Facies-related reservoir heterogeneity of grain-supported limestones: insight from the Early Cretaceous Yamama Formation, southern Iraq

Abstract

Sedimentological investigation of 150 m drill cores and well log analyses, including gamma-ray, resistivity, sonic, neutron, density logs, were conducted to constrain the impact of depositional facies on reservoir quality distribution in limestone succession of the Yamama Formation (Early Cretaceous), Nasiriya Oilfield, southern Iraq. Understanding the factors controlling reservoir heterogeneity in carbonate reservoirs is crucial for developing geological and reservoir models. Nine microfacies were identified: peloidal oncoidal grainstones-rudstones, skeletal cortoids packstones, skeletal dasyclads wackestones, pelletal packstones-grainstones, cortoidal peloidal grainstones, ooidal peloidal grainstones, skeletal grainstones, bioturbated dolomitic wackestones, and spiculitic skeletal mudstones-wackestones. The formation was deposited in open-marine shallow-water carbonate ramp, ranging from the intertidal to outer-ramp during the Berriasian-Valanginian. The depositional ramp was characterized by grainstones shoal barriers in the distal inner-ramp. Sea level fluctuations significantly influenced the vertical facies and reservoir quality distribution. The grain-supported, distal inner-ramp shoal facies formed the reservoir units, while the mud-supported, middle-outer-ramp facies are impervious units. Diagenetic processes, including dissolution of skeletal allochems, physical and chemical compaction, dolomitization, and cementation, have variably affected reservoir quality. Dissolution enhanced porosity by creating vuggs, while compaction and cementation often reduced porosity. Nevertheless, early diagenetic circumgranular calcite and small amount of scattered equant and syntaxial calcite overgrowths helped protecting the grain-supported limestones from physical compaction and thus preserved interparticle pores (≤ 22%) at depth (>3100 m). Conversely, equant calcite cement, which occurs in substantial amounts, has reduced porosity by filling the interparticle and moldic pores. Reservoir heterogeneity of the formation is attributed to depositional facies, which control the texture of the sediments, and to various types of diagenetic alterations.