ABSTRACT

The mid-Cretaceous Mauddud Formation is the main producing carbonate reservoir in the Raudhatain and Sabiriyah fields of northern Kuwait. Historical field information and results from waterflood pilots indicate that reservoir performance in these reservoirs is controlled by geological complexity at several scales. A detailed, integrated sedimentological and biostratigraphic investigation of the reservoirs, combined with dynamic reservoir data, have provided an understanding of Mauddud reservoir heterogeneity and of the principle controls on reservoir matrix behaviour. The largely carbonate Mauddud Formation overlies the Upper Burgan Member, a thick succession of fluvio-deltaic deposits, and consists of a diverse suite of carbonate facies deposited in low to high-energy, shallow-marine ramp settings. The basal part of the reservoir comprises mixed carbonate and siliciclastic sediments and reflects the establishment of a carbonate-dominated regime during waning supply of Burgan siliciclastic sediment. This system was eventually drowned and covered by the Wara Formation, a shaly offshore succession that is also the reservoir seal.

Sedimentary facies associations and microfossil assemblages within the reservoir are organised in a broadly upward-shallowing succession constructed of several transgressive-regressive cycles, which are defined by prominent, widely-correlatable flooding surfaces. Each cycle exhibits a characteristic internal stacking pattern of minor depositional cycles. Field-wide mapping and interpretation of facies within each cycle reveals a SW to NE, proximal to distal, trend consistent with regional seismic and palaeogeographic interpretations. The high-energy, inner to mid-ramp carbonate succession in the lower portion of the Mauddud reservoir is punctuated by siliciclastic incursions. Abrupt lateral facies changes, thickness variations, and local intra-reservoir erosion surfaces in this section suggest that deposition was influenced by subtle syndepositional tectonism. The upper part of the reservoir, in contrast, lacks significant siliciclastic influence and is made up of widely-correlatable, lower-energy carbonate facies, although local subtle facies variations show that the Raudhatain-Sabiriyah structures continued as palaeohighs during deposition. The contrast in quality between grain-dominated facies at the crests of the two structures and less grainy facies along their flanks was accentuated by carbonate cementation in the water legs of the reservoirs, largely in the form of calcite concretions of variable abundance. Cementation is most pronounced in low-energy wackestone facies, particularly in proximity to flooding surfaces where nodules may be amalgamated to form laterally continuous, cemented layers which are commonly fractured. Another significant, but contrasting, diagenetic modification within the reservoir was the generation of secondary macroporosity through dissolution of aragonitic skeletal components in a shallow to intermediate burial environment.

The stratigraphic evolution of the Mauddud reservoir, and its diagenetic overprint, in addition to post-depositional fracturing and faulting, created reservoir heterogeneities, which are critical to reservoir performance; one of the most significant of these is the relationship between horizontal and vertical permeability. Parasequences dominated by high-energy inner ramp grainstones, thin inner ramp rudist-bearing tempestites, and vuggy and fractured rudist floatstones and rudstones constitute thief zones that represent major challenges to reservoir management. In contrast, some cemented layers and flooding surfaces support pressure differentials of up to several hundred pounds/square inch (psi), thus complicating sweep and promoting reservoir compartmentalisation. The strong facies, diagenetic and stratigraphic controls on the distribution of thief zones and intra-reservoir baffles demonstrates how important it is to comprehensively understand reservoir sedimentology and stratigraphy when devising long-term development plans for reservoirs of this deceptively simple character.

More recent 3-D seismic data, production surveillance, and horizontal development wells show faults and fractures to be important heterogeneities in both reservoirs. Due to the immaturity of the water flood in the Mauddud reservoirs, the impact of these features on field and well behaviour is as yet unclear, but it is anticipated that the impact of such features on well and field performance will become more pronounced during later development.

INTRODUCTION

The Mauddud Formation reservoirs in the giant Raudhatain and Sabiriyah fields of northern Kuwait (Figure 1) were discovered about 50 years ago. Despite this long production history, these reservoirs are still at a relatively immature stage of development. As with many similar carbonate reservoirs in the Arabian Gulf, which have poor aquifers, it has eventually been necessary to implement water injection for pressure support (Abdul Azim et al., 2003). Under such circumstances, a detailed and accurate static description of the reservoir is a prerequisite for reservoir modelling and performance prediction, development planning, and effective reservoir management. This paper describes the reservoir characteristics of the Mauddud Formation in northern Kuwait, detailing in particular its sedimentological facies and the impact of diagenetic overprints. The facies distribution is described in a sequence stratigraphic framework and this provides an excellent foundation for a stratigraphic reservoir architecture, defining reservoir layers, and understanding permeability distribution.

The reservoir description presented here also illustrates the subtle complexities typical of carbonate reservoirs in the region, particularly with respect to the dynamic impact of depositional heterogeneities and porosity-permeability relationships.

FIELD HISTORY

Both Raudhatain and Sabiriyah fields contain multiple reservoir intervals, the principle producing zones being the fluvio-deltaic siliciclastic Zubair and Burgan formations and the shallow-marine carbonates of the Mauddud Formation (Brennan, 1991; Nemcsok et al., 1998; Al-Eidan et al., 2001). Although production to date has been dominated by the Lower Burgan Formation, the Mauddud Formation is likely to become the principal producing reservoir in both fields once full-field waterflood commences.

Oil has been produced from the Mauddud Formation reservoir in the Sabiriyah and Raudhatain fields since the late 1950s, under pressure depletion since there is little natural aquifer support. Offtake rates and predicted recovery factors have remained low under natural depletion and in order to increase offtake, pressure support is now being provided through the implementation of a full-field sea-water injection programme which commenced in late 1999 (Jones et al., 1997).

Prior to start-up of full-field waterflood in the Mauddud reservoirs, water injection was tested in pilot schemes in both fields in order to assess the displacement and sweep efficiencies of water (Al-Ajmi et al., 2000; Abdul Azim et al., 2003). From the start of the pilots, a comprehensive static and dynamic dataset was collected with which to monitor actual water movement between the injecting and producing wells. The directions and rates of water movement have provided valuable information on Mauddud reservoir anisotropy and has significantly improved understanding of internal lateral and vertical reservoir connectivity and of the role played by fractures and high-permeability layers in reservoir performance (Abdul Azim et al., 2003).

GEOLOGIC SETTING AND STRATIGRAPHY

The Raudhatain and Sabiriyah fields are situated in North Kuwait, 30–40 km northwest of Kuwait City (Figure 1). Both fields are domal structures, slightly offset from the northward plunging nose of the NS-trending Kuwait anticlinal arch, a Palaeozoic structure, which was periodically reactivated from the mid-Cretaceous to the Tertiary (Carman, 1996).

The Mauddud Formation, first defined in Qatar (Sugden and Standring, 1975), is a term that has been widely applied in the Middle East (Kuwait, Saudi Arabia, Bahrain and UAE) to the carbonate-dominated succession that resulted from the transgression that followed the deposition of the Burgan and Nahr Umr formations delta-related clastic-dominated sediments. Progradation of the Wara Formation delta-related depositional systems introduced the clastic-dominated sediments that overlie the Mauddud Formation in northern-central parts of the Gulf. Regional maximum flooding surface MFS 110 of Sharland et al. (2001; Figure 2; see also Davies et al., 2002) thus lies within the lower part of the Mauddud Formation dependant on position relative to transgressive retrogradation. The Wara Formation represents a flooding without a designated Arabian Plate MFS followed by MFS 120, which lies within the transgressive Ahmadi Formation and equivalents above the Wara Formation. Foraminifera recorded within the Mauddud Formation (e.g. Orbitolina seifini) indicate it is of essentially Late Albian age (Figure 2). The Mauddud Formation in Kuwait is thus contemporaneous with the lower part of the Sarvak Formation in adjacent Iran, the lower portion of the Natih Formation in Oman, and parts of the Harshiyat and Fartaq formations of the Yemen (Davies et al., 2002; van Buchem et al., 1996, 2002; Ellis et al., 1996).

