Sedimentary Responses to Subsalt Tectonic Evolution in the Southeastern Margin of the Precaspian Basin

The eastern margin of the Precaspian Basin is the main distribution area of oil and gas cooperation projects between China and Kazakhstan. The subsalt oil and gas resources in the basin are very rich. 85% of the proven oil and gas reserves found in the basin are located in the Upper Paleozoic Carboniferous carbonate formation. In this paper, a systematic study of the Upper Paleozoic in the southern part of the eastern margin of the Precaspian Basin was conducted to clarify the stratigraphic depositional characteristics, tectonic-depositional evolution process and establish the stratigraphic framework, with the aim of providing a scientific basis for exploration and development. The well-seismic comparison was carried out by combining the stratigraphic comparisons with the seismic stratigraphic delineation, and a unified stratigraphic framework was established for the well-seismic sequences in the area. The KT-II, MKT, and KT-I formations were divided into five secondary stratigraphic sequences (SSQ). On the basis of the secondary stratigraphic sequences, the KT-II, MKT, and KT-I formations were divided into nine tertiary stratigraphic sequences, and the study area was divided into the restricted platform intra-terrace beach facies, tidal facies, lagoon facies, open platform intra-terrace beach facies, inter-terrace depression facies, intra-terrace sea facies, and platform margin slope facies. The KT-II subsalt carbonate develops a sedimentary system of carbonate terrace-terrace front slope facies, with obvious east-west division of sedimentation, and develops near-shore confined terrace-barrier reef beach-open terrace-weak edge terrace-slope facies. There are differences between north and south. The KT-I carbonate develops the sedimentary system of carbonate terrace - terrace front slope facies. The whole carbonate terrace extends in southwest-northeast direction, and develops nearshore confined terrace - barrier reef beach - open terrace - weakly edged terrace margin - slope facies.