Geochemical Characteristics And Hydrocarbon Generation Potential Of Source Rocks In Lunpola Basin,Tibet

Lunpola basin is located in the junction zone of Qiangtang block and Gangdisi block.From west to east,it can be divided into three depressions:Jiangriacuo,Jiangjiacuo and Pacuo,with huge resource potential.This article selects the high quality hydrocarbon source rock of Eocene series the third member of Niubao formation(E₂n^3-3),the upper part of the second member of Niubao formation(E₂n^2-1)and the middle part of the second member of Niubao formation(E₂n^2-2).By geochemical experiments like TOC,rock pyrolysis,maceral,element analysis,the BP neural network model is used to evaluate the source rocks of the three longitudinal submembers of Lunpola Basin as well as the Jiangriacuo Sag,Pacuo Sag and Jiangjiacuo Sag on the plane,and the hydrocarbon generation is calculated.The following understanding is obtained:1.The organic geochemical characteristics of the source rocks of Niubao Formation in the study area are identified,and the evaluation criteria of source rocks suitable for Lunpola Basin are defined by comparing various evaluation indexes.The source rocks were evaluated by microscopic examination of kerogen type index,rock pyrolysis and other organic geochemical experiments,among which E₂n^3-3,E₂n^2-1and E₂n^2-2source rocks belong to high quality source rocks.The organic matter types showed that sapropelite and vitrinite were the main components.The organic matter types of E₂n^3-3were mainly type?₁,mixed with some type?.The organic matter types of E₂n^2-1and E₂n^2-2mingle with type?and type?₁,while a small amount of samples belong to type?₂.The maturity of organic matter in all three layers reached the threshold of hydrocarbon generation,and the maturity increased with the increase of depth.2.Biomarker compounds were used to analyze the source of organic matter and sedimentary environment characteristics of source rocks.In general,phytane is dominant in the samples and the content of gamacerane and thiofluorene is high,which reflects that the Niubao Formation was deposited in a high-salt reduction environment,in which E₂n^3-3has the highest degree of salinization and the highest reductivity.The content of tetracyclic terpanes is low,and the distribution of dimethylphenanthrene in aromatic hydrocarbons is V-shaped.In the steranes,C₂₈in E₂n^3-3is dominant,and C₂₉in E₂n^2-1and E₂n^2-2is dominant.The comprehensive analysis shows that the source of organic matter is algae and other lower aquatic organisms,mixed with some terrestrial plants.3.The neural network model was established to predict the organic carbon content,and the hydrocarbon generation of three sets of source rocks in the study area was calculated.SP,GR,AC,DEN and depth are selected as input layers.The results show that the training samples fit well with the test samples and overcome the situation of removing abnormal points artificially in the?log R method.The forecast results of organic carbon content show that the TOC content from the edge of the basin to the center increases between 0 and 1.5%,which is consistent with the measured data.Using the organic carbon method to calculate the basin hydrocarbon generation,the total oil generation of the three sets of source rocks is 1.179×10⁹t,and the total gas generation is 158.266×10⁹m³.The oil generation is high in Jiangriacuo sag and Jiangjiacuo sag,and the gas generation is also high in Jiangriacuo sag,which has the potential for further exploration.Longitudinally,the amount of oil generation and gas generation in E₂n^2-2has high oil and gas generation capacity,so it has greater hydrocarbon generation potential.