Reservoir forming conditions and favorable exploration zones of shale gas in the Weixin Sag, Dianqianbei Depression

Forming conditions of the shale gas reservoir of the Weixin Sag in the Dianqianbei Depression are studied and preferable sweet spots for shale gas exploration are proposed based on the results of geologic survey, drilling and seismic survey in the shale gas region. The black shale of Cambrian Niutitang Formation and Silurian Longmaxi Formation are the two principal exploration targets in the Weixin Sag, having the characteristics of big net thickness (>50m) and abundant organic matter, low porosity and permeability. Those two formations have favorable conditions for the formation of shale gas reservoirs and also can be easily hydraulic fractured in the later stages of production. The coexistence of the clastic rock layer on the top, shallow entire seal, and the absence of exposed faults are the three desirable factors for the preservation of shale gas reservoir. The shale core analysis shows that the major composition is methane, the content of which increases as the preservation conditions become better and the shale TOC content and formation pressure increase. After integrated evaluation and optimized selection of shale gas exploration zones, it is determined that the Junlian-Luobu zone located in the north of the sag is the most favorable sweet spot for shale gas exploration in the overlapping target layer of the Longmaxi Formation and Niutitang Formation, and the Huani-Weixin zone in the middle of the sag is proposed to be the sweet spot for shale gas exploration in the Niutitang Formation. It is recommended that importance be attached to the evaluation of shale gas exploration in the Lower Carboniferous Jiusi Formation and Upper Permian Leping formation.