Triassic Deposits in the Caspian Region: Structure, Tectonic Settings, Sedimentary Environments, and Oil-and-Gas Potential

Abstract

Both tectonic settings and sedimentary environments of the oil- and gas-bearing Triassic sequences in the Caspian region are examined. It is shown that the Scythian and West Turanian plates were located at an active margin of the East European continent in Triassic. A unified thick (up to 3–5 km) cover of sedimentary and volcanosedimentary (mainly tuffaceous) rocks without the characteristic features of rifting was formed. In the northern and eastern parts of the region, the Triassic is composed of variegated continental terrigenous rocks. In many areas of the region (eastern and southern parts of the Pre-Caspian Basin, Eastern Ciscaucasia, Mangyshlak, and Middle Caspian), the Lower Triassic (Olenekian) and Middle Triassic are represented by marine terrigenous–carbonate rocks. The structural features of sequences in the Pre-Caspian Basin are associated with salt tectonics. The maximum thickness (up to 4.5–5 km) of Triassic rocks within the Scythian Plate is recorded in the southwestern part of the Middle Caspian and southeastern part of the Karpinsky Ridge, where they fill up a series of NW-extending graben-shaped troughs. Their absence in the northwestern part of the region is related to the latest shear deformations, as confirmed by their discrepancy with the facies zonation of Triassic deposits. Currently, the Permian and Triassic deposits represent a pre-platform geodynamic seismic sequence unconformably overlying the pre-Kungurian deposits. Triassic deposits of the Scythian and West Turanian plates are disturbed by faults (amplitude up to 2 km) and dislocated in some areas. Folding with faults and strike-slip faults is especially intense on the Karpinsky Ridge. Increased dislocation and Late Triassic magmatism on the Scythian and West Turanian plates are associated with the processes of collision during the closure of the Paleotethys Ocean. Oil and gas prospects of the region are assessed positively.