INTRODUCTION

The GOCE satellite gravity mission was launched in 2009 to measure the gravity gradient with high accuracy and spatial resolution. GOCE gravity data may improve the understanding and modeling of the Earth’s interior and its dynamic processes, contributing to new insights into the geodynamics associated with the lithosphere, mantle composition and rheology, uplift and subduction processes. However, to achieve this challenging target, GOCE should be used in combination with additional data sources, such as in-situ gravimetric, magnetic, and seismic data sets.

We present a study in which it is proposed to invert satellite gravity and gravity gradients, and terrestrial gravity in the well explored and understood Northeast Atlantic Margin. The inversion outcome will be compared with results obtained by means of models based upon other sources like seismic data, magnetic field information or other in-situ data. This will provide improved information about the lithosphere and upper mantle.

The GOCE gravity gradients in the gradiometer reference frame will be used in forward and/or inverse modeling in the Northeast Atlantic Margin and the Arabian Peninsula. Not only will the original gravity gradients be used, but they will also be combined with other gravity data, e.g. from GRACE or terrestrial gravimetry (Figures 1 and 2). On the one hand, grids of gravity gradients will be computed, for example at mean GOCE altitude, on the other hand regional, high-resolution gravity fields will be computed using a multi-scale representation.

One of the outcomes of the Atlantic Margin study will be a sensitivity matrix that will be used as input to study the Arabian Peninsula, in general, and the Rub’ al-Khali Desert in Saudi Arabia, in particular. In terms of modeling and data availability this is a frontier area. Here GOCE gravity gradient data, in combination with other data, will be used to better identify the structure and composition of the crust and the lithospheres in the region. The improved model of the crust and the lithosphere will allow us to calculate the evolution of the basal heat flow in the region. Based on the improved heat flow model, the maturity of the main source rocks in the study area will be estimated which will improve our understanding of the petroleum systems in the region.

Application for the Arabian Peninsula

Gravity gradient data are generally sensitive to the density structure of the upper crust. It provides a better resolution of the edges of geological features (such as faults, lineaments and large intrusions). Gradient data from GOCE have the potential to identify the extent of different structures with varying densities in the lower crust in the Arabian Plate. Thus, it can help to identify density zonation in the basement and enhance structural boundaries within the crust on a regional (Terranes) scale (Figure 3). Using the gravity anomaly maps obtained from GOCE data, in combination with land-measured gravity data, gravimetrical backstripping of the basin will be performed in order to identify basement inhomogeneity and Moho topography. The outcome of this phase is a model of the crust and the lithosphere of the study area. This includes the thickness and composition of the crust and the lithosphere.

Heat Flow Modeling and Source Rock Maturity

The structural model of the crust and the lithosphere, obtained from the gravity data, is then used for modeling the basal heat flow within the basin. We use a grid-based stochastic tool (PetroProb) developed by TNO to model basal heat flow. It is based on the inversion of basin subsidence data to calculate the tectonic subsidence in the area and the tectonic heat flow (Figure 4). It takes into account the sedimentation, erosion and paleo-water depth history in the basin. It also incorporates the effect of sedimentation infill and heat production in the crust. The uncertainties in the crustal and lithospheric structures are included in the heat flow modeling tool. The tool allows the calibration of the model to measured temperatures and maturity in order to reduce the uncertainties in the input parameters.

The thickness, structure and properties of the crust and the lithosphere are essential inputs to PetroProb to model the tectonic heat flow (Figure 5). The model includes the radiogenic heat of the basement due to the effect of radiogenic elements in the crystalline basement. Different zonations in the basement can be assigned varying radiogenic element concentrations and thus varying heat flow contributions. The maturity of the source rocks in the basin are estimated based on the modeled heat flow and regional maturity maps are produced of the Paleozoic and Mesozoic plays in the Arabian Peninsula.

Data and Input Model

In addition to Bouguer anomaly grids and GOCE gravity gradient data, information on the lithostratigraphy of the study area is needed to build an initial geological model. Using the basement depth map, well information and regional cross-sections, a geological model can be constructed (Figure 6). The resolution of the input model will be adjusted to the resolution of the GOCE gravity data. Information on the evolution of the basin is required in order to construct our tectonic model which will make use of the gravity anomaly maps. Boundary conditions necessary for our models (such as paleo-water depth and surface water interface temperatures) are available from published literature. Validation of the models is done using temperature and maturity measurements in the region.

Outcome of the Study

The gravity gradient data will be used to build a new model of the crust and the lithosphere in the Arabian Peninsula (and the Rub’ al-Khali Desert). Based on that, a regional tectonic heat flow model will be calculated for various geological ages. Using the heat flow maps and the geological model of the area, new maturity maps will be generated for the main Paleozoic and Mesozoic source rock units in the region.

User Group

We intend to establish a User Group to ensure involvement of the user community in the study. On the one hand, the User Group could contribute to the definition of the required products and the outcome. For example, where do we want to focus the study, what interval and petroleum system to be looked into, etc. On the other hand, the User Group could be involved in the evaluation of the products that will be generated to assess their use for hydrocarbon exploration as well as Earth interior modeling. The User Group can involve, in addition to TNO and its partner in this project, universities, research organizations as well as oil companies in the region who are the potential users of the publically available data.

References

11716 The Khuff outcrop project in the Oman Mountains: Framework and research approach

Thomas Aigner (University of Tübingen, <t.aigner@uni-tuebingen.de>) and Michael Pöppelreiter (Shell, <m.poppelreiter@shell.com>)

Summary

This overview presentation introduces an extensive outcrop analog study that has been ongoing for more than three years on the Khuff-equivalent strata in the central Oman Mountains, Sultanate of Oman. This project is focussed on the sequence stratigraphy and the stratigraphic architecture of grainstone bodies as potential reservoir analogs for the subsurface. An integrated multi-dimensional, multi-disciplinary and multi-scale approach led to a series of nested reservoir models that allow to better understand controlling factors and to distill some general trends. The results may be directly used (1) to reduce uncertainties on near well- and field-scale correlation, (2) in reservoir modelling of analogous fields, and (3) as predictive rules in exploration. This overview is followed by detailed presentations on the various Khuff sequences.

Introduction

The Middle Permian to Lower Triassic Khuff Formation is one of the most prolific hydrocarbon producers all over the Middle East (e.g. Al-Jallal, 1995; Sharland et al., 2001). On a plate-wide scale, the Khuff Formation shows a largely “layer-cake”-type stratigraphic architecture. This is most likely a function of the almost flat epeiric carbonate ramp geometry (e.g. Sharland et al., 2001, 2004; Osterloff et al., 2004). On a field- or near well-scale (< 10 km), however, the apparently simple reservoir architecture strongly contrasts with large permeability ranges (i.e. 10s to 1,000s of mD) commonly observed in producing Khuff fields in the Gulf region (e.g. Esrafili-Dizaji and Rahimpour-Bonab, 2009). Current models suggest that observed permeability heterogeneities are caused by a combination of primary and secondary factors. Primary factors include composition, texture and geometry of depositional geobodies as well as their sequence-stratigraphic position. Secondary factors include diagenetic and tectonic overprints (e.g. Konert et al., 2001; Insalaco et al., 2006). Textures and geometries of depositional geobodies cannot be derived from current subsurface imaging techniques, and wellbores and cores only offer limited spatial information.

Therefore a research project was initiated between the University of Tübingen, Qatar Shell and Petroleum Development Oman to study Khuff grainstone bodies in outcrop analogs. The more than 3-year study involving several PhD and MSc students was carried out in the Al Jabal al-Akhdar area (Oman Mountains) in the Sultanate of Oman. Its aim was to unravel the Khuff stratigraphic architecture on hierarchical scales to understand geometries and textural variation of grainstones as potential reservoir bodies. Hierarchical static 3-D geological models were generated that visualise the sedimentary architecture of primary reservoir bodies using industry standard 3-D geomodelling software.

This presentation is an overview on the framework and research approach of the entire study. It is followed by a series of presentations that detail the specific project results through the various Khuff Sequences.

Research Approach

This project made an effort in integration of dimensions, disciplines and scales:

  • (1) Multi-dimensional approach. Each of the outcrop sections was worked from 1-D to 3-D analysis. The 1-D analysis included detailed sedimentological, microfacies and outcrop gamma-ray logging. In the 2-D analysis, outcrop sections were correlated by means of facies cross-sections. 3-D analysis involved the generation of three-dimensional geocellular models.

  • (2) Multi-disciplinary approach. Sedimentological data were integrated with biostratigraphy, chemostratigraphy, cyclo- and sequence stratigraphy, as well as petrophysical data.

  • (3) Multi-scale approach. A series of nested 3-D facies models (Petrel) were generated ranging from near well- (2x1 km) to field- (8x8 km) to subregional (60x40 km)-scale. Thus the distribution and geometries of grainstone reservoir bodies was quantified on a hierarchy of scales, from hundreds of meters to kilometers to tens of kilometers.

Main Results

  • (1) Primary reservoir heterogeneities with a particular emphasis on grainstone bodies could be documented in an approximately 750 m succession of Khuff-equivalent outcrops in the Al Jabal al-Akhdar area of the Oman Mountains.

  • (2) A sequence-stratigraphic framework could be established, subdividing the Khuff into 6 major depositional sequences. Each can be systematically subdivided into cycle sets and cycles. This hierarchical cyclicity provides surfaces for correlation and forms the ideal basis for layering schemes in geocellular models of various scales.

  • (3) Nested geocellular reservoir models allowed to distill some general rules and trends for the distribution and geometry of grainstone reservoir bodies.

  • (4) Key for this project was integration: integration of dimensions (from 1-D to 2-D to 3-D), disciplines (sedimentology, biostratigraphy, chemostratigraphy, sequence stratigraphy, petrophysics), and scales (from near-well, to field- to subregional scale).

  • (5) Close collaboration between oil companies, software vendors and the University ensured mutual knowledge transfer and training (via field trips, short courses, student internships, etc).

