Structural features and forecast of the oil and gas potentiality of carbonate and terrigenous sediment complexes in the east of the Caspian basin

Introduction. Modern ideas and characteristics of the regional structure of the subsalt Paleozoic complex in the section of the eastern side of the Caspian basin are determined by the degree of development of deposits of carbonate and terrigenous composition and their sharply differentiated distribution within the eastern frame as a whole. New materials and geological and geophysical data in recent years indicate new opportunities for predicting promising objects (oil and gas traps), separately within lithological-stratigraphic complexes of different composition, taking into account facies, features and nature of sedimentation.

Aim. Probable search objects, taking into account the development of zones of predominant accumulation of carbonate and terrigenous deposits, differ in morphology and type of traps. At the same time, the refinement of the features of the structure and distribution of sediment complexes of different composition makes it possible to identify more favorable zones for predicting new oil and gas deposits, taking into account the distribution of characteristic types of traps in them. In this regard, the purpose of the study and this article is a theoretical justification for identifying and predicting additional zones and intervals of productivity based on new capabilities of geophysical exploration methods (seismic exploration and production geophysical methods).

Materials and methods. The data and results of the generalization of prospecting work carried out in previous years of studying the eastern side of the Caspian basin are used. The solution of the set geological problems, based on the main goal of the work, is based and demonstrated taking into account the data of the analysis of various facies sedimentation settings with the identification of zones of predominant development of carbonate (Upper Devonian — Carboniferous) and terrigenous (Lower Permian) sections. A comparative analysis of the structure of the main tectonic elements was carried out: Zhanazhol-Tortkol and Temir carbonate development zones (shallow-water shelf framing), on the one hand, internal submerged areas (steps) of the basin with relatively deep-water sedimentation conditions.

Results. The main features of the structure of Paleozoic deposits are shown on the example of the Akzhar Vostochny area (relatively deep-water zone of the sea basin) and the Urikhtau group of structures within the Zhanazhol-Tortkol zone of uplifts (shallow-water carbonate shelf), which must be taken into account when assessing the hydrocarbon potential along the section and lateral. Favorable prerequisites for substantiating the high predictive hydrocarbon potential of the carbonate units KT-III, KT-II and KT-I in terms of the Upper Devonian and Carboniferous, as well as the terrigenous Lower Permian deposits (Artinsk, Sakmarian, Aselian age) are clarifi ed and substantiated. Practical recommendations are given for further study of the Paleozoic strata, taking into account the diff erentiated distribution of carbonate bundles in the section of the onboard zone and the development, directly, within the bundles of zones containing reservoir rocks. Also, the conditions and features of sedimentation at the Lower Permian stage of sedimentation are substantiated in order to fully assess the composition and completeness of the section in order to identify additional oil and gas saturated intervals and zones.

Conclusion. A more detailed study of the structure of the section of the eastern side with an emphasis on the zones of development of predominantly carbonate and terrigenous deposits makes it possible to expand the range of possibilities and improve the methods of geophysical research. New favorable aspects are outlined for identifying additional productivity intervals, evaluating and interpreting a section with a continuous development of carbonate deposits (bundles KT-II and KT-I), as well as a favorable effect of clay minerals and “bound” water in assessing the prospects of terrigenous Lower Permian deposits.

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