The genesis of H2S in the weiyuan gas field, Sichuan Basin and its evidence

The Sinian Dengying Formation gas pool in Weiyuan is the oldest large-scale sulfur-bearing gas field in China, which has a H2S content ranging from 0.8% to 1.4%. The Cambrian Xixiangchi Formation gas pool discovered recently above the Dengying Formation contains gas geochemical behaviors similar to those of Dengying Formation but different in sulfur isotopes of H2S. Investigations show that though these two Sinian and Cambrian gas pools are separate ones, they share the same Cambrian source rock. The higher dry coefficient, heavier carbon isotopes, sulfur isotopes of sulfide, lower filling of gas pools, formation water characteristics, reservoir properties and H2S distribution, indicate that H2S in both the Sinian and Cambrian gas pools originates from TSR. The sulfur isotopes of sulfates have shown that H2S was formed in respective pools, namely hydrocarbons charged into the pools reacted with the Dengying Formation and the Xixiangchi Formation gypsum (TSR), respectively, to form H2S. Compared with sulfur isotopes of sulfates in each pool, delta S-34 values of H2S are 8 parts per thousand lighter for the Dengying Formation pool and 12 parts per thousand lighter for the Xixiangchi Formation pool, respectively, which is attributed to the difference in temperatures of TSR occurrence. The reservoir temperature of the Xixiangchi Formation pool is about 40 degrees C lower than that of the Dengying Formation pool. Temperature plays a controlling role in both the sulfur isotopic fractionation and amounts of H2S generation during TSR.