Applying Inorganic Geochemical Approaches To Conduct Hydrocarbon Source Correlation Under Post-to Over-mature Conditions:A Case In The Sinian And The Lower Cambrian Giant Gas Accumulations,Sichuan Basin,Southwestern China

Determining the source of hydrocarbons in sedimentary basins under post-to over-mature conditions is one of the most difficult and frontier fields in petroleum geology and geochemistry for decades.However,along with the advancement of exploration in this field,this issue needs to be resolved urgently.Here,we conduct a case study in giant gas accumulations of the Sinian Dengying and the lower Cambrian Longwangmiao formations in the Sichuan Basin,southwestern China,which has obtained a breakthrough in gas exploration recently but has great debates on gas sources.Newly explored inorganic geochemical approaches are applied.The aims of this research are to provide new references for the debate of hydrocarbon-source correlation in the study area,to improve the understanding of the law and theory of hydrocarbon accumulation and exploration in the study area,and to provide a new example in post-to over-mature conditions in the frontier field of studying hydrocarbon accumulation by using inorganic geochemistry.Therefore,this study is expected to have significance for both the basic theory and the practical application.Based on the studies of traditional organic petrology and geochemistry,we evaluate the two categories and five sets of possible source rocks in the study area,from bottom to top including the lower Sinian Doushantuo mudstones,the upper Sinian Dengying algal dolomites,the mudstones of the third member of the Sinian Dengying Formation,the lower Cambrian Maidiping argillaceous dolomites,and the lower Cambrian Qiongzhusi mudstones and shales.The two categories mean that they can be divided to two categories as mudstone/shale and carbonate rocks.These five sets of source rocks have a broad range of organic abundance with present TOC value of 0.03-2.36%and vary with region and stratigraphy,and the Qiongzhusi mudstones and shales in the Chuangzhong area and the Weiyuan-Ziyang area rank the best.The rocks are all in post-to over-mature stages and are originally oil-prone having been deposited in marine reducing environments with little variations among areas and stratigraphic intervals.The organic matter in the rocks is derived predominantly from bacteria and algae with only different compositions spatially and stratigraphically.Thus,the five possible source rocks have different hydrocarbon generation capabilities,controlled by organic richness and distribution.The Qiongzhusi mudstones and shales are the best of these source rocks followed by the Dengying algal dolomites and then the other three sets of source rocks stratigraphically,and the Chuanzhong area and the Weiyuan-Ziyang area have better quality source rocks regionally.Therefore,the lower Cambrian Qiongzhusi mudstones and shales in the Chuanzhong area and the Weiyuan-Ziyang area rank the highest in terms of hydrocarbon generation capabilities.Based on the preliminary determination of the quality of the source rocks and their development background above,the inorganic(major,trace and rare earth elements)geochemical methods are further applied to evaluate the source rocks from the three controls of source rock development,e.g.,paleo-primary productivity,preservation environment and sedimentary rate.This aims to expand new methods in the evaluation of post-to over-mature source rocks,given few conventional organic geochemical parameters are still effective under this thermal stage.Results of the paleo-productivity show that the Doushantuo mudstones and the Qiongzhusi mudstones and shales are the highest,followed by the mudstones of the third member of the Dengying Formation and the Maidiping argillaceous dolomites,while the Dengying algal dolomites are the lowest.However,the paleo-productivity does not vary largely among areas.Results of preservation environment show that the Dengying Formation was deposited under generally oxic-suboxic conditions,while the other four units were deposited under anoxic conditions,i.e.,H2S restricted and non-euxinic conditions.The Qiongzhusi mudstones and shales of the Chuanzhong area and the Weiyuan-Ziyang area are the most anoxic.Results of sedimentary rate show that it increases as follows,the Dengying algal dolomites,the Maidiping argillaceous dolomites,the mudstones of the third member of the Dengying Formation and the Doushantuo mudstones,the Qiongzhusi mudstones and shales.This is common among different areas.Therefore,the lower Cambrian Qiongzhusi mudstones and shales in the Chuanzhong area and the Weiyuan-Ziyang area rank the highest in hydrocarbon generation capabilities.These improve the understanding obtained from conventional organic petrology and geochemistry from the perspective of inorganic geochemistry,and succeed in establishing inorganic geochemical methods for the evaluation of post-to over-mature source rocks.Further compared with TOC contents of source rock,we find that the preservation conditions are the main control of the organic richness,while the paleo-productivity and the sedimentary rate play secondary roles.This may be one of the typical characteristics of marine source rocks in the old Sinian-lower Cambrian petroleum