  • (6) Results of the study may be used to (a) improve near well- and field-scale correlation strategies in subsurface models of the Khuff reservoir, (b) to optimise placement of development wells in analogue Khuff fields, and (c) to support exploration and appraisal effort in the Khuff Formation in adjacent field areas.

Acknowledgements

This study is part of a research project of the University of Tübingen, sponsored by Shell and Petroleum Development Oman. We would like to thank the Shell and PDO focal points J. Amthor, A. Brandenburg, J.-M. Dawans, G. Forbes and J. Schreurs for financial support and Shell, PDO and the Omani Ministry of Oil and Gas for the permission to publish this abstract. We are grateful to S. Al Kindy (PDO), E. Adams (Shell) and S. Richoz (University of Graz) for help, assistance and useful ideas. The authors would also like to thank Holger Forke (Humboldt Museum Berlin) for biostratigraphic analysis. Shuram Oil and Gas (Muscat) is acknowledged for fieldwork logistics. Schlumberger is thanked for access to Petrel; ALT (Luxembourg) is thanked for access to WellCAD.

References

11738 Fracture mineralization and fluid flow evolution: An example from the Permian–Triassic carbonate successions of the United Arab Emirates

Ihsan S. Al-Aasm (The Petroleum Institute, UAE), Simone Fontana (University of Pavia, Italy <simone.fontana@unipv.it), Andrea Ceriani (University of Pavia, Italy), Sadoon Morad (The Petroleum Institute, UAE) and Fadi H. Nader (Ministry of Energy and Water, Lebanon)

Introduction

Fractures are often the primary conduits for pore fluids, and thus have an important impact on the diagenetic and reservoir quality evolution of the host rocks. Therefore, fluid flow can be used to develop conceptual models for the spatial and temporal distribution and connectivity of reservoir porosity and permeability on a regional and basinal scale. In the context of the hydrocarbon-rich Middle East, the Permian–Triassic Khuff Formation hosts huge gas accumulations in the subsurface of the Arabian Plate. Reservoir properties of this formation are strongly influenced by depositional facies and diagenetic evolution (Ehrenberg et al., 2007; Fontana et al., 2010; Moradpour et al., 2008). In the Emirate of Ras Al Khaimah (RAK), in the northeast of the United Arab Emirates (UAE), there are excellent outcrops of the Permian–Triassic rock sequence, which is partly correlatable with the Khuff Formation (Strohmenger et al., 2002; Maurer et al., 2008, 2009). In this study, which involves fieldwork, petrographic, fluid inclusion and isotope (O, C and Sr) investigations, outcrops of Permian–Triassic carbonate rocks have been studied to relate fracture mineralization to tectonic history of the area.

Geological Setting

The investigated Permian–Triassic carbonate platform successions, which crop out in the northern UAE, comprise the Ru’us Al Jibal Group (Bih, Hagil, and Ghail formations) and the Elphinstone Group (Milaha and Ghalilah formations) (Figure 1). The studied rocks were deposited on a stable platform at the passive margin of the Neo-Tethys Ocean. Since the Late Cretaceous, the sedimentary succession underwent uplift due to the Semail Ophiolite obduction to the south, and to the Zagros Orogeny to the northwest. This has resulted in the formation of the Northern Oman Mountains (Searle et al., 1983; Warrak, 1996). The carbonate successions have been affected by a complex diagenetic history, which is closely linked to the regional tectonics.

Materials and Methods

Over 100 representative rock samples and thin sections from the Upper Permian to Triassic successions were examined using binocular reflected-light and transmitted-light petrographic microscopes subsequent to staining with potassium ferricyanide and Alyzarine Red-S. Cold cathodoluminescence (CL) investigations were carried out in order to characterize the growth features and paragenetic relationships of the different mineral phases. δ18O, δ13C and 87Sr/86Sr isotope analyses were performed on microdrilled carbonate mineral samples. O and C stable-isotope data are presented in the d notation relative to the Vienna PDB and SMOW standards with precision better than 0.05‰. Fluid inclusion microthermometric analyses of 11 samples were performed using a Linkham THMSG 600 heating-freezing stage coupled with an Olympus BX60 microscope.

Results and Discussion

The fracturing styles and occurrence in the studied carbonate succession are mainly influenced by tectonics (i.e. accentuated fracturing of outcrops in the vicinity of major faults and folds). As an example, the rocks of the Bih Formation in the vicinity of the Haqil tectonic window are more intensively fractured than the same stratigraphic intervals outcropping in the tectonically less disturbed Wadi Shah. Similarly, the outcrops of the Ghalilah Formation in Wadi Sha’am are more fractured than the same rocks exposed in Wadi Milaha. On the basis of cross-cutting relationships and structural measurements, three major fracture sets were distinguished, including: (1) NS-oriented fractures occluded by non-planar to sub-planar fine- to medium-crystalline dolomite; (2) E-W fractures filled by dolomite (Dc2) and saddle dolomite (Ds); and (3) NNW-SSE fractures cemented by calcite (C1), which precipitated also in reactivated E-W fractures (Figure 2). The Bih, Hagil and Ghail formations are completely dolomitized, although the original limestone texture can, in some cases, be recognized (Figure 3a). The Milaha and Ghalilah formations are partially dolomitized and their calcareous composition and texture are locally preserved (Figure 3b).

Bed-parallel stylolites (BPS) post-date replacive dolomitization of the carbonate sediments. Post-BPS, syntectonic cemented fractures display a paragenesis characterized by dolomite, quartz (subordinate) and, finally, calcite cements (Figures 3c and d). Calcite cement (C2) of presumably telogenetic origin (i.e. formed during uplift and incursion of meteoric waters) is recognized.

The δ18O values of the matrix dolomite (Dm) range between -6.7‰ to -2.1‰ and δ13C from -0.6‰ to +4‰. Dolomite cements (Dc1 and Dc2) have δ18O values from -9.0‰ to -3.3‰ and δ13C values between +1.2‰ and +3.3 ‰; saddle dolomite (Ds) has δ18O values that range from -10.8‰ to -5.7 ‰ and δ13C values from +0.2‰ to +3.3‰. Calcite cement C1 has δ18O between -6.7‰ to -0.2‰ and δ13C values from -5.3‰ to -0.4‰ and the telogenetic calcite cement C2 has δ18O between -6.2‰ to -4.5‰ and δ13C values from -6.0‰ to -3.4‰ (Figure 3e).

Isotopic measurements of samples of the Ghalilah Formation in Wadi Sha’am are similar to the ones of Wadi Bih, with a major difference: while the Dm samples of the Bih, Hagil and Ghail formations of Wadi Bih area have O, C and Sr isotope values resembling the Upper Permian–Lower Triassic seawater signature, the isotopic values of Dm and late fracture-filling carbonate cements (Ds and C1) of the Ghalilah Formation in Wadi Sha’am (δ18O values around -9‰ and 87Sr/86Sr ratio from 0.709 to 0.711), suggest either Dm recrystallization or dolomitization at higher temperatures and the circulation of “exotic”, possibly hydrothermal, fluids through late diagenetic fractures. In the Bih Formation, fluid inclusion data of Dm display homogeneization temperatures (Th) ranging from 129 to 144°C with salinity ranging from 13 to 16% NaCl eq.; Dc1 has Th values of 110–150°C and salinity of 13–19%; Dc2 has Th values of 120–200°C (mean 172°C) and salinity of 21–23%; Ds has Th values of 170–210°C (mean 188°C) and salinity of 22–23%; Q has Th values of 187–237°C (mean 209°C) and salinity of 23%; C1 has Th values of 170–227°C (mean 197°C) and salinity of 21–23%; C2 shows highly variable temperatures (Th from 60° to 217°C) and nil salinity (Figure 3f). Fluid inclusion data on Ds, Q and C1 phases of the Ghalilah Formation in Wadi Sha’am show, similarly to Bih Formation samples, a general trend of increasing temperature and salinity following the paragenetic order, but they display more scattered, and generally cooler, Th values than the ones measured in the Bih Formation, although they maintain similar salinities. This fact may reflect the progressive cooling of the diagenetic fluids while circulating, through faults and/or fractures, from deeper to more shallow portions of the basin. The latest calcite cement C2 is presumably telogenetic and it derived by infiltration of meteoric waters during uplift.

Conclusions

Fieldwork, petrographic, isotopic, and fluid inclusion investigations of the Upper Permian–Triassic carbonates of northeastern UAE, which are partly equivalent to the Khuff, suggest that the diagenetic evolution is related to the regional tectonics and is summarized as follows:

  • The carbonate platform succession is characterized by a relatively consistent diagenetic history. The carbonates were dolomitized relatively soon after deposition by modified marine fluids. Post-compactional, hot and highly saline diagenetic fluids caused the precipitation of dolomite, quartz, and calcite cements in fractures and vugs. A final calcite cement generation has been formed by percolation of meteoric fluids during uplift.

  • The Ghalilah Formation is only partially dolomitized, locally hydrofractured and displays dominant bedding controlled dolostones, which are partly recrystallized. Such recrystallization has presumably occurred after Late Cretaceous (Semail Ophiolite obduction), by hot (hydrothermal) fluids, which also resulted in the formation of fracture-filling carbonate and quartz cements in hydrobrecciated zones.

  • The diagenetic minerals characterizing the Permian–Triassic rocks were formed in association to tectonically-linked fractures. Considering the regional/structural geology of the area, the high temperature fluid-fow and associated diagenesis are suggested to have occurred since the Late Cretaceous maximum burial conditions up to the Tertiary tectonic compression (mainly Oligocene– Miocene), along the Hagab Thrust and coeval structures associated with the Zagros Orogeny.

