Zubair Reservoir (Cretaceous) in Zuluf Field, Saudi Arabia: Depositional Facies and Facies Distribution

A.M. Wahbi¹

Abstract

The Lower Cretaceous Zubair clastic deposits in the subsurface of Saudi Arabia are stratigraphically constrained by the Buwaib Limestones - Hauterivian maximum flooding event (K50) at the base and clastic deposits of the Biyadh Sandstone at the upper boundary. This study examined over 1,400 ft. of core recovered from 5 wells that penetrate the Zubair reservoir in Zuluf field in the central offshore Arabian Gulf. A detailed core description allows the determination of the facies relationships and depositional environment that is necessary for reservoir development. This resulted in the identification of 7 depositional facies. Those facies comprise; (1) wispy- to ripple-laminated, fine- to very fine-grained sandstone, most probably indicating deposition in a mouth bar setting; (2) sandy heterolithite, representing fringing mouth bar deposits; (3) slumped and hummocky-stratified fine- to very fine-grained sandstone, indicative of remobilization and deposition under the influence of storms in a prodelta slope setting; (4) bioturbated fine- to very fine-grained sandstone, and (5) muddy heterolithite, both representing upper to middle prodelta slope deposits; (6) mudstone, representing middle to lower prodelta slope (to open marine?) deposits; and (7) bioclastic argillaceous limestone, representing storm deposits. The facies are present in naturally-occurring facies associations which were analyzed in order to further characterize the different depositional settings and ultimately their subsurface distribution, both vertical and lateral, in the context of the reservoir architecture. This led to the identification of 7 lithostratigraphic cycles that were correlated among all the cored wells and demonstrated lithological and thickness variations throughout the reservoir. A coarsening-upward trend is reflected within cycles and overall stacking pattern with an apparent gradual decrease in thickness of sandstone facies in the later cycles. The cycles reflect changing patterns of sedimentation and are bounded by recognizable marine flooding events. Data collected indicates an ultra-distal deltaic, storm-influenced depositional setting.

AAPG Search and Discovery Article #90188 ©GEO-2014, 11th Middle East Geosciences Conference and Exhibition, 10-12 March 2014, Manama, Bahrain