Devonian and Carboniferous Carbonate Platform Facies in the Bolshoi Karatau, Southern Kazakhstan: Outcrop Analogs for Coeval Carbonate Oil and Gas Fields in the North Caspian Basin
H. E. Cook, V. G. Zhemchuzhnikov, W. G. Zempolich, P. J. Lehmann, D. V. Alexeiev, V. Ya. Zhaimina, A. YE. Zorin, 2007. "Devonian and Carboniferous Carbonate Platform Facies in the Bolshoi Karatau, Southern Kazakhstan: Outcrop Analogs for Coeval Carbonate Oil and Gas Fields in the North Caspian Basin", Oil and Gas of the Greater Caspian Area, Pinar O. Yilmaz, Gary H. Isaksen
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The Bolshoi Karatau carbonates of southern Kazakhstan record development of a 4500-m (14,763-ft)-thick platform that evolved close to the North Caspian Basin of western Kazakhstan during the Late Devo-nian and Carboniferous (Cook et al., 1994). Carbonate facies in the Bolshoi Karatau mountains of south-ern Kazakhstan provide outcrop analogs for coeval reservoirs in supergiant oil and gas fields in the North Caspian Basin (Figure 1). The carbonate platforms 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 Upper Devonian and ended in the Bashkirian, a span of about 50–55 m.y.(Figure 2) (Lisovsky et al., 1992). Second, the strati-graphic thickness of the Bolshoi Karatau and the Tengiz oil field is similar (Figure 3). Third, the proven oil reserves in Tengiz occur in the Visean through Bashkirian, and these strata are well exposed in the Bolshoi Karatau (Figures 2, 3).
The seaward margin of the Bolshoi Karatau carbonate platform was probably structurally controlled by the rifted edges of a passive continental margin. The overall geometry of the carbonate platform was controlled by thermal subsidence and local tectonics. Over a 50–55-m.y. period of time, this passive margin underwent thermal subsidence, normal faulting, and numerous sea level fluctuations of varying amplitudes. Sedimentation rates suggest that subsidence decreased exponentially. Sediment accumulation rates ranged from 185 to 285 m/m.y. (606 to 935 ft/m.y.) during the Late Devonian, 60–100 m/m.y. (196–328 ft/m.y.) during the Tournaisian, 35–50 m/m.y. (114–164 ft/m.y.) during the Visean, 15–30 m/m.y. (49–98 ft/m.y.) during the Serpukhovian, and 20– 50 m/m.y. (66–164 ft/m.y.) during the lower Bash-kirian. The net result was a carbonate platform that evolved from reef and sand-shoal-rimmed platforms in the Devonian to deep-water ramps and skeletal mounds in the Tournaisian to ramps with skeletal mounds and rimmed margins in the Visean, Serpu-khovian, and Bashkirian (Figure 2).
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Throughout time, the greater Caspian area has maintained its position as one of the major petroleum provinces in the world. Its early history as a prolific producer of oil is well known. Caspian region exploration dates to the seventh century B.C., during the time of the Median Kingdom in today's southern Azerbaijan. Oil played an important role in the everyday lives of these ancient tribes of the region, and it is still a very important commodity today. The past two decades have seen many important advances in our knowledge of the geological evolution of hydrocarbon-bearing sedimentary basins. The success of modern exploration is, to a large extent, based on new advances in both deep and 3-D seismic imaging, as well as improved pressure-prediction and pre-drill oil and gas quality predictive methodologies, to mention just a few. Nevertheless, large areas of the greater Caspian region have remained unexplored due to a variety of factors such as deep-water conditions and zones with high pore-pressures in the South Caspian Sea and The Black Sea, and vast shallow-water regions with harsh winter ice conditions in the North Caspian Sea. This publication contains 12 extended abstracts and 6 full-length papers that discuss technology development, challenges in estimating proven and potential reserves, outcrop-based studies of potential reservoirs, regional tectonics and geodynamic evolution, and source rock and stratigraphic analyses of the greater Caspian area.