Reservoir Description of a Mid-Cretaceous Siliciclastic-Carbonate Ramp Reservoir: Mauddud Formation in the Raudhatain and Sabiriyah Fields, North Kuwait

GeoArabia, v. 15, no. 2, 2010, p. 17-50 Gulf PetroLink, Bahrain

Reservoir description of a mid-Cretaceous siliciclastic-carbonate ramp reservoir: Mauddud Formation in the Raudhatain and Sabiriyah fields, North Kuwait

Nigel Cross, Ian Goodall, Cathy Hollis, Trevor Burchette, Hussain Z. D. Al-Ajmi, Imelda Gorman Johnson, Raja Mukherjee, Mike Simmons and Roger Davies

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,

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Cross et al.

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).

18 18

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Mauddud Formation, Raudhatain and Sabiriyah fields, North Kuwait

47°E 47°30 48° 48°30 Umm Qasr 30°N Abdali Southeast Ratqa Raudhatain Ash-Shaham

Sabiriyah Bubiyan Island Mutriba

Bahrah

29°30 Failaka 29°30 KUWAIT Medina

Kuwait City Khashman

Dharif

Ahmadi Rugei Abduliyah

Minagish 29° 29° Burgan Umm Gudair

34°E 38° 42° 46° 50° 54° 58° 38°N TURKEY Caspian 38° Sea CYPRUS SYRIA N 34° LEBANON 0 300 34° Med Sea IRAQ SAUDI ARABIA km JORDAN 30° 30° Gulf KUWAIT IRAN of Suez South Umm Gudair BAHRAIN 26° SAUDI ARABIA QATAR SAUDI ARABIA- EGYPT KUWAIT Arabian UAE 22° Shield Partitioned OMAN N SUDAN Red 0 25 Neutral Zone Sea 18°

Wafra ERITREA YEMEN Arabian km Sea 28°30 14° 14° SOCOTRA ETHIOPIA Gulf of Aden 47°30 48° 34° 38° 42°48°3046° 50° 54° 58°

Figure 1: Simplified maps of the Arabian region showing Kuwait and the location of the Sabiriyah and Raudhatain Fields.

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) thuslies

19 19

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Cross et al.

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).

Chronostratigraphy MFS / TMS Eustasy Age Continent Shelf Preservation Age and Tectonics Southwest Northeast 3 2 1 (Ma) 200 m 100 0 Santonian Saudi Arabia Kuwait Zagros Con- Halul AP9 iacian 88 Aruma Laffan K150

90 Active northeast margin Tur- 90 Burgan Arch

Late onian 92 S 93 HST Mishrif Sarvak K140 Ceno- S 95 K130 manian Ahmadi Rumaila S 98 Wara Mauddud K120 100 a Figure 3 100 si 101 100 m K110 ethys Wa Burgan 3rd sand S AP8 Albian 106 TST K100 Kazhdumi 110 Balambo 110 K90 111

ACEOUS Burgan 4th sand HST 116 Atlantic opening

CRET Aptian K80 Shu'aiba Dariyan 120 120 K70

Early Zubair Gadvan 123 Barr- K60 emian TST 126 K50

Haut- 129 estern uplift due to 130 K40 130 erivian W Ratawi Shales Fahliyan

Val- Thamama HST anginian 136 Equatorial post-rift passive margin with Neo-T Habshan II, III K30 Ratawi Limestone 138 Minagish K20 Berr- 140 140 iasian Habshan I 143 Makhul/Garau S K10 147 Tithonian TST J110

Late 149 Oman rifting JURASSIC 150 Hith AP7 150 Figure 2: Schematic chronostratigraphic section of the early to mid-Cretaceous across the Arabian Gulf area (from Sharland et al., 2001; Davies et al., 2002). The Mauddud Formation is Late Albian in age and highlighted by the red box (Figure 3).

20 20

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Mauddud Formation, Raudhatain and Sabiriyah fields, North Kuwait

SEDIMENTOLOGICAL FRAMEWORK Facies Stratigraphy

Wara The foundation for the work reported here consists of detailed sedimentological and micropalaeontological MaA description and analysis of c. 3 km of Mauddud reservoir core from over 30 wells in both the Raudhatain and MaB Sabiriyah fields. This was augmented by analysis ofan extensive petrographic dataset compiled from selected MaC 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). MaD This reservoir layering scheme was also subsequently successfully applied to the Mauddud reservoir of the

Upper 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 MaE 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 MaF organised into predictive facies associations linked to specific depositional regimes (Figures 4 to 6). It should be MAUDDUD emphasised that the geometries of depositional units are beyond the resolution of available 3-D seismic data.

Many of the samples examined petrographically contain MaG 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 Lower palaeobathymetry (Banner and Simmons, 1994). Further ? MaH insight into the depositional tolerances of Mauddud

Mauddud Facies Associations ~10 MaI Restricted Inner Ramp (FA1) Outer Ramp (FA4) Inner Ramp (FA2) Clastic Shoreface (FA5)

m Mid Ramp (FA3) Offshore (FA6)

MaJ 0 Figure 3: Schematic reservoir stratigraphy of the Mauddud Formation in the Raudhatain and Sabiriyah fields. This stratigraphic column highlights the dominant facies Upper Burgan associations that make up the reservoir layers.

21 21

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Cross et al.

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.

Facies Description Characteristics Facies Association

Forms sub-metre scale sharp-based beds. Lf Rudist floatstone Rudists commonly cm-scale fragments.