Internally, the Mauddud succession comprises a number of smaller-scale stratigraphic cycles, an architecture which strongly impacts its reservoir behaviour. Mid-Cretaceous palaeogeographic reconstructions for the northern Gulf area show the Mauddud Formation to represent a north- or northeast-facing ramp at the southern margin of the Garau Basin, one of a number of intra-cratonic basins within the Arabian Plate, and to be coeval in its early stages with retreating shoreline and deltaic, shield-derived siliciclastic sediments of the Burgan Formation (Strohmenger et al., 2006).

In Raudhatain and Sabiriyah fields, the Mauddud Formation is c. 130 m thick and comprises a lower, interbedded mixed carbonate and clastic succession overlain by an upper, carbonate-dominated interval in which the bulk of the oil is reservoired (Figure 3).

SEDIMENTOLOGICAL FRAMEWORK

The foundation for the work reported here consists of detailed sedimentological and micropalaeontological description and analysis of c. 3 km of Mauddud reservoir core from over 30 wells in both the Raudhatain and Sabiriyah fields. This was augmented by analysis of an extensive petrographic dataset compiled from selected samples. This study allowed the construction of a generic, stratigraphically-based reservoir layering scheme for the Mauddud Formation in North Kuwait comprising 10 sedimentologically-distinct reservoir zones, defined here as layers MaJ to MaA in ascending order (Figure 3). This reservoir layering scheme was also subsequently successfully applied to the Mauddud reservoir of the Bahrah field to the south. The layering scheme is discussed later in the context of the sedimentological and stratigraphic framework.

Open-hole log and high-resolution borehole image data (FMI, STAR, EMI and CBIL) provided valuable sedimentological information in uncored wells. The descriptive, non-genetic facies scheme of Dunham (1962) and Embry and Kovan (1971), which is based on lithology, texture, and physical/biogenic components, was used to describe the cored Mauddud intervals. Integrated with detailed microfacies and biofacies analyses, and viewed in a stratigraphic context, the depositional facies were organised into predictive facies associations linked to specific depositional regimes (Figures 4 to 6). It should be emphasised that the geometries of depositional units are beyond the resolution of available 3-D seismic data.

Many of the samples examined petrographically contain microfossils, including foraminifera, calcareous algae and macrofossil (e.g rudist and echinoderm) fragments. These can be grouped into distinctive assemblages which, when associated with sedimentary microfacies, have been termed biofacies. Each biofacies can be interpreted in terms of depositional setting, principally with regard to palaeobathymetry (Banner and Simmons, 1994). Further insight into the depositional tolerances of Mauddud microfossils comes from interpretations based on morphological similarity to extant taxa and to known consistent sedimentary associations as summarised in Hughes (2000), Simmons et al. (2000), Davies et al. (2002) and references therein. Biofacies interpretation takes into account potential current and storm-related redeposition of assemblage components, but because of such factors, interpretations should always be treated as approximate.

The carbonate-dominated facies associations of the Mauddud Formation in northern Kuwait indicate deposition on a ramp or low-angle shelf, an interpretation which is consistent with other regional data and palaeogeographic reconstructions (van Buchem et al., 1996; Sharland et al., 2001; Strohmenger et al., 2006). Outer, mid- and inner ramp sub-environments can be recognised, although the depositional facies associations actually form a continuous facies spectrum. The inner ramp environment is defined as the zone above fair-weather wave-base; the mid-ramp environment is recognised between fair and storm weather wave-base; and the outer ramp environment occurs below storm wave-base (cf. Burchette and Wright, 1992). The offshore environment is characterised by hemipelagic deposition. Most of these settings can be identified within the Mauddud Formation, while the facies associations are comparable with many of those identified in the Natih Formation of the eastern UAE and northern Oman (van Buchem et al., 1996, 2002).

Siliciclastic shoreface deposits occur within the lower part of the Mauddud Formation succession and are interpreted as progradation of Burgan-style fluvio-deltaic systems (Al-Eidan et al., 2001; Strohmenger et al., 2006). Again, a depositional continuum is recognised, which represents the transition from offshore to lower shoreface and possibly upper shoreface environments. The term mudrock is used here for fine-grained siliciclastic sediments to distinguish them from fine, mud-grade carbonate sediments for which the textural term mudstone is used.

The key characteristics of the principal facies associations are summarised below.

Restricted Inner ramp Facies Association (FA1)

Facies Association FA1, unique to layer MaC near to the top of the Mauddud reservoir section (Figure 4), is characterised by highly bioturbated, clay-bearing skeletal wackestones interbedded with less common packstones and rare rudistid grainstones and floatstones (Figure 6a). The microbiota is particularly distinctive and comprises a moderate- to high-abundance, low-diversity assemblage, typically made-up of Ovalveolina, miliolid, nezzazatid and small textularid foraminifera, together with ostracods, Trocholina and Permocalculus algae. Bioturbation is expressed as a distinct and pervasive Thalassinoides ichnofabric which concentrates a weak nodular diagenetic overprint and largely obscures bedding. Locally within this association, thin beds (typically < 60 cm thick) of high abundance, low diversity rudistid-rich floatstones and grainstones with abrupt bed contacts are developed.

The dominance of micrite-rich facies suggests deposition in a non-turbulent environment, while the low biotic diversity, dominated by miliolids, alveolinids and calcareous algae indicates shallow water depths of probably less than 10 m together with some degree of environmental restriction and elevated salinity. The intense bioturbation overprint is consistent with low sedimentation rates. Grainstone layers and rudist floatstones are considered to be the products of storm reworking of rudist biostromes or build-ups in an otherwise low energy, lagoonal, setting.

Inner ramp Facies Association (FA2)

This association comprises massively-bedded, bioturbated or rarely flat-laminated and cross-stratified peloidal-skeletal packstones and grainstones (Figure 6b). The skeletal assemblage is dominated by conical (i.e. high-spired) Orbitolina foraminifera, together with smaller benthic foraminifera (including miliolids), coralline algae (commonly Lithophyllum), dasycladacean algae, Permocalculus, and fragmented and abraded mollusc debris.

The grain-dominated nature of these deposits, coupled with a high-abundance, high-diversity biota that includes green algae, conical (as opposed to discoidal) orbitolinids and miliolids is consistent with relatively high-energy shallow water conditions. The predominantly massive and bioturbated fabrics, with only rarely preserved mechanical stratification, suggests that turbulence was only periodic and that sediment was frequently reworked by an active infauna. These deposits are common throughout much of the reservoir and are often form the terminal parts of upward-coarsening or shallowing parasequences where they are penetrated by complex Thalassinoides networks. The cleaner grain-rich lithologies of this association are best developed in the middle and lower parts of the Mauddud succession (MaE to MaG, and MaI) while packstone textures dominate in the upper part of the succession (MaD and MaB; Figure 5).

The peloidal-skeletal packstones and grainstones are occasionally intercalated with units of massive rudist floatstone up to c. 12 m thick, which are virtually devoid of micritic matrix and therefore form a key reservoir lithology (e.g. MaD). The biota in these sediments consists entirely of bored and reworked, cm- to dm-scale, thick-walled rudistid fragments, which form both the larger clasts and the supporting, sand-grade matrix. In-place rudists are absent from core samples of these sediments, which suggests that they may represent poorly preserved biostromes or sand sheets generated by storms in areas adjacent to build-ups. No evidence for large build-ups or depositional margins is resolvable on available 3-D seismic data.

