The microbial precipitation of lacustrine dolomite from Permian Formation, Urumchi, Xinjiang, China

Middle Permian Lucaogou Formation is characterized as lacustrine dark gray or gray carbonate rocks, interbeded with sandstone, thick balck mudstone and oil shale in Yangniuchang section, Urumchi, Xinjiang Province. The carbonate rocks were mainly composed of dark gray dolomicrite, then gray micrite. The dolomicrite comprised micro-crystalline dolomite, ankerite and calcite with mixture of clay-sized quartz and enrichment of organic matter. Three micromorphologies of dolomite were found under SEM, including microspheroids (9μm in diameter), micro-rods (2μm) and micron-sized (<5μm) anhedral dolomite, and the micro-anhedral dolomite held the majority in dolomicrite. There were also found large amount of nanoparticles (70~150nm in diameter) in the micrite with shape very similar to micrococcus. Compared with the micrite, dolomicrite has higher Sr value, Sr/Ca ratio, Mn value, and δ18O value, which indicate that the dolomicrite is formed in deeper anoxic water with higher salinity than micrite, and the dolomicrite lacks of replacement textures from micrite. The δ13CPDB of dolomicrite ranged from 9.2 ‰ to 15.6‰. The higher δ13C value may be the result of isotope fractionation led by methanogens. Such characteristics indicate that the dolomite precipitation had close relationship with the bacterial methanogenesis in methane-producing zone.