Interpretation of crustal structure and hydrocarbon potential of the South Caspian and Kura basins, Azerbaijan 

Abstract

The South Caspian Basin and Kura basin have had a complex tectonic and stratigraphic history and characterized by different thermal regimes. The basins are a genetically linked system created in a Mesozoic extensional setting with a complex Cenozoic sedimentary filling. The study presents a new interpretation of the regional geodynamic history and crustal structure based on the new geological and geophysical data. New insights at the South Caspian Basin and Kura basin formation, evolution, and hydrocarbon potential were achieved by integrating published structural maps into the tectonostratigraphic framework delineating these basins and geothermal data, including onshore and offshore borehole temperature measurements, geothermal gradients, and heat flow data. The gravity and magnetic data were used to understand the regional geological model. For the first time geological evolution of the offshore the South Caspian Basin and onshore Kura Basin were linked within a single map set. Delineating and linking these basins allow novel understanding the geodynamic history of the Black Sea and Caspian regions. The study reveals several specific regions including “cold” South Caspian basin with a 20 km thick sedimentary succession and less than 10 km crustal thickness, “intermediate” Lower Kura basin, and “warm” Kura basin (including Yevlakh Agjabadi depression) with less than 10 km thick sedimentary succession and the crustal thickness of 20 to 25 km. According to the proposed evolution history the basins adjacent to the South Caspian basin involves Mesozoic island arc extension origin followed by subsequent development in Jurassic, with possible additional rifting in Eocene and flexural overprint in Tertiary. The South Caspian basin contains the dynamic petroleum systems with the prolific Oligocene-Miocene source rocks characterized with proved hydrocarbon potential increasing basinwards. Inherited tectonic boundaries between the South Caspian and Kura Basins such as the West Caspian Fault zone serve as markers for hydrocarbon prospectivity. The crustal parameters control the distribution of temperature gradients within the basins and hence hydrocarbon generation. Isothermal surfaces are displaced: depth of the surfaces changes across the boundary between the continental crust of the onshore Kura Basin and the different “oceanic-type” crust of the South Caspian basin. This boundary is located at around 500 km where isothermal values are abruptly displaced downwards by about 4 km. A sharp increase in depth of the 120°C isotherm along the boundary has significant implications for the thermal maturity of the source rocks. Rapid burial rates of the offshore South Caspian basin together with the low geothermal gradient have delayed the maturation of organic matter in the source rocks, making the South Caspian basin the location of one of the world’s deepest active petroleum systems. Thus, in deep and prospective offshore South Caspian hydrocarbon generation occurs at greater depth compared to onshore areas, characterized by a more limited hydrocarbon potential. The difference in maturity of onshore and offshore source rocks could play a role in segregating hydrocarbon prospective areas.