Mid-Ramp Facies Association (FA3)

Highly bioturbated, commonly clay-rich packstones and rare wackestones dominate this facies association (Figure 6c). These sediments contain a high-abundance, high-diversity biota that is dominated by discoidal (i.e. low-spired) Orbitolina, and smaller quantities of small benthic foraminifera, algae (coralline, dasycladacean and Permocalculus) as well as mollusc, echinoid and bryozoan debris. Detrital clay is concentrated in thin seams that define a weak, dm- to m-scale bedding. Bioturbation, while less prominent than in other facies associations, has obliterated other signs of stratification, although its impact is enhanced by a commonly intense nodular calcite cement overprint. The bed-bounding clay seams are compacted around these early post-depositional nodules.

The discoidal morphology of many of the Orbitolina in this facies suggests decreasing penetration of light (Simmons et al., 2000), and implies either elevated turbidity and/or an increase in water depth compared with facies association FA2. The matrix clay and moderate bioturbation are consistent with higher turbidity, suggesting a mid ramp depositional setting in which the grain-dominated material was deposited by storm-related processes and mixed with finer-grained lithologies through bioturbation.

Outer ramp Facies Association (FA4)

Clay-rich, highly bioturbated wackestones and mudstones containing a biota dominated by abundant discoidal Orbitolina with a minor assemblage of planktonic foraminifera, variable echinoderm, coralline algal and bivalve debris make-up this facies association (Figure 6d). The low-spired (highly flattened) morphology of the Orbitolina suggests low levels of light penetration, (Simmons et al., 2000), probably related to greater water depth (as indicated by the presence of planktonic foraminifera) compared with the mid-ramp facies association (FA3). Intense bioturbation has overprinted primary depositional textures. This association forms the bases to most upward shallowing parasequences and is particularly common within the basal (MaI to MaJ) and terminal, transgressive parts of the reservoir (MaA), immediately below the overlying package of offshore Wara Formation mudrocks (Figure 4). The intense bioturbation suggests low-energy conditions and the predominantly finegrained textures suggest deposition beyond the influence of all but major storms.

Siliciclastic Shoreface Facies Association (FA5)

This association is made up of bioturbated, mud-prone, fine-grained, typically glauconitic and clean, medium-grained sandstones (Figure 6e). The bioclastic component is sparse, but includes echinoderm and mollusc debris, quartz-agglutinating Orbitolina, and corralline algae. Planolites, Chondrites and Asterosoma burrows are abundant in the finer-grained silts and sands, whereas complex assemblages of Planolites and Teichichnus overprinted by Rhizocorallium and deep-penetrating Thalassinoides typify the cleaner sandstones. The coarsest sandstones occasionally exhibit weakly-defined, horizontal to gently-inclined parallel stratification.

These deposits are developed in two contrasting motifs:

  • Upward-coarsening/cleaning sections up to 20 m thick, suggesting progradation and shallowing from an outer-shoreface/offshore to possibly upper-shoreface/foreshore setting.

  • Fine-grained, glauconitic and skeletal-rich sandstones that form upward-fining, transgressive beds up to a few metres thick above sharp, commonly erosional contacts.

The progradational sandstones of this association are limited to the middle parts of the reservoir (MaH and MaF). The transgressive, reworked sandstones are common at the base and top of the reservoir, but also occur locally throughout the reservoir (Figure 4).

Offshore Facies Association (FA6)

This facies association is dominated by very sparsely skeletal, laminated and weakly bioturbated mudrocks (Figure 6f), which form the toesets to the progradational clastic shoreface deposits discussed above (Figure 4). In the uppermost Mauddud Formation, bored phosphatic, sideritic and, locally, glauconitic pebbles are concentrated. Borings are often early cemented. The mineralogy of the pebbles and abundance of early-cemented borings suggests that the pebbles represent reworked hardgrounds deposited on lag surfaces.

DIAGENESIS

The Mauddud reservoir has undergone variable, but often significant, diagenetic modification, which can be divided into three main phases:

Shallow-marine Diagenesis

The most significant diagenetic modifications to the Mauddud reservoir took place during and immediately following deposition. They are characterised by carbonate cements that predate mechanical and chemical compaction, and which often have a major impact on reservoir properties by reducing both pore volumes and transmissibility. Two key cement types are recognised:

Firmgrounds and hardgrounds, characterized by pore filling calcite and dolomite cements that are concentrated at parasequence tops, most commonly within inner and mid-ramp packstones and grainstones (FA1-3). Typically, these cemented bed tops are <1 m thick and are dissected by Glossifungites burrow networks, which can themselves contain glauconite. The cements preserve interparticle volumes, suggesting a pre-compactional origin, and the association between these layers and Glossifungites burrows is consistent with compaction and cementation during periods of reduced sedimentation. Evidence of reworked hardgrounds is provided by bored phosphate nodules in which borings are commonly infilled by dolomite (Figure 7).

Carbonate nodules, which are typically 2–15 cm in diameter and occur throughout the Mauddud reservoir, particularly within bioturbated mid- and outer ramp packstones and wackestones (FA3 and FA4). They are composed of microcrystalline calcite, which replaced the original micrite matrix and occluded micropores (Figure 7). The nodules have coalesced and amalgamated, locally forming carbonate-cemented layers, which in extreme cases may be more than 5m thick, but are of unknown lateral extent. Fabrics around the nodules indicate a pre-compactional origin, most likely at a shallow depth beneath the sediment-water interface, where large volumes of seawater were able to circulate. The nodules commonly overprint biotubation, suggesting that redistribution and sorting of the matrix by burrowing organisms provided flow pathways and nucleation points for the nodules. The predominance of nodular carbonate in deeper water facies implies that slow rates of sediment accumulation facilitated carbonate precipitation (cf. Claris and Martire, 1996; Mullins et al., 1980). Concentration of nodules at the bases of cycles, i.e. above flooding surfaces, supports this conclusion.

Shallow Burial Diagenesis

The shallow-burial realm is taken to encompass all processes that took place beneath the sediment-water interface, following the onset of mechanical compaction, but prior to significant chemical compaction. Petrographically defined paragenetic relationships indicated significant dissolution of allochems and matrix. Biomoulds after molluscs, benthic foraminifera and algal debris are common (Figure 8) and suggest preferential dissolution of aragonitic allochems. This would necessitate circulation of significant volumes of carbonate-understaurated fluid, whilst the nature of pore occluding cements implies that dissolution took place relatively early in the burial history. Potential fluid sources include evolved marine pore-water and aquifer-derived meteoric pore-waters which could have been driven downdip from the Arabian Shield or and modified by fluid-rock interaction. The non-luminescence of the innermost zone within pore filling cements would be supportive of such a source (Figure 7f). Although it is difficult to draw a clear relationship between stratigraphy and leaching, it is noteworthy that leaching and pore filling sparry calcite cements often concentrate at the tops of upward-cleaning parasequences within the inner ramp facies assemblage in Layers MaD to MaF, but can be difficult to correlate over even short interwell distances (< 250 m).

Deeper Burial Diagenesis

Deep burial diagenesis in the Mauddud reservoir embraces all processes that took place after the reservoir had been buried deep below the influence of circulating marine and meteoric pore waters. Diagenetic modification took place through cementation, fracturing and chemical compaction, with cementation being the most significant. Pore-occluding, coarsely-crystalline, drusy calcite cement occludes primary and secondary macropores and under cathodoluminescence and conventional staining, shows an evolution from a non-ferroan to ferroan mineralogy (dull to bright luminescence; Figure 7). This suggests precipitation from fluids that became increasingly reducing (oxygen-depleted) with time and is characteristic of fluid evolution during the transition from a shallow to a deeper burial environment. It is also consistent with dissolution of allochems and matrix early in the burial history. It is difficult to distinguish matrix and fracture pore-occluding cements visually (Figure 7), suggesting fluid circulation via a combined matrix and fracture network during the latter stages of cementation.

