and M.A. Kavoosi

The Upper Albian – Turonian deposits in the Lurestan area are attributed to the Garau and Sarvak formations. This interval is dominantly composed of carbonate facies and forms the second most important hydrocarbon reservoir in southwest Iran after the Oligocene – Lower Miocene deposits (Asmari Formation). The Lurestan area is located in the NW-most part of the Zagros Foreland Fold-Thrust Belt. The succession is gradationally underlain by Lower Cretaceous sediments of the Garau Formation, known as an excellent source rock in the area, and disconformably overlain by the Coniacian shales/marls of the Surgah Formation grading partially to, the Sarvak Formation.

Fifteen surface sections and ten wells are organised in four regional transects. The results are considered in the field observations and microscopic investigations of the surface and subsurface sections along with eight paleogeographic and isopach maps. Field observations and facies analysis indicate absence and presence of quartz grains and calciturbidites respectively, which reflect the different tectonic regimes. The calciturbidites are composed mainly of non-skeletal grain such as intraclast and peloids that distally interfinger with the pelagic facies. Based on the correlation, paleogeographic and isopach maps, sedimentation have been deposited in a rimmed-shelf and isolated platforms. It is believed that deposition on the rimmed-shelves and carbonate ramps are mainly controlled by regional and local tectonism, but our results suggest considerable facies and thickness variation of depositional sequences in the Lurestan area that are related to both tectonic and sea-level changes; whereas eustasy played the main role from Middle Cenomanian onwards. Dolomitization, Fe-staining, pyritization and glauconitzation are the common diagenetic features on the top of Middle Cenomanian carbonates that indicate the presence of an unconformity. Sudden establishment of upper Cenomanian pyritic and glauconitic pelagic and hemipelagic facies continaing packed ammonites, even on the top of paleohighs, most probably suggest drowning of the carbonate platform in the study area that was accompanied by more open-marine facies in Fars and Dezful embayments. Progressive deepening of facies continued during Turonian in the Lurestan area, whereas restricted inner platform carbonates and even non-depostion occurred in the Dezful embayment and Fars area respectively at the same time. All the data resulted in the understanding of both tectonic and sea-level changes, which controlled deposition in Lurestan that is compatible with the maximum of sea-level rise during Turonian.

Plate tectonic models and regional structural frameworks bridging the gap by palaeo-stress inferences in the Middle East region

Koopman, A.

Generally subtle, but locally significant effects of tectonic activity recurrently interplayed with the global eustacy record across sectors of the Arabian shelf and its passive margin towards the Neo-Tethys Ocean. Plate-tectonic models are generally considered helpful and consulted, in order to improve regional structural frameworks of hydrocarbon plays in the Middle East region. The near-field, far-field, and remote effects of primary plate boundary forces should profoundly control the (re-) activation of (inherited) fault trends, critically affecting structural relays, tectono-stratigraphic trends, carbonate factories, and patterns of sediment dispersal or differential erosion. A set of simple inferences based on elementary concepts and assumptions concluded from the plate-tectonic models may help to assess regional-scale palaeo-stress conditions, to constrain structural framework models across the Middle East region.

Fracture-controlled dolomite reservoirs in Late Cretaceous Carbonates of the Sarvak Formation, Anaran Anticline, Islamic Republic of Iran

Lapponi, F., R. Swennen, G. Casini, A. Amilibia Cabeza, T. Needham, J. Garland, W. Blendingen, T.L. Scarrott, P. Gillespie, I. Sharp and D. Hunt

Spectacularly exposed late burial dolomite bodies of hydrothermal origin replace Cretaceous carbonate shelf sediments of Albian to Turonian age (Sarvak Formation) of the Anaran Anticline, in the Simply Folded Belt of the Zagros Mountains, Islamic Republic of Iran. The outcrops have been mapped, logged and sampled in order to evaluate the effect of hydrothermal dolomitisation process on the reservoir characteristics. In addition, a LiDAR photorealistic model of the representative dolomite bodies has been interpreted (geobody shape and size, fault interpretation, fracture analyses) and the data incorporated into a 3-D outcrop model.

Dolomite replaces carbonate rocks characterised by different facies. The typical dolomite geometries are massive (600 m thick x 1,500 m long), domal and pipe-like bodies (150 m diameter x 200 m thick), usually associated with highly fractured and faulted zones, and stratabound bodies (30–50 m thick x 300 m long), which emanate from the massive dolomite body and follow permeable layers or are located beneath cycle-capping mudstones.

Typical petrographic associations consist of abundant void-filling saddle dolomite phase and a more volumetrically important matrix-replacive finer grain dolomite phase. Preservation of precursor sedimentary structures (i.e. bedding planes) is in general poor in the “core” of the dolomite bodies, but variable, with primary facies and fabrics identifiable locally in clean river sections. Fabric preserving dolomite is more prevalent towards the edges of dolomite bodies. Development of vuggy porosity is also variable and typically facies controlled. Preferential development of vuggy porosity below cycle-capping mudstones is typical in platform top facies. Breccia-type textures associated to fault zones are also common and testify to a complex dissolution/cementation fluid flow and diagenetic history. Crackle, mosaic and chaotic fabrics are common.

Field evidence points to a strong control of ENE-WSW and WNW-ESE fault systems on the dolomitisation fluid flow. These structures are related to the pre- to syn-folding phase of the Zagros and are often associated with a dolomitisation halo.

The distribution of dolomite decreases vertically with three major discrete breaks corresponding to three main aquitard intervals: (1) the early Cenomanian basinal micritic limestones and shales of the Ahmadi Member, which marks the transition between Lower (massive plus stratabound dolomite) and Upper Sarvak Formation(pipes); (2) the Middle Turonian basinal micritic limestones and shales of the Ghir-ab Member, which mark the Cenomanian/Turonian boundary; (3) Coniacian Surgah Formation claystones, above which no dolomite has been observed.

Extensive systematic sampling across the different dolomite body types (stratabound, massive, pipes) and adjacent undolomitised precursor facies has been also carried out in order to capture lateral and vertical variation in the poro-permeability values and quantify the impact on the reservoir quality. In general, porosity values are low to moderate in both dolomite and limestone, with the dolomitised grainy facies of the Lower Sarvak Formation showing better reservoir quality. Permeability is also moderate but distinctly higher in the dolomitised lithologies when compared to the undolomitised counterpart, implying a redistribution of the pore system network related to dolomitisation. Fractures seem to have a major control in the highest permeability values. Different fracture and matrix porosity models for dolomitised and undolomitised lithologies are applied and incorporated in 3-D geological conceptual model.

The middle Cretaceous: A case for special treatment?

