Geochemical Investigation On Evolution Of Organic Matter In Alkaline Lacustrine Sediments Of The Fengcheng Formation In The Junggar Basin,Northwest China

The evolution of organic matter in saline source rocks has a great significance in understanding the biogeochemical process and hydrocarbon-generation mechanism of organic macromolecules.Given the complexity of organic macromolecules,the related research is a challengable hotspot and frontier scientific issue in the field of organic geochemistry.This issue is urgent to solve with the progression of oil-gas exploration and exploitation in salined basin.Here,we conduct a case study in the Lower Permian Fengcheng alkaline petroleum systems in the Junggar Basin,northwest China,which has obtained a breakthrough in oil exploration.Newly explored organic-inorganic geochemical approaches based on the level of molecular scale are applied.The aims of this research are to provide a new example for the frontier field of the evolution of organic macromolecules in saline source rocks and to explore new research methods,and to provide new references for the unclear hydrocarbon-generation mechanism of source rocks in the study area to deepen the understanding of hydrocarbon generation rule and theory of alkaline source rocks to serve for exploration deployment.Therefore,this study is expected to have significance for both the basic theory and practical application.First,based on the analysis of heteroatomic compounds in the organic matter of Fengcheng Formation and its adjacent LPIA sediments by using negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry(i.e.,(-)ESI FT-ICR MS),the evolutionary paths of organic matter in different depositional environments was compared to preliminarily determine the influence of extreme alkaline conditions on the evolution of organic matter.Results indicate that,compared with the low salinity sedimentary paleo-environment of organic matter in the Carboniferous(C),Lower Permian Jiamuhe Formation(P₁j)and Fengcheng Formation first(P₁f₁)and third members(P₁f₃),the high salinity depositional setting enriched with alkali minerals in Fengcheng Formation second member(P₁f₂)can hinder the aromatization and polymerization of organic macromolecular at mature to highly mature stages,as a result,a large number of alkylated compounds existing in the highly mature stage provide a foundation for the oil generation.Based on the preliminary determination of the hydrocarbon-generation behavior of alkaline source rocks,the full-component geochemical analysis of crude oils was further used to reveal its composition becase oils can record the evolutionary process of organic matter in different stages of source rocks during its geological history.Results indicate that alkaline oils contain three end-members,corresponding to low-mature oil,mature oil,and high-mature oil.Alkaline sedimentary organic matter earlier enters the oil generation window(Ro?0.7%),and then reachs the mature oil generation stage(Ro?1.0%).When Ro>1.3%,the adsorption of alkali minerals and input of halotolerant algae(e.g.,most likely Dunaliella salina)extend oil generation window,resulting in the generation of highly mature oils after the normal oil window and improving oil generation efficiency.Compared with nitrogen-containing heteroatomic compounds in the brackish(sulfate-type)organic matter of the Middle Permian Lucaogou Formation in the Jimusar Sag,it was found that the biological nitrogen cycle in the Fengcheng Formation alkaline organic matter reatively weaken due to lacking of biological nitrogen precursors,particular for the fragments of higher plant.The reduction of biological nitrogen cycle makes the content of heteroatomic compounds in crude oil relatively decrease,while the light components mainly composed of saturated and aromatic hydrocarbons relatively increase,making the alkaline oils have higher quality,cleaner,and more environmentally friendly.This may be an important attribute of hydrocarbon-generation from such source rocks.Based on the above heteroatomic geochemical analysis to determine the evolution characteristics of alkaline organic matter,the facies change of organic matter evolution were further constrained by using newly defined Fourier-transform infrared(i.e.,FTIR)structural parameters.Results indicate,based on the 13 FTIR structural parameters defined in this thesis,the alkaline system mainly contain central alkaline lacustrine sedimentary,transitional saline lacustrine sedimentary,and marginal brackish–freshwater lacustrine sedimentary.The FTIR structural parameters show that alkaline crude oils can be mainly divided into three types.The first end-member oil is characterized by high-salinity,anoxic,and aquatic organic matter input,which was sourced from the organic matter-rich argillaceous dolomite or dolomitic mudstone.The third end-member oil is characterized by low-salinity,aboxic,and increasing terrestrial organic matter input,which was derived from brackish–freshwater organic matter-rich mudstone.In contrast,the second transitional end-member oil has characteristics between the both and was originated from saline organic matter-rich peperite.The elemental geochemical analysis of source rocks and crude oils reveals the inheritance and differential enrichment of inorganic elements during the hydrocarbon generation and expulsion process of alkaline source rocks.Results indicate,from the source rock to oil,the abundance of inorganic elements decrease sharply,which is mainly controlled by the element differentiation effect and thermal evolution of organic matter.Rare earth and non-redox sensitive elements are mainly affected by the differentiation effect,showing regular and systematic changes from source to oil.However,the related parameters of redox sensitive elements reveal the variation interval and trend of oil and source are consistent,and the differential enrichment is mainly affected by the thermal evolution of organic matter,while the impact of sedimentary paleo-environment is relatively weak.Considering the alkaline environment hinder the polymerization of organic macromolecules,the abundance of trace element of oil samples collected from the center of the alkaline deposition is still high at highly mature stage.In summary,the alkaline organic facies chage greatly and can affect the evolution of organic matter,which is characterized by that alkaline environments hinder the polymerization of macromolecular compounds and extend the oil generation window with“multi-stage oil generation”.The combination of(-)ESI FT-ICR MS,FTIR and inorganic(trace and rare earth)element geochemistry privides new analytical indeas and methods for the studying of organic matter.Accordingly,salined basin,especially alkaline basin may contain abundant oil reserves in deep reservoirs.