RESERVOIR PROPERTIES

The properties of the Mauddud reservoir are controlled substantially by depositional texture, which for individual facies associations has been preferentially modified by cementation and dissolution (Figure 8). Both porosity and permeability are wide ranging (1–35% and 0.01 to > 1,000 mD), over the entire dataset and for individual facies associations (Figure 9). In general, much of the reservoir is characterised by permeabilities of < 10 mD with permeability contrasts of more than two orders of magnitude across many bed boundaries providing key challenges to reservoir management.

The primary control on both porosity and permeability is depositional texture. Within an idealised parasequence (Figure 10), facies reveal an upward-evolution from microporous mudstones and wackestones to macroporous packstones and grainstones, with a corresponding increase in effective porosity. In general, cementation, especially carbonate nodules, is concentrated within mid- to outer ramp facies (FA3-4), degrading their porosity and permeability further. In contrast, solution enhancement of the matrix pore network is often best developed in mid- to inner ramp packstones and grainstones (FA1-3), increasing permeabilities to > 100 mD. However, compaction, cementation, dissolution and fracturing all influence core plug porosity and permeability generating the data ranges presented in Figure 9. Modal analysis of the petrographical dataset assessed the relative importance of each of these processes, the most significant of which is cementation and replacement by calcite (Figure 9), which reduces pore volumes and hence permeability.

Fractures, which are best identified on image logs, do not clearly influence permeability at the bed scale over most of the Mauddud reservoirs in Raudhatain and Sabiriyah fields, but the role of faults and fractures in controlling gross reservoir flow remains uncertain. The coarse, rubbled nature of core near the top of fourth-order coarsening-upwards intervals in the rudist floatstone facies (FA2) has so far precluded accurate core analysis; the core poroperm dataset consequently does not include measurement of the highest quality carbonate facies. The importance of the rudist floatstone to flow is best evaluated through comparison of well test KH data and PLT results with conventional core poroperm data. This suggests that it is not always necessary to invoke fracturing to explain well behaviour.

RESERVOIR LAYERING

The facies associations defined in the cored intervals of the Mauddud reservoirs have been related to open-hole log responses and to features resolved by high-resolution micro-resistivity and acoustic borehole image logs (FMI, STAR, EMI and CBIL). Detailed calibration to logs is problematic, however, because many of the depositional facies have been defined using variations in textures and biota, rather than petrophysical characteristics, and because the intense local diagenetic overprint (particularly nodular carbonate cementation) reduces the contrasts necessary for good facies discrimination on open-hole logs (Figure 5).

Depositional stacking patterns (e.g. transgressive versus regressive) were interpreted from the vertical distribution of key facies associations within each well and mapped within a framework of correlatable stratigraphic surfaces. For the most part, key correlatable surfaces represent distinct breaks in the stratigraphy, which are interpreted as flooding events and manifested by abrupt landward facies shifts, or field-wide clastic incursions. The recognition and correlation of major cycles within the Mauddud was enhanced by a high-resolution biozonation based largely on foraminiferal assemblages. The physical characteristics of the field-wide correlatable cycles were assessed by integrating core analysis, petrophysical, and production log data, to provide a dynamic reservoir layering scheme. On this basis, the Mauddud reservoirs of both Raudhatain and Sabiriyah fields can be subdivided into ten reservoir layers, labelled MaJ to MaA up-section, which although depositional in nature have a predictable reservoir quality character.

Reservoir Layer MaJ

Layer MaJ is 8–14m thick and comprises an interbedded succession of mudrocks and outer ramp wackestones (FA5), which record the progressive ‘start-up’ of carbonate production in an offshore position (Figure 12a). In the southern parts of both Raudhatain and Sabiriyah fields this layer contains stacked, upward-shallowing units of clastic mudrock and heterolithic facies, locally capped by thin, poor-quality sandstone layers which are the distal parts to backstepping clastic shorefaces (Al-Eidan et al. 2001; Davies et al. 2002). This layer forms the basal zone of Mauddud reservoirs in North Kuwait and so overlies the fluvio-deltaic Burgan reservoir. The uppermost portion of the Burgan Formation records waning supply of siliciclastic sediment from the Arabian Shield, expressed as regional backstepping of the Burgan delta. The thick interval of mudrocks, skeletal wackestones, and carbonate-cemented packstones within layers MaJ and MaI, and MaH together form the seal for the underlying Burgan reservoir and support a pressure differential of greater than 700 psi.

Reservoir Layer MaI

This unit varies in thickness from around 16 m in the northwest to around 6 m in the southeast and comprises a broadly upward-shallowing succession of facies, in which basal offshore siliciclastic mudrocks, pass upward via outer ramp wackestones and mid-ramp packstones to inner ramp packstones (FA5 to FA2). This interval is constructed of at least two smaller scale (c. 3 m), upward-shallowing cycles characterised by discrete basinward facies offsets and correlatable Glossifunigites surfaces. There is a general proximal-to-distal distribution of facies across each of the two fields, with a greater proportion of mid- to outer ramp facies in the north and northeast, whereas mid-to inner ramp facies occupy the southern part of the area (Figure 12b). Although a broadly linear NW-SE distribution of facies characterises the Raudhatain field the shallow-water facies are more prominently developed towards the crest of the field.

Reservoir Layer MaH

Layer MaH is a prominent, siliciclastic interval with a field-wide distribution that punctuates an otherwise carbonate-dominated reservoir succession which records the final significant northward progradation of the Burgan fluvio-deltaic system into northern Kuwait. In southern areas the layer comprises up to 20 m of mudstones, siltstones and bioturbated to weakly-laminated sandstones (FA5 and 6) organised as a broad, punctuated, upward-coarsening shoreface succession, but it thins northwards in both fields to become dominated by offshore deposits (FA6; Figure 12c). The uppermost metre of the section is composed of increasingly calcareous sandstone and terminates in a prominent Glossifungites burrowed cap. In the southern part of the crest of Raudhatain and the southern flank of Sabiriyah Field, the zone has been thinned by erosion to less than c. 3 m and the layer consists of a thin mudrock interval (Figure 12c), and the sand-prone, upward coarsening succession is absent.

The thickest section of this layer occurs within the Sabiriyah field, where the proximal sandstones are also best developed. The local erosion of this layer implies early development of the Raudhatain structure during or immediately after deposition. The local nature of the erosion, which does not appear to tie with any depositional trend, supports a tectonic interpretation rather than any more regional (e.g. eustatic sea-level) control on its development.

Reservoir Layer MaG

This layer comprises 8–25 m of inner ramp packstones and grainstones and mid-ramp packstones (FA2 to FA4), with rare outer ramp wackestones and shoreface sandstones. These form an upward-shallowing trend which culminates in a surface of maximum progradation since the overlying succession is retrogradational and fines upwards into deeper water facies. Within the Sabiriyah field there is a broad north to south, proximal to distal, facies distribution so that along the southern flank of this structure, the succession thins over the inferred palaeohigh, while to the north, additional localised abrupt deepening across the inferred paleostrike suggests that faulted lows within the otherwise gentle ramp profile influenced sedimentation (Figure 12d).

In the Raudhatain field, a more complex MaG facies distribution suggests a pronounced control on facies distribution by the evolving structure. The northern flanks of this field show thicker MaG successions, comprising largely deeper water facies (FA4 and FA6), whereas shallow water facies (FA2) dominate around the crest (Figure 12d). The latter include abundant rudistid dominated lithologies indicating that build-ups may have developed preferentially in this area, possibly along the footwall highs to syndepositional extensional faults. The presence too of relatively deeper water deposits in the MaG interval along the eastern margin of the Raudhatain field provide additional evidence that the two fields existed as discrete positive structures during Mauddud deposition. Siliciclastic shoreface deposits in this interval along the southern margin of the Raudhatain field represent the continued, episodic progradation of shoreface/delta sands from the Burgan area to the south.

