Lithofacies types, reservoir characteristics, and hydrocarbon potential of the lacustrine organic-rich fine-grained rocks affected by tephra of the permian Lucaogou formation, Santanghu basin, western China

Abstract

Volcanisms were widespread during the Permian periods in the Santanghu Basin. However, information on the classification of the lithofacies of the Lucaogou Formation (Lucaogou Fm) is still lacking, and the hydrocarbon potential of the organic-rich fine-grained rocks (FGRs) remains uncertain. Therefore, this study determines the lithofacies classification standard of the Lucaogou Fm FGRs, clarifies the advantageous lithofacies types, and evaluates the unconventional resource potential based on organic geochemical analysis, macroscopic core observation, core routine test, rock mineral identification, and pore structure characterization. The Lucaogou Fm FGRs are mainly composed of felsic minerals and dolomite, which can be divided into four rock types with high organic matter (OM) content and laminated structure. Based on TOC, rock type, and sedimentary structure, ten types of lithofacies were determined: organic-rich massive tuff (ORMT), organic-medium massive tuff (OMMT), organic-medium massive dolomite (OMMD), organic-poor massive dolomite (OPMD), organic-rich laminated tuffaceous dolomite (ORLTD), organic-medium laminated tuffaceous dolomite (OMLTD), organic-poor laminated tuffaceous dolomite (OPLTD), organic-rich laminated dolomitic tuff (ORLDT), organic-medium laminated dolomitic tuff (OMLDT), organic-poor laminated dolomitic tuff (OPLDT). There is a large amount of algal OM in the Lucaogou Fm FGRs because of the deposition of tephra, but they seriously hinder the development of pore space and the flow of pore fluid due to the relatively low maturity, resulting in poor pore structure and fluid mobility. Among them, ORMT, OPMD, OMMT, and OMMD have been transformed by diagenesis, with various types and scales of pore space, and obviously good physical properties, oiliness, pore structure, as well as fluid mobility. Followed by OPLTD, ORLTD, and OMLTD, while OPLDT, OPLDT, and OMLDT are the worst. The advantageous lithofacies of the Lucaogou Fm FGRs are influenced by both rock types and OM abundance, and are also related to sedimentary structures. In addition, the Lucaogou Fm FGRs have a large thickness, high-quality OM, high content of brittle minerals, low content of clay minerals, developed micro-nanopores network, and certain producible oil, indicating that they have significant hydrocarbon potential.