Аннотация
Abstract The Bolshoi Karatau carbonates of southern Kazakhstan record the development of a 4,500–m–thick platform that evolved close to the North Caspian Basin of western Kazakhstan during the Late Devonian and Carboniferous. Carbonate facies in the Bolshoi Karatau Mountains provide outcrop analogs for coeval reservoirs in supergiant oil and gas fields in the North Caspian Basin. 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 Late Devonian and ended in the Bashkirian, a span of about 50–55 Myr. Second, the stratigraphic thickness and 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 very well exposed in the Bolshoi Karatau. 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 Myr 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 were 185-285 m/Myr during the Late Devonian, 60-100 m/Myr during the Tournaisian, 35-50 m/Myr during the Visean, 15-30 m/Myr during the Serpukhovian, and 20-50 m/Myr during the Early Bashkirian. 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 and Early Visean and to skeletal mounds and sand-shoal-rimmed margins in the Middle-Late Visean, Serpukhovian, and Bashkirian. Depositional sequences, sequence boundaries, and facies were controlled by relative sea-level oscillations, sedimentation rate, climate, subsidence, and biotic changes through time. Relative sea-level changes were responsible for shelf-margin flooding and backstepping, multiple stacked sequence sets in shelf interiors, paleosols, extensive meteoric diagenesis, and karst and collapse breccia. Five depositional supersequences (second-order cycles), of seismic stratigraphic scale, are recognized in the Bolshoi Karatau: (1) Supersequence #1 includes Frasnian and Famennian Girvanella-Renalcis algae-bryozoan-sponge boundstone and cementstone reef-rimmed facies. Shelf interiors contain skeletal mud mounds, carbonate sands, cryptalgal laminites, evaporitic laminites, and a 90-m-thick regionally extensive karst and evaporite collapse breccia. Basin margins contain carbonate turbidites and megabreccia debris-flow aprons. (2) Supersequence #2 comprises Tournaisian-Lower Visean ramps of brachiopod-crinoid biostromes and abundant tidal-flat facies. Seaward ramp settings contain Waulsortian skeletal mud mounds and bioclastic turbidite aprons. (3) Supersequence #3 comprises Lower Visean-Upper Visean carbonates that have both Waulsortian skeletal mud mound shelf margins and shoal-rimmed shelf margins. Waulsortian skeletal mud mound margins and upper and lower slopes are sponge-Tubiphytes-algal-bryozoan boundstone and cementstone. Deeper-water parts of these slopes contain carbonate turbidite aprons. Shelf-interior facies consist of interbedded ooid and bioclastic sands. Shoal-rimmed platform margins are dominated by cross-bedded ooid-bioclastic sands and turbidite aprons. (4) Supersequence #4 comprises Upper Visean-Lower Bashkirian basinal carbonate turbidites and upper-slope Waulsortian skeletal mud mounds made up of Donezella algae-Tubiphytes-sponge-bryozoan boundstone and algal rudstone. These upper-slope mounds developed in a setting that was receiving abundant carbonate turbidites and debris flows. Shelf margins and shelf-interior facies consist of interbedded ooid and bioclastic sands and phylloid algae-rich sands. (5) Supersequence #5 encompasses Lower Bashkirian Waulsortian skeletal mud mounds that formed on upper-slope and drowned-shelf-lagoon settings. Waulsortian mound facies are dominated by phylloid algae-Donezellid algae-Archaeolithoporella algae-gastropod-brachiopod boundstone and cementstone facies. The boundstone facies are typically interbedded with algal rudstone and possible pisoid facies. Drowned shelf-lagoon facies comprise lime mudstones and carbonate turbidites derived from an ooid-bearing shelf margin. In the Bolshoi Karatau, thick stacks of upward-shallowing cycles of dolomitized and karsted shelf-margin and shelf-interior ooid-bioclastic sands form potential reservoirs, whereas lower-slope and upper-slope Waulsortian skeletal mud mounds contain abundant marine cement and are relatively tight. Reservoir enhancement is related to early dolomitization and meteoric diagenesis. These geometric and diagenetic patterns are analogous to some reservoirs in the North Caspian Basin such as in the Karachaganak and Tengiz fields and the newest supergiant, the Kashagan oil field. Bolshoi Karatau studies provide important data on the heterogeneity of the reservoirs in terms of their facies types, their cyclicity and stacking patterns, the origins of these stacking patterns, porosity-enhancing conditions associated with this cyclicity, and the spatial distribution of these reservoirs in the platform. These outcrop studies should be valuable for better understanding and predicting the characteristics and development of the North Caspian Basin oil and gas reservoirs as well as the oil and gas reservoirs in the Timan-Pechora and Volga Ural basins of Russia.