Clay-prone skeletal wackestone 1 Lwc(o) containing restricted foraminiferal Foraminifera include Ovalveolina, FA fauna nezzazatids, miliolids and textularids. Large Thalassinoides often concentrate Clay-prone skeletal packstone a weak nodular cement. Lpc(o) containing restricted foraminiferal

Restricted Inner Ramp fauna

Lgl Laminated skeletal grainstone

Commonly peloidal within the grainstone

2 textures and includes a diverse open Lgb Bioturbated skeletal grainstone marine biota, including conical Orbitolina, FA coralline alage (Lithophyllum), dasycladacean algae, Permocalculus. Inner Ramp

Lpb Bioturbated skeletal packstone

Clay-prone skeletal packstone Lpc(d) containing mixed conical to Typified by strong Thalassinodes

3 discoidal Orbitolina ichnofabrics, which concentrate nodular

FA cements and are surrounded by deformed

Mid-Ramp cm-scale clay partings. Both conical and Clay-prone skeletal packstone discoidal Orbitolina common. Lpc(p) containing discoidal Orbitolina

Nodular cement often coalesced to Lwc Clay-prone skeletal wackestone form completed cemented beds. 4 Massive to weakly laminated, and within FA common Oyster and Orbitolina storm lags.

Outer Ramp Lm Mudstone Occasionally contain reworked phosphatic hardgrounds bored by Trypanites.

This facies is typified by upward-fining Sglb Glauconitic sandstone motifs and is generally bioturbated (Thalassinoides) and skeletal (Oysters).

Laminations are planar and generally Sl Laminated sandstone weak, with disruption from Thalassinoides

5 and Ophiomorpha ichnofabrics. FA Sb Bioturbated sandstone

Siliciclastic Dominated by Rhizocorralian and Asterosoma, with common Planolites, Thalassinoides and Chondrites. Sbm Clay-prone bioturbated sandstone

Mb Bioturbated mudrock Contains weak Planolites ichnofabric and

6 occasional carbonaceous debris.Locally common siderite. fshore FA Of Ml Laminated mudrock

Figure 4: Summary of the principal carbonate and clastic facies within the Mauddud Formation reservoir. This section illustrates the sedimentological character of the facies associations rather than a particular piece of reservoir stratigraphy. For key to typical sedimentological log see Figure 5.

22 22

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Mauddud Formation, Raudhatain and Sabiriyah fields, North Kuwait

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).

23 23

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 on 01 October 2021 by guest Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf sedimentological facies make-up and characteristic gamma-ray, neutron-density and resistivity motifs. Core permeability and porosity data is also also is data porosity and permeability Core motifs. resistivity and neutron-density displayed on therelevantlogdisplay. Depositional cyclesarealsohighlighted. gamma-ray, characteristic and make-up facies sedimentological the zones, reservoir of distribution the highlightning Formation Mauddud the of display log petrophysical and sedimentological example An 5: Figure Wara Upper Burgan Mauddud Formation Lithostratigraphy Hydrocarbon stain Porosity Increasing Formation

Fracture Body cavity Mouldic MaJ MaI MaH MaG MaF MaE MaD MaC MaB MaA Reservoir Zones 0.0 6.0 Gamma/Caliper Gamma-Ray Caliper Cement Qualifers si py Glauconite unless otherwisestated) Disseminated (allcarbonate Diffuse nodular Nodular Siderite Pyrite 100.0 16.0

Hydrocarbon Stain Cements

Porosity

Boundstone Carbonate Texture Facies Associations Floatstone Grainstone Offshore FA6 Siliciclastic ShorefaceFA5 Outer RampFA4 Mid-Ramp FA3 Inner RampFA2 Restricted InnerRampFA1 Packstone Wackestone Mudstone

Lithology rock Mud- Grain Sizeand Clay Sedimentrary Rubble Rubble Silt Structures Very Fine Sand Rubble Skeletal allochems Rubble Rubble Rubble Rubble Rubble Rubble Fine Echinoid Coral Gastropod Ovalveolina Miliolid Rudist Molluscan debris Discoidal Conical Medium Coarse Orbitolina Orbitolina Facies Associations 1.95 0.45 0.45 140.0

Neutron/Density/Sonic/ Core Porosity(pu) Density (g/cm3) Core Porosity Neutron (pu) Sonic (µs/ft) Trace fossils Chondrites Asterosoma Teichichnus Glossifungites Rhizocorralian Planolites Thalassinoides -0.15 -0.15 40.0 2.95 1.0E-02 0.2 0.2 0.2 Depositional cycles Resistivity/Permeability Core Permeability(mD) Transgressive cycle Regressive cycle Shallow (ohm.m) Medium (ohm.m) Deep (ohm.m) 2,000.0 2,000.0 2,000.0 1,000.0 Vertical scale 5m 0

High Order Depositional Cycles Low Order Cross et al.

(a) (c) (d) r r O O

ca r

1cm 10cm 10cm T 1mm Clay-rich skeletal wackestone in which abundant compactionally modified 10cm r r Clay-prone skeletal packstone containing 10cm clay-partings contain large discoidal abundant discoidal Orbitolina (O) and less Orbitolina (O). Pervasive Thalassinoides common coralline algae (ca). concentrates strong early nodular cement.

n (e) (f)

Rudist-dominated grainstone/floatstone (left) and formaniferal wacke- /packstone (right) containing nezzazatid (n) and textularid (t) foraminifera. Both facies contain common cm-scale rudistid fragments in core (r). t Note the strong Thalassinoides (T) ichnofabric in the foraminiferal wacke-/packstone.

