Organic Petrology Characteristics And Hydrocarbon Generation Behavior Of Black Shale In The Doushantuo Formation

The Neoproterozoic Doushantuo Formation is a rich organic sedimentary deposit in the Late Neoproterozoic strata in China,and it is considered to be an important ancient and ultra-deep source rock in the South.This paper takes the Doushantuo Formation in the southeastern Chongqing on the southeastern margin of the Sichuan Basin as an example.Using the combination of organic petrology and organic geochemistry,and with the comparative study,the hydrocarbon generation behavior of the Late Proterozoic source rocks is discussed.The Doushantuo Formation experienced complex tectonic evolution and high thermal evolution of organic matter.It is one of the typical representatives of ancient and ultra-deep source rocks in southern China.The macerals analysis shows that the source rocks of the Doushantuo Formation have a long history and high maturity,which leads to the obvious convergence of the optical properties of the macerals.The residual genetic characteristics are very vague.At present,the maceral composition is relatively simple and can be divided into residual bitumen and inertinite.The genesis of residual solid asphalt in source is related to the thermal evolution of liquid hydrocarbon in source rock.The carbon isotope composition of kerogen shows the characteristics of the contribution of Marine environment bacteria and algae,including the input of eukaryotic biological materials.The thermal simulation experiment was carried out on the rich organic shale in the Late Neoproterozoic Salt Range Formation in Pakistan.The source rocks of the Salt Range Formation shale and the Doushantuo Formation have similarities in sedimentary age,sedimentary environment,organic matter source and lithology.In which the macerals of the algae source are very rich,and the maturity of organic matter is only in the early stage of the oil-producing window,which solves the problem that the lack of low maturity of the Late Neoproterozoic source rocks in China cannot be credible in the laboratory.Re-enactment of the thermal evolution of these ancient sedimentary organic matter.The Salt Range Formation shale was heated and pressurized in closed and open systems,and the macerals evolution and hydrocarbon generation behavior of the source rocks of the Doushantuo Formation were simulated.The simulation experiment reveals the thermal evolution characteristics of the organic matter of the Precambrian source rocks.Whether it is lamalginite or original asphalt with photoperiodicity resembling vitrinite,obvious hydrocarbon generation and discharge occurs during the thermal simulation process,and various characteristic light evolutions are exhibited.Among them,the reflectivity evolution of the original asphalt is consistent with the theoretical change law of the vitrinite reflectance according to Easy Ro.The results verify that the original asphalt reflectivity can be used as the reliability of the maturity parameters of the Precambrian source rock.It can be seen from the hydrocarbon generation product composition and its variation characteristics that the hydrocarbon generation process of ancient source rocks is similar to the well-known source rocks since the Late Paleozoic,showing obvious oil generation threshold,oil generation window and oil generation peak.The kinetics of hydrocarbon generation reaction shows that the Precambrian source rock has the characteristics of large hydrocarbon generation,wide range of activation energy distribution,and the activation energy distribution can be extended to a higher reaction temperature stage,which may mean late.The nature or evolution of the macerals of the Neoproterozoic source rocks may be heterogeneous,resulting in a certain hydrocarbon generation potential in the high-over maturity stage.The apparent activation energy distribution of the oil-generating window of Late Neoproterozoic source rocks is 329.64?517.46KJ/mol,and the corresponding Easy%Ro range is 0.6?1.18%.The apparent activation energy at the peak of oil production is 480 KJ/mol,hydrocarbon conversion rate is about 31%;In the high maturity stage,the apparent activation energy distribution range is 520-660KJ/mol,and the hydrocarbon conversion rate accounts for only about 4% of the total.The simulation experiments reveal that the hydrocarbon generation contribution of Late Neoproterozoic source rocks similar to the Doushantuo Formation is mainly in the oil-storage stage,and the kerogen hydrocarbon generation products are mainly liquid hydrocarbons;At high maturity stage,kerogen may still have certain hydrocarbon-producing contribution,although its proportion of kerogen hydrocarbons is very low,but because of the huge hydrocarbon generation(potential)of such source rocks,even It is a small proportion and it may also be a contribution that cannot be ignored.It is worth emphasizing that the hydrocarbon generation contribution of the Late Neoproterozoic source rocks is mainly in the oil-storage stage and is dominated by liquid hydrocarbons.The natural gas associated with the contribution of these source rocks should be residual in the source rocks.Neutralization and cleavage of liquid hydrocarbons that are driven into the reservoir.Liquid hydrocarbon cracking should be related to high temperature processes caused by deep burial or other factors,which means that the contribution of ultra-deep source rocks in the Neoproterozoic is mainly “oil cracking gas”.