Reservoir Layer MaF

This layer, ranging from 5–25 m thick, is characterised by a complex mosaic of mid- to inner ramp facies, offshore mudrocks, and locally shoreface sandstones deposited in a minor regressive-transgressive cycle (Figure 12e). Facies distribution in layer MaF shows predominantly carbonate deposition across both fields, although the inner ramp/lagoon was bordered to the south by shoreface sandstones and offshore mudrocks (Figure 12e). Possible synsedimentary faulting near the crest of the Raudhatain field led locally to a deeper-water environment represented by a thicker mud-dominated succession. Conversely, in the Sabiriyah field a syndepositional high is implied by a preponderance of inner ramp facies at the crest of the field.

Reservoir Layer MaE

Layer MaE is a 12–20 m thick, overall upward-shallowing, succession of mostly inner to mid-ramp deposits (FA2 and FA3). These facies are distributed on a broad southwest to northeast deepening ramp, although the shallowest water facies are biased towards the field crests (Figure 12f).

Rapid facies variation in this layer along the southern part of the Sabiriyah field implies local changes in palaeobathymetry, possibly as a result of syndepositional tectonism. Rudist floatstones are well developed in the southern part of Sabiriyah and suggest proximity to local build-ups, perhaps associated with faulted highs. Such deposits are juxtaposed against deeper water mid- to outer ramp facies (Figure 12f).

Reservoir Layer MaD

This layer is volumetrically the most significant reservoir zone. It comprises up to 28 m of mid-to inner ramp packstones and sparse grainstones (FA2 and FA3) in a stacked succession of poorly-defined, upward-coarsening and fining trends. In the top few metres, rudistid debris becomes more abundant, a phenomenon associated with gradational transition to the restricted inner ramp facies of layer MaC.

Spatially, with the exception of localised thickening into the hangingwalls of inferred syndepositional faults, this reservoir zone displays little palaeobathymetric variation across either the Raudhatain or Sabiriyah fields. Towards the north and northeast, minor deepening from inner to mid-ramp environments occurs (Figure 12g).

Reservoir Layer MaC

This layer comprises 3–10 m of restricted inner ramp wackestones, packstones and rare, interbedded rudistid grainstones/floatstones (FA1). In core, this layer appears to be relatively trendless, although gamma-ray log responses suggests an upward increase in clay content, culminating in a correlatable ‘hot’ spike (Figures 5, 11 and 12). The abundance of coarse- grained, rudist debris-rich beds increases towards the northern parts of both fields (Figure 12h).

In the Bahrah field (10 km to the south of the Raudhatain and Sabiriyah fields), this reservoir layer comprises interlaminated mudstones and miliolid grainstones. The upper, highest gamma portion of this trend records a minor shoreface sandstone incursion.

Reservoir Layer MaB

This layer, which varies in thickness from 1–10 m, comprises fairly homogeneous inner to mid-ramp packstones with a trendless aspect (FA3). Facies deepen generally towards the north and northeast, and as with other layers the shallowest water facies are concentrated around the field crests (Figure 12i). Outer ramp deposits occur along the southeast margin of the Sabiriyah field and hint at a structural control on deposition.

Reservoir Layer MaA

Layer MaA is a c. 7 m thick layer of outer ramp skeletal wackestones and laminated mudstones (FA4 and FA5), which form stacked, metre-scale upward-shallowing and upward deepening cycles (Figure 12j). At the top of the layer and immediately beneath the capping Wara offshore mudrocks are several correlatable Ostrea-encrusted sideritic and phosphatic hardgrounds.

DEPOSITIONAL MODEL

The Mauddud Formation of North Kuwait represents deposition on an unrestricted, northward-deepening carbonate ramp or low-angle shelf across which there were periodic proximal siliciclastic incursions from the retreating Burgan paralic system (Strohmenger et al., 2006). The lateral coexistence of, and interplay between, carbonate and siliciclastic facies in this setting is reminiscent of other Mesozoic depositional systems such as the Pinda Limestone of west Africa (Eichenseer et al., 1999) and the Neocomian of the Neuquen Basin in Argentina, and some modern systems in southeast Asia (see e.g. Tudehope and Scoffin, 1994; Wilson and Lokier, 2002). Similarly, evolution of the Mauddud platform architecture was governed by variations in accommodation space generated by the periodic interplay between eustatic sea-level change and clastic sediment input during a period of continuous subsidence. Although both fields are covered by high-quality 3-D seismic surveys, northward-directed clinoforms, which might be attributable to progradation of the ramp, have not been observed. Sediments in the Mauddud Formation were pervasively bioturbated and, consequently, preservation of current-generated sedimentary structures or bedding is rare. Rates of sedimentation appear to have been relatively slow and the dominance of micritic lithologies, particularly in the upper section, suggests that for much of the time deposition took place in a relatively low-energy setting. The principle inner and outer ramp dominated intervals promote a natural two-part subdivision of the Mauddud succession into a high-energy, mixed carbonate-clastic ramp in the lower Mauddud (Figure 13a; MaJ-MaF) and a low-energy carbonate ramp in the upper Mauddud (Figure 13b; MaA-MaE).

Grainstone-dominated facies, locally with preserved cross-stratification, are most prominent in the lower part of the Mauddud succession, suggesting more open, higher-energy conditions during that period. There are also several siliciclastic intercalations in this interval, pointing again to the close proximity of the Burgan siliciclastic sediment source. Relatively thick, but localised, rudist grainstones and floatstones are present in layers MaE and MaG, indicating the likely presence of rudist build-ups at this level. Unit MaH in this lower Mauddud section represents an ‘out-of-sequence’ clastic incursion which is truncated in places by a prominent erosion surface. Within this part of the reservoir, abrupt lateral thickness and facies variations hint that syn-depositional faulting or compaction influenced an otherwise uniform, shallow-water depositional system.

In the upper part, inner and mid-ramp facies alternate in numerous, minor shoaling cycles or parasequences, culminating in restricted inner ramp facies in Reservoir Layer MaC prior to the onset of platform drowning recorded by Layers MaB and MaA. However, the lack of well-defined vertical facies trends suggests that, from initiation, the Mauddud ramp experienced only relatively low rates of accommodation increase, and it appears to have accreted mostly vertically rather than being strongly pro- or retrogradational. More significant backstepping may have occurred towards the tops of reservoir layers MaG and MaI. In contrast to the lower sections, all the carbonate facies are strongly micritic and the shallow-water facies, in particular, lack widespread grainstone textures. The cyclic stacking of facies indicates a strong control by small-scale relative sea-level changes and the facies range that deposition occurred in a lower-energy environment. No significant high-energy shoal belt has been mapped within the field, although there is evidence for local rudist build-ups in the study area. Lateral facies transitions in the upper Mauddud are gradual and stratigraphic geometries appear to be layer cake, and largely uninfluenced by syndepositional topography.

SEQUENCE STRATIGRAPHY

Regional Sequence Stratigraphic Context

The low-order transgressive-regressive sequence stratigraphic interpretation of the Mauddud Formation is consistent with the interpretation of Davies et al. (2002), and supported by the observations of Strohmenger et al. (2002). Davies et al. (2002) suggest that the diminishing supply of siliciclastic sediment within the lower part of the Mauddud was due to transgression and the landward migration of facies belts. This gradual reduction in clastic supply from the Burgan delta permitted vertical growth of the Mauddud Formation carbonates. Within this broad sequence stratigraphic context, minor base-level shifts are expressed in the Mauddud succession as stacked shoaling cycles.

During deposition of the uppermost Mauddud (reservoir layers MaA and MaB), the ramp was outpaced by rising relative sea level, and finally drowned by the overlying Wara Formation offshore mudrocks, which represent pro-deltaic sediments originating from a rejuvenated Burgan delta. Regional facies mapping suggests that for much of this period carbonate and siliciclastic facies coexisted within cycles, with carbonate facies preferentially developed in offshore areas and siliciclastic shoreline sediments proximally (Davies et al., 2002).