1mm a (b)

10cm 10cm

O r

Mud-prone bioturbated sandstone with well developed deposit-feeding Massive mudrock ichnofabric including Asterosoma (a) containing abundant and Rhizocorallian (r) siderite nodules 1mm

10cm p 1mm e O e Massively bedded peloidal and skeletal packstone and grainstone containing abundant peloids (p), conical Orbitolina (O) and echinoid debris (e) Figure 6: Facies associations of the Mauddud reservoir. Core photograph and relevant micrographs of the (a) restricted inner ramp association (FA1), (b) inner ramp association (FA2), (c) mid-ramp association (FA3), (d) Outer ramp association (FA4), (e) offshore association (FA5), and (f) siliciclastic shoreface association (FA6).

24 24

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Mauddud Formation, Raudhatain and Sabiriyah fields, North Kuwait

(a) (c) (d) r r O O

ca r

1mm a (b)

10cm 10cm

O r

Mud-prone bioturbated sandstone with well developed deposit-feeding Massive mudrock ichnofabric including Asterosoma (a) containing abundant and Rhizocorallian (r) siderite nodules 1mm Figure 6: See facing page for caption.

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 10cm form a key reservoir lithology (e.g. MaD). The biota in these sediments consists entirely of bored and p reworked, cm- to dm-scale, thick-walled rudistid fragments, which form both the larger clasts and 1mm the supporting, sand-grade matrix. In-place rudists are absent from core samples of these sediments, e which suggests that they may represent poorly preserved biostromes or sand sheets generated by O e storms in areas adjacent to build-ups. No evidence for large build-ups or depositional margins is Massively bedded peloidal and skeletal resolvable on available 3-D seismic data. packstone and grainstone containing abundant peloids (p), conical Orbitolina (O) and echinoid debris (e) 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.

25 25

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Cross et al.

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 theOrbitolina 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 fine- grained 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:

26 26

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Mauddud Formation, Raudhatain and Sabiriyah fields, North Kuwait

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 ofpore 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

27 27

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Cross et al.

a b

a a b

Bored phosphatic nodules (a), commonly located Boring are commonly infilled by ferroan (a) and non above hardgrounds. ferroan (b) dolomite.

c d a

Centimetre scale a nodules (a), overprinting bioturbation.

Nodules composed of microcrystalline non ferroan calcite (a). Clay seams indicate a precompactional origin.

e f

Increasing Burial Depth b

a a c

Grainstones pervasively cemented by non ferroan Grainstone under cathodoluminescence, showing calcite which is grain fringing (a), syntaxially evolution from dull to bright luminescence in pore filling overgrowing (b) and pore filling (c). cements (a).

a `f Figure 7: Characteristics of principal diagenetic phases within the Mauddud Formation reservoir.

Non ferroan calcite occluding secondary macropore and fractures (f).

28 28

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Mauddud Formation, Raudhatain and Sabiriyah fields, North Kuwait

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.

29 29

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Cross et al.

a b a a

Depositional Fabric; Clean (inner ramp) grainstones with Depositional Fabric; Packstones (mid-ramp) in which primary interparticle macropores that have often been primary macropores are only preserved within forams (a) enhanced by matrix allochem dissolution. and solid solution modification is negligible.

c d

Cementation; Dissolution of benthic foraminifera, algal debris and matrix increases porosity. Permeability is only increased where there is connectivity via the primary or secondary pore network.

Cementation; (Left) Calcite nodules reduce porosity and permeability. Nodules can be isolated or amalgamated to form 'dense' layers.

45 120 40 e f 100 35 30 80 25 60 20 15 40 10 tal Replacive and Pore tal Replacive and Pore Filling Carbonate (%) Filling Carbonate (%) 20 5 Effect of compaction To To 0 0 0.0 10.0 20.0 30.0 40.0 0.0 0.1 1.0 10.0 100.0 1000.0 Helium porosity (%) Horizontal permeability (mD) Volume of pore filling and matrix replacive calcite (dominantly nodules) has a dominant control on porosity and permeability by reducing pore volume and connectivity. Figure 8: Characteristics of principal pore types within the Mauddud Formation reservoir.

30 30

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Mauddud Formation, Raudhatain and Sabiriyah fields, North Kuwait

(a) Restricted inner ramp association Inner ramp association 10,000 Increasing secondary Fracturing 1,000 macroporosity 100 10

(mD) 1 Lf Lgb 0.1 Increasing clay Increasing primary compaction and Lpc(o) and secondary Lgl 0.01 cement Lwc(o) macroporosity Lpb

Horizontal permeability 0.001 Dominated by microporous packstones and wackestones Grainstones and clean packstones with good primary with consequently suppressed permeabilities. Porosity and depositional reservoir properties. Poroperm degradation is permeability are degraded by cementation and compaction. by cementation and compaction. Grain and matrix dissolution improves reservoir properties.

Mid-ramp association Outer ramp association y 10,000 Increasing primary and 1,000 secondary macroporosity Fracturing 100 Increase in poorly 10 connected porosity

(mD) 1 Lpm 0.1 Increasing clay Lpc(d) Lwc compaction and cement 0.01 Lpc(p) Lwm 0.001 Horizontal permeabilit

Packstones and wackestones with mixed to microporous Wackestones and mudstones with very poor primary networks. Locally dissolution of allochems and matrix has reservoir properties, which have usually been degraded improved reservoir properties, but often nodular further by pervasive nodular calcite cementation. cementation significantly reduces poroperm.

