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, 17 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

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