Thinning of the Mauddud Formation towards the south of Kuwait, illustrated by Kirby et al. (1998), has been attributed by Strohmenger et al. (2002) to Mauddud erosion prior to deposition of the Wara Shale. Multiple hardgrounds exist at the top of the Mauddud succession (in reservoir layer MaA) in Raudhatain and Sabiriyah fields and indicate at least slow deposition with no evidence for erosion. It thus seems more likely that the thinning seen between northern Kuwait and the Burgan field is probably stratigraphic and related to the existence of a thicker, more proximal succession of fluvio-deltaic siliciclastic sediments in the Burgan Formation of the Burgan field to the south. In the Burgan area, continuous siliciclastic sediment supply is considered to have inhibited carbonate productivity, such that the Mauddud Formation is thinner, and on the whole younger, than in the north (Kirby et al. 1998). The southward backstepping of the Burgan shoreline siliciclastic facies belt is also consistent with the interpreted time-transgressive base to the Mauddud Formation (e.g. Davies et al., 2002; Strohmenger et al., 2002). Conversely, the demonstrably diachronous lithostratigraphic relationship between the Mauddud Formation and overlying Wara Formation (Davies et al., 2002) suggests that the Mauddud carbonate facies belts retreated towards the north as the supply of finegrained siliciclastic material resumed.

Sequence Stratigraphy versus Reservoir Layers

There is close correspondence between the depositional evolution of the Mauddud Formation and the reservoir layer framework discussed above since the reservoir layers mostly represent individual minor cycles or sequences. Lateral facies changes within the reservoir layers derive from the interplay between relative sea-level changes and growth of the incipient structures of the Raudhatain and Sabiriyah fields.

The offshore (?prodelta) mudrocks and outer ramp wackestones of reservoir layer MaJ at the base of the Mauddud form a strong contrast to the shoreface sandstones of the underlying Upper Burgan reservoir and record a significant reduction in the supply of siliciclastic sediment from the Arabian Shield following the regional K110 flooding event (Sharland et. al., 2001). Accommodation space created during this initial transgressive trend was infilled in reservoir layer MaI by a shoaling-upward carbonate depositional system which culminated in mid- to inner ramp packstones and grainstones. The MaI/MaH boundary is marked by a significant influx of fine-grained siliciclastic sediment, also corresponding to a minor base-level rise, which drowned the incipient ramp, followed by clastic shoreface progradation. In reservoir layer MaG, carbonate facies appear to have substituted for siliciclastic sediments without change in relative sea level, suggesting that the carbonate ramp environment re-established as siliciclastic supply waned.

The upper part of layer MaG exhibits facies backstepping in response to a relative sea-level rise, an event which culminated in a widely correlatable flooding surface at the base of MaF. This regressive-transgressive motif continues through reservoir layers MaE and MaD, as siliciclastic sediments retreated to the southernmost limit of the Raudhatain and Sabiriyah fields. Layers MaB and MaA display an upward fining and deepening trend, reflecting ramp backstepping, and represent a prelude to the deeper-water, pro-deltaic environment of the Wara Formation.

RESERVOIR HETEROGENEITIES AND DYNAMIC BEHAVIOUR OF THE MAUDDUD RESERVOIR

This study represents the first detailed geological analysis of the Mauddud reservoir in Raudhatain and Sabiriyah fields. Integration with a substantial well and production database has highlighted the important influence that depositional and diagenetic heterogeneities exert on fluid flow within the reservoirs. Data have been acquired over around 20 years of primary depletion. More recently, a 5-spot waterflood pilot in each of the fields has assessed the recovery efficiency provided by water injection (Al-Ajmi and Chetri, 2000; Al-Ajmi et al., 2000). The pilots have provided comprehensive static and dynamic data for these limited areas, including conventional open-hole log suites in all wells, continuous cores for the entire Mauddud Formation in five wells and RFT, PLT, and conventional plug kh data from all wells. These have permitted detailed calibration of the static interpretation within the pilot areas and, in particular, have allowed the geological basis for kh anisotropy and vertical water movement to be assessed.

Small-Scale Depositional Facies Controls

Original carbonate depositional textures in the Mauddud reservoirs have been variably overprinted by diagenesis, but continue to exert a significant control on reservoir quality and lateral heterogeneity. In contrast, vertical reservoir heterogeneity and dynamic reservoir layering is largely the product of relative sea-level changes and the accompanying broad-scale shifts in depositional environment, including the interplay between laterally co-existing carbonate and siliciclastic systems. The principle depositional heterogeneities and their impact on reservoir performance are outlined below (Figures 14 and 15).

High-permeability Layers

Until commencement of the waterflood pilot schemes, the Raudhatain and Sabiriyah reservoirs had been produced by primary depletion so that the impact of reservoir heterogeneities on production had not been clear. The pilots have demonstrated that high-permeability layers of various kinds exert particular control on the rate of horizontal fluid movement in the Mauddud reservoirs. They form thief zones through which injection water breaks through to production wells much more quickly than predicted by intial simulation modelling. It became critical, therefore, to determine the origin, distribution, and geometries of these high-permeability intervals in order to be able to provide forecasts of reservoir performance. High-permeability layers, or “super-k zones”, in the Mauddud reservoirs, have the following origins:

Rudist storm deposits: Within reservoir layer MaC several discrete, highly-permeable, rudist-fragment grainstones and floatstones occur. Each is less than 1 m thick, caps a fourth-order depositional cycle, and so is sandwiched between poorer quality lagoonal pack/wackestones. As indicated, such beds are more common in the northern parts of both fields. They may represent, in part, storm-generated spillovers or sheets from an inner ramp facies belt located to the north of the Raudhatain and Sabiriyah fields. Some layers may have been derived through the reworking of adjacent rudist biostromes. Inter-well correlations suggest that these beds are laterally extensive on a km scale, rather than field-wide, often linking pairs or small groups of wells.

Such layers are particularly important since they occur within a laterally continuous reservoir interval high in the reservoir. They are commonly the sites of first water appearance in producing wells located up to 0.75 km from injection wells, with breakthrough sometimes occurring within a matter of days. In particular, PLT data from the waterflood pilots shows that water inflow is often very good where these layers occur in injection wells. Moreover, marked increases in post-stimulation injection rates, suggest that acid treatments disproportionately enhance near-wellbore permeabilities in these macroporous, vuggy beds.

Rudist biostromes and bioherms: Trendless packages up to 12 m thick of extremely high-permeability rudistid floatstone and grainstone are developed locally in both the Raudhatain and Sabiriyah fields in reservoir layers MaG, MaF and MaE. These facies are interpreted to represent storm deposits and material reworked from so far uncored rudistid build-ups. They appear to have relatively restricted (< 1 km) correlation distances. High rates of injection-water inflow and oil production are experienced in these zones and, in closely-spaced development wells, they are associated with faster rates of water movement.

PLT logs show that the highest part of reservoir layer MaD makes a consistent, significant contribution to the vertical production profile of the Mauddud reservoirs. However, cores from the upper part of this interval are commonly unrecovered or are reduced to rubble. The high abundance of rudist fragments within the rubble intervals suggests that such sections represent porous, vuggy rudist-dominated patches or local build-ups. This section occurs within the transition from a high-energy inner ramp setting (FA2) to a lower-energy, possibly lagoonal environment (FA1) in the overlying reservoir layer MaC.

