Source Rocks And Hydrocarbon Geochemical Characteristics Of Songnan Low Uplift In Southeast Hainan

With the sharp increase in global energy demand and the increasing difficulty of oil and gas exploration and development in land and shallow sea areas,countries pay more and more attention to deep-water oil and gas exploration.The Yongle 8 gas field discovered in Songnan low uplift area in recent years has opened up the oil and gas development in deep water area and is a favorable area for natural gas exploration in the next step.Limited by the difficulty of sampling in deep-sea exploration,the number of core cores and other samples is small,and the debris samples are seriously polluted by oil-based mud,which seriously restricts the analysis and research of oil and gas geochemistry.In this paper,the patent method is used to scientifically and effectively decontaminate the contaminated samples in the deep water area of Qiongdongnan Basin,and then a complete set of oil and gas geochemical analysis and test are carried out on the source rock and crude oil samples of Songnan low uplift area and its surrounding wells.Subsequently,the nature and hydrocarbon generation potential of the source rocks of the Oligocene Lingshui Formation and Yacheng Formation were qualitatively evaluated.By studying the biomarker characteristics of saturated hydrocarbon and aromatic hydrocarbon in source rock extracts,the formation environment and biogenic composition of source rock are analyzed.The composition,physical properties,carbon isotope and biomarker characteristics of oil,gas and water in the study area were analyzed to explore the causes of oil and gas.The organic matter abundance of source rocks in Songnan low uplift area is generally low,and most of the total organic carbon content is less than 1.0%.Lingshui Formation and Yacheng Formation are medium-good source rocks.Lingshui Formation source rock sapropel+shell group content close to 90%,vitrinite content is very low.The source rocks of Yacheng Formation are mainly humic source rocks,and their macerals are mainly vitrinite,mainly gas generation.The organic matter types are mainly II2–III.The source rocks of Yacheng Formation mature to mature stage,Lingshui Formation basically entered the low mature threshold,deep can reach mature stage.The Ts/(Ts+Tm)value of Yacheng Formation mudstone drilled in Songnan low uplift is 0.2,and the thermal evolution degree is low,which is related to the shallow burial depth.The content of‘T'type compounds of bistrustane series is low,and the Oletane/C₃₀hopane value is only 0.04.The input of terrestrial angiosperms is limited,which is consistent with the transitional environment of sea and land where Yacheng Formation is mainly deposited in the swamp facies.The distribution characteristics of trifluorene series in aromatic compounds reflect that the sedimentary environment of most source rocks is a reduction environment with high salinity.The crude oil in the study area has the characteristics of low density,low wax content and low viscosity,which is light oil.Crude oil parent material is humic organic matter,in normal to mature crude oil stage.The crude oil supplied by the Yacheng Formation mainly from terrestrial sources has the characteristics of low density and light carbon isotope.Natural gas is mainly composed of hydrocarbon gas,CO2 and N2.The hydrocarbon gas is mainly methane,the heavy hydrocarbon gas content is low,and the natural gas is mainly dry gas.Carbon isotope characteristics show that natural gas in some wells migrates from deep thermogenic gas to shallow layer.The Songnan low uplift gas field is naturally mainly derived from marine source rocks of Yacheng Formation in the southern slope area and Baodao Sag,with light methane carbon isotope.The hydrocarbon gas in the study area is organic origin and CO2 is inorganic origin.However,partial inversion of isotope in some samples may be caused by mixed source gas or microbial degradation.