References

11769 Sequence stratigraphy and reservoir development of the Khuff Formation of the Arabian Plate

Abdulrahman Alsharhan (Middle East Geological and Environmental Establishment, <sharhana@emirates.net.ae) and Christian J. Strohmenger (ExxonMobil Research Qatar, <christian.j.strohmenger@exxonmobil.com)

The Middle Permian (Capitanian) to Early Triassic (Induan) Khuff Formation is a prolific gas reservoir of the Arabian Plate. It is composed of mainly shallow-water carbonates and evaporites, representing a second-order transgressive-regressive sequence, which is composed of seven third-order composite sequences (KS1–KS7). The maximum flooding surface of Khuff Sequence KS4 (Khuff MFS4 at ca. 258 Ma) is interpreted to also represent Khuff second-order maximum flooding. The transgressive system set of the Khuff second-order sequence starts with Khuff Sequence KS7 and ends at the Khuff maximum flooding surface of Khuff Sequence KS4 (Khuff MFS4). The highstand systems set starts on top of the Khuff second-order maximum flooding surface and ends at the Top Khuff Formation (top of Khuff Sequence KS1).

The transgressive systems set (Khuff sequences KS7, KS6, KS5 and transgressive systems tract of Khuff Sequence KS4) is characterized by relatively thick anhydrite intercalations. The highstand systems set (Khuff Sequences KS1, KS2, KS3, and highstand systems tract of Khuff Sequence KS4) is more carbonate dominated and anhydrite intercalations are thinner, less common, and mostly restricted to Khuff Sequence KS1. Widespread layers of salina-dominated (subaqueous) anhydrite can locally and regionally (Middle Anhydrite) be used as marker horizons. This marker anhydrite may also act as a barrier to vertical fluid flow, and also be used in many areas to subdivide the Khuff into informal parts or members. The Khuff Formation is a complex sequence of dolomite, limestone and anhydrite. Dolomite is the dominant lithology of the formation. The development of the systems of fractures in many reservoirs in the Gulf region has significantly enhanced permeability values. Local thickness changes of the Khuff Formation may be attributed, at least in part, to infra-Cambrian salt movements, while lithofacies variation represent major transgressive-regressive cycles related to climatic and eustatic sea-level fluctuation.

11717 The Arabian Plate and the IGC Programme 572 (Permian–Triassic extinction and recovery): Results from the Muscat - Gutech (German University of Technology in Oman) field meeting (February, 2010)

Aymon Baud (BGC, <aymon.baud@unil.ch), Michaela Bernecker (German University of Technology in Oman, <michaela.bernecker@gutech.edu.om), Leopold Krystyn (Vienna University, Austria), Sylvain Richoz (University of Graz), Oliver Weidlich (Wintershall), Benoit Beauchamp (University of Calgary, Canada), Fabrice Cordey (Lyon University, France), Stephen Grasby (Geological Survey of Canada) and Charles Henderson (University of Calgary)

Summary

The Permian–Triassic transition has been surveyed in the Oman Mountains and new detailed sections have been presented (Baud and Bernecker, 2010), from autochthonous shallow-water units (Saih Hatat and Al Jabal al-Akhdar) to slope deposits in the Jabal Sumeini area (Wadi Maqam units), from distal tilted block (Wadi Wasit) to oceanic deep-water deposits (Buday’ah).

Middle Permian Transgression

At the dawn of the Wordian (Middle Permian), the “Fusulinid Sea” transgressed over most of Oman with the exception of Jabal Ja’alan and the Huqf-Dhofar High. This transgression enabled the establishment of a vast carbonate platform in Al Jabal al-Akhdar, a 700 m-thick succession of cyclic shallow-marine carbonate, the Saiq Formation (Middle and Late Permian, basal Triassic (Baud et al., 2001a, b, 2005; Richoz et al., 2005; Richoz, 2006)). A similar succession occurs in Saih Hatat (Le Métour, 1988; Weidlich and Bernecker, 2003; Chauvet, 2007), in the Musandam (Bih Formation; Maurer et al., 2009), as well as in Interior Oman and in the Haushi area (Khuff Formation; Angiolini et al. 1998, 2003). Clearly, for us, this transgression was the result of the break-up of the Neo-Tethyan rift and the associated thermal subsidence.

Following the peak of the thermal subsidence in the Wordian–Capitanian, a stable carbonate platform became established on the Arabian Peninsula. The Saiq, Khuff and Hagil formations show a strong regressive tendency at the end of the Guadalupian (Middle Permian), with restricted environment facies and a reduced biophase, mainly associated with a global fall in sea level at this time and climate changes (Isozaki, 2009). During the Lopingian (Late Permian), the subsidence as recorded in the Saiq mega-cycle B (up to 300 m of shallowing-upward cycles) was still well active.

The most striking effect of the climax of the Neo-Tethyan extension was the formation of a continental slope (Sumeini) and a basin (Hawasina) that constituted with the adjacent Arabian Platform, the southern continental passive margin of the Neo-Tethys Ocean. Furthermore early-rifted blocks detached from the edge of the Arabian Shield formed isolated proximal platforms along the continental slope (later they were incorporated in the Hawasina Nappes). The continental margin slope deposits are clearly identified (with slumps and intra-formational breccia) in the northwestern part of the Oman Mountains (Jabal Sumeini), where they form the basal part of the Maqam Formation dated as Roadian (Middle Permian).

The distal isolated platform identified as nappes in Baid and Jabal Qamar areas by Béchennec (1988), Béchennec et al. (1992), Pillevuit (1993) and Pillevuit et al. (1997) are mainly made of Middle–Late Permian open-shelf carbonates. The Jabal Qamar unit includes a fragment of the pre-Middle Permian Basement (Rann, Ayim and Asfar formations; Pillevuit, 1993) overlain in unconformity by the late Early to early Middle Permian shallow-marine carbonate Qamar Formation with its quartz-sandstone basal member. The Baid unit is truncated at the base and is made of about 100 m of the Middle– Late Permian (Capitanian–Wuchiapingian) shallow-marine carbonate (Baid Formation; Béchennec, 1988; Pillevuit, 1993; Pillevuit et al., 1997; Baud et al., 2001b). The distal paleogeographic position of these Permian tilted blocks in regard to the Arabian Platform is documented by: (1) the differences in terms of facies (open marine with ammonoids) with those restricted to the others parts of the Oman Mountains (Al Jabal al-Akhdar, Saih Hatat and Musandam); and (2) the presence of reworked boulders originating from these isolated platforms in the calcirudites of the proximal units of the basinal Hawasina Nappes.

Basinal facies of the Middle Permian are present in the Hawasina Nappes at the base of numerous tectonics units, made up of formations from the Hamrat Duru Group. These successions generally start with thick volcanic sequences (Al Jil and Buday’ah formations). These volcanic rocks are either of MORB type or alkali basalt-related (Maury et al., 2003; Lapierre et al., 2004). The volcanic succession is filled and overlain by red ammonoid limestones dated as Middle Permian (Capitanian) followed by radiolarian chert and shales newly dated as Lopingian in Buday’ah. In the Wadi Wasit area, the volcanic series is capped by red cephalopod-bearing carbonate, dated Middle Permian (Wordian; Blendinger et al., 1992; Pillevuit et al., 1997; Baud et al., 2001b), by shales and breccia with reworked blocks of Middle Permian to basal Triassic platform carbonate (Béchennec et al., 1992b; Pillevuit, 1993; Pillevuit et al., 1997; Krystyn et al., 2003; Weidlich and Bernecker, 2007).

Near Nahkl the volcanic series includes blocks of Middle Permian shallow-marine carbonate and is overlain by pelagic limestone (Weidlich, 2007). In the Rustaq area the volcanic succession is also capped by a condensed carbonate sequence (Hallstatt facies type) dated as Middle Permian (Wordian; Blendinger et al., 1992; Pillevuit et al., 1997; Baud et al. 2001b; Richoz et al., 2005).

Different types of deep-water black limestones are also identified in the basinal units of the Batain Plain (southeastern part of the Oman Mountains), the “Qarari Limestone” with a base dated as Roadian (Middle Permian; Immenhauser et al., 1998) and the top as Changhsingian.

From the Permian/Triassic Transition to the Basal Triassic (Figure 1)

At the end of the Permian (top of KS3 Sequence of Koehrer et al., 2010), regressive conditions up locally to emersion (?) are recorded as well on the Arabian carbonate platform (Al Jabal al-Akhdar, Saih Hatat and Musandam). On the slope of the continental margin, we observe a shallowing in the Sumeini unit deposits.

Shallow tidal influenced carbonate platform is the main component of the Induan dolomitized deposit in the Al Jabal al-Akhdar (Units C1 to C4 of the Saiq Formation in Baud and Bernecker, 2010, correlated with Khuff sequences KS2–KS1) that is now dated by conodonts. During the Dienerian, part of the margin was affected by a renewed extensional regime, tilting and drowning resulting in erosive deposition and accumulation of carbonate breccia (Unit C2 of the Saiq Formation) followed in the Al Jabal al-Akhdar by high-energy, partly oolitic dolomitized shallow-water deposits, Dienerian in age (Unit C3) and renewed breccias (Unit C4). The Saiq-Mahil transition (correlated with the Khuff-Sudair transition) is probably of late Induan age (chemostratigraphical correlations; Richoz, 2006).

On the slope of the continental margin, a continuous carbonate deposition and shale has been recently precisely dated from Changhsingian to Spathian. Overlying the Wuchiapingian–Changhsingian, deep-water chert and dolostone (upper Member B of the Maqam Formation), we note the deposition of upper Changhsingian shallowing siliceous strongly bioturbated lime mudstones. A major facies change occurs with the Griesbachian papery, laminated calcimicrobial mudstone overlying the boundary clay (base of C1c Member of the Maqam Formation). The calcarenite, calcirudite turbidites and avalanches with shallow water Upper Permian lime clasts start in the Dienerian (instability period). The incredible thickness of the Smithian deposits (platy limestones, shales and megabreccia up to 900 m of thickness, middle and upper Member C of the Maqam Formation) indicate high carbonate productivity on the platform and a very active subsidence at the base of slope (Watts, 1985; Baud et al., 2001b; Richoz et al., 2005; Richoz, 2006).

On the Baid Exotic block, after karstification of part of the tilted Permian carbonate platform and the Dienerian drowning event, the Dienerian–Smithian deep-water red ammonoid limestone filled fissures and cavities (Hallstatt breccia) and was deposited over the Permian limestones (Tozer and Calon, 1990; Pillevuit, 1993; Pillevuit et al., 1997; Baud et al., 2001a; Richoz, 2006; Wood and Baud, 2008).

