The Lower Triassic Jialingjiang Formation reservoirs are distributed widely in the East Sichuan Basin,which are composed mainly of fractured reservoirs.However,natural gas with high concentration of H_2S,ranging from 4% to 7%,was discovered in the Woionghe Gas pool consisting primarily of porous reservoirs,while the other over 20 fractured gas reservoirs have comparatively low,tiny and even no H_2S within natural gases.Researches have proved the H_2S of the above reservoirs are all from the TSR origin.Most of the Jialingjiang Formation natural gases are mainly generated from Lower Permian carbonate rocks,the Wolonghe gas pool's natural gases are from the Upper Permian Longtan Formation,and the natural gases of the Huangcaoxia and Fuchengzhai gas pools are all from Lower Silurian mudstone.The formation of H_2S is controlled by the characteristics and temperature of reservoirs,and is not necessarily related with gas sources.The Jialingjiang Formation in East Sichuan is buried deeply and its reservoir temperature has ever attained the condition of the TSR reaction.Due to poor reservoir potential,most of the gas pools do not have enough room for hydrocarbon reaction except for the Wolonghe gas pool,and thus natural gases with high H_2S concentration are difficult to be generated abundantly.The south part of East Sichuan did not generate natural gases with high H_2S concentration because the reservoir was buried relatively shallow,and did not suffer high temperature.Hence,while predicting the distribution of H_2S,the characteristics and temperature of reservoirs are the necessary factors to be considerd besides the existence of anhydrite.
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, δ34S values of H2S are 8‰ lighter for the Dengying Formation pool and 12‰ 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℃ 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.