Levell, B., H. Droste and V. Vahrenkamp

The middle Cretaceous (Base Aptian to Top Turonian) was “different”. Do our analogues allow us to understand it? The middle Cretaceous is characterized by: (1) a period of consistent sea-level rise leading up to a perhaps unprecedented large flooding of the continental crust; (2) an open equatorial world ocean connection; (3) the largest recorded rate of oceanic crust production and the highest frequency of Large Igneous Provinces with its profound impact on climate warming via CO2 degassing; (4) near-record, sea-surface temperatures; (5) possibly also near-record storm intensity; (6) the anomalous C34n SuperChron or Cretaceous magnetic quiet zone; and (7) the evolutionary explosion of flowering plants.

These characteristics are clearly linked one to another via deeper causal mechanisms, which are actively being diagnosed. Nonetheless the symptoms are clear: (1) creating ideal conditions for the accumulation of thick carbonate sequences of predominantly calcitic mineralogy in a wide equatorial belt. (2) Carbonate platforms of extreme width trapping huge quantities of carbonate mud to form sequences dominated by wackestones and mud-dominated packstones. (3) Increased weathering of cratons with consequential periodic shedding of fine clastics over the distal outreaches of the vast carbonate platforms to form regional seals (e.g. Nahr Umr Formation). (4) A sluggish oceanic circulation with bad waters and possibly the highest frequency ever seen of Oceanic Anoxic Events, in turn leading to the widespread deposition of organic-rich strata with source rock potential.

But the uniqueness doesn’t stop with depositional environments: the situation had to reverse towards collision of the initially quietly drifting plates with their carbonate deposition at the leading edges. The subsequent upheaval in the Late Cretaceous and later, caused the creation of structures, the maturation of source rocks and pathways for the circulation of porosity creating and destroying burial fluids ahead of oil charges.

This paper explores the implications of the uniqueness of this period on understanding the creation of some of the great reservoirs of the Arabian Plate in the context of clearly “non actualistic” conditions. How much does the uniqueness of the period really need to be taken into account in predicting the static and dynamic properties of oil and gas fields?

3-D forward stratigraphic modeling of the Rub’ Al Khali Wasia Formation: A Case Study

Leyrer, K. and P.J. Van Laer

Since a few years, the vast area of the Rub’ Al Khali enjoys increased attention in the search for hydrocarbons. Despite the fact that huge amounts of new data have been acquired and new seismic and drilling activity is still ongoing, the overall quantity of data is rather sparse compared to the size of the area to be explored. In addition to this, the available legacy data is mostly sketchy and sometimes has quality issues to it.

To overcome the data limitations in this frontier basin, it was decided to perform 3-D forward stratigraphic modeling using the software DIONISOSTM and consequently to support exploration activities on a regional scale.

Available internal as well as published geophysical, petrophysical and geological data sets have been used as input and contributed considerably to reduce uncertainty in the modeling results. Continued cross-checks with the latest results from all geoscience disciplines involved, ensure high-quality modeling products which, in combination with the results of other analytical approaches, can be used to guide upcoming exploration activities.

The 3-D forward modeling is not only able to reproduce temporal and spatial occurrences of source rocks, reservoirs and seals and therefore complements existing knowledge but also gives valuable insights into so far unexplored areas of the Rub’ Al Khali. It allows for the identification and subsequent tracking of individual facies belts and consequently adds to the generation of reliable geological models. Furthermore, the reconstruction and quantification of crucial depositional parameters allows for the testing of existing models and enables the generation of sequence stratigraphic frameworks, which can be used to improve the understanding and prediction of exploration-relevant geobodies in a regional context and scale.

Mid Cretaceous stratigraphy, facies and carbon-isotope curves of northwest Qatar

Lubeseder, S., J.Kuss and M. Zahran

The mid Cretaceous (Albian to Coniacian) stratigraphy and facies development of the Mauddud, Mishrif and Laffan formations of northwest Qatar are discussed and placed into a southern Gulf regional framework.

Core, cutting and thin section analysis show the studied area was located at the transition zone between a sand/clay-contaminated carbonate platform interior sub-basin in the southwest (Saudi Arabia and Bahrain) and a platform interior setting in the east (northeast to eastern Qatar). At this transition, the Qatar Arch served as a barrier for the terrigenous detrital influx coming from the southwest. Further to the east, the platform interior facies is bounded by more common rudist shoals on the platform-margin towards the Shilaif Basin in the United Arab Emirates.

A simple analysis of spectral gamma-ray curves is being used to differentiate between authigenic Uranium and detrital Uranium to locate sections with increased terrigenous clay input. The results impact the sequence interpretation and general depositional model and highlight the transgressive nature of many of the carbonate beds in the mixed ramps.

Three medium-scale sequences are recognized in the studied interval, of which the lower two (Mauddud and Lower Mishrif sensu Qatar stratigraphy) are bounded by important exposure surfaces and the third is truncated by the top Upper Mishrif (sensu Qatar stratigraphy) unconformity below the overlying Laffan shales.

The modified mid Cretaceous depositional and sequence model will be discussed and compared to mid Cretaceous oceanic events following newly obtained carbon-isotope curves and biostratigraphic data.

New exploration play in basal Burgan ’Unnamed Clastics’, Kuwait

Manower Ahmed, A.M., S.S. Thakur, A. Al-Darmi and M. Kumpass

Unnamed clastics at the base of Burgan offer a hitherto unexplored hydrocarbon play in onshore Kuwait. It is a sub-unconformity play and comprise a composite sequence with three systems tracts upwards, namely; transgressive, highstand and followed by another transgressive package. Thickness of package varies between 10–145 ft (generally between 37–60 ft, net pay 40 ft). Sand bodies in the transgressive system tracts are discontinuous laterally and vertically, and that of the highstand system tracts are fairly continuous sheets and shale-out in the distal settings. Thin channel sand bodies of lower transgressive and highstand units entrap oil whenever upper transgressive unit provides an effective top seal. The effectiveness of the top seal depends on the vertical limits of incision by overlying lowstand Burgan clastics. The trapping is controlled by both stratigraphic and structural components. In most of the wells targeting Lower Cretaceous or deeper, casings are generally set within this unit, often proper logs are not run across the section. However, some of the wells have partial suit of logs and well cutting. The resource remained largely unaccounted and unexplored. The play has been identified in Ratqa, Mutriba, Medina and Raudhatain areas. A case study is presented from the Raudhatain area.

The Albian – Cenomanian in the Middle East: New element for carbonate platform stratigraphy

Masse, P.J.L.

