Carbon isotope evidence for shale gas preservation conditions and large methane release over million years:A case study of shale gas reservoirs of Wufeng and Longmaxi Formations in the Sichuan Basin

Although carbon isotope reversal and its reasons in shale gas reservoirs have been widely recognized,the application of the reversal is yet to be investigated.A study on high-maturity shale from Wufeng and Longmaxi Formations in the Sichuan Basin not only reveals the relationship between the degree of isotopes inversion and the production capacity(e.g.,estimated ultimate recovery(EUR))of the gas well but also indicates the preservation conditions of shale gas reservoirs.(1)Although there are differences in gas isotopes in different shale gas reservoirs,the isotope fractionation of shale gas is small during the production stage of gas wells,even when the wellbore pressure drops to zero.The main cause of the difference in carbon isotopes and their inversion degree can be the uplift time during the Yanshan period and the formation pressure relief degree of shale gas reservoirs in distinct structural positions.Thus,carbon isotope inversion is a good indicator of shale gas preservation condition and EUR of shale gas wells.(2)The degree of carbon isotope inversion correlates strongly with shale gas content and EUR.The calculation formula of shale-gas recoverable reserves was established using△δ^(13)C(δC_(1)-δC_(2))and EUR.(3)The gas loss rate and total loss amount can be estimated using the dynamic reserves and isotopic difference values of gas wells in various shale gas fields,which also reflects the current methane loss,thereby demonstrating great potential for evaluating global methane loss in shales.

Enrichment mechanism and resource potential of shale-type helium:A case study of Wufeng Formation-Longmaxi Formation in Sichuan Basin

China’s helium resource is highly dependent on overseas imports.Organic-rich and U/Th-rich shale reservoirs generally contain helium,and although the helium content is low,the total reserve is large.Therefore,the effective development and utilisation of shale-type helium resources is a realistic way to improve the security of helium resources in China.In this study,the generation mechanism,helium source and content,migration modes and pathways,controlling factors of enrichment,distribution pattern,and resource potential of the helium were analysed,using the Wufeng-Longmaxi shale in the Sichuan Basin and its periphery.Furthermore,countermeasures were proposed for shale-type helium exploration and development.The results show that the Wufeng-Longmaxi shale has a high content of U and Th and a good ability to generate helium.The helium is generated by a typical crustal source of helium and is characterised by self-generation,self-storage,and wide distribution.The helium resource potential is a product of its content and the resources of the associated natural gas.The continuous supply of helium and effective preservation are the main geological factors that control the enrichment of shale-type helium.The preliminary evaluation results show that the reserves of helium in proven shale gas reserves are 10.8×10^(8)m^(3)in the Sichuan Basin and its periphery,where the extra-large helium fields are likely to be discovered.Additionally,0.0912×10^(8)m^(3)of helium was produced,along with the annual production of shale gas.To avoid the waste of helium and to improve the self-supply ability,it is suggested that research on the resource potential,enrichment mechanism,and distribution pattern of shale-type helium should be carried out as soon as possible,and helium extraction techniques for helium-bearing natural gas should be studied.