Basinal/offshore association Clastic shoreface association 10,000 Failed plugs Compaction and 1,000 Heterogeneous plugs cementation 100 10

(mD) 1 H 0.1 Sb Mb Increasing grain size 0.01 and primary macroporosity Sglb Ml Sl

Horizontal permeability 0.001 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 Helium porosity (%) Helium porosity (%) Mudrocks with no reservoir quality. Sandstones in which primary depositional properties (grain size, sorting, and clay volume) control porosity and 10,000 permeability. 1,000 Total dataset plotted. 100 10 Restricted inner ramp association Inner ramp association

(mD) 1 Mid-ramp association 0.1 Outer ramp association 0.01 Basinal/offshore association

Horizontal permeability 0.001 Clastic shoreface association 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 Helium porosity (%)

(b) 45.0 40.0 Min 35.0 Mean 30.0 25.0 Max 20.0 Stan dev 15.0 10.0 Helium porosity (%) 5.0 0.0 10,000 1,000 100 10 Figure 9: Porosity and 1 permeability of the 0.1 Mauddud Formation, 0.01 by facies association and layer.

Horizontal permeability (mD) 0.001 1 2 3 4 5 6 Facies association 31 31

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Cross et al.

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

Inner ramp facies association. a Large, clean, primary interparticle macropores. Some post-depositional solution enhancement. y

c Inner ramp facies association. Clean packstones with primary interparticle macropores, often contaminated by micrite. Many exhibit post- depositional solution enhancement. , coarser textures increasing reservoir qualit

d . Cleaner

a Mid-ramp facies association. Packstones with primary intraparticle macropores b (a), and interparticle microporosity (b).

650 mm

Upward increase in depositional energy e

Outer ramp facies association. Wackestones with only primary interparticle micropores, here destroyed by matrix replacive calcite. Flooding surface

Figure 10: Distribution of porosity and permeability within an idealised parasequence.

32 32

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Mauddud Formation, Raudhatain and Sabiriyah fields, North Kuwait

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.

North South

Gamma- Sonic 100 Ray Gamma- Sonic Ray 0 100 140 40

Gamma- Sonic 0 100 140 40 Ray Wara 0 ft 0 100 140 40

MaA Upper Mauddud MaB Relatively tramline MaC reservoir architecture produced by correlatable carbonate-dominated MaD shallowing upward cycles bounded by minor flooding events.

MaE

MaF Lower Mauddud MaG Highly heterogeneous reservoir architecture defined by complex MaH stacking of mixed carbonate-siliciclastic stratigraphy. Broad increase in siliciclastic component towards the south reflecting MaI increasing proximity to the Burgan delta source.

MaJ

Burgan

Mudrock Undifferentiated limestone Sand Figure 11: North-South correlation of the Mauddud Reservoir across the Raudhatain field.

33 33

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Cross et al.

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).

34 34

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Mauddud Formation, Raudhatain and Sabiriyah fields, North Kuwait

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 ofmid- 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

35 35

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Cross et al.

Reservoir Layer MaJ Reservoir Layer MaI Reservoir Layer MaH Reservoir Layer MaG 3317500 3317500 a b c d 3315000 3315000

3312500 3312500

3310000 3310000

3307500 3307500

3305000 3305000

3302500 3302500

3300000 3300000

3297500 3297500

3295000 3295000

3292500 3292500 752500 757500 762500 767500 772500 752500 757500 762500 767500 772500 752500 757500 762500 767500 772500 752500 757500 762500 767500 772500

Reservoir Layer MaF Reservoir Layer MaE Reservoir Layer MaD Reservoir Layer MaC 3317500 3317500 e f g h 3315000 3315000

3312500 3312500

3310000 3310000

? 3307500 3307500 ?

3305000 3305000

3302500 3302500

3300000 3300000

3297500 3297500

3295000 3295000

3292500 3292500 752500 757500 762500 767500 772500 752500 757500 762500 767500 772500 752500 757500 762500 767500 772500 752500 757500 762500 767500 772500

Rudistid sandsheet Erosion Reservoir Layer MaB Reservoir Layer MaA Restricted inner ramp (FA1) Transitional mid- to outer ramp 3317500 Syn-depositional high km i j Inner ramp (FA2) Outer ramp (FA4) 0 5 3315000 Transitional inner to mid-ramp Siliciclastic shoreface (FA5) Syn-depositional low Possible fault (orientations inferred from Mid-ramp (FA3) Offshore (FA6) 3312500 field-scale structural grain)

3310000

3307500

3305000

3302500

3300000

3297500

3295000

3292500 752500 757500 762500 767500 772500 752500 757500 762500 767500 772500

36 36

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Mauddud Formation, Raudhatain and Sabiriyah fields, North Kuwait

Reservoir Layer MaJ Reservoir Layer MaI Reservoir Layer MaH Reservoir Layer MaG 3317500 3317500 a b c d 3315000 3315000

3312500 3312500

3310000 3310000

3307500 3307500

3305000 3305000

3302500 3302500

3300000 3300000

3297500 3297500

3295000 3295000

3292500 3292500 752500 757500 762500 767500 772500 752500 757500 762500 767500 772500 752500 757500 762500 767500 772500 752500 757500 762500 767500 772500

Reservoir Layer MaF Reservoir Layer MaE Reservoir Layer MaD Reservoir Layer MaC 3317500 3317500 e f g h 3315000 3315000

3312500 3312500

3310000 3310000

? 3307500 3307500 ?