In the Middle East, the Albian – Cenomanian comprises a prolific lithostratigraphic nomenclature. From Iraq to Oman, numerous formations were defined: Dokan, Mahilban, Maotsi, Fahad, Mishrif, Rumaila, Tuba, Dair, Mauddud, Natih and other formal or informal names for various lithologies of the carbonate platform during this period. In-house studies on subsurface (Iran, Iraq, Kuwait, Qatar, Oman) and outcrops sections (Oman, Iran) led Total biostratigraphers to use foraminifers charts for datings and paleoenvironments definition. Those charts were compared to published charts, based on western Tethys studies. Western Tethys charts do not include the endemic species of the Gulf, defined by Henson (1948), or figured here. We use species of the genus Charentia, Neoiraqia, Orbitolina, Dictyoconus, Carinoconus, ?Lacazina, Cycledomia, Orbitolinella, Asterohedbergella, to construct a distribution chart. Calibration on ammonite zones is possible in the Late Albian – Early Cenomanian and Mid Cenomanian of Oman.

Further work of biostratigraphers could focus on a characterization of the “top Natih” or “top Mishrif” carbonate platform with foraminiferal assemblages located at formation top or below the first Hippuritids and a research of new sections at platform-basin transition, with planktonic foraminifera (Khatiyah or Zagros intra-shelf basins). This will help in the evaluation of the degree of truncation of the “Mishrif/Natih” formations below the Turonian – Coniacian unconformity.

Albian – Turonian carbonate ramp and platform evolution in the Al Shaheen field, offshore Qatar

Maurer, F., F.S.P. van Buchem, R. Lang, E. Hoch, K. Habib, M. Emang and N. Bounoua

In offshore Qatar two sedimentary systems are distinguished in the Albian to Turonian interval. The stratigraphically older one comprises a low-angle mixed marl/carbonate ramp system, corresponding to the Nahr Umr Formation and the lower part of the Mauddud Formation developed during the Albian. This ramp system is overlain by a carbonate platform to intra-shelf basin system, corresponding to the upper part of the Mauddud and the Khatiyah formations, of Cenomanian – Turonian age.

In the Al Shaheen feld the geometry and facies of the two systems have been constrained by 3-D seismic and the study of cores and thin sections. The Mauddud and Khatiyah formations have significantly different sedimentary characteristics and facies, and have been differently affected by tectonics. The Mauddud Formation is dominated by packstone and grainstone with a rudistic platform barrier, and a topographic relief expressed in seismically resolvable clinoforms. Deposition in the Mauddud is influenced by antecedent topography and syn-sedimentary differential subsidence, leading to more accommodation in the South. The Khatiyah facies is mud-dominated, has marly interbeds, and has a more layer-cake architecture. Synsedimentary tectonic uplift became increasingly important during the Cenomanian – Turonian, eventually leading to significant erosion of the Khatiyah in the northern part of the Al Shaheen field.

Well correlation and seismic point at stratigraphic hiatuses between the Lower and Upper Mauddud and between the Khatiyah and the overlying Laffan Formation. The extent of the stratigraphic gaps and the correlation of the Middle – Upper Cretaceous formations offshore Qatar with age-equivalent sequences elsewhere in the Middle East is under investigation.

A transition from a passive to a tectonically active margin and foreland basin development in the Late Cretaceous of the Fars Area and Offshore (Zagros)

Piryaei, A.R., J. Reijmer, J.Borgomano and F.S.P. van Buchem

The northeastern margin of the Arabian Plate in the Fars Area was influenced by a series of tectono-sedimentary events during the Late Cretaceous. The sedimentary successions show an evolution from a passive to a tectonically active margin. This evolution could be linked to uplift and erosion of the Arabian platform, followed by tilting of the platform and the formation of a foreland basin. This process is well documented in the sedimentary succession studied by significant variations in sedimentary facies and sedimentation patterns, changes in sediment thicknesses, changes in accommodation space and shifting depocentres. The presence of igneous dikes at the top of Cenomanian platform is the other important issue, which is reported in the Zagros by this study for the first time. These intrusives show a zonation with a clear metamorphosed areole.

In total fourteen outcrop sections were measured, and then combined with data from numerous, re-interpreted wells. The sedimentary sequences are dated based on foraminifera, but also on new age dating of nannoplankton, radiolaria and ammonites. Strontium isotope stratigraphy provided additional information for a few intervals. The datasets are organised in the nine regional transects parallel and perpendicular to the Zagros trend. These transects show three tectono-sedimentary phases: Phase I (Late Albian to Turonian), Phase II (Coniacian to Late Campanian) and Phase III (Late Campanian to Maastrichtian).

Phase I is characterised by shallow-water carbonate platforms bordering intra-shelf basins. The platform facies consists of a rudist and benthic foraminifera-bearing facies, while the intra-shelf basins contain the ‘oligosteginid’ and other pelagic facies. Eustatic sea-level variations can be interpreted as the main factor controlling the accommodation space in parts of the area, whereas to the southeast the role of the regional and salt tectonic are more dominant.

Phase II is marked by major changes in the depositional environments and sedimentary facies, as a result of obduction and foreland basin creation. This phase commences with the deposition of sediments in an isolated platform carbonate setting (Santonian) that grade laterally and vertically to pelagic marls of the Santonian and Campanian. In the NW Fars the pelagic marls host large volumes of carbonate and siliciclastic gravity flows and far-traveled radiolarites and ophiolites as well as thrust slices of older carbonates. In the SE Fars, Phase II contains far less siliciclastic gravity flows and radiolarite/ophiolitic materials, which are only present at a few localities. Proposed as controlling factor on the sedimentary system are regional tectonic processes associated with local salt tectonic processes.

Phase III is dominated by pelagic facies that, in the NW Fars, interfingers with autochthonous shallow-water Omphalocyclus and Loftusia-bearing facies (Tarbur Formation). In the SE Fars, at the top of the interval, Phase III occurs associated with slumped conglomeratic/brecciated units with clasts of the underlying carbonate platform.

An outcrop reference model for the Late Albian – Turonian carbonate platforms of the Arabian Plate, Sarvak Formation, Iran

Razin, P., F. Taati and F.S.P. van Buchem

A high-resolution sequence stratigraphic model has been constructed for the mid-Cretaceous Sarvak Formation in the High Zagros region of southwest Iran, which was located close to the eastern margin of the Arabian Plate. The exceptional outcrop quality, displaying the detailed geometrical and facies patterns in the transition zone from carbonate platform to intra-shelf basin, offered the rare opportunity to distinguish between the relative control of carbonate sediment supply (S) and accommodation (A) on the depositional geometries of third-order and fourth-order depositional sequences.