In the proximal deep-water basin (Wadi Wasit units) the Lopingian allodapic limestones are partly eroded by a submarine avalanche breccia (Dienerian) containing Permian to basal Triassic mega-blocks. One of these blocks with a unique Permian–basal Triassic record has been analyzed in Krystyn et al. (2003). Upper Dienerian–Smithian deep-water platy limestone overlie the Lower Dienerian mega-block breccia.

In the distal Hawasina Basin (Buday’ah), the Upper Permian radiolarian chert deposits are overlain by Changhsingian siliceous shales and calcareous shales followed by Griesbachian laminated platy limestones and shales and Dienerian–Smithian papery limestones.

Conclusions

The Neo-Tethys Ocean opened with the northward drifting of the Iran/Mega-Lhasa microcontinent followed a rifting extensional phase in the Roadian–Wordian. Thermal subsidence, with the development of the continental margin, is well recorded in the Wordian–Capitanian carbonate succession and continued during the Lopingian. Tectonic instability of the margin, with block tilting, platform drowning and (fault) breccia deposits started at the dawn of the Triassic with the main climax during the Dienerian and the Smithian.

References

12033 The opening of Neo-Tethys and the formation of the Khuff passive margin

Andy Bell (Shell, <Andy.Bell@Shell.com) and Pieter Spaak (Shell)

Any investigation of regional geology and palaeomagnetism in the Middle East will show that in the Permian a series of terranes separated from Gondwana and drifted north, opening the Neo-Tethys Ocean in their wake. To the north of these terranes, the Palaeo-Tethys Ocean closed and was largely subducted. Eventually in a non-synchronous movement but largely in Late Triassic and Early Jurassic times, these terranes docked with the northern margin of the former Palaeo-Tethys during the Cimmerian Orogeny.

The opening of the Neo-Tethys in Arabia does not follow the classic pattern of continental break-up resulting in oceanic crust. Classically we should expect thermal up-doming, followed by the onset of syn-rift deposition, frequently but not always associated with volcanism. The formation of en-echelon systems of rotated fault blocks are also characteristic, followed by a break-up unconformity and the formation of the first oceanic crust. What we see in Arabia as a consequence of the opening of Neo-Tethys exhibits few of these features. Distinguishing thermal up-doming from the uplift associated with the ‘Hercynian’ event in Arabia, coupled with the glacial sculpting of the Permian–Carboniferous Unayzah Formation is fraught with difficulty. Although locally volcanics of Permian age are known from Oman, they are absent over most of Arabia.

The real difference with the classic view of rifting, however, is the almost complete absence of rotated fault blocks with associated thickened sections of syn-rift sediments. While it is possible that some of the features have been obscured by glacial-related features or lie outboard of the Arabian Peninsula and currently underlie the Zagros, it is unlikely to be the only reason for their apparent absence. By analysing the subsidence histories of points along the Arabian margin and detailing the isopachs of the Permian and Triassic sediments it is possible to interpret a different model of continental breakup that has exploited possibly older weaknesses in the crust and allowed a “soft” separation of the Cimmerian terranes. This opening style has significant implications for the play development; not only for the morphology of reservoir bodies and structures, but also for the thermal and structural history of this section of the Tethyan margin.

11704 Khuff Sequence KS6: Paleorelief-influenced facies and sequence patterns in the Lower Khuff Formation, Sultanate of Oman

Daniel Bendias (University of Tübingen, <daniel.bendias@gmail.com), Thomas Aigner (University of Tübingen), Michael Pöppelreiter (Shell) and Bastian Köhrer (Wintershall)

This outcrop analog study of Lower Khuff Sequence KS6 (Saiq Formation) aims to capture lateral reservoir facies distribution during the initial phase of basin-fill. The Hercynian tectonic event triggered the erosion of Cambrian to Carboniferous strata in Oman and formed the topography of the Sub-Khuff Unconformity. When the Neo-Tethys Ocean flooded the Arabian Shield clastic sediments, the so-called “Basal Khuff Clastics” became preferentially deposited in possible paleo-lows. One main objective of this study was to unravel the effect of this paleo-relief on reservoir facies distribution within the overlying sediments.

The lowermost Khuff Sequence 6 (KS 6) can be subdivided into four different facies associations (backshoal, shoal, foreshoal and offshoal) with distinct sedimentological characteristics and reservoir potential. The KS6 represents one transgressive-regressive cycle. In contrast to younger Khuff sequences (KS4 to KS1) the underlying paleo-relief seems to strongly affect the thickness and facies composition of the KS6 and the Basal Khuff Clastics.

Workflow and Methods

The concept of dynamic stratigraphy (Aigner, 1985; Aigner et al., 1998; Kerans and Tinker, 1997) guided this study from 1-D sedimentological observations in the field to 2-D correlation to the final 3-D model (Figure 1).

  • 1-D: Three months of intense fieldwork led to detailed logs of five sections of the KS6 Sequence. The 1-D dataset includes Dunham textures, outcrop gamma-ray and microfacies data provided by more than 200 thin sections (Figure 1a).

  • 2-D: Facies cycles, outcrop gamma-ray and biostratigraphy were used to create several correlation scenarios (Figure 1b).

  • 3-D: In order to delineate strengths and weaknesses of the different 3-D geocellular modelling approaches, a broad spectrum of modelling methods was tested. Finally a range of models was generated based on the “Truncated Gaussian with trends” algorithm using different correlation scenarios and varying lateral extents of reservoir facies types (Figure 1c).

Conclusions

The initial paleo-relief apparently controlled the thickness of the initial Khuff clastics, and that of the overlying KS6 carbonates as well as their composition. The correlation strategy to follow paleo-landscape surfaces using all available data resulted in a “pseudo-layer cake” stratigraphic architecture with subtle shingles. This study revealed potential reservoir units in the KS6, commonly regarded as non-reservoir in the subsurface of Oman. In the transgressive part of the investigated sequence, the predominant reservoir facies are bioclastic crinoidal grainstones with only poor diagenetic potential, concentrated around the margin of a paleohigh. Oolitic/peloidal grainstones in the upper regressive part have a much higher diagenetic potential and are areally much more widespread. Abundance and lateral extent of individual grainstones strongly vary with stratigraphic position. Within the palaeogeographic framework of a shoal to offshoal setting, the thickest and laterally most persistent shoal bodies occur during peak regression.

References

11789 Petrophysical rock types of the Khuff C, Eastern Saudi Arabia

Edward Clerke (Saudi Aramco, <edward.clerke@aramco.com)

Petrophysical rock types (PRTs) can be defined as objects in the three dimensional space defined by Thomeer pore system parameters. Classifications in the Thomeer parameter space must also be accompanied by relationships to convenient well bore measurements (commonly well logs) and geologic parameters to create economic value in the petroleum business. Recent and detailed core-and-log-based high-frequency, sequence-stratigraphic characterization of the Khuff-C by Eid, Dukhayyil and Tawil are now complete and this work builds on that important geological foundation. Hence, any effort to define petrophysical rock types must include and utilize a host of multi-disciplinary data.

We acquired additional core plug data for petrophysical rock typing of the Khuff-C using detailed petrophysical core description and Archie rock type description at the centimeter level. These plugs were selected to explore and sample various aspects of the pore systems after extensive and detailed core-log integration. Detailed petrographic data was generated at King Fahd University of Petroleum and Minerals to aid in petrophysical rock typing, with mercury injection capillary pressure (MICP) data (Thomeer analyzed) and with Qemscan mineralogical images. Samples displayed a wide variety of textures and mineralogies. Wide variation in pore types was observed from intercrystalline (limestone and dolomite) to interparticle, intraparticle and moldic pore types. Despite this, the MICP data was strongly (86%) monomodal. The prevalence of MICP monomodality and multiple pore body types implies that the pore throats (doorways) and not the pore bodies (rooms) control the flow of fluids in these rocks.

Six PRT trends are determined, named by their dominant mineralogy and dominant pore type (Dolomite_Moldic_Intraparticle_Intraskeletal, Dolomite_Interparticle, Dolomite_Intercrystalline, Calcite_Moldic_Intraparticle_Intraskeletal, Aerial, Mix) in which the maximum pore throat diameter is directly related to the sample porosity and within which the Thomeer Pore Geometrical Factor, G, is relatively constant. These six PRTs are then demonstrated to be in excellent accord with the Eid-Dukhayyil-Tawil depofacies (Exposure/Paleosol, Supratidal, Salina, Tidal Flat Complex, Burrowed Shallow Subtidal, Zoophycos Subtidal, Restricted Lagoon, Shoal Flanks, High Energy Shoal and High Energy Shoal with Clasts).

These results and the agreement with depofacies is a key result, which enables improved and geological net pay and permeability calculations and Thomeer-based saturation-height models. The results also demonstrate that certain stratal architectures result in lateral stratigraphic traps. The optimum implementation of these petrophysical rock types is possible in wells where full geochemical well log data has been acquired.

11699 Sequence stratigraphy, reservoirs, diagenesis and hydrocarbon contents in the Khuff Formation and its equivalents: Independent or linked?

Roger B. Davies (Neftex Petroleum Consultants Ltd, <roger.davies@neftex.com)

Thirty years ago, the author undertook a detailed sedimentological, petrographic and reservoir quality evaluation of extensive cores through the Khuff Formation from Well Zakum-182 in offshore Abu Dhabi. Many new observations, including detailed depositional and early diagenetic fabrics preserved during dolomite replacement and the presence of sulphur as a late diagenetic phase infilling the cores of anhydrite nodules, vugs and fractures, ignited a life-long interest in this fascinating formation and its equivalents, notably the Dalan and Kangan formations of Iran. This was followed by a much larger project in which the author, as part of a multi-disciplinary team, was responsible for a regional analysis of the sedimentology, diagenesis and reservoir quality of the Khuff Formation over a wide area ranging from subsurface sections in Abu Dhabi, Dubai and Qatar to outcrops in Saudi Arabia, Oman and the Musandam Peninsula. Studies since then have ranged from a sequence-stratigraphic analysis over the entire Arabian Plate to analyses of depositional sedimentology, diagenesis, reservoir architecture, reservoir quality and relationships between geology and gas contents. As a result of working this range of studies, the author proposes that, even though masked by complex depositional and post-depositional factors, there are significant relationships between the sequence-stratigraphic organisation of the Khuff Formation and its mineralogy, diagenesis, reservoir architecture, reservoir properties, compartmentalisation and even gas composition, all of which impact the value of its enormous gas resources. This presentation reviews each of these relationships and how our understanding of the Khuff Formation has evolved over the last 30 years.