3305000 3305000

3302500 3302500

3300000 3300000

3297500 3297500

3295000 3295000

3292500 3292500 752500 757500 762500 767500 772500 752500 757500 762500 767500 772500 752500 757500 762500 767500 772500 752500 757500 762500 767500 772500

3310000 Figure 12: Depositional facies maps for the reservoir layers MaJ (a), MaI (b), MaH (c), MaG (d),

3307500 MaF (e), MaE (f), MaD (g), MaC (h) and MaB (i) in Raudhatain and Sabiriyah fields.

3305000 interplay between eustatic sea-level change and clastic sediment input during a period of continuous 3302500 subsidence. Although both fields are covered by high-quality 3-D seismic surveys, northward-directed 3300000 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- 3297500 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 3295000 for much of the time deposition took place in a relatively low-energy setting. The principle inner and

3292500 752500 757500 762500 767500 772500 752500 757500 762500 767500 772500

37 37

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Cross et al.

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 co- existed 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

38 38

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Mauddud Formation, Raudhatain and Sabiriyah fields, North Kuwait

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 fine- grained 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).

39 39

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Cross et al. 1 Restricted Dominant) (Low-energy 2 Inner Ramp Energy) (Moderate Upward-coarsening/cleaning mid-ramp package. Massively-bedded high-energy inner ramp package, with subordinate rudist floatstone Upward-coarsening/cleaning shallow mid-ramp to inner ramp package. Restricted inner ramp storm- infuenced trendless package. Packstone storm bed. Rudist floatstone storm bed. Increasing energy 3 Grainstones. Deposited in a High-energy inner ramp setting Upward-coarsening/ cleaning mid-ramp package Mid-Ramp Facies Facies Facies Facies Facies 2 3 2 3 3 2 2 3 1 associations associations associations associations associations A6) A5 and F 4 Outer Ramp Upward-coarsening/ cleaning mid-ramp package Thin intervals of glauconitic sandstone/ limestone defining the tops/ bases of upward cleaning package

A6). C Ma gl Inferred rudist and coral patch reef, with a rudist floatstone apron.

A5 and F

Sub- , MaE , gl Facies Sea-level FWWB SWWB associations environment Facies (b2) 2 3 2 3 dally-influenced intrashoal

associations Ti cross-bedded grainstones. MaG, MaF MaG,

Direction of possible siliciclastic influx (F

Figure 13: Depositional models for the Mauddud Formation reservoir in Raudhatain and Sabiriyah fields: High-energy mixed carbonate- clastic shelf/ramp model for lower Mauddud layers MaJ to MaE (a), and low energy carbonate ramp model for the Upper Mauddud layers (b). FWWB = Fair weather wave base, SWWB Storm MaD to MaA weather wave base.

Direction of possible siliciclastic influx (F MaB and (micritised) peloidal grains. MaD

, restricted circulation inner ramp lagoon. MaI1 Orbitolina High-energy inner ramp. Shoreline-attached grainstone shoals containing abundant conical Low-energy Low-diversity foraminifera wackestone/micritic packstones and rare grainstones. -dominated packstone shoals deposited at or close to 2 Inner Ramp (High Energy)

Outer ramp wackestones /mudstones Inter calated basinal/pro-delta bioturbated/laminated mudrocks

Moderately high-energy shallow mid-ramp. Shore-detached conical Orbitolina FWWN, across a broad area. MaA J

Discontinuous high-energy rudist biostrome/ bioherm. Locally abundant, but areally restricted (as seen in MaE and MaG). 4 Facies Ma associations

Orbitolina , shallow mid-ramp. MaI2 3 Increasing energy Inferred discontinuous high-energy rudist biostrome/bioherm. Developed over highly unstable packstone-dominated shoals. Mid-Ramp

vel

-Level -L

ea ea Lower-energy conditions, within an intra-packstone shoal channel. FWWB SWWB S S

FWWB SWWB Moderate-energy Clay-poor skeletal packstones containing conical and storm lags. and storm lags 4

Outer Ramp s Orbitolina , shallow mid-ramp. scarce storm lags. scarce storm lags Basin. Pyritic mudrocks Sub- Facies , sediment-starved Sea-level FWWB SWWB and

, sediment-starved Basin. Pyritic mudrock associations environment and (a2) Moderate-energy Clay-poor skeletal packstones containing conical Relatively low-energy deep mid-ramp. Clay-rich skeletal packstones and wackestones containing discoidal Orbitolina Relatively low-energy deep mid-ramp. Clay-rich skeletal packstones and wackestones containing discoidal Orbitolina Low-energy outer ramp. Clay-rich skeletal wackestones and mudstones Low-energy outer ramp. Clay-rich skeletal wackestones and mudstones.

(a) (b)

40 40

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Mauddud Formation, Raudhatain and Sabiriyah fields, North Kuwait 1 Restricted Dominant) (Low-energy 2 Inner Ramp Energy) (Moderate Upward-coarsening/cleaning mid-ramp package. Massively-bedded high-energy inner ramp package, with subordinate rudist floatstone Upward-coarsening/cleaning shallow mid-ramp to inner ramp package. Restricted inner ramp storm- infuenced trendless package. Packstone storm bed. Rudist floatstone storm bed. Increasing energy 3 Grainstones. Deposited in a High-energy inner ramp setting Upward-coarsening/ cleaning mid-ramp package Mid-Ramp Facies Facies Facies Facies Facies 2 3 2 3 3 2 2 3 1 associations associations associations associations associations A6) A5 and F 4 Outer Ramp Upward-coarsening/ cleaning mid-ramp package Thin intervals of glauconitic sandstone/ limestone defining the tops/ bases of upward cleaning package

A6). C Ma gl Inferred rudist and coral patch reef, with a rudist floatstone apron.

