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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.
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.
Detailed well-log interpretations, including gamma-ray, density, neutron, and resistivity, alongside petrographic analysis of 100 samples over 170 m of drill cores, have revealed factors influencing reservoir heterogeneity in the Yamama Formation, Ah’Dimah Oilfield, southern Iraq. The formation comprises four reservoir units (YA-YD) separated by four non-reservoir units (BA-BD). The reservoir units are subdivided into subunits. YB2, YB3, and YC demonstrate the best reservoir quality, while YD2 is water-bearing. Seven microfacies were identified within both reservoir and non-reservoir units, deposited in a shallow carbonate ramp. These include bioclastic wackestone, Lithocodium-Bacinella float/boundstone, peloidal cortoid intraclast grainstone, reefal bioclastic rudstone, bioclastic foraminiferal wacke/packstone, miliolidal pack/grainstone, and spiculitic foraminiferal wackestone. Despite the deep burial depth of the formation (> 4000 m), it maintained good porosity values in most intervals, reaching up to 20%. Early isopachous cement protected porosity and dissolution enhanced porosity, while cementation, compaction, and pyritization reduced it. The reservoir units correlate with depositional environments, being deposited in the shoal area, while non-reservoir units were deposited in lagoon, middle, and outer-ramp settings. The Lithocodium-Bacinella float/boundstone and reefal bioclastic rudstone facies, forming reefal patches and build-ups within the shoal, dominated in YB2 and YC. Targeting these patches northeast of Ah’Dimah Oilfield is promising for field development.
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