--> ABSTRACT: Distribution of Reservoir Facies in Devonian and Carboniferous Carbonates of Southern Kazakhstan: The Roles of Early Cementation, Dolomitization and Meteoric Diagenetic Processes, by W. G. Zempolich, H. E. Cook, V. G. Zhemchuzhnikov; #91020 (1995).

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Distribution of Reservoir Facies in Devonian and Carboniferous Carbonates of Southern Kazakhstan: The Roles of Early Cementation, Dolomitization and Meteoric Diagenetic Processes

W. G. Zempolich, H. E. Cook, V. G. Zhemchuzhnikov

Field mapping, facies analysis and laboratory study of Late Devonian and Carboniferous carbonates of the Bolshoi Karatau Mountains, southern Kazakhstan, suggest that the distribution of reservoir-grade porosity is strongly influenced by early-marine cementation, dolomitization and meteoric-related diagenetic processes. These results establish predictable diagenetic patterns that are comparable to the distribution of reservoirs in giant carbonate fields of similar age found in the subsurface of the North Caspian Basin.

Marine cement is particularly abundant within resedimented upper slope and reefal bounds tone facies at platform margins where it occurs as thick isopachous linings and as frame-filling cementstone. Isotopic compositions of the early marine cement are enriched in 13C and 18O and are in agreement with secular trends defined by other Devonian and Carboniferous carbonates. Thus, marine cementation plays an important role in both constructing these lithofacies and in reducing most primary depositional porosity. As a result, porosity-enhancing diagenetic processes in the platform margin (e.g., early marine-related dolomitization) are limited to the partial alteration of skeletal fabrics and micritic matrix.

The relative absence of marine cement in shelf margin grainstone and shelf interior lithologies allows for the extensive development of porosity-enhancing diagenetic processes that are related to small- and large-scale relative sealevel fluctuations. Diagenetic processes which have been mapped and identified by petrographic and isotopic study include marine-related dolomitization, mixing-zone dolomitization and meteoric leaching. In the field, reservoir development is expressed as: 1) early marine-related dolomitization of shelf-margin oolite and interior lagoonal sediments which results in formation of intracrystalline and moldic porosity, and thin stratal reservoirs at a scale of meters to tens of meters; and 2) karsting of platform interior lithologies which results in formation of vuggy and moldic porosity, and thin and thick stratal reservoirs at the scale of meters to <=100 meters.

The sequential deposition of Late Devonian through Carboniferous carbonates results in a thick stack of relatively-tight lithologies at platform margins and porous lithologies within shelf interiors. Comparisons of predicted diagenetic patterns and reservoir geometries with those at Karachaganak and Tengiz fields suggest that similar processes were operative in the North Caspian Basin.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995