Origin of shale gas in the Triassic Yanchang Formation, Ordos Basin, China

The Chang 7 Member shale gas of the Upper Triassic Yanchang Formation, Ordos Basin, is a representative of continental shale gas in China. The Chang 7 shale is currently in the oil window, suggesting it should primarily be producing oil, but an exploratory well found high levels of gas production. In addition, the methane carbon isotopic composition (delta C-13(1)) of Chang 7 shale gas is generally depleted in C-13 relative to that calculated by Ro, according to the delta C-13-Ro equation. It is uncertain whether Chang 7 shale gas has mixed with biogenic gas or early thermogenic gas. In this study, over 100 samples of fresh shale cores were collected from the Yanchang Formation, and on-site gas desorption experiments and experimental analysis were carried out. Measured shale gas content was considerable, and was found to be proportional to the abundance of organic matter. Through comprehensive evaluation of the gas composition and delta C-13(1), it was concluded that there was no biogenic gas contribution to the shale gas in the study area. The negative delta C-13(1) of shale gas was due to the different formation mechanisms of shale gas and conventional natural gas. Methane in shale gas reservoirs is the result of methane accumulation at all stages, which increases the shale gas content.