Mineralogical Characteristics of Carbonate Rocks of the Upper Triassic Bolila Formation in the Eastern Part of the North Qiangtang Basin

Carbonate rocks of the upper Triassic Bolila Formation in the eastern area of the North Qiangtang Basin are important tight oil and gas reservoirs. Due to the influence of sedimentation and diagenesis, the mineral particles are mostly less than 1mm. Analytical methods such as modular automated processing system (MAPS) based on the backscattered electron and automatic mineral identification system (QEMSCAN) were used to analyze the minerals and to research their composition, morphology and structure on a microscopic scale for four samples. The vertical variation of the minerals shows that at the bottom, the mineral composition is dominated by quartz and calcite, and the roundness of mineral particles is poor. The quartz content is relatively high in the middle. The terrigenous minerals and calcite bands are staggered. At the top, the carbonate mineral content is more than 90%, and layered dolomitization occurs locally. Mineral characteristics indicate that the source of Borila Formation sediments is affected by both shallow marine and terrigenous clastics. The supply of terrigenous clastics was relatively sufficient in the early stage. Subsequently, the supply of terrigenous clastics decreased, and the content of carbonate minerals increased gradually. The sedimentary environment was dominated by carbonate deposits at the late period of the Borila Formation.