Mechanism of organic-rich shale formation and shale gas enrichment in the Carboniferous Tian’eping Formation from the Xiangzhong Depression

[Objective] According to study on the shale formation and shale gas enrichment mechanism, the current study aims at finding out main factor controlling shale gas reservoir in the Carboniferous Tian'eping Formation in the Xiangzhong Depression as well as its enrichment patterns.[Methods] Carbonates carbon and oxygen isotopes as well as shale trace elements and major elements were analyzed at Xiangxindi 4 core in order to recover paleo-environment and investigate the origin for the shale formation. Combined with the thermal evolution simulation of medium-low maturity shale gas reservoirs, the mechanism for shale gas enrichment is identified through petromineralogy, organic geochemistry, physical properties of shale gas reservoirs, existence forms of shale gas and tectonic preservation condition.[Results] Our results show that (1) the organic-rich shale in Lower Carboniferous Tian'eping Formation were formed due to seawater stratification and seabed anoxia caused by the intensive climatic fluctuations in the Early Carboniferous, (2) the extensive and intense magmatic events in central Hunan led to the increase of locally paleogeothermal gradient and further caused secondary hydrocarbon generation in the Lower Carboniferous organic-rich shale. Shale gas in the Tian'eping Formation was formed owing to crude oil cracking and secondary hydrocarbon generation of organic matter, and (3) shale gas preservation were promoted due to decollement in the Ceshi Formation of the lower Carboniferous blocking the vertical escaping channel of the shale gas from the underlying Tian'eping Formation.[Conclusions] In conclusion, the shale gas of Lower Carboniferous Tian-ping Formation in central Hunan Depression is the common result of favorable facies zone controlling the total organic carbon content, magmatic thermogenesis controlling the reservoir physical properties and detachment structure controlling the preservation.