Sinian gas sources and effectiveness of primary gas-bearing system in Sichuan Basin, SW China

Based on correlation between geochemical characteristics of Sinian and Cambrian source rocks and discovered gas reservoirs,paleoand the analysis on geological conditions of reservoir formation,the sources of natural gas in the Sinian of Sichuan Basin have been discussed to sort out the contribution of Sinian source rocks to the gas reservoirs and effectiveness of Sinian primary gas-bearing system.Through the analysis of natural gas composition,carbon and hydrogen isotopes and effectiveness of Sinian accumulation assemblages,it is concluded that:(1)The natural gas derived from the Sinian source rock is characterized by low ethane content,heavy ethane carbon isotope and light methane hydrogen isotope,and obviously different from the gas generated by the Cambrian source rock.(2)The gas reservoirs discovered in Sinian Dengying Formation are sourced by Sinian and Cambrian source rocks,and the Sinian source rock contributes different proportions to the gas in the 4th member and the 2nd member of the Dengying Formation,specifically,39%and 55%to the 4th member in marginal zone and intra-platform,54%and 68%to the 2th member in the marginal zone and intra-platform respectively.(3)The effectiveness of the Sinian primary gas-bearing system depends on the gas generating effectiveness of the source kitchen,reservoir and combination of gas accumulation elements.For high-over mature marine source rocks at the Ro of less than 3.5%,besides gas generated from the thermal cracking of liquid hydrocarbon,the kerogen still has some gas generation potential by thermal degradation.In addition,the Sinian microbial dolomite still preserves relatively good-quality reservoirs despite large burial depths,which match well with other basic conditions for gas accumulation in central Sichuan paleo-uplift,increasing the possibility of Sinian primary gas-bearing system.The research results confirm that the Sinian primary gas-bearing system is likely to form large-scale accumulation.

An analysis of the types and distribution characteristics of natural gas reservoirs in China

The natural gas reservoir beds of different areas in China can be divided into three kinds, clastic natural gas reservoir bed, carbonate natural gas reservoir bed and special natural gas reservoir bed. They have different combination patterns controlled by deposition, diagenesis and tectonism. Our analysis indicates that the natural gas reservoirs are mainly distributed in the Precambrian, Palaeozoic, Mesozoic, and Tertiary-Quaternary. Craton basin, foreland basin and intracontinental rift basin which contain most of natural gas in China have special geological features and favorable accumulation conditions, and will be important exploration areas in the future.