Geochemical characteristics of water-dissolved gases and implications on gas origin of Sinian to Cambrian reservoirs of Anyue gas field in Sichuan Basin, China

Abstract

The Anyue gas field is a supergiant gas field which was discovered recently in the Sichuan Basin, China where the Sinian Dengying Formation and the Cambrian Longwangmiao Formation are principal gas reservoirs. Gas in the Anyue field contains mainly methane (>98%) and a low content of heavy hydrocarbons, with minor H2S component. Reservoirs commonly contain bitumen and produce formation water. It is currently suggested that natural gas in the Anyue field is derived from the secondary cracking of crude oil. However, carbon isotopic ratio of methane is significantly less negative than that of bitumen, which contradicts the isotopic signature oil cracking process would predict. Besides, this phenomenon also cannot be explained by the process of thermochemical sulphate reduction (TSR), because there is no remarkable correlation between the δ13C1 values and H2S contents as suggested in previous studies. Both free gas and water-dissolved gas from the Anyue gas field were analyzed in this study for their stable isotopes. Results show that the δ13C1 of the gas derived from the formation water is much less negative than that of free gas. Therefore, it is inferred that the less negative δ13C1 values in the free gas reservoir are attributable to addition of natural gas that is previously dissolved in formation water. Geological setting of the gas reservoirs in Dengying and Longwangmiao formations of the Anyue gas field is favorable for the formation and preservation of water-dissolved gas. In addition, both formations had experienced significant structural uplifting before the gas reservoirs were formed. Reduced temperature and pressure in the formations by geological uplifting could cause super-saturation of methane in the formation water, and as a consequence, water-dissolved methane gas could exsolve from water phase into gas phase. Therefore, it is concluded that natural gas in the Anyue field is a mixture of free gas and gas exsolved from the formation water. As a result, the carbon isotope of methane gas in the Anyue field became less negative than that of a single free gas component would predict.