The Characteristics And Accumulation Mechanism Of Organic Matter Of Shallow Carbonate Platform

This study shows the research approach and method to identify the characteristics of organic matter and its accumulation mechanism in the shallow carbonate platform through taking the Ordovician sub-salt carbonates in the central-eastern part of Ordos Basin as an example.A variety of geological methods were used,such as core observation,logging analysis,and experimental test,combining inorganic geochemistry and organic geochemistry.The organic matter enrichment model of the shallow carbonate platform is established.The characteristics of organic matter abundance show that the organic abundance of the Ordovician subsalt carbonates in the middle and eastern part of the Ordos Basin in the study area is lower,which is not in the category of high-quality source rocks.Based on the relationship between log gamma ray curve(GR)and organic carbon abundance(TOC),a prediction formula for organic carbon abundance was established.The research results showed that the organic carbon content of the sixth and eighth submembers were higher than that of other strata;the areas with higher organic carbon content distributed along the edge of the study area;the dolomite flat,gypsum-bearing and gypsum-salt basin had higher organic carbon contents than other microfacies.Kerogen maceral in the study area is mainly composed of amorphous masses and sapropelic group,and kerogen type belongs to type-?and type-?₁.The organic geochemical characteristics of saturated hydrocarbon indicate that the main parent materials are from low aquatic organisms such as bacteria and algae.According to the characteristics of pyrolysis analysis,bitumen reflectance identification and biomarkers for saturated hydrocarbons,the thermal evolution of organic matter of the study area is higher,and most of them have reached the stage of high-maturity to over-maturity.The factors influencing organic matter enrichment,such as terrigenous detrital input,primary productivity,paleo-climate,paleo-salinity,redox condition,water seal degree and hydrodynamic condition,were firstly analyzed to reconstruct the original sedimentary conditions.The research shows:The terrigenous detrital input is more likely to occur in areas close to the paleocontinent,while in areas far away from the paleocontinent,terrigenous detrital input was less.The productivity of the study area is generally low,but there are also areas with medium primary productivity.The paleo-climate in the study area is hot and arid,which may be not conducive for biological reproduction.The paleo-salinity of the study area is high,illustrating the marine sedimentary environment.Both oxic and anoxic environments exist in the study area.Most of the subsalt carbonates in the study area deposited in the open marine environment without limitation,and some developed in the strongly restricted marine environment,which is always under suboxic-anoxic conditions.Through partial correlation analysis,it is found that there is no obvious correlation between the influencing factors of organic matter enrichment and the results of organic matter enrichment.Based on the comprehensive analysis,the accumulation mechanism of organic matter in the shallow carbonate platform is proposed:preservation mode is the mainly accumulation mode of organic matter,which is the result of synergistic effect and interactions of the influencing factors of organic matter enrichment.