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Abstract

Tengiz Field is a steep-sided, isolated carbonate platform in the Precaspian Basin, Kazakhstan, with hydrocarbon production from Carboniferous platform and slope facies. Systematic differences in reservoir pressure decline during production indicate that this reservoir consists of three subcompartments or material balance regions: (1) a central “platform reservoir” made up of cyclic platform-top facies that acts like a single, stratified, multistory reservoir; (2) a “wedge reservoir” formed by a prograding margin containing upper slope microbial facies; and (3) an “apron reservoir” containing allochthonous facies deposited in deep water around the base of the buildup. The facies in the apron reservoir accumulated during an early depositional stage and were subsequently partly to fully buried by prograding microbial slope facies of the wedge reservoir. The wedge and apron reservoirs together form a succession 800 to 1000 m thick within the Tengiz oil column.

The wedge reservoir shows uniform pressure decline with time and is well connected. Field data (cores and well logs) are insufficient to determine internal continuity of lithofacies and depositional environments or to quantify the pore network responsible for the high connectivity. An outcrop analog (Asturias, Spain) with facies matching those observed in Tengiz cores was used to predict that the microbial lithofacies form a distinct and continuous mechanical unit within the wedge reservoir. Tengiz microbial facies contain a high concentration of solution-enlarged, syndepositional and other early fractures oriented parallel and normal to depositional strike. Borehole image logs provide data on enlarged fracture apertures and local fracture density, but no data related to fracture height or length. An outcrop analog with early fractures in similar facies (Windjana Gorge, Australia) was used to obtain large-scale height and spacing data for solution-enlarged syndepositional fractures. Dissolution processes in the outcrop are different from those of Tengiz, but the fracture aperture and cavern sizes are comparable to their known counterparts in the Tengiz wedge reservoir, and application of the outcrop height data to geologic models of the Tengiz wedge subcompartment can account for its dynamic behavior. The apron reservoir shows a nonsystematic pressure decline with time and is less depleted than the wedge reservoir. The irregular decline indicates reduced internal connectivity within the apron reservoir, which is corroborated by core and borehole image data indicating high lithofacies heterogeneity and the absence of continuous microbial facies responsible for reservoir continuity in the wedge reservoir. A reservoir pressure increase of 1700 psi from the wedge reservoir to the apron reservoir observed in a single well penetration suggests reservoir communication between them may be reduced across a stratigraphic baffle.

The wedge and apron reservoirs both contain a late burial matrix diagenetic overprint represented mainly by co-precipitated bitumen and calcite cement and local development of matrix microporosity. Enlargement of the early fractures in the wedge reservoir also occurred during burial diagenesis based on the presence of diagenetic halos containing the burial overprint around the fractures and based on the presence of co-precipitated bitumen and calcite in the fractures. Scenarios and mechanisms for fracture enlargement are evaluated against the observations from field data and the outcrop analogs.

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