A5 and F

Sub- , MaE , gl Facies Sea-level FWWB SWWB associations environment Facies (b2) 2 3 2 3 dally-influenced intrashoal

associations Ti cross-bedded grainstones. MaG, MaF MaG,

Direction of possible siliciclastic influx (F

Direction of possible siliciclastic influx (F MaB and (micritised) peloidal grains. MaD

Outer ramp wackestones /mudstones Inter calated basinal/pro-delta bioturbated/laminated mudrocks

Moderately high-energy shallow mid-ramp. Shore-detached conical Orbitolina FWWN, across a broad area. MaA J

Discontinuous high-energy rudist biostrome/ bioherm. Locally abundant, but areally restricted (as seen in MaE and MaG). 4 Facies Ma associations

Orbitolina , shallow mid-ramp. MaI2 3 Increasing energy Inferred discontinuous high-energy rudist biostrome/bioherm. Developed over highly unstable packstone-dominated shoals. Mid-Ramp

vel

-Level -L

ea ea Lower-energy conditions, within an intra-packstone shoal channel. FWWB SWWB S S

FWWB SWWB Moderate-energy Clay-poor skeletal packstones containing conical and storm lags. and storm lags 4

Outer Ramp

s See facing page for caption. Orbitolina , shallow mid-ramp. scarce storm lags. scarce storm lags Basin. Pyritic mudrocks Sub- Facies Figure 13: , sediment-starved Sea-level FWWB SWWB and

(a) (b)

41 41

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Cross et al.

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.

42 42

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Mauddud Formation, Raudhatain and Sabiriyah fields, North Kuwait

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

43 43

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021

Cross et al.

al., 2002) al.,

(Davies et et (Davies

Low Frequency Low High Frequency High MaI MaJ MaF MaA MaB MaE MaC MaD MaH MaG Upper

Burgan Ahmadi Southwest Raudhatain Field Sabiriyah Field fields showing the broad distribution of facies associations. Northeast ) A1 ) South A5 ) ) ) A4 A2 ) Associations A3 A6 Figure 14: Schematic reservoir architecture of the Mauddud Formation across Raudhatain and Sabiriyah Burgan ara W fshore (F ransgressive) Restricted Inner Ramp (F Inner Ramp (F Mid-Ramp (F Outer Ramp (F Clastic Shoreface (F Of Rudist Dominated Facies s Progradational (Regressive) Backstepping (T rend T Dominated Facies Mauddud North

44 44

A1) form high permeability A3-4) overprinted by Highly cemented zones fect of cemented grainstone/packstone A2-3) combine to produce major barrier A5), cemented outer ramp wackestones (F High permeability zones A2) overlain by cemented mud-prone wackestone (F A5-6). Darcy-quality upper shoreface sandstones at the top have variable Sub-m-scale rudist grainstone/floatstone beds (F streaks that act as potential thief zones and channelise flow The boundaries of stacked upward shallowing cycles baf movement due to the combined ef tops (F flooding surface. Relatively thick inner ramp grainstones and packstones contribute strongly to the production profile and have potential focus fluid flow Stacked outer to mid-ramp-dominated facies (F commonly amalgamated nodular calcite cements. intense within transgressive outer ramp facies where low sedimentation rates results in intense cementation. Local rudist build-ups or grainstone-dominated shoals (F permeability contrasts with enveloping facies and will form flow layers on an interwell scale. Highly layered permeability profile within progradational shoreface sandbody (F distribution due to local erosion over faulted highs, whilst base of zone is field-wide mudrock. Stacked mudrocks (F mid to inner ramp packstones (F to vertical fluid movement at the base of reservoir between Upper Burgan and the sandstones in MaH. Pressure Breaks <5 psi <5 psi 50 to 1,500 psi 700 psi Southwest ? ? ? ? ? Raudhatain Field ? ? showing the broad distribution of potential baffles and barriers to fluid movement. ? ? ? Sabiriyah Field Figure 15: Schematic reservoir architecture of the Mauddud Formation across the Raudhatain and Sabiriyah Fields Figure 15: Schematic reservoir architecture of the Mauddud Formation across Raudhatain and Sabiriyah Fields ? ? Northeast ara MaI MaJ MaF MaB MaE MaA MaC MaD MaH MaG W Upper Burgan

45 45

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Cross et al.

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

46 46

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Mauddud Formation, Raudhatain and Sabiriyah fields, North Kuwait

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.

