Hydrocarbon Generation And Shale Gas Enrichment Characteristics Of Fine-grained Sediments Of Shanxi Formation In Yan'an Exploration Area

Shale gas is a type of unconventional gas.It is the main replacement for natural gas in China in the future,with the characteristic of "continuous" accumulation.There are abundant shale gas resources in Shanxi Formation in Yan 'an area of Ordos Basin in China,but the accumulation pattern of shale gas in this area has not been established yet.In this paper,a core column with a diameter of 2.5cm was drilled from a large block of mudstone sample of Shanxi Formation in Yan'an area.The hydrocarbon generation simulation experiment was carried out on the core column by WYMN-3 temperature pressure hydrocarbon generation simulator to study its hydrocarbon generation and expulsion potential,then scanning electron microscopy and nitrogen adsorption method were used to analyze the pores of Shanxi Formation reservoirs in Yan'an.We combine the findings with actual geological conditions to explore the main factors affecting shale gas enrichment in the study area,to establish accumulation pattern of shale gas reservoir.In this paper,the Shanxi Formation is divided into small layers and sedimentary microfacies according to logging curves and stratigraphic standards.It is believed that the Shanxi Formation in the study area as a whole is in the delta sedimentary facies,and the mudstone thicknesses of the Shan 1 and Shan 2 members are generally between 20 and 40 m.On this basis,trace element analysis and test data are used to analyze and restore the Shanxi Formation.The paleo-sedimentary environment and paleo-productivity of small layers show that the whole Shanxi Formation was underwater during the deposition period,basically in a warm and humid reductive environment.The paleo-productivity of the deposits and the organic carbon content were high,and the maturity of organic matter reached High-maturity to over-mature stage,a large number of type III kerogens that generates shale gas were developed.The results of the hydrocarbon generation simulation experiment show that the different lithological combinations of the Shanxi Formation have an impact on the hydrocarbon generation and expulsion at different mature stages.At present,the Shanxi Formation has three lithologies: mud-sand-mud,coal-mud-sand and coal-mud.In combination,when the organic matter maturity is too high,the hydrocarbons generated in the mudstone are retained in the mudstone formation,which can form a higher overpressure and can be discharged in a large amount.The results of scanning electron microscopy and nitrogen adsorption experiments show that the area is mainly developed with mesoporous intergranular pores and intragranular pores,which are the main channels for oil and gas migration and adsorption.Based on the study of shale burial geological conditions,and relating to the hydrocarbon generation intensity and pore characteristics of the study area,two shale gas accumulation pattern of the Shanxi Formation in the Yan'an area have been established.They are the thick mudstone intercalated with thin sand layers pattern and the gas-rich mode of thick mud shale and silt sandstone pattern.