Four third-order sequences are distinguished in the Sarvak Formation, with a duration varying between 1.5 and 3 My, and a thickness varying between 50 to 150 m. These are in turn composed of fourth-order and fifth-order sequences that form the stratigraphic building blocks of this carbonate system. A significant distinction has been made in the third-order sequences between the early transgression (e-TST) when the system is still flat, and corresponds to a ramp setting, and the late transgression (l-TST) when a carbonate platform to intra-shelf basin topography is created. The rate of accommodation creation is identified as the dominant factor controlling the morphology of the depositional profile, and, as such, the driving motor behind the dynamics of this type of carbonate system. The dip angle of the depositional profile has a major influence on: (1) the hydrodynamics of the system, (2) the type of carbonate sediment, and (3) the volume of carbonate sediment produced. A good correlation with the third-order sequences of the Natih Formation in Oman is demonstrated, which supports a dominant control by eustatic sea-level changes and a similar response of the carbonate system to changes in the rate of sea level rise on the southern part of the Arabian Plate.

This outcrop analogue can be considered as a good reference model for Cenomanian – Turonian carbonate platform margins of the Arabian Plate, but also as a textbook example of the response of carbonate systems to sea-level fluctuations (relative influence of accommodation and sediment supply).

North Oman structural characterisation from regional to borehole scale, Natih Formation

Richard, P., K. Shuaili, A. Farmer and G. Engen

The production of some of the largest fields in Oman comes from the Natih Formation. The recovery from these reservoirs is largely by gas oil gravity drainage (GOGD), the success of which is a well connected fracture network. In order to support the development of these fields, fracture characterisation and modelling studies have been carried out. The key pillars of these study workflows are, on the geological side, a thorough understanding of the regional geology at all scales, and on the technology side, a well established integrated fracture characterization and modeling software SVS-Fracture Solutions, developed by the Carbonate Team, in Shell EP-Research.

We will first illustrate how the structural evolution of the field fits into the regional context. We will use seismic, outcrop and sandbox models to demonstrate the structural style and fault geometries at the field scale. We will then zoom on the well scale to illustrate how core and borehole image data, combined with the regional understanding and the dynamic behaviour of the field can be used to elaborate well constrained conceptual fracture models. In these models, the concept of fracture corridors and the definition of mechanical units and their impact on the fracture development are essential to the delivery of the final product, which will be used in the dynamic simulation world.

This paper illustrates the added value of a consistent structural characterisation (deformation history, fault and fractures geometries) approach to support the development strategy of a field. We place particular emphasis on demonstrating the impact of data integration at all scales as well as the use of analogues.

Fracture heterogeneity in the Natih Formation, Jabal Madar, Sultanate of Oman

Sarg, J.F., B.L. Blake, T.R. Birdsall, J. Claringbould and B.Trudgill

The Late Cretaceous carbonates of the Middle East include many prolific reservoirs. However, elements of these reservoirs are still poorly constrained. This study provides an initial evaluation of fracture heterogeneities and connectivity within carbonate deposits of the Natih Formation in a salt cored dome that crops out 140 km south of Muscat, in the Adam Foothills of northern Oman. The Natih E is a significant producing unit throughout the Middle East. Natih shallow- water carbonates were deposited on a Cenomanian carbonate platform, and in the Jabal Madar area are comprised of skeletal peloid wackestones, packstones and grainstones. High-resolution SEM analysis (QEMSCAN®) reveals that matrix porosity is dominantly patchy meso-vugular and mudstone microporosity. Permeability of potential reservoir intervals is thought to be largely dependent on an extensive fracture network, comprised of multiple generations of fractures.

This study examines the complex outcrop fracture pattern of the Natih Formation through field mapping and photorealistic LiDAR (Light Detection and Ranging) image analysis. An evaluation of the mechanical stratigraphy reveals that bed-bounded fractures occur largely in wackestone layers, which are connected by through-going fractures and fracture swarms. The dominant fracture orientations at Jabal Madar are NNW-SSE and NNE-SSW, and the secondary orientations are NW-SE and ENE-SSE.

Three tectonic events are largely responsible for the development of the fracture system at Jabal Madar: obduction of the Semail Ophiolite (Campanian), uplift and exhumation of the Oman Mountains (Miocene), and localized salt doming (Late Cretaceous and Miocene). Fractures at Jabal Madar are primarily radial and concentric in orientation and are interpreted to have developed predominantly through folding associated with active salt diapirism.

Tectonic record of ophiolite obduction in the sedimentary sequence of the Pust-e Kuh Arc, Simply Folded Belt, Zagros Mountains (Lurestan, Iran)

Saura, E., J. Vergés, J.-C, Embry, S. Homke, D. Hunt and I.R. Sharp

The emplacement of ophiolite and radiolarite obduction complexes (e.g. Kermanshah and Neyriz) on top of the north-eastern Arabian margin started during the Cenomanian – Turonian in most of the Zagros Belt. In the Pusht-e Kuh Arc (north-west Zagros), where the latest thrust emplacement has been dated as Early Maastrichtian, the effects of the concomitant lithospheric loading can be observed in the Late Cretaceous – Early Eocene sedimentary record. Lithofacies study of the carbonate-dominated Barremian – Aptian Khami Group and Albian – Campanian Bangestan Group, deposited during the transition from passive to active margin, show their evolution is associated to 13 major eustatic cycles.

The deposition of the Cenomanian – Turonian Upper Sarvak Formation is characterised by a northwards progadational platform-basin transition. Detailed paleofacies maps of this interval reveal a SE-elongated paleohigh with at least four periods characterized by normal faulting followed by karstification related to flexing of the foreland margin along the forebulge domain. This period is also characterized by incipient folding as determined in the Khorramabad region where the Upper Sarvak platforms show karstification and progradation away from the culmination of the anticline. Subsequently, the Campanian – Maastrichtian Gurpi Formation recorded higher subsidence and filled the entire foreland basin.

The Amiran fan deltas in the inner part of the basin also show deflection around the growing Khorramabad anticline during Maastrichtian time. The Maastricthian to Early Eocene infill of the Amiran basin wedges out towards the southwest, with a maximum thickness of 1,500 m in its central part. This shallowing upwards mixed clastic-carbonatic basin infill is diachronous and progrades south-westward at a rate of c. 5.5 mm/year. The sedimentary record analyzed in Lurestan may be compared to the early foreland basin successions in Oman where chronology related to obduction and emplacement of the Semail Ophiolite is well-known.

Fracture and matrix heterogeneity, vuggy porosity and reservoir characterisation in the Upper Sarvak Formation, Chenareh River Gorge, Iran

Scarrott, T.L., G. Casini, J.-C. Embry, S. Homke, D. Hunt, I. Sharp, P. Gillespie, F. Livbjerg, A. Janapanhah, M. Jalali and M. Jamaledini

The Chenareh Anticline, Lurestan, Iran, represents a particularly well-exposed “type” section through naturally fractured Cenomanian – Turonian neritic carbonates of the Sarvak Formation. A comprehensive reservoir study of fracture and matrix heterogeneity is made possible through extensive fieldwork observations, including intensive and systematic sampling along a measured section in the Chenareh Gorge, interpretation of QuickBird satellite imagery (0.7 m resolution) and high-resolution 3-D photorealistic models based on LiDAR technology.

