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Harry E. Cook1, Viacheslav G. Zhemchuzhnikov2, William G. Zempolich3, Patrick J. Lehmann4, Dmitry V. Alexeiev5, Valentina Ya. Zhaimina2
(1) USGS, Menlo Park, CA
(2) JSC Eezdenees, Almaty, Kazakhstan
(3) Mobil, Dallas, TX
(4) Exxon, Houston, TX
(5) Institute of Oceanology, Moscow, Russia

Abstract: Paleozoic Carbonate Platforms in the Bolshoi Karatau of Southern Kazakstan: Outcrop Analogs for Coeval Carbonate Reservoirs in the North Caspian Basin

The Bolshoi Karatau carbonates of southern Kazakstan record development of a 4,500 m-thick passive margin carbonate platform which evolved close to the North Caspian Basin during the Devonian-Carboniferous. Carbonate facies provide outcrop analogs for coeval reservoirs in supergiant oil and gas fields in the North Caspian Basin. The carbonates in the Bolshoi Karatau and the North Caspian Basin are similar in several important ways. First, both the Bolshoi Karatau and the Tengiz oil field carbonate platforms were initiated in the Late Devonian and ended in the Bashkirian. Second, the stratigraphic thickness and sedimentary facies of the Bolshoi Karatau and the Tengiz oil field are similar. Third, the proven oil reserves in Tengiz occur in the Visean through Bashkirian and these strata are well exposed in the Bolshoi Karatau.

The Bolshoi Karatau platform underwent thermal subsidence and numerous sea-level fluctuations of varying amplitudes. The net result was a platform that evolved from reef and sand-shoal rimmed margins in the Late Devonian to deep-water ramps and skeletal mounds in the Tournaisian to ramps with skeletal mounds and rimmed margins in the Visean-Bashkirian. Upward-shallowing cycles of dolomitized and karsted shelf margin and shelf interior ooid-bioclastic sands form potential reservoirs whereas slope skeletal mud mounds contain abundant marine cement and are relatively tight. Reservoir enhancement is related to early dolomitization and multiple episodes of meteoric diagenesis.

Bolshoi Karatau studies provide data on reservoir heterogeneity in terms of their facies, cyclicity and stacking patterns, origins of the stacking patterns, porosity enhancing conditions associated with reservoir cyclicity, and reservoir location within the platform. These outcrop studies should be valuable for better understanding and predicting the characteristics and development of North Caspian Basin oil and gas reservoirs.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana