Devonian and Carboniferous Carbonates of Southern Kazakhstan: Depositional Analogs for Coeval Giant Oil and Gas Fields in the North Caspian Basin, Western Kazakhstan

H. E. Cook, V. G. Zhemchuzhnikov, W. G. Zempolich, V. Ya. Zhaimina, L. V. Sergeieva, V. M. Buvtyshkin, D. V. Alexeiev

The Bolshoi Karatau Mountains of southern Kazakhstan contain carbonate rocks that range in age from the latest Frasnian (?) through the early Bashkirian. These rocks record the development of a 4,500 m thick passive-margin carbonate platform which evolved close to the oil and gas rich North Caspian Basin during the Late Devonian and Carboniferous.

We are developing a series of time-stratigraphic models to serve as predictive analogs for coeval subsurface carbonate oil and gas reservoirs in the North Caspian Basin and in other stratigraphically similar basins of the former USSR: (1) Latest Frasnian (?) and Fammenian carbonates are characterized by reef-rimmed platform margins composed of algal (Renalcis?)-stromatoporoid-calcisponge (?) boundstone; platform interiors contain stromatoporoid biostromes, large Waulsortian bioherms and abundant peritidal carbonate sands. Basin margin settings contain megabreccia debris flow deposits that fort slope and base-of-slope aprons. (2) Tournaisian platform types are poorly understood but may consist of ramps of low-angle shoal-rimmed platform margins dominated by brachiopod-crinoid biostromes. (3) Visean and Serpukhovian carbonates developed both reef and shoal-rimmed platform margins. Reef-rimmed platform margins and upper slopes are characterized by sponge-Tubiphytes- lgae-mud boundstone and flanking facts s of crinoid-bryozoan-coral-brachiopod-mud boundstone and cementstone. Deeper water parts of these slopes contain crinoid-rich Waulsortian bioherms and mass-flow carbonates deposited as slope aprons. Platform interior buildups consist of phylloid-algae-bryozoan-mud bioherms and interbedded ooid and bioclastic sands. Shoal-rimmed platform margins are dominated by cross-bedded ooid-bioclastic sands and slope aprons. (4) Bashkirian carbonates are reef-rimmed platform margins comprised of algae-brachiopod boundstone and cementstone. These four platform margin types exhibit phases of backstepping and progradation as a result of relative sealevel rising and lowering events.

These results indicate that Late Devonian and Carboniferous reefs and platforms exhibit distinct morphologic, biologic and lithologic changes through time. Depositional models developed through this study predict: (1) the different rock types, the overall size and shape of the rock bodies, and where the different rock bodies occur within these carbonate platforms and (2) which types of rock bodies in the subsurface could form potential oil and gas reservoirs.

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