Matrix heterogeneity, including variations in pore type and porosity-permeability, has been captured using qualitative and quantitative techniques performed on thin sections collected in the measured section of the gorge. Each thin section is assigned a porosity value through the use of image analysis and permeability is predicted using poroperm cross plots based on empirical data from an extensive global dataset. Fracture heterogeneity has been studied systematically along the section applying a scanline sampling technique. Extensive, quantitative analysis of fracture and fault network geometry has also been performed on high-resolution 3-D photo-realistic models.

Porosity values are highly variable, particularly on small scales due to the effects of primary sedimentary facies, diagenesis and cementation. Matrix porosity and permeability range from 1.0–18.3% and 0.001–25 mD, respectively, with the best reservoir potential in the cherty slope facies and high-energy rudist shoals along the platform margin. Consistently high porosities exist in dolomitised intervals and units associated with secondary solution vug porosity.

Vuggy geobodies are recorded at a range of scales using outcrop digitiser software to distinguish their spatial distribution at outcrop scale. At higher resolutions shape parameters, aspect ratio, pore space patch density and porosity and can be defined for individual vugs and their relative sample area as a method of analysing the representative elementary area (REA). Realisations are thereafter based on 3-D transformation of 2-D data, supported by a statistical analysis of vug type distribution.

Fractures and faults reveal complex geometry and timing relationship. All units exhibit either completely or partially cemented stratabound fractures. Partly cemented to open non-stratabound fractures reactivate pre-existing stratabound fractures. Normal faults, thrusts and strike-slip faults also affect the section. Sedimentary facies and diagenesis are a key control on fault/fracture distribution and network geometry.

Sequence stratigraphic, facies and reservoir framework for the Bangestan Group, Lurestan, Zagros Mountains, Iran

Sharp, I.R, J.-C. Embry, D.W. Hunt, D. Morsalnejhad, N. Bang, M. Jalali, F. Keyvani, F. Livbjerg, C. Dons, A. Lønøy, T. Samuelsberg, S. Packer and E. Caus

The Albian – Campanian aged Bangestan Group (Kazhdumi, Mauddud, Sarvak, Surgah and Ilam formations) contains some of the most prolific reservoir-source-seal units of the Zagros hydrocarbon province of Iran. These units crop out extensively in the Zagros Mountains, affording a unique opportunity for facies, sequence stratigraphic and reservoir characterisation as an aid to understanding datasets in the nearby subsurface. In this paper we present the results of a multi-year research study in Lurestan Province, Iran, which has resulted in a thorough re-examination of the existing lithostratigraphic and chronostratigraphic scheme, and the establishment of a new sequence stratigraphic, facies and reservoir-source-seal scheme. This scheme has been correlated to age equivalent reservoir units of the Arabian Plate (Iraq, Qatar, United Arab Emirates and Oman).

The base of the Bangestan Group is marked by the Albian-aged Kazhdumi Formation sitting unconformably on the Early Aptian-aged Dariyan Formation. The Late Aptian is absent, and where well exposed the unconformity is seen to be angular in nature, indicating a period of tilting/normal faulting prior to deposition of the Kazhdumi Formation. The Kazhdumi Formation can be divided into three depositional sequences (Kz1, Kz2, Kz3), which represent a relatively complex mixed carbonate-clastic, intra-shelf basin succession, which passes vertically and laterally (towards the northeast) into a low-angle Orbitolina-dominated muddy carbonate ramp/shoal (Mauddud Member – Bala Rud Shoal). In the revised scheme the Mauddud Member is included in depositional sequence Kz3 (Kazhdumi Formation), and is dated as latest Albian. The Mauddud Member is capped by a regionally extensive karst of latest Albian – earliest Cenomanian age, and is unconformably overlain by the Sarvak Formation.

The Sarvak Formation comprises both low-angle ramp and steeper dipping (5–10°) carbonate shelf/platform systems. In Lurestan, the Sarvak Formation can be divided into six depositional sequences. Sequences Sa1 and Sa2 are entirely of Early Cenomanian age, and collectively define the Lower Sarvak Formation (300 m+ thick). The Upper Sarvak Formation is 270 m thick and divided into four depositional sequences (Sa3, Sa4, Sa5 and Sa6). Sa3 and Sa4 are Early – Mid Cenomanian and Late Cenomanian in age respectively. Sa3 equates to the Rumalia Formation, and Sa4 to the strongly progradational Mishrif Formation.

Three regionally extensive karst surfaces are developed in the latest Cenomanian – Turonian interval, and are interpreted to be related to flexure of the Arabian Plate margin due to the initiation of intra-oceanic deformation. The first of these karsts (Sb-Sa4) is proceeded by a forced regressive basin-ward stepping wedge, and is overlain by Sa5. Sa5 is almost always strongly truncated by SB-Sa5, which is represented by a fracture controlled karst. The karst corresponds to the Cenomanian/Turonian boundary, and the Lower Turonian is typically missing. Depositional sequence Sa6 is of late Middle to Late Turonian age, and is overlain by the Coniacian-aged Surgah Formation.

In proximal locations this contact is an exposure surface (SB-Sa6), whilst in basinal locations the contact is conformable. Neritic carbonates of the Ilam Formation conformably overlie the Surgah Formation and pass retrogradationally into hemipelagic carbonates of the Gurpi Formation. The Surgah and Ilam formations represent clastic and muddy carbonate ramp depositional systems respectively.

Reservoir development within the Sarvak Formation is optimal within fractured HST skeletal (rudist) foram-grain margin facies, whilst TST non-skeletal shoals tend to be early marine cemented and of reduced reservoir quality. Reservoir heterogeneity is marked however, related both to primary facies variation and to late-stage fracture-controlled diagenesis.

Mid-Cretaceous sequences of the Arabian Plate and their global expression

Simmons, M.D., R.B. Davies, A. Godet and P.R. Sharland

The Albian – early Turonian succession of the Arabian Plate has long been regarded as a second-order sedimentary cycle, containing higher-frequency (third-order) cycles. These third-order cycles are the products of relative sea-level change and, given the general tectonic quiescence of the Arabian Plate during the mid-Cretaceous, are therefore likely to be eustatic in origin.

