Geochemical characteristics and its geological application of over-mature Longmaxi shale gas in the northern Guizhou area, China

The organic-rich marine shale of the Lower Silurian Longmaxi formation in the northern Guizhou area, China, is a significant target for shale gas development, because of its high TOC values and high gas contents. However, the geochemical characteristics of the Longmaxi shale gas in this area are unclear. In this study, molecular and isotopic compositions of 7 gas samples from the Longmaxi shale were analyzed, and their applications in reservoir evaluation were discussed. The results show that this shale gas is thermogenic origin and oil-derived, and the over-mature evolution (2.18–3.12%) resulted in the reverse of carbon isotopic compositions (δ13C1 > δ13C2). The shale gas is rich in non-hydrocarbon gases, and the abnormal enrichment of nitrogen is the result of tectonic movements and hydrodynamic conditions. The prospective targets for shale gas development should be far away from large fault zones in the northern Guizhou area, where the preserve condition for the Longmaxi shale gas is slightly poorer than the Sichuan basin.