Triassic Hydrocarbon System of the Middle-Caspian Oil and Gas Basin
Authors: Sokolov Denis , Delengov Mikhail , Sabirianova Regina , Musikhin Konstantin , Bogdanov Oleg 
Published: October 2021

The objective of this paper is to assess the generation potential of the Triassic hydrocarbon (HC) source rocks for the petroleum potential of the Middle-Caspian Oil and Gas Basin. Tectonically the study area belongs to the Epigercine Scythian-Turanian plate, which includes the following major tectonic elements: the Karpinsk-Mangyshlak ridge and the Prikum-Central Caspian system of troughs and uplifts.

Comprehensive research approach to identify the main features of the Triassic hydrocarbon system, such as the interpretation of seismic data, laboratory geochemical analysis and evolution restoration of the hydrocarbon systems elements by 3D basin modeling techniques. The geochemical research includes pyrolytic analysis of potential Triassic source rocks, determination of the vitrinite reflectance values, chromatography-Mass Spectrometry studies of molecular structure. The subsequent basin modeling made it possible to integrate a wide range of geological and geophysical information into a single complex. These basin modeling results provide a better understanding of evolution restoration of the hydrocarbon systems elements and make it possible to foresee hydrocarbon accumulation's localization.

The combined set of research helped to identify the boundaries of the Triassic HC source rocks in the Middle-Caspian Basin. It also allowed characterizing in detail their geochemical parameters and evaluating the volume of its generation potential. Triassic deposits are part of the taphrogenic intermediate complex of the Scythian platform and mainly located in graben structures. The increased content of organic matter is associated with the Lower Triassic Neftekum Formation of the Olenek Stage, which is composed of clayey limestones and mudstones.

The content of total organic carbon (TOC) in limestones in the Eastern Fore-Caucasus area averages 0.1 % in some layers up to 2.05 %. The average TOC content in mudstones is 0.43 %. This parameter is up to 0.9 % in the direction of the East-Manych trough. Triassic deposits were penetrated by 5 wells, the average content of organic carbon was 1.22% in mudstones. The samples show a migrant presence of bitumen.

According to 3D basin modeling results, the Olenek HC source rock has practically exhausted its potential in on most of the except the southern part of the study area, as well as the eastern flank.

The Olenek stage's Neftekum HC source rock in the southern part of the model is identified with MK1-3 catagenesis gradations, this grade represents the main zone of oil generation. At the northern part of study area, those strata of source rock are positioned at a great depth. The hydrocarbon source rock in those areas have reached the grades of catagenesis MK5-AK1, which represents the zones of condensate and gas formation.

The research results allow to take a new look at the influence of the Triassic hydrocarbon system and its prospects of petroleum potential of the Middle-Caspian Oil and Gas Basin. It also allows evaluating the generation parameters variance of the Olenek stage's Neftekum HC source rock regarding its area and time period.