Six maximum flooding surfaces (K90 – K140) of Albian – Early Turonian age were recognised across the Arabian Plate by Sharland et al. (2001, 2004) (see also Davies et al., 2002 and Simmons et al., 2007). These MFS together with intervening sequence boundaries/correlative conformities and maximum regression surfaces can be reasonably well age calibrated by biostratigraphy (especially ammonites, planktonic foraminifera and nannofossils) and isotope data. This allows us to correlate these surfaces outside of the Arabian Plate and to test their eustatic origin by demonstrating their occurrence on a global scale.

We have found evidence for the K90 – K140 sequences in locations ranging from Europe, Arctic North America, through the western interior of the USA to offshore Brazil, the west coast of Africa, India and Australia. At all these location the expression of these surfaces is biostratigraphically calibrated and hence we can be confident in a eustatic driver for the K90 – K140 sequences.

It is interesting to speculate on the causes of eustacy in the mid-Cretaceous. Although the mid-Cretaceous is a time of the emplacement of large igneous provinces within the oceans, these are unlikely to have created sea-level change at the required pace and amplitude. Instead it seems more likely that a growing body of direct and proxy evidence points to a coincidence of climatic fluctuation and eustacy, suggesting that melting and creation of ephemeral polar ice may be a causal mechanism, even in what is commonly regarded as a “greenhouse” time.

A robust third-order sequence stratigraphic model for the mid-Cretaceous of the Arabian Plate is a valuable tool for regional correlation and mapping, and the recognition of exploration analogues, as well as placing existing reservoirs and source rocks in regional context.

Regional sequence stratigraphic correlation of the Burgan and Mauddud formations (Lower Cretaceous), Kuwait

Strohmenger, C., G. Al-Sahlan, P.E. Patterson, J.C Mitchell, R. Wellner, H.R. Feldman, T.M. Demko, R.W. Wellner, P.J. Lehmann, G. Glen McCrimmon, R.W. Broomhall and N. Al-Ajmi

A regional sequence stratigraphic study has been carried out on the Lower Cretaceous Burgan and Mauddud formations, integrating core and well-log data from giant oil fields of Kuwait.

The Burgan and Mauddud formations form two second-order composite sequences. The older composite sequence consists of the lowstand, transgressive, and highstand sequence sets of the Burgan Formation. It is subdivided into 15 high-frequency, third-order depositional sequences, which are characterized by tide-influenced, marginal-marine deposits in northeast Kuwait that grade into more fluvial-dominated, continental deposits to the southwest. The younger composite sequence consists of the lowstand sequence set of the uppermost Burgan Formation and the transgressive and highstand sequence sets of the overlying Mauddud Formation. These composite sequence deposits are siliciclastic-prone in south and southwestern Kuwait and are carbonate-prone in north and northeastern Kuwait. A major, second-order marine flooding surface at the top of the Burgan Formation in Kuwait is a regional chronostratigraphic boundary that can be correlated throughout the country.

The Mauddud transgressive and highstand sequence sets are subdivided into seven high-frequency third-order depositional sequences. The lower Mauddud transgressive sequence set displays a lateral change in lithology from limestone in northern Kuwait to siliciclastic deposits in southern Kuwait. The upper Mauddud highstand sequence set is carbonate-prone and thins southward due to depositional thinning.

The traditional lithostratigraphic Burgan-Mauddud contact is time-transgressive. Significant post-depositional erosion occurs at the contact between the Mauddud and the overlying Wara Formation.

Diagenetic patterns and rock properties of the Natih Formation in a carbonate oil field, North Oman

Taberner, C., V. Vahrenkamp, C. Hollis, M. Esteban and PDO/Shell Study Team

Rock properties of the Natih Formation in a carbonate oil field from North Oman were strongly modified as a result of diagenesis. The paragenetic sequence is well defined, and typical for a carbonate field from this geographic location and time period. It exhibits an interplay between early diagenetic processes, faulting and the tectonically controlled circulation of deep burial fluids. Conclusions are supported by detailed petrographic and geochemical data including isotopes, fluid inclusions and elemental compositions.

The earliest paragenetic phase was dominated by localized meteoric processes at paleoexposure surfaces, including both cementation at some depositional cycle tops and third-order sequence boundaries. Both meteoric cementation and leaching were muted by the apparent lack of chemical reactivity of the predominantly (low magnesium) calcitic chemistry that is typical for this time period. This lack of reactivity helped preserve a range of primary depositional pore types into the burial realm.

The lengthy paragenetic phase during early to intermediate burial was dominated by competing processes of burial leaching and burial compaction/cementation. These processes accentuated depositional reservoir heterogeneity. In particular, long-lived, selective burial leaching caused significant enhancement of pore throats and thus increased permeability in more hydrologically open intervals. Compaction and burial cementation also occurred repeatedly over a long period of time, producing intervals with lower porosity and permeability – particularly those associated with more closed hydrological intervals and/or increased admixtures of clastics.

Brines derived from the Ediacaran – Early cambrian Ara Group evaporites reached the reservoir via deep-rooted and reactivated fault systems repeatedly from the onset of early burial diagenesis (middle Late Cretaceous – First Alpine Event) causing both cementation as well as leaching.

The final phase of calcite cementation is coeval with a first phase of hydrocarbon migration, with precipitation at temperatures of at least 70–80°C. The final paragenetic stage was dominated by at least one episode of leaching, creating matrix porosities exceeding 35%. Porosity creations was focused upon but not limited to solution-enhancement of micro-porosity in micrite. Based on petrographic evidence, leaching clearly post-dated pressure-dissolution and is interpreted to be related to extensive mixing of undersaturated mildly acidic hypersaline brines with evolved connate waters. Organically-derived CO2 and organic acids released during source rock maturation and pre-charge fluid circulation maintained the acidic conditions over a time period that allowed significant matrix dissolution. Finally, hydrocarbons moved into the structure and retarded further diagenetic processes in the oil leg.

Our fluid evolution model proposes the circulation of burial fluids upwards along faults and upon hitting a sealing trap soaking the adjacent matrix rock above a structural spill point. It explains a general trend of increased reservoir heterogeneity associated with preferential leaching up-dip in the structure. Key to this preferential leaching is the bounding fault to the field, along which carbonate-undersaturated fluids were introduced to the structure. A progressive increase in carbonate saturation, through rock:water interaction, away from this fault would have resulted in decreased volumes of leaching down-dip.

3-D seismic characterization of a heterogeneous Cenomanian carbonate reservoir containing “channels”

Takahashi, I. and R. Matsui

This work discusses an integrated characterization of a shallow-marine carbonate oil reservoir of Cenomanian age, taking advantage of high-quality and high-resolution 3-D seismic data. The reservoir resides in the uppermost part of the thick limestone sequence, having the net thicknesses of 200–350 m and the average porosity of 5–15% with large vertical/lateral variations.