systems.Research on the origins of reservoir bitumens in the Dengying and Longwangmiao formations and their genetic relationships with the natural gas is conducted to clarify whether it is possible to determine the source of post-to over-mature hydrocarbons by analyzing the reservoir bitumens.Organic petrology results show that there are difference between the Dengying bitumens and the Longwangmiao bitumens.The Dengying reservoir only has solid bitumens in post-mature stage,while the Longwangmiao reservoir has two types of bitumens,the solid bitumens in post-mature stage but lower than that of the Dengying reservoir and the oil bitumens in mature stage with yellow-green fluorescence.Combined with the natural gas compositions and stable isotopes,the gases in the Dengying and the Longwangmiao reservoirs are believed to be oil-cracking with the Dengying gases in higher maturity.In the Dengying reservoirs,the bitumens are the residual product of oil cracking into gas.However,the origin of the bitumens in the Longwangmiao reservoir is a little complex.Part of them were formed by oil-cracking,while part of them were formed after the gas from the Dengying reservoirs migrated into the Longwangmiao reservoirs along faults and gas washing happened.Besides,the oil bitumens in the Longwangmiao reservoir may be sourced from the Longmaxi source rocks.According to these understandings,it is suggested that the analysis of bitumens in the Dengying Formation can determine the source of the natural gases in both the Dengying and the Longwangmiao formations.The bitumens play as a bridge for determining the source of post-to over-mature gases.Therefore,in the practice,it is requested to know the origin of bitumens and its relationship with oil and gas first.Therefore,inorganic geochemical studies are performed to determine the source of post-to over-mature hydrocarbons through reservoir bitumens.First,trace and rare earth elements were applied.It is showed that the rare earth elements can be used to clarify the source rocks,while the trace elements cannot because of its complex characteristics.Source of the Dengying solid bitumens is determined through REE distribution pattern,cluster hierarchal analysis,correlation coefficient and elemental ratios.Results show that the bitumen sources vary among areas.The Dengying bitumens in the Weiyuan-Ziyang area are sourced mainly from the Qiongzhusi mudstones and shales and affected by the Doushantuo mudstones a little.The source of the Dengying bitumens in the Chuanzhong area is complex,including the Qiongzhusi mudstones and shales,the mudstones of the third member of the Dengying Formation,the Dengying algal dolomites and the Doushantuo mudstones.The Dengying bitumens in the Chuanxinan area are sourced from the Maidiping argillaceous dolomites and the Dengying algal dolomites,with little contribution from the Qiongzhusi mudstones and shales,which is corresponding with the source rock development background.The Re-Os isotope geochemistry is further applied to determine the source of post-to over-mature hydrocarbons.Re-Os dating yield two ages of 414 ± 44 Ma with 187Os/187Osi = 1.36±0.19 and 78 ± 37 Ma with 187Os/188Osi= 3.21 ± 0.17.By comparing with the stratigraphic burial and thermal history in the study area,we propose that the age of 414 Ma represents the timing of first oil generation,whereas the age of 78 Ma marks the time of oil cracking processes and the formation of natural gas and reservoir bitumen.This clarifies the critical moment of oil and gas accumulation.In addition,Re-Os results confirm that the bitumens in the Dengying Formation are mainly sourced from the Qiongzhusi mudstones and shales,which is consistent with the conclusions from the study of rare and earth element geochemistry study above.We further address the Re-Os isotope geochemical behavior,and it shows that hydrocarbon accumulation process may affect the Re-Os isotope systems,while thermal evolution and biodegradation likely had few effects on the Re-Os isotopic data.These conclusions improve our understanding of the geochemical behavior of the Re-Os isotopic system during the formation and evolution of oil and gas(especially under post-to over-mature conditions),and indicate the validity of applying Re-Os isotopes in determining the source and accumulation geochronology of post-to over-mature hydrocarbons.Finally,the evolution of oil and gas accumulation process is reconstructed by integrating all the results obtained in this study and the geological background.Results show that the accumulation process is complex and might have at least three stages,including the paleo-oil reservoir formation,paleo-oil cracking and gas formation,and gas reservoir adjustment.The accumulation process varies in areas,which includes three types:"in-situ cracking and external accumulation","in-situ cracking and mixed accumulation",and "in-situ cracking and in-situ accumulation".Among them,the "in-situ cracking and in-situ accumulation" and "in-situ cracking and mixed accumulation" may be the most favorable models for the formation and preservation of gas reservoirs.Thus,the post-to over-mature natural gas exploration in the future should focus on these models,which may be of general significance to the deep carbonate gas accumulations in post-to over-mature stages like in this study.