REFERENCES

Abdul Azim, S., H.Z. Al-Ajmi, C. Rice, D. Bond, S. Abdullah and B. Laughlin 2003. Reservoir description and static model build in heterogeneous Mauddud carbonates: Raudhatain field, North Kuwait. 12th Middle East Oil Show and Conference, Bahrain 5-8th April, 2004. SPE 81524. 9 p. Akbar, M., S. Chakravorty, S.D. Russel, M. Al-Deeb, R.S. Efnik, R. Thawyer and H. Karakhanian 2000. Unconventional approach to resolving primary and secondary porosity in Gulf carbonates from conventional logs and borehole images. SPE-ADIPEC-0929. Ninth ADIPEC Conference, Abu Dhabi, UAE, 15-18 October 2002. Al Ajmi, H.Z. and H.B. Chetri 2000. Integrated approach to infer fractures and manage the waterflood project: Mauddud Reservoir, Raudhatain and Sabiriyah fields, North Kuwait. Society of Petroleum Engineers, SPE 63142. Al Ajmi, H.Z., R. Mukherjee and H.B. Chetri 2000. Integration of dynamic data of pilot waterflood with rock fabric – implications for field waterflood startup in Mauddud reservoir, North Kuwait. Society of Petroleum Engineers, SPE 62991. Al-Eidan, A., W.B. Wethington and R.B. Davies 2001. Upper Burgan reservoir description, northern Kuwait: Impact on reservoir development. GeoArabia, v. 6, no. 2, p. 179-208. Banner, F.T. and M.D. Simmons 1994. Calcareous algae and foraminifera as water depth indicators: An example from the Early Cretaceous carbonates of Northeast Arabia. In M.D. Simmons (Ed.), Micropalaeontology and hydrocarbon exploration in the Middle East. Chapman & Hall, London, p. 243-252. Brennan, P. 1991. Rhaudhatain field – Kuwait, Arabian Basin. In E.A. Beaumont and N.H. Foster (Eds.), Treatise of Petroleum Geology, Atlas of oil and gas fields, American Association of Petroleum Geologists, p. 187-210. Burchette, T.P. and V.P. Wright 1992. Carbonate ramp depositional systems, Sedimentary Geology, v. 79, p. 3-57. Carman, G. 1996. Structural elements of onshore Kuwait. GeoArabia, v. 1, no. 2, p. 239-266. Claris, P.A. and L. Martire 1996. Interplay of cementation, mechanical compaction and chemical compaction in nodular limestones of the Rosso Ammonitico Veronese (Middle-Upper Jurassic, northeastern Italy). Journal of Sedimentary Research, v. 66A, p. 447-458 47 47

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Cross et al.

