Controls of sedimentary environments on shale gas reservoirs: A case study of the Wufeng and Longmaxi Formations in northeastern Chongqing

The controls of sedimentary environments on shale gas reservoirs in the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in northeastern Chongqing are treated in this study on the basis of field investigation, laboratory data and previous results of research. The organic-rich shales occur mostly in the stagnant neritic shelf environment, and have a thickness of 70-80 m at the depositional center. The organic carbon contents amount to 7.56%, with an average of 3.09%. The rock types in the static anoxic environment include organic-rich carboniferous-silty mudstones and shales, carboniferous-siliceous mudstones and shales, siliceousbearing mudstones and shales, carboniferous mudstones and shales, silty mudstones and shales, and siliceous mudstones and shales. The mineral compositions in the shales contain quartz (62.76% in average) and clay minerals(22.61% in average). The intergranular pores in pyrite created in the anoxic-reduction environments, interlayer micropores in clay minerals, micropores created by hydrocarbon generation in organic matter, and microcracks created by brittle minerals have provided excellent reservoir spaces for the shale gas. The study of reservoir characteristics in sedimentary environments will offer some thoughts on the shale distribution, organic matter abundance, reservoir spaces and evaluation of favourable areas in future shale gas exploration and development.