11735 Reservoir rock characterization, classification and pore typing in the Dalan/Kangan carbonates (Khuff analogues), South Pars Field

Behrooz Esrafili Dizaji (University of Tehran, MAPSA Co., Iran <ESRAFILY@khayam.ut.ac.ir>), H. Rahimpour-Bonab (University of Tehran) and F. Kiani Harchegani (Azad University of Khorasgan, Iran)

The South Pars Field, discovered in 1990, is part of the world’s largest single gas accumulation located in the Gulf. The Iranian part of this immense gas accumulation accounts for 5% of the world’s and 60% of Iran’s total gas reserves. This field produces from Dalan/Kangan carbonates (Khuff analogues). These Permian–Triassic carbonate reservoirs in the field are highly stratified in nature and display layer-cake geometry. Combined core analysis and detailed thin section studies are used for facies analysis. Accordingly, 14 major facies were recognized in these carbonates. Facies analysis shows that their depositional setting was located along the inner part of an epiric carbonate system that extended from a peritidal setting to a shallow subtidal zone (back-shoal setting), passing over to a high-energy shoal and fore-shoal facies (Figure 1).

Petrographical and geochemical evidences indicate that these facies were mainly exposed to a shallow diagenesis and minor subsequent burial. Major diagenetic processes and events affecting the Dalan/Kangan carbonates include: micritization and marine cementation, anhydrite nodule formation, early dolomitization and dolomite neomorphism, dissolution and/or neomorphism of aragonite, anhydrite plugging and calcite cementations, mechanical and chemical compaction, fracturing. Hypersaline and meteoric diagenetic realms were two well-identified zones in the reservoir intervals. Three major diagenetic environments affected reservoir intervals.

Generally, carbonate reservoirs consist of different rock types and various fluid-flow units. Significance of reservoir rock classification for heterogeneity characterization in these reservoirs is now widely recognized. Our studies indicated that depositional facies and early diagenesis (intensity and extent of dolomitization, dissolution and cementation) has controlled the first-order reservoir heterogeneity in these reservoirs (Figure 1).

In this research a new reservoir rock classification scheme is established, which is based on poroperm controlling factors (Table 1). By integration of petrographic examination and petrophysic data, twelve major rock types are recognized. These rock types are ranked in various reservoir qualities (from poor to good). Therefore, the best reservoir quality can be found commonly in dolomitized and non-dolomitized grain-dominated rock types. Other good reservoir rocks are sucrosic and recrystallized dolomite (with high intercrystalline poroperm) and fractured rock types but these rock types are not common in the reservoir.

Furthermore, pore-typing analyses are carried out using thin sections and image analysis, SEM, MICP, X-ray CT, core poroperm data and flow unit equations (FZI, RQI and R35). Some 152 samples are selected from reservoir intervals that have commercial poroperm values (units with porosity > 5% and K > 0.1 mD) for production. Image analysis (2-D in thin section photographs) of these samples indicated seven basic pore types. These pore types and their relative percentages are intergrain (14%), fenestral (3%), moldic (43%), intercrystalline (8%), fracture, vuggy or solution enlarged (16%), fracture (13%), and stylolitic (3%). Relative proportions of these pore types for each sample is plotted on the ternary diagenetic gradient (Figures 2 and 3). Pore scale examinations showed that pore system properties were modified during diagenesis to varying degrees. Considering general diagenetic history and porosity evolution of this reservoir, three main porosity generation stages are recognized as pre-meteoric, meteoric and post-meteoric. Although a significant amount of secondary porosity has formed during meteoric diagenesis, later diagenesis has effectively improved reservoir quality. The results reveal that relative abundance of moldic pores has the main effect on pore system connection in these carbonate rocks. In conclusion, localized late diagenetic alterations played a key role in the porosity evolution and imposing reservoir heterogeneity (particularly in the pore scale).

11740 Sequence stratigraphy of the Permian–Triassic Upper Khuff carbonates, Ghawar Field, Saudi Arabia

Raed K. Al-Dukhayyil (Saudi Aramco, <dukhayrk@vt.edu, <dukhayyilrk@yahoo.com), James F. Read (University of Vermont, USA), and Aus A. Al-Tawil (Saudi Aramco, <aus.tawil@aramco.com)

Over 1,500 meters of Upper Permian Khuff B/C and Early Triassic Khuff A and B cores and wire-line logs from 16 wells in Ghawar Field were studied. The carbonate-evaporite successions are interpreted to be arid epeiric ramp facies, which include: subaqueous laminated-and supratidal nodular- to massive anhydrite, tidal flat laminites, lagoonal mudstone, (with thrombolites restricted to Triassic), ooid-peloid shoal complexes, and subtidal off-shoal mudstone. Facies are packaged into meter-scale parasequences, often separated by exposure surfaces at sequence boundaries.

The Khuff B/C interval (up to 105 m thick) is a third-order sequence with nine high-frequency sequences (HFS) capped by the Permian–Triassic (P/T) unconformity. The transgressive systems tracts (TST) of most of the HFS are grainstone-dominated, while the highstand systems tract (HST) deposits are dolomudstone, laminite and/or paleosols. Carbon-isotope values commonly decrease beneath the sequence boundaries, whereas the oxygen-isotope values increase. The P/T boundary in Ghawar is marked by regional exposure with a marked increase in total gamma-ray beneath the surface and a significant decrease above it. The P/T boundary has a significant negative carbon excursion (reflecting global input of light carbon) and the overlying Triassic section has an abrupt decrease in faunal content (following the P/T extinction).

The Early Triassic Khuff A and B carbonates and evaporites are up to 105 m thick. The Khuff B sequence is composed of four high-frequency sequences, comprised of cyclic off-shoal lime-mudstone and oolitic grainstone (TST) and HST grainstone and peritidal carbonate-evaporite deposits, evaporites including laminated and nodular types (HFS 3), and laminated types only (HFS 4). The lower Khuff A sequence, contains a basal laminated anhydrite, and has three high-frequency sequences composed of evaporites, peritidal and oolitic carbonates (TST), and an HST that is oolite-prone and capped by laminites and local exposure breccia. The upper Khuff A sequence has digitate stromatolites at the base and is dominated by peritidal carbonates with thin grainstone, but systems tracts are difficult to define.

11796 High-resolution sequence stratigraphy of the Late Permian Khuff-C in Hawiyah, Ghawar Field

Ghazi Al Eid (Saudi Aramco, <ghazi.eid@aramco.com) and Aus Al Tawil (Saudi Aramco)

The Late Permian Khuff-C carbonates of the Khuff Formation in Ghawar Field are composed of four high-frequency sequences, Khuff-C1 to Khuff-C4 (KC1 to KC4), with numerous mappable small-scale shallowing-upward cycles that vertically partition the reservoir. Each sequence is bounded by well-defined sequence boundaries, observed in core and logs, with well-defined transgressive systems tract (TST) and a highstand systems tract (HST), separated by a mappable maximum flooding surface (MFS).

KC1 is made up of 10 mappable cycles, where the TST of each cycle starts with flooding grain- to bioclast-rich storm beds and restricted dolo-mudstones that shallow-up to a regional muddy tidal flat cap. The MFS is a totally dolomitized distal lime-mudstone, passing into cross-bedded skeletal-peloid-ooid grainstones. The HST is made up of prograding grainstone overlain by restricted muddy to peloidal wackestone lagoon and tidal flat mudstones.

KC2 consists of seven cycles and is defined by an initial transgressive set of restricted mud-dominated cycles with clast-rich storm-influenced facies deposited over the thick, extensive tidal-flat sediments of KC1, which extend across a large area of the Ghawar Field. The middle cycles are made up of intraclastic storm beds and intensely burrowed shallow sub-tidal pellet packstones, deepening to dominantly open-marine lime-mudstone (MFS), and shallowing-up to sand-flat, low-angle, cross-bedded to burrowed peloid packstones. The upper cycle is made up of shallow sub-tidal embayment lime-mudstone, shallowing-up to sections that later became exposed and aerially altered (i.e., breccias, paleosols, mud-cracks, etc.), which mark the sequence boundary of KC3.

KC3 is composed of 10 cycles with four initial transgressive peri-tidal cycles of lagoonal dolo-mudstone capped by crinkly laminated to mud-cracked dolo-mudstone tidal flat facies. These four cycles are overlain by a back-stepping, cycle-set of grainstone with their distal bryozoan lime-mudstone equivalent (MFS). The HST of KC3 is marked by shallow sub-tidal sand flat facies prograding over distal open-marine lime-mudstone. KC3 is marked at the top by other signs of aerial exposure: pedogenic features, roots/paleosols, which mark the KC4 sequence boundary.

KC4 consists of five mud-dominated cycles. The base of this sequence is bounded by an exposure surface of KC3. The TST is represented by transgressive peri-tidal muddy cycles to shallow sub-tidal sand-flats to burrowed fine peloid cycles, formed under low to moderate energy, while the prograding grainstones and the overlying anhydrite define the HST.

High-resolution sequence stratigraphy of the Khuff-C carbonates will provide the framework for future reservoir characterization studies and geological modeling of this reservoir.

