Abstract

The Upper Devonian and Lower and Middle Carboniferous passive-margin carbonate platform in the Bolshoi Karatau Mountains of southern Kazakhstan is geologically similar to the coeval carbonate platform that contains the supergiant Tengiz oil field and other giant carbonate oil and gas fields in the North Caspian basin. The Bolshoi Karatau carbonate platform overlies Devonian siliciclastic rocks; the Tengiz field is interpreted to also overlie Devonian siliciclastic rocks. The two platforms are similar in size, as both range from about 2 to 4 km thick, 100 to 250 km wide and 100’s of kms long. During the Late Devonian and Carboniferous these coeval carbonate platforms evolved near one another on separate paleotectonic plates at low paleolatitudes in the same warm tropical ocean.

Distinct mappable facies belts comprise the Bolshoi Karatau carbonate platform. These belts contain several types of bioherms, reefs and carbonate sandstone bodies that are analogous to some of the oil and gas reservoir facies in the North Caspian basin. Twelve carbonate sequences are recognized over a stratigraphic interval of about 4.5 km which ranges in age from Late Devonian (Famennian) through Middle Carboniferous (Bashkirian). Sea level lowstand, transgressive and highstand systems tracts have been identified in some sequences at the platform margin. Shoal water platform environments were exposed to vadose and phreatic conditions during relative sea level lows as expressed by horizons of karstification and dolomitization. These diagenetically altered carbonates contain several kinds of secondary porosity and form potential oil and gas reservoir facies.

Studies of the Bolshoi Karatau carbonate platform provide new data on the stratigraphic and sedimentologic evolution of Upper Devonian and Carboniferous carbonate platforms in southern Kazakhstan. Preliminary depositional and stratigraphic models of this outcrop belt are herein presented which will hopefully serve as useful analogs for making subsurface predictions and for interpreting the stratigraphic cyclicity and distribution of potential reservoir, seal and source rock facies in carbonate platform areas of the North Caspian basin.