3-D seismic attribute and AVO inversion analysis were applied and revealed field-wide reservoir heterogeneity. The rock physics analysis using elastic logs and ultrasonic core velocity data enabled correlating seismic attributes, i.e. acoustic impedance (AI) and Vp/Vs, with petrophysical properties, i.e. porosity and shaliness. The seismic AI predicted generally more porous properties to the northwest, consistently with paleo-bathymetry estimated from structural restoration.

Most noticeably, 3-D slices of seismic attributes visualized gigantic channel-shaped signatures that traverse across the field, with their widths and thicknesses reaching upto 3 km and over 100 m, respectively. In the 3-D data, the “channels” appeared to be sharply bounded and erosional in nature with relatively straight patterns. Seismic inversion predicted the channel-fill sediments are commonly less porous and more argillaceous than the surroundings. Moreover, porous properties were often observed below the channel bottoms. A well intersected a small-sized channel having the above seismic characteristics and its log interpretation suggested the channel-fill sediments to be low-porosity karst-related breccia and the underlying interval is productive in the well test.

Integration of these seismic and well observations led to a unified interpretation that the channels are formed by erosional incision during Turonian regional exposure and that the channel-fills are likely to be muddy/tight sediments often accompanied by diagenetic enhancement below them. Due to the tight property, the channels were considered as possible cause of reservoir compartmentalization identified from well data. Throughout this work, 3-D seismic was utilized as the key driver of the integrated evaluation and was confirmed to successfully reveal critical carbonate reservoir characteristics.

Application of a fourth-order stratigraphic framework for the Mishrif Member: A basis for mapping lithofacies variability and petroleum systems in the Rub’ Al-Khali Basin of Saudi Arabia

Taylor, D.P., G.W. Hughes and D.L. Ternes

The Cenomanian – Turonian-age Mishrif Member of the Wasia Formation in Saudi Arabia represents a time of shallow-water carbonate progradation directed northeasterly into an intra-shelf basin. Up to 150 km of lateral progradation is observed on the more windward western side of the intra-shelf basin, terminating with a rimmed carbonate shoal platform with up to 900 ft of relief. New biostratigraphic interpretations have provided a basis for identifying third- and fourth-order cycles within the Mishrif Member, and can be tied to log and seismic data, allowing construction of chronostratigraphic lithofacies maps. Higher-frequency depositional cyclicity is also observed, and it is possible to interpret individual depositional assemblages, comprising bioclastic shoals and rudist-bank facies, in areas with 3-D seismic coverage and well control.

Based on new micropaleontological data, the Mishrif Member consists of up to four fourth-order depositional sequences. Each sequence commences with a planktonic foraminiferal-dominated biofacies that represents deep-marine conditions related to the transgressive system tract (TST). Highstand system tract (HST) associated foraminiferal and rudist biofacies are represented by shallow-marine carbonates typically deposited in shoal and localised rudist-bank settings. These deepening and shallowing cycles have been correlated across the eastern Rub’ Al Khali basin, and designated Mishrif TST1-HST1, TST2-HST2, TST3-HST3 and TST4-HST4 in ascending order. Mishrif source rocks correspond to the Mishrif TST1 sequence (Natih B equivalent of Oman). The overlying Mishrif HST1 reservoir sequence (Natih A equivalent) is sealed by the next transgressive cycle, Mishrif TST2. This reservoir-seal cyclicity continues in some places up to TST4-HST4, which is ultimately sealed by regionally extensive shales of the Aruma Formation.

There are two proven petroleum systems within the Wasia Formation, the Safaniya-Mauddud system and the self-sourcing Mishrif petroleum system. Safaniya lime-mudstone source rocks were deposited in a regionally extensive intra-shelf basin where anoxic conditions existed. Kerogens are oil-prone Type II-Sulphur requiring a low activation energy equivalent to a vitrinite reflectance of 0.60%. Based on thermal maturation modeling, the Safaniya entered the oil window 65 million years ago and is currently in the oil generation window. Oil sourced from the Safaniya is typically 30° API. The Safaniya conformably underlies the Mauddud Reservoir. Mishrif source rocks were deposited in a similar setting to the Safaniya but are marlier and contain less net source thickness. Mishrif source rocks are presently within the oil generation window, and reservoired oil is typically 38° API.

Seismic sequence stratigraphy of the Wasia Formation, Rub’ Al-Khali Basin, Saudi Arabia

Ternes, D. and D. Taylor

New 3-D seismic data acquired in the Rub’Al-Khali Basin of Saudi Arabia during 2008 and 2009, provide the opportunity to apply a sequence stratigraphic framework to constrain the seismic interpretation. Core and cuttings-based micropaleontological and sedimentological study of selected wells enabled chronostratigraphic calibration of the seismic interpretation. Further analysis identified key sequence boundaries and maximum flooding surfaces within the Wasia Formation. The subsequent log interpretation guided the correlation of regional sequence boundaries and marker horizons within the seismic data volume. The sequence framework described in this study pertains to the area of the 3D seismic survey, and is not intended to represent the entire Rub’ Al-Khali Basin.

Third-order scale geometries are clearly imaged on the seismic data, with regionally correlated sequence boundaries, while fourth-order sequences can be identified locally but are difficult to spatially correlate due to seismic resolution limitations. The internal seismic reflection geometries have been characterized and classified into several lithofacies associated with a rimmed shelf margin that prograded into an intrashelf basin to the northeast. Seismic character was calibrated to lithofacies derived from sparse wellbore data. Four primary seismic geometries have been categorized: (1) parallel platform geometries correlated to lagoonal and back-bank facies, (2) prograding units associated with shoal and slope facies, (3) mounded geometries associated with shoals and rudist-banks, and (4) parallel basinal reflection geometries to low angle clinoforms associated with deeper water intrashelf basins. The stacking pattern and scale of the clinoforms are attributes that help determine paleogeography and water depth.

The Wasia Formation is subdivided into four third-order seismic sequences spanning the Albian to Early Turonian. The first sequence deposited during the Early Albian includes the Khafji Member or Nahr Umr Shale. This sequence is comprised of shales that were deposited with uniform thickness during a marine transgression. The internal reflection character of the Khafji shale consists of parallel to sub-parallel dim reflectors. The top of this sequence is a depositional hiatus defined by a prominent seismic horizon that can be regionally correlated.