11741 Review of the Permian–Triassic gas play in the Arabian Gulf region

Mohammad I. Faqira (Saudi Aramco, <mohammad.faqira@aramco.com), Abdel Fattah Bakhiet (Saudi Aramco) and Abdel Ghayoum Ahmed (Saudi Aramco)

Significant amounts of non-associated gas reserves are proven from the Permian–Triassic Khuff-Dalan play across the Arabian Gulf region (Figure 1). This play is part of the Paleozoic petroleum system that is mainly sourced by the base Qusaiba hot shale of the Silurian Qalibah Formation. It is sealed by the Triassic Sudair-Aghar shales (Figure 2) and trapped in high relief, north-trending anticlines or salt domes.

The Khuff Formation was deposited on a broad shallow-marine restricted shelf across the Arabian Platform (Figure 3) and consists of interbedded carbonates and evaporites. Four reservoirs, named Khuff-A, B, C and D in descending order (Figure 4), are present within the carbonates and their porosity ranges between 3% and 12%. The Khuff reservoirs tend to thicken with improved development toward the east.

Several long N-trending folds were initiated during the Permian–Triassic across the Arabian Plate (Faqira et al., 2004). These folds, together with salt-cored structures, played a significant role in reservoir development through diagenetic processes. Better reservoir development and communication occurs in the salt dome structures, whereas the anticlinal trends tend to have less reservoir development and no communication across the various Khuff reservoirs.

The hydrocarbon gases of the Khuff reservoirs were sourced from the base Qusaiba hot shale of the Silurian Qalibah Formation. The distribution of these rocks across the Arabian Plate has been influenced by the mid-Carboniferous Hercynian unconformity, in which the Silurian source rocks are preserved in the mid-Carboniferous basins and eroded from the intervening arches (Figure 5).

Hydrocarbon migration is the most critical element of the Khuff play. We believe that this migration requires faults with sufficient throw or high intensity fractures to connect the base Qusaiba hot shale source rocks to the Khuff reservoirs (Figures 6 and 7). The Khuff hydrocarbons accumulate in either long north-trending basement-cored high relief forced fold structures controlled by deep-seated faults or salt domes. Afifi (2010) believes that the hydrocarbon migration is attributed to the breaching of thick shale/carbonate/anhydrite seals at the base of the Khuff Formation by faults that propagated upwards into the Khuff during the initial development of high-relief salt domes, followed by migration up-dip on fill-and-spill bases.

The quality of the Khuff gas varies with depth in response to increased reservoir temperature. The non-hydrocarbon gas components found are usually H2S, CO2 and N2. The H2S is generated in the Khuff reservoirs following a reaction between the hydrocarbons and anhydrites. Geochemistry and petrography have confirmed that H2S in the Khuff reservoirs is due to thermo-chemical sulphate reduction (TSR), in which anhydrite is replaced by calcite and sulfur.

References

11706 Diagenesis control on reservoir properties of a Permian–Triassic carbonate reservoir, North Oman

Sulaiman Zahran Al Farqani (Petroleum Development Oman, PDO, <Suleiman.Z.Farqani@pdo.co.om), Irene Gomez Perez (PDO), Michele Claps (PDO) and Henning Peters (Shell)

Introduction

The subsurface Permian–Triassic Khuff carbonates in North Oman are sour reservoirs with variable amounts of H2S and CO2. A diagenetic study was part of an integrated multi-disciplinary project to understand the origin and distribution of H2S, attributed either to bacterial or thermochemical sulphate reduction, and its relation to the hydrocarbon charge history for these reservoirs.

Methodology

The petrological study was based on the study of about 150 samples from 5 cored wells. The resulting paragenetic sequence was constrained from detailed transmitted, fluorescence and cathodoluminescence microscopy, together with stable-isotope and fluid-inclusions analyses.

Results

The study revealed that reservoir quality was significantly enhanced by early dissolution that created moldic and vuggy porosity, and pervasive dolomitization, which created intercrystalline porosity, both occurring in near-surface conditions. Different stages of calcite, dolomite and anhydrite cementation, formed in shallow to late burial diagenetic environments, played major roles in reservoir deterioration.

Late (saddle) dolomite and late spar calcite cements occur in insignificant amounts, but could locally be important in reducing reservoir connectivity. Occurrence of different phases of solid bitumen, in conjunction with basin modeling, suggest at least two events of hydrocarbon charge: a first oil charge starting after early dolomitization and a later charge post-dating late anhydrite cements.

Stable carbon, oxygen and strontium isotopes were bulk analysed on micrites and dolomicrites through the different depositional sequences in the main reservoirs, and microsampled on specific cement phases throughout the paragenetic sequence. In the upper, Early Triassic reservoir, data indicate more negative signatures of δ13C versus δ18O than in the lower, Late Permian reservoir. This could be interpreted as a result of changing secular trends. Oxygen isotopes in cements follow a typical burial trend with a shift towards more negative values, supporting a gradual increase in temperature and burial depth. Trends towards more negative values in carbon isotopes compared with Permian seawater compositions can be attributed to carbon sources from either meteoric water or from migrated hydrocarbons.

Fluid inclusion petrography and microthermometry results indicate high homogenization temperatures in dolomites, late calcites and anhydrites. The results further supports two hydrocarbon-charging events (or continuous charging), at two relatively high-temperature windows.

Conclusions

The Khuff subsurface reservoirs in North Oman underwent different diagenetic conditions from near-subsurface to deep burial. Reservoir properties were significantly enhanced by early dissolution and dolomitization. Calcite, dolomite and anhydrite cements deteriorated porosity. There is no compelling petrographic or isotopic evidence of mineral products resulting from thermochemical sulphate reduction (TSR) (e.g. calcification of anhydrite). However, some high homogenization temperatures recorded from fluid inclusions keep open the possibility of incipient TSR-derived H2S. Deep-seated faults could have acted as conduits of hydrothermal fluids infiltrating the grain-dominated layers of the Upper Khuff reservoir, as basin history does not support high burial depth/temperatures.

Acknowledgements

Thanks to the Ministry of Oil and Gas and Petroleum Development Oman for allowing presentation of this work. Thanks to Badley Asthon for core sedimentology and fluid inclusion analysis, Shell Global Solutions International B.V. for their support in using the luminoscope, and University of Amsterdam for undertaking isotope analyses.

References

11728 Breccia beds in the Khuff (Permian–Triassic) in Ras Al Khaimah, United Arab Emirates: Collapse or transgressive in origin?

Simone Fontana (University of Pavia, Italy, <simone.fontana@unipv.it), Sadoon Morad (The Petroleum Institute, Abu Dhabi), Fadi H. Nader (Ministry of Energy and Water, Lebanon), Andrea Ceriani (University of Pavia, Italy) and Ihsan S. Al-Aasm (The Petroleum Institute, Abu Dhabi)

Introduction

The laterally extensive, so-called mid-Bih breccia beds occur in carbonate successions of the Upper Permian–Lower Triassic Khuff-equivalent Bih Formation in Ras Al Khaimah, United Arab Emirates (UAE). These carbonates have been deposited on a stable platform setting at the passive margin of the Neo-Tethys Ocean. The breccia beds have previously been interpreted to be formed by dissolution of sulphate beds by groundwater followed by collapse of overlying carbonate beds (Strohmenger et al., 2002; Fontana et al., 2010). Contrary to this earlier interpretation, we present several lines of field, petrographic, isotopic and fluid inclusion evidence suggesting that the “breccias” are intra-formational conglomerates representing a major marine transgressive surface.

Geological Setting

The ca. 1,700 m thick Permian–Triassic carbonate platform succession outcropping in the Musandam Peninsula, Ras Al Khaimah, UAE, includes the Ru’us Al Jibal Group (Bih, Hagil and Ghail formations) and the Elphinstone Group (Milaha and Ghalilah formations; Figure 1) (Ricateau and Riché, 1980).

The area was subjected to two main orogenic events separated by a period of tectonic relaxation. These events include the Late Cretaceous thin-skinned tectonics associated to the ophiolite obduction of the Oman Mountains and Dibba Zone, and the Cenozoic thrusting linked to the Zagros Orogeny, which took place mostly during Oligocene–Miocene (Searle et al., 1983). During the Cambrian to Permian, the area formed part of the Gondwana passive margin. During Middle Permian, rifting started in the eastern border of the Arabian Plate, resulting in the opening of Neo-Tethys Ocean (Angiolini et al., 2003). From Middle Permian to Early Cretaceous over 3 km of carbonate sediments were deposited on a carbonate platform. During middle Cretaceous an eastward-directed subduction arose (Glennie, 2005) and, in the Late Cretaceous (Loosveld et al., 1996), the ocean closed and the Semail Ophiolite, the volcanic complex, the slope deposits and the basin sediments, were obducted on the autochthonous Arabian Platform (Searle, 1985). A foreland basin developed to the west of the uplifted area, as a flexural response of the Arabian lithosphere to the ophiolites obduction (Patton and O’Connor, 1988). The Cenozoic deformation caused folding and thrusting in the Northern Oman Mountains. The Hagab Thrust transported approximately 15 km westwards the complete 3,500 m thick shelf carbonate succession, carrying also in a piggy-back style the Upper Cretaceous structures (Searle, 1988b). In the Hagil tectonic window, the Musandam platform carbonates tectonically overlie the Hawasina rocks that were previously (i.e. in the Late Cretaceous) tectonically overlying the shelf (Robertson et al., 1990).

Material and Methods

Rock slabs from representative samples of the Bih Formation were examined using binocular microscope subsequent to polishing and staining with Potassium Ferricyanide Blue and Alizarine Red-S to distinguish calcite and dolomite and their ferroan equivalents. Petrographic study of thirty-seven thin sections was carried out through conventional and cathodoluminescence (CL) observations. Stable carbon and oxygen isotope analyses were performed on the dolostone matrix as well as on the dolomite and calcite vug- and fracture-filling cements, sampled through microdrilling technique. Petrographic and microthermometric analyses of fluid inclusion in wafers from five samples of the Bih Formation (including two samples from the conglomerate) were conducted to highlight temperature and salinity of the fluids responsible for dolomitization and carbonate cementation.

