Origin of Organic Matter,Fluid Transport and Geological Significance in Stylolites
[Objective]As a typical structure of carbonate rocks,the formation of stylolites is of great significance for the enrichment of organic matter and fluid migration within the carbonate rock itself.However,there is currently no in-depth research on the source of organic matter and the mechanism of fluid transport in stylolites.[Methods]This study collected carbonate core samples containing stylolites from the Dachigan structural belt in the eastern Sichuan Basin.The stylolites and matrix were respectively subjected to organic carbon and pyrolysis(Rock-Eval),common thin section observation,thin section observation under fluorescent light,whole rock X-ray diffraction,carbon and oxygen isotopes and scanning electron microscopy(SEM)and other testing items.[Results and Conclu-sions]The organic matter occurred in the stylolites is mainly in the form of three types including soluble organic mat-ter,pyrobitumen,and bitumen,indicating that the organic matter in the stylolites is mainly the secondary migrated organic matter.Secondary dolomite and pyrite are well developed in the stylolites that the intercrystalline pore spaces and fractures are retained,thereby to provide good conditions for the accumulation and migration of oil and gas.The strong cementation on the contact between the stylolites and matrix and the formation of secondary minerals nearby may be the results of fractionation when the hydrocarbon fluid migrates from the matrix to stylolites.Through analysis,it is believed that the strong cementation at the contact interface between the suture and the matrix,as well as the formation of nearby secondary minerals,are the reasons for the fractionation effect of hydrocarbon fluids when migrating from the matrix to the suture.Through this study,it was found that during the development of sutures,the organic matter in the sutures will crack and form natural gas during the high evolution stage.When mixed with natural gas of other origins,it will affect the identification of natural gas sources;When the suture line serves as a migration channel for crude oil,the fractionation effect on crude oil can also cause certain difficulties in identifying the source of crude oil.Therefore,appropriate biomarker comparison parameters should be selected to analyze the oil source relationship,and the spatial variation pattern of crude oil properties should also be considered.
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