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).