Geochemical characteristics and origin of natural gas from Wufeng-Longmaxi shales of the Fuling gas field, Sichuan Basin (China)

As the first giant shale gas field in China, the Fuling gas field has recently been regarded as one of the most important regions for natural gas exploration and production in the Sichuan Basin. However, the origin of natural gas from Upper Ordovician Wufeng and the bottom of Lower Silurian Longmaxi (WL) shales in the Fuling gas field is poorly understood to limit a comprehensive understanding of gas generation, accumulation and exploration. In this work, based on molecular and stable carbon isotopic composition of a total of 24 gas samples from five shale gas wells in the Fuling field, we analyzed the geochemical characteristics and gas origin, and discussed the cause for the geochemical anomalies (carbon isotopic reversals). Molecular composition results show that gases from the Fuling gas field are dry and mainly composed of methane (97.9-98.9%), with a very low level of ethane (C2H6), propane (C3H8) and non-hydrocarbon gases (mainly CO2 and N-2). These dry gases are classified as oil-associated gas and mainly derived from secondary cracking. Due to the lack of gas samples across a maturation gradient from immature to late mature, the WL gases in the Fuling field show an undear evolution trend between the 8(13)C(2) and wetness values; however, all these samples are located in the isotopic reversal zone. Carbon isotopes of gaseous alkanes clearly display full isotopic reversals (8(13)C(1)> 8(13)C(2)> 8(13)C(3)), which is indicative of a relatively high thermal maturity and consistent with the measured vitrinite reflectance and modeled values (similar to 3.0%R-0.).The observed complete carbon isotopic reversals in the WL gases are caused by a combination of several mechanisms, in which isotope exchange at high temperature is the primary controlling factor. Other secondary factors include Rayleigh-type fractionation of C2H6 and C3H8, secondary cracking and gas diffusion mixing of gases at different thermal maturity levels. (C) 2016 Elsevier B.V. All rights reserved.