Main geological factors controlling shale gas reservior in the Cambrian Shuijingtuo Formation in Yichang of Hubei Province as well as its and enrichment patterns

According to the stratigraphic framework across late Ediacaran-Early Cambrian boundary in Yichang, western Hubei, there existed uplifts alternating with sags during the Precambrian-Cambrian transition. The sags were favorable for the deposition of the rich organic shale of the Lower member of Cambrian Shuijingtuo Formation. V/Cr, Ni/Co, V/(V+Ni) and chemical indexes of alteration (CIA) indicate that these shales were formed in an anoxic or euxinic environment. However, a relatively oxic water and a synchronous decrease in total organic carbon (TOC) and biogenic Mo (Moxs) were found in the Lower Member Shuijingtuo Formation, which demonstrates that TOC was affected by both redox environment and marine organic carbon flux. The correlation between TOC and Moxs is much better than that of the TOC and biogenic Ni(Nixs). This phenomenon, together with the composition of the δ¹³Ccarb and δ¹³Corg in the same interval strata, indicates that there might exist dissociation of gas hydrate or release of methane. The high correlation between TOC and shale gas content and brittle minerals, (e.g. quartz, pyrite and some other minerals) indicates that TOC is one of the most important factors for evaluating the quality of shale gas reservoirs. Thus the lowermiddle part of the Lower member of Shuijingtuo Formation characterized by high TOC should be the best reservoir of the shale gas in the Shuijingtuo Formation. The study of burial and evolution history of the marine sedimentary in Yichang area indicates that the source rock of the Lower Cambrian had not gone through secondary hydrocarbon generation after reaching its gas generation peak in middle Late Triassic because of the restriction from the Huangling uplift, and thus avoided the excessive thermal evolution of the organic matter and the destruction of the shale gas reservoirs. Enrichment of the shale gas in the Cambrian Shuijingtuo Formation in Yichang was controlled by TOC, Huangling uplift and its evolution.