High-energy, inner ramp packstone/grainstone layers: These facies are well developed in reservoir layers MaG to MaE where they form high porosity and permeability caps to upward-shallowing cycles. Permeabilities in such layers are at least an order of magnitude higher than those of the enveloping mid-ramp packstone facies. During waterflooding these deposits perform in a similar fashion to the rudist biostromes described above. Water uptake in these layers in injector wells is pronounced. Water breakthrough in the closest producers is often earlier than anticipated, particularly in reservoir layer MaE in Sabiriyah field where peloidal- skeletal grainstones form part of a c.10 m-thick high-energy inner ramp sand shoal belt. Where rudist floatstone layers in the upper part of the succession are not laterally continuous, injected water appears to slump into underlying layers and, on reaching the peloidal-skeletal grainstone may be channelled laterally, reducing sweep efficiency. Northward facies transitions into a lower-energy mid-ramp packstone-dominated succession reduce the potential of these deposits to form thief zones in the northern parts of both fields.

Clastic shoreface deposits: In the lower portion of the Mauddud reservoir, the permeable siliciclastic shoreface succession within reservoir layer MaH represents a significant contrast to the encasing carbonate and mudrock lithologies. In Sabiriyah field, in particular, clean, upper shoreface sandstone beds close to the tops of stacked, metre-scale progradational parasequences form conduits which preferentially channel flow. Lateral facies changes to the north into more distal, mud-prone lower shoreface sandstones and siltstones mitigate the effects of such layers. The local erosion of this layer across syn-depositional highs in the south east of Raudhatain field has also removed these potential thief zones in this area. Since reservoir layer MaH often remains unperforated, and it is separated from the nearest perforated zone by a permeability barrier, the clastic shoreface deposits are poorly swept and so are not considered to represent significant thief zones in either field.

Intra-reservoir Baffles

Flow baffles in the North Kuwait Mauddud reservoirs are defined here as layers that are correlatable between wells, typically over distances of up to 1km, across which original pressure differentials (ie. at the onset of production) of >5 psi have been observed in production data. Such layers, while not necessarily capable of supporting very large pressure differentials, may form inconvenient baffles or barriers on the production timescale, complicating reservoir sweep and the provision of pressure support.

While potential baffles can be predicted in part using stratigraphic and depositional principles, detection has only been possible through integration of core observations with RFT data. Flow barriers are also detected using geochemical (residual salt analysis) techniques. Barriers to vertical permeability have been detected primarily within the main reservoir interval (layers MaB to MaE) where they can be classified into three main types:

Skeletal wackestone layers deposited in a restricted inner ramp setting (FA1) can be correlated at least locally (e.g. within the waterflood pilot area), and are typically microporous with poor permeability. Permeability has often been further degraded by nodular calcite cementation.

Carbonate cemented layers, including diffuse zones of nodular carbonate and carbonate-cemented parasequence tops; this phenomenon typically affects grainstone and matrix-poor packstone lithologies in a stratiform fashion, reducing the properties of the better quality reservoir rocks to the extent that they form vertical baffles. The distribution of these cements varies on a well to well basis and the correlation length of such features is unknown.

Flooding surfaces: Decametre-scale transgressive-regressive cycles form the principal components of the reservoir architecture. Each exhibits an upward increase in reservoir quality, and therefore flow capacity related to the change in style of porosity from micro- to macropore networks. Pressure data shows that the boundaries between such parasequences, particularly where diagenetically enhanced, commonly act as baffles to vertical fluid movement and that these may support small pressure differentials. Such layers exhibit the widest correlation lengths in the Mauddud reservoirs and have the potential to form field-wide baffles or at least baffles over significant areas.

Siliclastic mudrocks within layers MaG and MaH form a major barrier to fluid movement, as revealed by pressure differentials of up to 1,500 psi.

Diagenetic Baffles and Barriers

A principle control on Mauddud reservoir quality is early-diagenetic nodular carbonate cementation. Such nodules can coalesce to form layers several metres thick (see section Diagenesis), the thickest of which are found in two principal locations:

Within deep-water, mud-prone intervals and slowly deposited layers: Nodule growth appears to have been promoted by low rates of sedimentation (see Diagenesis), and probably occurred at the seafloor or just below the sediment-water interface. Geochemical (Iatroscan) data show separate asphaltene trends above and below reservoir layer MaH, suggesting that it acted as a major flow barrier during hydrocarbon emplacement, and thus supporting an early burial origin. In reservoir layer MaF, stacked, upward-shallowing parasequences contain decimetre-scale layers of nodular calcite cements. In the waterflood pilot areas, this zone clearly supports pressure differentials of up to 50 psi in Sabriyah and contributes to differences of up to 1,500 psi in the Raudhatain field.

Beneath the oil-water contact: Both fields exhibit more intense carbonate cementation with depth within the Mauddud reservoirs, and towards the field flanks. This suggests that nodular carbonate precipitation initiated during early burial continued in the aquifer into the burial realm, but was terminated as the reservoir charged. The resultant down-dip degradation in Mauddud reservoir quality in both fields suggests that patterned waterflood is likely to be a more effective development strategy than peripheral waterflood.

Impact of Fracturing on Reservoir Behaviour

A detailed, core-based fracture study of the Mauddud reservoir in both Raudhatain and Sabiriyah fields suggest that, while fractures exist, open fractures are sparse (c. 0.1 per metre) and their impact on dynamic reservoir performance remains unclear. Most fractures observed in core are of short length (1–70 mm) and concentrated within and around competent carbonate nodules and rudist shells; most are below the resolution of the borehole imaging tools. Such fractures do, however, increase the measured permeability in some core plugs from carbonate concretions to unrepresentative values and so necessitate careful editing of conventional core analysis data.

The most recent data from 3-D seismic interpretation, from production surveillance, and from new horizontal wells suggest that fault density is higher than previously anticipated and represents a significant influence on well and reservoir performance. The impact of reservoir heterogeneities typically becomes more pronounced as production rates and field depletion increase and where recovery processes with several fluid phases are involved. The historically low depletion rate for the Mauddud reservoirs in these two North Kuwait fields means that important heterogeneities, such as faults and fractures which have so far been relatively unproblematic, may yet resolve as significant production issues.

APPLICATION TO OTHER MIDDLE EASTERN RESERVOIRS

Many of the reservoir characteristics and production-related issues discussed above with respect to the Mauddud Formation in Raudhatain and Sabiriyah fields are common to Cretaceous reservoirs throughout the Middle East and can be regarded as generic problems affecting reservoirs of this age in the region. Less typical of the region are the siliciclastic incursions that occur within the basal Mauddud reservoirs as a result of their proximity to the Arabian Shield source area. Principle reservoir features are controlled by the stratigraphic architecture of the mid-Cretaceous depositional systems, broad, ramp-like and pervasively cyclic carbonate platforms which have developed over the whole of the Arabian passive margin. In this intra-shelf setting, depositional environments were characterised by low-turbulence, shallow depositional slopes, and facies boundaries that are gradational over large distances. The absence of abundant frame-building organisms at this time also meant that organic sediment production, which was dominated by rudist bivalves and the erosional products of their skeletons, was mostly particulate and fine grained. Post-depositional compaction and cementation, often expressed as a decrease in reservoir quality towards the oil- or gas-water contacts, is a function of the relatively shallow burial depth of the Cretaceous reservoir formations at the time of regional hydrocarbon migration and charging during the latest Cretaceous and early Tertiary. While diagenesis was largely terminated in the hydrocarbon-filled reservoir pore spaces, cementation and compaction continued in the aquifers around many fields during further burial (cf. Dunnington, 1967; Neilson et. al., 1998).