Field Observations

The breccia beds, which are hereafter called conglomerate, are laterally extensive (over 100s of sq km) (Figure 2a). The conglomerate occurs as a single bed (ca. 1 m thick) or an interval of multiple beds (1–2 m thick) separated by decimetre-thick dolograinstone and dolopackstone layers (Figure 2b). The conglomerate beds have irregular (i.e. erosive) lower boundary, which cuts a few metres down into the underlying dolostone beds (Figure 2c). The conglomerate intervals display also normal grading (Figure 2d) and vague low-angle cross stratification (Figure 2e). The conglomerate beds are overall polymictic composed mostly of subrounded to subangular, dolomudstone and dolopackstones pebbles ranging in size from mm to several cm; fully angular pebbles are rare (Figure 2f). The pebbles are similar in composition and texture to the associated dolostone beds. In rare cases, the pebbles are made of coral fragments (Figure 3a). These features of the conglomerate suggest formation by a single or multiple, temporally related events, which have presumably operated over most or the entire carbonate platform. A most likely event would be marine transgression, which occurred subsequent to relative sea-level fall (Campbell, 1962; Husinec and Jelaska, 2006; McLaughlin and Brett, 2007). Evidence of deposition by marine transgression rather than formation by collapse due to dissolution of sulphate beds includes: (i) the highly erosive boundary between the conglomerates and underlying beds, (ii) the presence of cross stratification and normal grading in the conglomerate beds, (iii) the overall dominance of subrounded shape of the pebbles, and (iv) the similarity in lithology of the pebbles to that of associated beds. Further evidence excluding a collapse origin includes the total absence of sulphate deposits or their relicts.

Diagenesis of the Conglomerates

The conglomerate bed, as well the entire Bih Formation, underwent replacive dolomitization, prior to the development of bed-parallel stylolites; dolomitization has in some cases preserved the textural features of the original limestone. The cement of the conglomerates consists mainly of relatively uniform, fine- to medium-crystalline non-planar to sub-planar dolomite (Figure 3b), which most likely precludes collapse origin of the conglomerate, because such origin would instead imply: (i) formation of large vugs rather than interparticle porosity, and (ii) cementation by coarse-crystalline equant calcite cement. Late diagenetic events include partial recrystallization of the host rock and fracturing with related cementation by dolomite, minor quartz, and calcite (Figure 3c).

Carbon, Oxygen, and Strontium Isotopes

The δ13CVPDB values of the dolomite matrix constituting the conglomerate fragments vary from -6‰ to -3.7‰ and those of δ18OVPDB from +1.3‰ to +1.8‰, whilst the δ13CVPDB values of the matrix dolomite of the entire Bih Formation vary from +0.5‰ to +4.0‰ and those of δ18OVPDB from -3.6‰ to -6.7‰ (Figure 3d). The 87Sr/86Sr ratio in two breccia samples is 0.7078 and 0.7080 for the dolomite matrix and the early diagenetic dolomite cement, respectively. The 87Sr/86Sr ratio of a dolomite matrix sample of the top part of the Bih Formation is 0.7084 (Figure 3e). These ratios are similar to O, C, and Sr inferred for Permian–Triassic seawater (Veizer et al., 1999), suggesting a marine origin for the dolomitizing fluids. Similar isotopic values of the dolostone pebbles and adjacent dolostone beds provide further support to the hypothesis that the conglomerate pebbles originate from erosion and subsequent re-deposition of the submarine carbonate sediments.

Fluid-inclusion Microthermometry

Homogeneization temperature (Th) and final ice melting temperatures (Tmice) of fluid inclusions hosted in the dolomite matrix and in the vug- and fracture-filling diagenetic minerals indicate that the diagenetic fluids had variable temperature and salinity through time, evolving from Th of 110–160°C and salinity of 12–18% during early dolomitization and early dolomite cementation up to Th of 160–220°C and salinity of 21–23% during precipitation of late diagenetic mineral (Figure 3f). Since the available data do not enable evaluation of the possible difference between the conglomerate and the rest of the Bih Formation, they give information more on the late diagenetic processes rather than on the comprehension of the mechanisms governing the formation of the conglomerate.

Conclusions

The so-called mid-Bih “breccia” in the Permian–Triassic Bih Formation, which crops out in Ras Al Khaimah, United Arab Emirates, is considered here as intraformational conglomerate formed by major marine transgressions. The great lateral extent of the conglomerate beds suggests that transgression resulted in erosion and re-deposition of carbonate sediments across the carbonate platform.

References

11720 Integrated biostratigraphy of the Saiq Formation (Al Jabal al-Akhdar, Oman Mountains) and its implication for the regional correlation of Khuff time-equivalent deposits

Holger Forke (Museum of Natural History Berlin, <holger.forke@gmx.de), Michael Pöppelreiter (Shell), Thomas Aigner (University of Tübingen), Bastian Koehrer (University of Tübingen), Lisa Walz (University of Tübingen), Daniel Bendias (University of Tübingen) and Marlene Haase (University of Tübingen)

We present a multi-disciplinary approach (sequence-biostratigraphy, palaeoenvironmental analysis, gamma-ray, and carbon isotopes) for the stratigraphic subdivision of Khuff time-equivalent deposits in Oman (Saiq and lower Mahil Formation, Al Jabal al-Akhdar). The Khuff is herein subdivided into six third-order depositional sequences. According to the proposed biostratigraphic zonation, the KS6 to KS5 is Middle Permian (Roadian?–Wordian–Capitanian), the KS4 to basal KS2 is Late Permian and the remaining part of KS2 and KS1 is Early Triassic. Major biotic crises occurred during the upper KS5 (“end-Guadalupian faunal crisis”) and in the basal KS2 (“end-Permian faunal extinction”). The transition from Middle to Late Permian is further accompanied by a palaeoenvironmental shift from a differentiated bioclastic ramp to more uniform microbial-mediated platform deposits. The biozonation and biotic events have been applied to correlate outcrop sections and subsurface on a regional scale across the Arabian Platform.

The Middle Permian to Lower Triassic Khuff Formation and its time-equivalent deposits represent a major target for hydrocarbon exploration in the Middle East. In order to enhance reservoir models, integration of data from outcrop sections and subsurface wells and from multiple disciplines (sedimentology, biostratigraphy, geochemistry and geophysics) are needed.

The Khuff Formation covers most of the Arabian Platform and represents over large areas a flat epeiric mixed carbonate-evaporitic ramp. Bio- and ecostratigraphic analysis of benthic foraminifera (e.g. Gaillot and Vachard, 2007; Hughes, 2009) has been applied for stratigraphic subdivision of the Khuff Formation due to the predominantly shallow water, restricted to open-marine environments.

Within the framework of a multidisciplinary outcrop to subsurface study, five sections on the Saiq Plateau and in the Wadis of the Al Jabal al-Akhdar area (Oman Mountains) have been investigated spanning the Khuff time-equivalent interval (Saiq and lower Mahil formations) in Oman. Stratigraphic subdivision herein is mainly based on the FOD of foraminiferal species, but biotic marker beds, palaeoenvironmental shifts as well as geochemical (δ13C) and geophysical (GR) data are integrated to correlate the sections on a local scale (10–50 km) (Figure 1).

Biostratigraphy and Biofacies in Oman Mountains

The basal part of the Saiq Formation displays open-marine, bioclastic ramp deposits with a diversified macro- and microfauna. Larger benthic foraminifera are present showing a much higher biodiversity than described elsewhere from the Arabian Platform with several species previously unrecognised. Verbeekinid, schwagerinid, schubertellid, and staffellid species are particularily common during the initial transgression, right above the eroded Pre-Khuff basement. The foraminiferal assemblage with Afghanella tereshkovae, Verbeekina grabaui etc. point to a Murgabian (Roadian?–Wordian) age.

Diversity decreases rapidly in the overlying muddy interval interpreted as the maximum flooding zone of the KS6. Fauna and flora recovers on top of the KS6 and becomes more diversified during the lower KS5, which is characterised by large bivalves (Alatoconchidae), cerioid corals and poorly preserved larger benthic foraminifera. Local variations of facies and fossil assemblages indicate a topographic relief from shallower to deeper palaeoenvironments in a W-E direction. The presence of the enigmatic fossil Sphairionia sikuoides indicates a Midian (Capitanian) age. The upper KS5 witnesses the gradual demise of the latter three groups (“end-Guadalupian faunal crisis”, Isozaki and Aljinovic, 2009; Bond et al., 2010) accompanied by a distinct shift from bioclastic ramp deposits towards more uniform platform deposits dominantly composed of non-skeletal grains with a high amount of microbial-mediated precipitation. The first occurrence of the miliolid foraminifer Shanita amosi and several other genera (e.g. Paraglobivalvulina, Rectostipulina) testify the biotic turnover in benthic foraminiferal assemblages during the latest Capitanian and represent one of the biostratigraphic key markers in the Khuff.

Late Permian deposits (KS4-basal KS2) are characterised by thick oolitic/peloidal/cortoidal shoal-related grainstone bodies in the Wuchiapingian with a poorly diversified fauna and flora. Furthermore, the extensive dolomitization hampers a precise taxonomic determination. Most of the occurring species are present already in the upper KS5, but the appearance of Neomillerella mirablis might serve as a potential marker within the KS4. The fauna becomes more diversified during the uppermost KS4 through basal KS2 with several Changhsingian marker species (Glomomidiellopsis uenoi, Paremiratella robusta and Paradagmarita monodi). Additionally, the macrofauna contains abundant fasciculate (Waagenophyllum) and solitary corals (Iranophyllum?) forming distinct marker beds, which can be followed laterally throughout the area.

The Permian/Triassic boundary can be traced in all sections by the abrupt loss of biota (“end-Permian faunal extinction”), the δ13C and the GR signals. The interval is accompanied by a rapid shift towards muddy offshoal deposits. Fossils remain very scarce throughout the Lower Mahil member (upper KS2 - KS1, Lower Triassic).

Regional Framework

In order to integrate the biostratigraphic scheme into a regional framework, the data were correlated with existing stratigraphic subdivisions and palaeoenvironmental interpretations from other parts of the Arabian Platfom (Insalaco et al., 2006; Maurer et al., 2009; Kolodka et al., 2011).

