Abstract
Purpose. To refine the stratigraphic subdivision, lithological composition, and sedimentation environments of the Upper Triassic deposits within the Seghendyk Depression (Southern Mangyshlak) using geophysical data and core materials from exploration wells. Methodology. The study is based on an integrated analysis of geological and geophysical data, including gamma-ray logging, neutron-gamma logging, electrical resistivity logging, and spontaneous potential measurements, complemented by descriptions of core samples. Findings. Stratigraphic correlation was conducted for the Saura-Seghendyk, Zhangeldy, and Northern Karagie areas, enabling the identification of lithostratigraphic subdivisions within the Triassic sequence. Within the Upper Triassic succession, two formations were identified: the North Rakushechnaya Formation and the Bokand Formation, which differ in lithological composition and geophysical characteristics. The North Rakushechnaya Formation is subdivided into four members: argillite-gravelite, argillite-sandstone, argillite, and carbonate-terrigenous, representing an incomplete third-order transgressive sedimentary cycle. The Bokand Formation comprises two members, argillite-sandstone and sandstone-argillite, forming another incomplete cycle. In the Seghendyk Depression, a regular increase in the thickness of the North Rakushechnaya Formation (up to 865 m) was observed, primarily due to the thickening of the lower gravelite member, which exceeds the values recorded in adjacent tectonic zones. Based on the interpretation of geophysical data, a correlation stratigraphic scheme was developed uniting the Saura-Seghendyk, Zhangeldy, and Northern Karagie areas. Originality. For the first time, the internal structure of the Upper Triassic complex of the Seghendyk Depression has been detailed, identifying six lithostratigraphic members that reflect two incomplete sedimentation cycles. A refined stratigraphic framework is proposed, linking the Seghendyk Depression to the North Karagie Saddle and aligning it with the regional stratigraphic scale of Mangyshlak. Practical value. The developed stratigraphic model enables more accurate correlation of the Triassic sections in Southern Mangyshlak, improving the prediction of reservoir horizons within the promising Saura-Seghendyk, Zhangeldy, and Karagie areas during oil and gas exploration.