Late Jurassic manganese microbial carbonates of West Siberian oil and gas basin: Diagnostic features, environmental sedimentation, secondary transformations

This study focuses on the investigation of Late Oxfordian–Early Tithonian manganese microbial carbonates (MC) found at the top of the Georgiev regional stage in the Western Siberian oil and gas basin. In some isolated areas, these MC are oil reservoirs. However, these rocks are still one of the least studied sediments in the carbonate interval at the boundary of the Bazhenov and Georgiev subregional stage. We have described their geochemical, lithological and mineralogical composition in detail. Based on the characteristics of MC, we assumed their formation settings. The elevated MnO content, associated with microbial processes, and the high concentration of biophilic elements, both suggest the active involvement of organic matter in the formation and development of microbial sediments. The isotopic composition of carbon in the microbial carbonates further supports a biogenic source of carbon dioxide playing an important role in the formation of these deposits. We have established that the Late Jurassic manganese MC were intensely secondary altered, partially recrystallized and calcified by catagenetic processes. Isometric pyrite crystal aggregates with kutnohorite fragments were formed at the contacts between kutnohorite and calcite zones. We suggest that the presence of caverns and high reservoir potential in the microbial carbonates from the Yem-Egovskaya field of the Krasnoleninsky arch may be attributed to intense secondary catagenetic alterations. • For the first time, the approximate age of microbial carbonates has been established from paleontological finds below and above the section. • Based on the complex of geochemical, lithological and mineralogical features, diagnostic criteria of microbial carbonates were revealed, which will allow identifying the rock in the core. • Microbial carbonate accumulation environments were identified. Two main paleogeographic hypotheses of sedimentogenesis of the studied rocks were proposed. • Secondary transformations of the investigated microbial carbonate rocks were identified and described, which led to the improvement of its reservoir properties.