Biostratigraphic data from the Lower Khuff (KS6–KS5) across the Arabian Platform are rather scarce in the literature. However, existing data from Saudi Arabia, Oman and Iran show considerable variations in facies and fossil assemblages pointing to a stronger differentation of palaeoenvironments during this time, which can be seen even on a local scale. In the inner parts of the Arabian Platform, the lower part of the Khuff is represented by predominantly clastic deposits with reduced thicknesses. In some areas (e.g. Oman, parts of Saudi Arabia), the lower Khuff is dominated by mixed heterozoan-photozoan (bryonoderm-extended) faunal assemblages. Outcrop data from the outer platform in Iran (e.g. Kolodka et al., 2011) show similar faunal assemblages and facies in general, but foraminiferal species composition and stratigraphic range seems to vary locally.

The upper KS5 seems to represent a turning point towards more uniform foraminiferal assemblages. The Shanita amosi Zone has been recognized in several sections across the Arabian Platform and first occurs just below the deposition of the Median Anhydrite.

Several marker species serve as tie points in the Middle/Upper Khuff (Late Permian) to correlate the sequence-stratigraphic framework and confirm the uniform, layer-cake structure during this interval (Figure 2). Palaeoenvironmental interpretations of the fossil assemblages further enable to delineate subtle facies variations within the sections and regionally across parts of the Arabian Platform.

References

11827 The Khuff Formation in the Middle East: New insight into regional stratigraphy and palaeoenvironmental reconstruction using bio-assemblages and facies analysis

Jérémie Gaillot (Total), Aurélien Virgone (Total), Bruno Caline (Total, <bruno.caline@total.com), Grégory Frébourg (University of Geneva, Switzerland) and Franck Gisquet (University of Provence, France)

A multi-disciplinary synthesis of outcrop and subsurface data of the carbonates and evaporites of the Late Permian Khuff Formation was carried out in order to constrain the spatial and stratigraphic distribution of the depositional facies.

A robust biostratigraphic framework was built based on detailed description of alga-foraminiferal biotic content (Gaillot, 2006). The material studied includes 6,000 thin sections prepared from 16 cored wells in Iran, Qatar, Abu Dhabi and from four outcrop rock samples in Iran and southeastern Turkey. Comparison of microfossil distribution, mainly calcareous algae and foraminifers, within this large dataset has resulted in subdivision of the Middle and Late Permian Khuff deposits into eight units. Each unit shows a turnover level corresponding to significant reshaping of biotic assemblages characterized by higher biodiversity, which preferably occurs during the maximum accommodation phase of cycles before being isolated again during phase of lower accommodation in protected intra-shelf ‘lows’ (Figure 1). This suggests that the periods of higher biodiversity likely correspond to the flattest configurations of the Khuff platform, when the intra-shelf ‘lows’ are filled, and when extensive lateral shifts of biofacies are favored.

The Permian–Triassic transition is well constrained by sedimentological, biostratigraphical, geochemical and wireline log data over the studied area. The succession of events suggests that the platform-top experienced a major flooding event immediately before its near-complete emersion. During the latest Permian, the assemblages progressively migrated towards palaeohighs before the final drowning and demise of the green algal margin reef in the Zagros area. The rapid progradation of restricted facies belts seaward during the early transgression through the Permian/Triassic boundary allowed the microbial communities to overcome the latest Palaeozoic algal/foraminiferal assemblages in the studied area.

The detailed sedimentological description of the cores and outcrops has resulted in definition and characterization of 16 main depositional facies (Insalaco et al., 2006). These facies were used to interpret the depositional environment at a regional scale and for correlation of the Upper Khuff depositional systems across the entire epeiric shelf. Conceptual depositional models have been constructed for the main stratigraphic intervals after integration of the facies succession and biostratigraphic interpretations. These models evidence significant changes in platform type, subsidence regime, geometry, facies organization and climate within the four cycles (K4 to K1) of the Upper Khuff reservoir interval. The correlation and stratigraphic analysis suggests that the major stratigraphic trends and large-scale stratigraphic architecture are relatively isopacheous at production scale due to the almost flat platform geometry. At a larger scale, significant changes in thickness occur: either thickening towards palaeodepocentres or thinning with onlap towards palaeohighs. Palaeoecological results show that the structurally controlled palaeohighs are successively drowned and that the system evolves progressively from a rimmed platform towards an almost uniformly flat ramp.

The major oolite units developed within high-subsiding areas by sediment volume funneling, mainly during the late Wuchiapingian (upper K4 Sequence) and Early Triassic (K2 Sequence). Recent core examination has further constrained the interpretation of the uppermost K4 and the lowermost K3 intervals. Thin section examination under cathodoluminescence microscopy has evidenced a major subaerial exposure surface characterizing the sequence boundary of K4 Sequence. In addition, an oomouldic interval deposited at the top of the K4 unit has been recently re-interpreted based on detailed sedimentological core description using CT scanner imaging (Frébourg et al., 2010). Conversely to initial interpretation, the oomouldic interval does not show unequivocal criteria indicating marine deposition. Sedimentary and diagenetic features, which include pinstripe lamination, cross-stratification with angles > 15° and rhizolite traces, reveal aeolian deposition (Figure 2).

The depositional and stratigraphic interpretation of this multidisciplinary study has provided a consistent sedimentological and sequence-stratigraphic framework for the Upper Khuff in the Arabian Plate area. This coherent geological framework is used for regional exploration as well as prospect evaluation and field development. This framework has been also applied to reservoir-scale diagenetic and reservoir quality studies.

References

11893 The application of phytolith stratigraphy to ‘barren’ red-beds

Elisa Guasti (TNO, The Netherlands <elisa.guasti@tno.nl), Roel Verreussel (TNO), Timme Donders (TNO), Tom van Hoof (TNO), Renske Kirchholtes (University of California, Berkeley), Linda Garming (TNO) and Holger Cremer (TNO)

Significant oil and gas reserves occur in so-called “barren red-beds” in several regions around the world. Although red-beds occur throughout the geological timescale, examples with an economic interest are the Late Carboniferous, Permian and Early Triassic of Europe (Barren Measures, Rotliegendes, Bunter; Doornenbal and Stevenson, 2010), the Triassic of the United States, and the Permian–Carboniferous of the Middle East. These sediments are generally deposited in non-marine environments under arid climate conditions. In terrestrial basins, biostratigraphic analysis of plant microfossils (pollen and spores) is often effective in constraining static geological models. Unfortunately, such organic-walled microfossils are not preserved in red-bed deposits due to oxidation, hampering stratigraphic correlation on both local (field) and regional scale.

In our research, a group of biogenic siliceous microfossils, termed phytoliths, has been applied as biostratigraphic tool in the Permian–Carboniferous red-bed sequences. Phytoliths, or biogenic silica particles (BSPs), originate in the cells of higher land plants. After organic matter decomposition, phytoliths can be preserved as siliceous bodies.

The first steps toward applicable fossil phytolith taxonomy have been established by Garming et al. (2010). The combination of BSP stratigraphy based on range tops and bottoms and on abundance change provides the possibility to improve existing regional geological correlations, position regional unconformities and refine reservoir architecture models. Encouraging results are obtained in the Permian Upper Rotliegend Formation in the Groningeng Gas Field, The Netherlands. While further studies are needed to evaluate the BSP-based correlations and explore the origin of the observed cycles, strong evidence suggests that BSPs have potential to provide the basis for better stratigraphic correlations in “barren” deposits.

BSPs have been also discovered in the subsurface sediments of the Permian–Carboniferous Unayzah Formation in Saudi Arabia. Also in these gas-bearing sandstones, BSPs are a potential tool for subdividing and correlating the reservoir units (Garming et al., 2010; Cantrell et al., 2011).

In the Upper Rotliegend in the Southern North Sea Basin, BSP stratigraphy has been applied with the aim of refining the basin infill model of the Upper Rotliegend and to detect the position of the Base Permian Unconformity (BPU), which are critical aspects for E&P activities in the Southern North Sea. Significant changes in the quantitative composition of the BSP assemblages are recorded in all wells, and provide the most direct biostratigraphic information. Traditionally, the infill for the Upper Rotliegend sediments in the Dutch sector of the Southern North Sea basin is modelled with a general southwards thinning towards the basin margin (van Ojik et al., in press). One of the discussions in the Rotliegend basin development is whether the basin infill reflects an onlap situation, where lithological units step onto and over the basin edge as the basin fills, or a wedge shaped model, where layer after layer is stacked on top of each other over the entire basin area.

The BSP results indicate that most BSPs have long stratigraphic ranges and that numerical methods are needed to interpret the data. The quantitative results show significant shifts in the BSP composition that readily allow the distinction of three Assemblage Zones. Further statistical analysis on the dataset using Principal Component Analysis reveals probable cyclity. Spectral analyses on the PCA values indeed reveal cyclicity. In total, four to five cycles are recognized offering a still higher resolution to subdivide the aforementioned Assemblage Zones. The statistically significant trends and groupings are further supported by stable isotope analyses on the bulk samples. The oxidized rocks contain just enough organic matter to carry out δ13C-organic carbon isotope measurements. The isotope trends are used to corroborate the correlations based on BSP analyses. The combined results enable a stratigraphic correlation in these wells that a clearly overstepping of the basin margin (Figure 1) and prove that is possible to do biostratigraphy on ‘barren’ red-bed deposits.

Conclusions

This new biostratigraphic tool is a powerful technique within the context of understanding reservoir architecture and the ongoing exploration for gas fields in barren red-bed deposits. Further challenges include the coupling of the BSPs to parent plants for palaeoenvironmental interpretation, increasing taxonomic and stratigraphic resolution, and enhancing quantitative data interpretation, all to establish a high-resolution regional standard stratigraphy. Furthermore, additional studies outside the North Sea basin have shown that the tool holds great potential in other Permian–Carboniferous red-bed sediments in the Middle East, and in the Oligocene deposits of South America. In principle, the application of BSPs can be extended to any other geological time period with “barren” deposits.

References