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Diagenesis, Facies, and Reservoir Quality in a Sequence Stratigraphic Framework: The Visean-A Platform in Tengiz Field, Republic of Kazakhstan

Abstract

The Visean-A reservoir was deposited on the Tengiz isolated platform during the transition from greenhouse to icehouse conditions in the Mississippian. Reservoir quality on the platform top reflects facies and the strong overprint of meteoric diagenesis. However, predicting diagenesis is a vexing problem in carbonate geology. We used a new approach combining facies and diagenetic fabrics identified in core with reservoir quality maps to make predictions of diagenetic fabrics away from core control. Visean-A stratigraphy consists of a largely aggradational stack of m-scale, exposure-bound, high-frequency sequences (HFS). Visean HFSs have a mixture of heterozoan and photozoan facies, typically with heterozoan (crinoid & brachiopod)-dominated transgressive systems tracts and photozoan (foram-algal)-dominated highstand systems tracts. Incipiently coated, micritized grainstones cap HFSs and are typically subject to vadose diagenesis. Visean-A stratigraphy includes a nested hierarchy of 28 HFSs, which stack into 7 composite sequences, and three lower-order packages. Diagenesis is dominated by meteoric leaching and cementation, along with marine cements, compaction, and bitumen. Unlike the Tengiz slope, few fractures and little late burial dissolution and cementation are observed. Four important trends in reservoir quality are observed in the Visean-A platform. (1) HFS boundaries correspond to baffles caused by a combination of meteoric cementation at cycle caps, thin tuffs at HFS boundaries, and bitumen-rich, muddy transgressive facies. The most effective baffles occur at lower-order sequence boundaries. (2) The impact of meteoric leaching reflects cycle stacking and is enhanced during highstands of composite sequences. (3) The southern platform area has overall poorer reservoir quality, due mostly to less meteoric dissolution and more meteoric cementation. (4) Disseminated ash occurs in primary and secondary porosity in the upper 40m of the Visean and in the overlying Serpukhovian. We infer that ash filled diffuse karst associated with the overlying Serpukhovian Super-Sequence Boundary. During burial, ash nucleated abundant pressure solution seams, causing a series of baffles. This study illustrates the combined facies and diagenetic controls on reservoir quality that may be typical of carbonate platforms during icehouse times. Though non-unique, this approach provides a pathway to assessing diagenetic geobodies in carbonate platforms.