Generation And Accumulation Of Deep-seated Oil And Gas In The Western Central Depression Of Junggar Basin

Based on well logging and seismic data,combined with geochemical analysis data such as hydrocarbon generation simulation,biomarker compounds,isotopes,and inclusion analysis and hydrocarbon migration and accumulation physical simulation,generation and accumulation of deep-seated oil and gas in the Western Central Depression of the Junggar Basin were studied.And the hydrocarbon generation potential of source rocks were evaluated,the phase state of deep-seated oil and gas generation potential was discussed,main controlling factors of deep-seated oil and gas accumulation were defined,the deep-seated oil and gas accumulation models were established,which provide theoretical guidance for deep hydrocarbon exploration.Four sets of source rocks are mainly developed in the Western Central Depression of Junggar Basin,including Carboniferous and Permian Jiamuhe Formation,Fengcheng Formation and Lower Wuerhe Formation.Among them,Carboniferous source rocks were drilled in some wells of the northwest margin fault zone,while Permian Jiamuhe,Fengcheng and Lower Wuerhe source rocks were drilled in Mahu Sag and the periphery of Zhongguai Uplift.Due to the large buried depth and low drilling degree in the Pen-1 Well Western Sag and Shawan Sag,the Carboniferous and Middle-Lower Permian source rocks have not been drilled.According to the comprehensive evaluation of drilled source rocks,Fengcheng Formation mainly develope medium to good quality source rocks and the types of organic matter are mainly II₁ and I,the source rocks of lower Wuerhe Formation is mainly medium to good quality and the types of organic matter are mainly II₂ and III.It is inferred that the above two sets of source rocks are distributed in Mahu Sag,west of Pen-1 Well Western Sag and Shawan Sag,the oil and gas in the study area mainly comes from these two sets of source rocks.The organic matter abundance of Jiamuhe Formation source rocks is mainly medium with a small portion of good source rocks.The main types of organic matter are type II₂ and III,which are mainly distributed in the western of Mahu Sag and Shawan Sag.Natural gas from Jiamuhe Formation has been found around Zhongguai uplift.The abundance of Carboniferous source rocks is low,categorized as poor to medium,and the organic matter types are mainly II₂ and III.Through the correlation of oil and gas sources,it is found that the characteristics of biomarkers of Carboniferous oil and gas in Shixi Uplift,are obviously different from those from Permian source rocks in adjacent areas.Based on the latest study of basin uplift and sag structure and the analysis of Carboniferous sedimentary structure evolution,the Carboniferous is a residual marine sedimentary environment,and a series of Carboniferous fault depressions are developed in the sag.It is speculated that there may be a certain scale of source rocks developed in the Carboniferous Formation in the west depression of Pen-1 Well Western Sag and its periphery,which has a high oil and gas generating capacity and has a certain contribution to the deep Carboniferous and Permian oil and gas reservoir formation.The early exploration mainly focused on the bulge and shallow layer around the sag,the discovery was mainly oil and the evolution degree of Permian source rocks drilled in Mahu Sag was low.Therefore,it was generally believed that the Permian source rocks in Junggar Basin was mainly oil generating.In recent years,the exploration has gradually expanded to the sag area and its surrounding areas,many gas reservoirs have been found in the west of Pen-1 Well Western Sag,Shawan Sag area and its surrounding areas.Through the genetic analysis and gas source correlation of natural gas,it is found that the natural gas is mainly oil type gas,and also coal type gas,which is mainly caused by kerogen degradation.The gas source is mainly from Fengcheng Formation and lower Wuerhe Formation of Permian.Through the analysis of natural gas origin and gas source comparison,the natural gas is mainly oil type gas and coal type gas,which is mainly caused by kerogen degradation.The gas source is mainly from Fengcheng Formation and lower Wuerhe Formation of Permian.In order to further clarify the hydrocarbon generation evolution and deep-seated oil-gas phase behavior of Fengcheng Formation and Lower Wuerhe Formation,the simulation experiment of hydrocarbon generation was carried out on source rock samples.According to the relationship between experimental temperature and R_o,the experimental temperature of Fengcheng Formation source rock is 400～425℃,corresponding to R_o of 1.45%～1.72%.Overpressure is developed under the depth of 4000m,and R_o is obviously inhibited.Under the control of weak overpressure（pressure coefficient lesser than 1.3）,over the depth of 6100～6400m can reach the standard of large to medium sized gas field（20×10⁸m³/km²）.Under the highest overpressure（pressure coefficient greater than 2.0）,over the depth of 6850～7000m can reach the standard of large to medium sized gas field,which can enter the new understanding of large-scale gas generation stage.Based on the analysis of burial depth and evolution of two sets of main source rocks in Fengcheng Formation and Lower Wuerhe Formation of three sags,it is considered that the main burial depth of Mahu Sag is less than 6500m,and the main body is in the stage of oil generation,while the main burial depth of Permian source rocks in the Pen-1 Well Western Sag and Shawan Sag is more than 7000m,reaching the stage of large-scale gas generation.It is clear that the low geothermal gradient and abnormal high pressure in the central depression inhibit the hydrocarbon generation evolution of the source rocks,which are the main reason of the gas generation depth of the source rocks is greater than that of the traditional understanding.Through the comparison with the source rocks in Bozhong Sag,it is considered that the oil type source rocks also have the potential to generate large to medium sized gas fields.It is pointed out that the deep layers of Shawan Sag and Pen-1 Well Western Sag have the conditions to form large-scale natural gas reservoirs.Based on the analysis of discovered oil and gas reservoirs and failure wells in the westerndeep layer,it is considered that there are three stages of oil and gas charging and one stage of oil and gas adjustment in the Carboniferous and Permian layers in the Western Central Depression.The oil and gas charging occurred in the middle to late Triassic,Cretaceous and Neogene,the oil and gas upward adjustment occurred in the middle to late Jurassic during the Yanshan movement.Among them,there are two-stage charging and one-stage adjustment in Shixi Uplift,one-stage charging and one-stage adjustment in Moshen 1 Well of Mosuowan Uplift,two-stage charging and one-stage adjustment in Dabasung Xiayan Uplift,two-stage charging and one-stage adjustment in Mahu Sag and three-stage charging and one-stage adjustment in Zhongguai Uplift.The mechanism of deep-seated oil and gas reservoir formation is revealed.It is believed that the formation of deep-seated oil and gas reservoir is mainly controlled by late filling and later preservation.Large span fault connection between source and reservoir is conducive to continuous hydrocarbon charging and accumulation,oil and gas are enriched in the near source nose convex structural belt.In addition,the hydrocarbon accumulation model of“new generation and paleo reservoir,late filling,lateral hydrocarbon supply and paleo convex accumulation”is established,which is useful to provide theoretical guidance for deep-seated oil and gas exploration.