The second seismic sequence is defined by a package of clinoforms with downlap at the base and toplap at the top. It was initiated by a major flooding event and the deposition of the Safaniya Member, followed by a shallowing upward cycle and deposition of the Mauddud Member. The Safaniya shale includes organic-rich source rocks and is characterized by clearly imaged low angle clinoforms. The overlying Mauddud Member includes wackestones, to grainstones and rudist banks. High angle, small scale clinoforms are observed near the shelf margin in this highstand deposit that culminated with a marine regression. The isochron map of this sequence depicts Mauddud carbonate buildups along the rimmed shelf margin. Isochron thins are interpreted to represent lagoonal deposits located inboard of the shelf margin, and condensed basinal deposits outboard of the shelf margin.

The third seismic sequence includes the Ahmadi Member, and is relatively thin. The carbonate factory appears to be suppressed due to an influx of siliciclastics into the depositional system from the southwest. The reflection character is variable along depositional dip, with sub-parallel reflectors inboard of the shelf margin, grading to clinoforms and localized mounding along the rimmed margin.

The fourth and final seismic sequence began with a marine transgression and deposition of the Rumaila Member, characterized by sub-parallel reflections and low angle clinoforms. The overlying Mishrif Member has been subdivided into four fourth-order TST-HST cycles based on micropaleontological data. The seismic character is sub-parallel with subtle clinoform and mounding geometry near the shelf margin. The top of the Mishrif was an exposure surface with evidence of erosion from sub-cropped reflections. This exposure surface, also known as the Pre-Aruma unconformity, represents the top of the Wasia Formation.

Utilizing the seismic data to understand the sequence stratigraphic framework has been vital to evaluating petroleum systems within the Wasia Formation. It enabled the extrapolation of source, reservoir, and seal facies identified from well bores, into areas with additional exploration opportunities.

Sequence stratigraphy of the Albian – Turonian interval of southwest Iran based on outcrop and subsurface data

van Buchem, F.S.P., D. Baghbani, L. Bulot, M. Caron, A. Hosseini and B. Vincent

This study is based on a large regional dataset, consisting of 100 wells and newly studied outcrop sections, which cover offshore and onshore southwest Iran (coastal Fars, Khuzestan and Lurestan). An age revision of some of the classic lithographic units (Kazhdumi, Mauddud and Sarvak formations) is proposed based on the analysis of benthic and planktonic foraminifera and ammonites. The Albian succession shows an overall transgressive trend, gradually flooding the previously exposed coastal Fars area, until in the Late Albian marine sedimentation was re-established in the entire study area, with shallow-water muddy ramp systems (Orbitolina Limestones, Mauddud Formation) surrounding a deeper, organic-matter rich intrashelf basin (Kazhdumi Basin). The Cenomanian – Turonian succession was deposited in a depositional environment characterized by the presence of several intra-shelf basins, which shifted location during the third-order depositional sequences. Fine- to very coarse-grained platform rims were formed, which consisted dominantly of rudist debris. At the regional scale, very significant variations in accommodation are observed at that time. In coastal Fars, Cenomanian and Turonian deposits are locally completely absent, whereas further to the northwest, in Khuzestan shallow-water deposits of this age may reach a thickness of 1,200 m. Tectonic control at different scales is invoked to explain this period of instability, including local salt tectonics, regional subsidence along old lineaments (e.g. Kazerun Fault), and areas of persistent slow subsidence (coastal Fars).

The improved time control and large scale of this study allowed to analyse regional stratigraphic trends, which showed an overall important tectonic control, but also a clearly recognizable eustatic sea-level imprint on the sedimentation pattern.

Albian to Turonian chemostratigraphy of the eastern Arabian Plate

Vahrenkamp, V.C.

A chemostratigraphic curve has been established for the middle Cretaceous Albian to Turonian argillaceous/carbonate sequences of the eastern Arabian Plate. The curve consists of more than 550 d13C values collated from outcrop and subsurface core data. It is anchored by biostratigraphic control points and compares favorably with the character of well dated and published curves from the northern Neo-Tethys realm. Data range between 0‰ and 6‰, tracking the evolution over time of d13C in seawater established elsewhere in pelagic carbonate sequences. Major trends in the isotope profile can readily be correlated with major oceanic anoxic events (OAE1b, OAE2) as well as some minor events (e.g. Albian/Cenomanian boundary and mid-Cenomanian event). The correlation suggests an Albian age for the base Natih Formation. with the Albian/Cenomanian boundary coinciding with the top Natih F unit. The Natih C/D sequence is associated with the mid-Cenomanian carbon-isotope event. While the Natih Formation in the Oman Mountains reaches into the Turonian based on biostratigraphy this level is not reached in the subsurface. In a major oil field in Oman the top Natih carbon isotope signature clearly approaches but does not reach the Cenomanian/Turonian boundary event. This is due most likely to erosion on a fore deep bulge prior to deposition of the overlying Fiqa shales. The stratigraphic tie-points provide an excellent insight into rate of deposition and pace of platform development.

A major source rock interval (the Natih B sequence of Central Oman) which is associated with the infill of an intra-shelf basin has introduced a significant departure of the carbon isotope curve from the global marine trend. This carbon isotope fluctuation may have been caused by a diagenetic overprint or more likely during deposition as a consequence of significant isolation of the intra-shelf basin from normal marine seawater circulation.

Regional unconformities in the Cenomanian and Turonian limestone of southwest Iran (Sarvak Formation): Sub-aerial exposures, diagenetic patterns and impact on reservoir properties

Vincent, B., R.Swennen, M. Jalali, D. Baghbani and F.S.P. van Buchem

The Cenomanian – Turonian shallow-water deposits of the Sarvak Formation constitute one of the main reservoirs in southwest Iran. Heterogeneities in these reservoirs are controlled by lateral facies changes, caused by intra-shelf basin to platform transitions, and by diagenetic overprint related to three (regional) exposure surfaces that subdivide the succession in third-order depositional sequences. This diagenetic imprint has been studied in superb exposures of this formation in the outcrops of coastal Fars, which are considered as good outcrop analogues for the nearby oilfields.

The diagenetic study focuses on: (1) the effect of exposure occurring in the Middle Cenomanian on the reservoir properties of large-scale, bioclastic grainstone sandwaves, which are impregnated with paleo-oil. Observations show that meteoric water influx lead to an early stabilization of the mineralogy, which prevented later diagenetic evolution during burial and thus a partial preservation of primary pore-space. (2) Two exposure surfaces, one at the top of the Cenomanian, and one in the Turonian, with well-developed palaeosoils, including pisolith formation, and fluvial conglomerate and sandstone deposits, reworking palaeosoil elements. The Turonian surface displays evidence for erosion, and possibly (subtle) tectonic control. Observations show that despite the preservation of these palaeosoils the carbon-isotope signal of underlying altered carbonates is modified only a few meters below the surface. This is of primary importance for who aims to use the stable carbon isotope of carbonates as a correlation tool for sequence stratigraphy.