In particular, the low-permeability, microporous matrix properties of these Mauddud reservoirs are not only similar to Mauddud reservoirs elsewhere, but to many other early and mid-Cretaceous reservoirs in the Thamama, Bangestan and Wasia Groups and their equivalents in the UAE (O’Hanlon et al., 1996; Grötsch et al., 1998), Bahrain (Shehabi and Kollourii, 1987; Wolf et al., 1993), Oman (Harris and Frost, 1984); Saudi Arabia, Iraq and Iran. The upper, shallowest-water parts of shoaling-upwards depositional cycles commonly contain distinctive, coarse-grained, vuggy rudistid rudstones with significantly higher measured permeabilities than the background matrix and strongly contrasting relative permeability characteristics (cf. Dabbouk et al., 2002). The coarse, vuggy facies are distributed variably as reservoir zones up to 20 m thick at the tops of progradational highstand intervals, or as thin beds alternating cyclically with low-permeability matrix in more transitional mid-ramp or slope reservoir sections. Individual coarse-grained layers originated variably as biostromes, isolated or amalgamated tempestites, or as the bases of high-frequency depositional cycles, and typically can be correlated at the inter-well scale for distances of only hundreds of metres to a few few kilometres. Outcrop studies confirm lateral facies variations of this scale order for such beds in the Kharaib Formation (van Buchem et al., 2002).

As in the Raudhatain and Sabiriyah Mauddud, the high permeability contrasts between coarser, vuggy layers and background matrix properties can be up to several orders of magnitude in all such reservoirs. Fracture permeability, and the role which fractures play in reservoir performance, is also often underestimated (see e.g. Abdul Azim et al., 2003). This often complicates recovery during water or gas flood by promoting early breakthrough of injection fluids and, if not predicted, necessitating shut-in of producing wells or the “premature” introduction of water or gas-handling capabilities. The small thicknesses of such beds (often as little as 15 cm) means that, although easily identifiable in cores, in many cases they cannot be reliably detected on conventional wireline logs in uncored wells (see e.g. Akbar et al., 2000). Since the log databases for many reservoirs in the region with 15–20 years of developmental history typically contain only a small proportion of modern, high-resolution image logs, the distribution of such zones can be difficult to characterise accurately. The application of statistical modelling techniques in static reservoir descriptions is often required. Caution is also necessary in upscaling static models for simulation (see e.g. Giot et al., 2000) since close attention is required to the vertical reservoir layering. It is often important to discretely retain much of the vertical stratigraphic reservoir heterogeneity in order to obtain acceptable matches to well production history, water/gas breakthrough times, and the well-bore in-flow profiles indicated by production logs.

ACKNOWLEDGEMENTS

This paper is based on a proprietary studies of the Raudhatain and Sabiriyah Mauddud reservoirs commissioned by Kuwait Oil Company and BP, which were undertaken while Nigel Cross, Ian Goodall, Cathy Hollis and Imelda Gorman Johnson were employed by Badley Ashton and Associates Ltd. The authors would like to acknowledge additional support by Nigel Rothwell, Martin Smith, Dirk Bodnar, Bob Jones, Craig Rice, Jim Lantz, and Jeff Wedgewood. The authors are grateful to Kuwait Oil Company and BP Exploration Operating Co. plc. for permission to publish this work. Thanks are extended to 2 anonymous reviewers who’s suggestions greatly improved the original manuscript, and to Niño Buhay of GeoArabia for drafting and design of the final published manuscript.

ABOUT THE AUTHORS

Nigel Cross is a Geological Advisor with BG Group, where he has worked within their Egyptian and Trinidadian assets since 2004. Prior to BG, Nigel was a Development Geologist with Hess in the UK, and Petro-Canada (UK) Ltd, and specialised in the development of carbonate reservoirs in North Africa and the Far East. Nigel started his career as a Sedimentologist with Badley Ashton and Associates between 1996 and 2000.

nigel.cross@bg-group.com

Ian Goodall is an Independent Consultant Geologist with Goodall GeoScience Ltd based in Lincolnshire, UK, which he founded in 2000. Ian has over 20 years global oil industry experience and specialises in the application of high-resolution borehole image logs to the development of geologically realistic reservoir models. Ian has a research background in arid zone carbonate-evaporite depositional systems and began his career as a Reservoir Geologist with Badley Ashton and Associates.

ian@goodall-geoscience.co.uk

Cathy Hollis is a Senior Lecturer in Production Geoscience and Petrophysics at the University of Manchester in the UK. Before that, she was a production Geologist with Shell International Exploration and Production (SIEP) in The Netherlands, where she led the Carbonate Research Team. She joined Shell from Badley Ashton and Associates in the UK and Abu Dhabi. Cathy specialises in carbonate diagenesis and pore system analysis, and she has worked extensively in Kuwait, Oman and Abu Dhabi.

cathy.hollis@manchester.ac.uk

Trevor Burchette has a background in sedimentology and stratigraphy and 33 years of global experience in the exploration for, and the characterisation and development of, carbonate reservoirs of all ages. He currently advises in this field in BP Exploration’s Middle East and South Asia Strategic Production Unit in Sunbury, UK. Trevor has also developed BP’s carbonate training programme for many years and administers an internal BP carbonate reservoir network.

trevor.burchette@uk.bp.com

Hussain Z. D. Al-Ajmi is team leader of field development for Greater Burgan, South Kuwait, Kuwait Oil Company (KOC). He spent most of his career at KOC as Reservoir Engineer. Prior to his current assignment he was Senior Reservoir Engineer working on the Raudhatain field, in KOC’s North Kuwait Field Development Group.

hussain@kockw.com

Imelda Gorman Johnson is a Senior Exploration Geologist with ExxonMobil. Imelda began her career with Badley Ashton and Associates in 1999 and moved to ExxonMobil in 2002. Her work has focused on the impact stratigraphic hierarchy, tectonics, and diagenesis have had on reservoir development in carbonate depositional sequences; in the Middle East, west Texas, sub-Saharan Africa and northern Europe.

imelda.g.johnson@exxonmobil.com

Raja Mukherjee is a Senior Production Geologist with Shell EP. From 1995 to 2003, he worked with the north Kuwait field development team in Kuwait Oil Company. During this period he was extensively involved with reservoir characterization and development planning of Mauddud reservoir of north Kuwait fields. In 2003 he moved to Petroleum Development Oman (PDO), where he worked in the Study Centre and carried out number of different reservoir characterization studies of clastic and carbonate reservoirs for water flood and EOR projects. Raja is currently working in the Gas Directorate of PDO as production geology discipline lead. He holds an M.Tech. degree in Applied Geology from University of Saugar, India. He began his career with Oil and Natural Gas Corporation (ONGC) of India in 1983 and worked in various capacities on exploration and development projects. His field of interests are reservoir characterization and reservoir management of clastic and carbonate reservoirs.

raja.mukherjee@pdo.co.om

Mike Simmons is Director of Earth Model at Neftex Petroleum Consultants Ltd., a consultancy specializing in global sequence stratigraphy and its applications to hydrocarbon exploration and production. He has responsibilities for the development of the Neftex Sequence Stratigraphic Model. Mike oversees the application of the model in Neftex’s regional studies and carries out sequence stratigraphic studies for clients. Previously he was Director and Chief Geologist of CASP at Cambridge University and the Head of the Department of Geology and Petroleum Geology at the University of Aberdeen in Scotland. Mike spent 11 years with BP Exploration working as a Senior Geologist/Biostratigrapher, specializing in the Middle East and Former Soviet Union regions. He holds BSc and PhD degrees from Plymouth University, UK. Mike is the Neftex Editor for GeoArabia.

mike.simmons@neftex.com

Roger Davies is Projects Director and co-founder of Neftex Petroleum Consultants Ltd. Roger has over 25 years of oil industry experience starting with BP and working as an independent geoscience consultant before co-founding Neftex in 2001. Within Neftex, Roger leads projects for clients and has a fundamental role in the development and application of the Neftex Sequence Stratigraphic Model. He has a PhD in Carbonate Sedimentology and Micropalaeontology from Southampton University, UK, and a BSc in Geology from Bristol University, UK. His early career was spent as a Sedimentologist working worldwide on carbonate and clastic reservoirs for BP.

roger.davies@neftex.com