Comparison of source rocks from the Lower Silurian Longmaxi Formation in the Yangzi Platform and the Upper Devonian Semiluksk Formation in East European Platform

Abstract

Abstract Due to the depletion of conventional hydrocarbon resources, both China and Russia are giving more attention to the exploration and production of unconventional oil and gas resources, especially those generated and accumulated within source rocks. In an attempt to further understand the mechanisms of these resources, detailed mineralogical, lithological, and geochemical studies were performed to compare source rock samples from i) the Longmaxi Formation of the Lower Silurian in the Sichuan Basin (China), and ii) the Semiluksk Formation of the Frasnian Stage of the Upper Devonian in the Volga-Ural region of the East European Platform (Russia). The results showed that the main mineral of the source rocks of both formations is chalcedony that formed during an outpouring of deep fluids onto the sea bed. Simultaneous thermal analyses of this mineral indicated similar thermophysical properties to those of the hydrothermal-sedimentary chalcedony from jasper. Moreover, a direct relationship between the chalcedony content and the total organic matter content in the samples from the two formations was established. The presence of biophilic chemical elements in the siliceous component of the source rock samples indicate that high total organic content values were likely due to the presence of biophilic chemical elements in deep fluids, which led to the large-scale development of biota and subsequent accumulation of organic matter during sedimentation. The findings also revealed that the organic matter in the source rocks of the two regions was at different stages of maturity. The organic matter in samples from the Volga-Ural region was less mature and only at the early stage of oil generation, whereas the organic matter in samples from the Sichuan Basin reached both oil and gas generation stages. The Longmaxi Formation is already in the shale gas exploration and development stage, and the Semiluksk Formation could also be regarded as an exploration target for shale oil reservoirs.