Dabbouk, C., A. Liaqat, G. Williams and G. Beattie 2002. Waterflood in vuggy layer of a Middle Eastern reservoir – displacement physics understood. SPE-78530, 10th ADIPEC Conference, Abu Dhabi, UAE, 12-16 October 2002. 12 p. Davies, R.B., D.M. Casey, A.D. Horbury, P.R. Sharland and M.D. Simmons 2002. Early to mid-Cretaceous mixed carbonate-siliciclastic shelfal systems: Examples, issues and models from the Arabian Plate. GeoArabia, v. 7, no. 3. p. 541-598. Dunham, R.J. 1962. Classification of carbonate rocks according to depositional texture. In W.E. Ham (Ed.), Classification of Carbonate Rocks. American Association of Petroleum Geologists Memoir 1, p. 108-121. Dunnington, H.V. 1967. Aspects of diagenesis and shape change in stylolitic limestone reservoirs. Proceedings of the Seventh World Petroleum Congress, Mexico, v. 2, p. 339-352. Eichenseer, H.T, F.R. Walgenwitz and P.J. Biondi 1999. Stratigraphic control on facies and diagenesis of dolomitized oolitic siliciclastic ramp sequences (Pinda Group, Albian, Offshore Angola. American Association of Petroleum Geologists Bulletin, v. 83, p. 1729-1758. Ellis, A.C., H.M. Kerr, C.P. Cornwell and D.O. Williams 1996. A tectono-stratigraphic framework for Yemen and its implications for hydrocarbon potential. Petroleum Geoscience, v. 2, p. 29-42. Embry, A.F. and J.E. Klovan 1971. A late Devonian reef tract on northeastern Banks Island, Northwest Territories, Bulletin of Canadian Petroleum Geology, v. 19, p. 730-781. Giot, D., J-M. Dawans, R. King, P. Lehman, E. Shaw and F. El Wazeer, 2000. Tracking permeability in a major limestone reservoir: From rock observation to 3-D modelling. SPE-ADIPEC-0901. 9th ADIPEC Conference, Abu Dhabi, UAE, 15th-18th October 2002. Grötsch, J., O. Al-Jeelani and Y. Al-Mehairi 1998. Integrated reservoir characterization of a giant Cretaceous oil field, Abu Dhabi, U.A.E. 8th Abu Dhabi International Exhibition and Conference. Abu Dhabi, U.A.E., 11th-14th October 1998. Harris, P.M. and S.H. Frost 1984. Middle Cretaceous carbonate reservoirs, Fahud field and northwestern Oman. American Association of Petroleum Geologists Bulletin, v. 68, no. 5, p. 649-658. Hughes, G.W. 2000. Bioecostratigraphy of the Shu’aiba Formation, Shaybah field, Saudi Arabia. GeoArabia, v. 5, no. 4, p. 545-578. Jones, A.D.W., S. Al-Qabandi, C.E. Reddick and S.A. Anderson 1997. Rapid assessment of pattern waterflooding uncertainity in a giant oil reservoir. Society of Petroleum Engineers, SPE 38890. Kirby, R.H., B.S. Carr, J. Al-Humoud, A. Al Safar, D. Al-Matar and W. Naser 1998. Characterisation of a vertically compartmentalised reservoir in a supergiant field: Burgan Formation, Greater Burgan field, Kuwait, Part1: Stratigraphy and water encroachment. Society of Petroleum Engineers SPE 49215, p. 509-520. Mullins, H., A.C. Neumann, R.J. Wilber and M.R. Boardman 1980. Nodular carbonate sediment on Bahamian slopes: possible precursors to nodular limestones. Journal of Sedimentary Petrology, v. 50, p. 117-121 Neilson, J.E., N.H. Oxtoby, M.D. Simmons, I.R. Simpson and N.K. Fortunatova 1998. The relationship between petroleum emplacement and carbonate reservoir quality: Examples from Abu Dhabi and the Amu Darya Basin. Marine and Petroleum Geology, 15, 57-72. Nemcsok, S., N.H. Morrison, A. Crruthers and S. Abdullah 1998. Sedimentary interpretation of a multilayered clastic oil reservoir: Impact on development plans for the Zubair reservoir, Raudhatain field. SPE Paper 48972, p. 177-187. O’Hanlon, M.E.O., C.J.J. Black, K.J. Webb, G. Bin-Daaer and A. El-Tawil 1996. Identifying the controls on waterflood performance in a giant carbonate reservoir. SPE-36209, 7th ADIPEC Conference, Abu Dhabi, UAE, 12th-16th October 1996. Sharland, P.R., R. Archer, D.M. Casey, R.B. Davies, S.H. Hall, A.P. Heward, A.D. Horbury and M.D. Simmons 2001. Arabian Plate sequence stratigraphy. GeoArabia, Special Publication, no. 2, 371 p. Shehabi, J.A.N. and S.R. Kollouri 1987. Anomalous fluid saturation and fluids contact in Mauddud Reservoir, Bahrain field. Proceedings of the 5th Society of Petroleum Engineers Middle East Oil Show, Bahrain, March 7-10, 1987, SPE 15758, p. 575-584 Simmons, M.D., J.E. Whittaker and R.W. Jones 2000. Orbitolinids from Cretaceous sediments of the Middle East – A revision of the F.R.S. Henson & Associates Collection. In M.B. Hart, M.A. Kaminski and C.W. Smart (Eds). Proceedings of the Fifth International Workshop on Agglutinating Foraminifera. Grzybowski Foundation Special Publication 7, p. 411-437. Sugden, W. and J.J. Standring 1975. Qatar peninsula. Lexique Stratigraphique Internationale, III, 10b3. Strohmenger, C.J., T. Demko, J. Mitchell, P. Lehmann, H. Feldman, G. Alsahlan and H. Al-Enezi 2002. Regional sequence stratigraphic framework for the Burgan and Mauddud formations (Lower Cretaceous, Kuwait): Implications for reservoir distribution and quality. GeoArabia, v. 7, no. 2, p. 304-305.. Strohmenger, C.J., P.E. Patterson, G. Al-Sahlan, J.C. Mitchell, H.R. Feldman, T.M. Demko, R.W. Wellner, P.J. Lehmann, G.G. McCrimmon, R.W. Broomhall and N. Al-Ajmi 2006. Sequence stratigraphy and reservoir architecture of the Burgan and Mauddud formations (Lower Cretaceous), Kuwait. In P.M. Harris and L.J. Weber (Eds.), Giant Hydrocarbon Reservoirs of the World: From Rocks to Reservoir Characterization and Modeling. American Association of Petroleum Geologists Memoir 88, p. 213-245. Tudhope, A.W. and T.P. Scoffin 1994. Growth and structure of fringing reefs in a muddy environment, South Thailand. Journal Sedimentary Resarch, A64, p. 752-764. van Buchem, F.S.P., P. Razin, P.W. Homewood, L.M. Philip, G.P. Eberli, J-P. Platel, J. Roger, R., Eschard, T. Desaubliaux, T. Boisseau, J-P. Leduc, R. Labourdette and S. Cantaloube 1996. High-resolution sequence stratigraphy of the Natih Formation (Cenomanian/Turonian) in northern Oman: Distribution of source rocks and reservoir facies. GeoArabia, v.1, no. 1, p. 65-91. van Buchem, F.S.P., B. Pittet, A. Hillgärtner, A. Al Mansouri, I. Billing, H. Droste, J. Grötsch and H. Oterdoom 2002. High-resolution sequence stratigraphic architecture of Barremian/Aptian carbonate systems in northern Oman and the United Arab Emirates (Kharaib and Shu’aiba Formations). GeoArabia, v. 7, no. 3, p. 461-500.

48 48

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Mauddud Formation, Raudhatain and Sabiriyah fields, North Kuwait

Wilson, M.E.J. and S.W Lokier 2002. Siliciclastic and volcaniclastic influences on equatorial carbonates: Insights from the Neogene of Indonesia. Sedimentology, v. 49, no. 3, p. 583-601. Wolf, M., A.M. Al-Jalahma and P.F. Hook 1993. Log Determination of residual oil saturation in the Mauddud Zone, Bahrain field. Proceedings of the Society of Petroleum Engineers Middle East Oil Technical Conference and Exhibition, Bahrain, 3-6 April, 1993, SPE 25648, p. 431-443.

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. [email protected]

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. [email protected]

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. [email protected]

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. [email protected]

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. [email protected]

49 49

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021 Cross et al.

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. [email protected]

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. [email protected]

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. [email protected]

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. [email protected]

Manuscript received August 20, 2006 Revised September 15, 2009 Accepted October 21, 2009 Pre-press version proofread by authors November 24, 2009

50 50

Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/2/17/4567411/cross.pdf by guest on 01 October 2021