Source Rocks, Reservoir Conditions And Hydrocarbon Migrations In The Northern Qianjiang Depression, Jianghan Basin

As an important hydrocarbon accumulation area in the Jianghan Basin,the northern Qianjiang Depression is rich in oil and gas resources.In this thesis,conventional source rock evaluation was done to clarify the characteristics of different source rocks and the distribution of effective source rocks was determined.Through the analysis of the sandstone characteristics and reservoir diagenesis,the diagenetic sequence and pore evolution model were reconstructed and the distribution of high-quality reservoirs is predicted.Based on the oil-source rock correlation,analysis of nitrogen compounds and paleo-oil potential simulation,the parent source rocks of hydrocarbons and hydrocarbon migration directions were identified.We therefore could provide the theoretical basis for hydrocarbon exploration in this area by conducting this systematic research on hydrocarbon generation,migration and accumulation.The main understanding and achievements of the thesis are as follows:1.The source rocks of Qianjiang Formation have high organic matter abundance and the kerogen types are mainly types?and?₁.The organic matter is from immature to medium mature,and there are some differences in different formations and tectonic units.Vertically,the depth of the first member to the upper third member of Qianjiang source rocks is shallow and their organic matter maturity is relatively low,while the lower third member to the fourth member of Qianjiang source rocks are buried deeper and their maturity is relatively high.Laterally,the source rock in the Banghu subdepression reaches the hydrocarbon generation threshold first and the hydrocarbon generation intensity is the largest;the Zhouji subdepression and the Wangchang subdepression are relatively late in hydrocarbon generation,and the hydrocarbon generation intensity is weaker.The distribution of source rocks in this area is obviously controlled by the faults and sediments.The thickness of the source rock is distributed in a ring-shaped zone,centered on the Banghu-Zhouji syncline zone,and gradually gets thinned as going outer.The effective source rock mainly developed at the relatively desalting stage,including the first member,the third member and the upper fourth member,and was mainly distributed in the light-salty saline transitional facies of the deep Banghu subdepression.In general,the dark mudstones in the lower third member and the upper fourth member of the Qianjiang Formation in Banghu subdepression are the major source rocks in this area.In addition,the simulation of thermal maturation shows that the forming time of the first member in Qianjiang Formation to the Jinghezhen Formation is the main hydrocarbon generation period.2.The sandstone reservoirs of Qianjiang Formation are mainly in the middle diagenetic B stage currently,and the diagenetic sequence is:Mechanical compaction?early carbonate cementation?dissolution of feldspar,lithic fragment and early carbonate cement,and siliceous cementation??oil filling??late Fe-carbonate cementation and quartz dissolution?anhydrite cementation and replacement.Among them,the early carbonate cementation mainly consists of micrite calcite and sparry carbonate cement,which inhibites the compaction and secondary growth of quartz,and provides the material basis for later dissolution.The late carbonate cement is mainly Fe-carbonate cementation which attaches to the edge of carbonate fragments or sediments as plaques and is an obvious porosity reduction process.Patchy-intergrown anhydrite occurs later and its material sources include paleo-sedimentary water,plagioclase dissolution,albitization and carbonate cement dissolution.3.Two high porosity zone developed at 1600m and 3200m respectively in sandstone reservoirs of Qianjiang Formation.The first high porosity zone is mainly primary pores,but accompanied by small fraction of secondary alteration which is related to the dissolution of early carbonate cements,feldspar and carbonate fragments due to organic acids derived from organic matter evolution.In contrast,the secondary porosity zone is mainly secondary pores which is related to hydrocarbon expulsion.Depending on the characteristic of diagenesis,the Qianjiang sandstones are divided into6 diagenetic facies,including weak-moderate compaction-strong dissolution-weak carbonate cementation facies,weak-moderate compaction-moderate dissolution-strong carbonate cementation facies,weak-moderate compaction-strong dissolution-moderate carbonate and sulfate cementation facies,moderate compaction-weak dissolution-moderate carbonate cementation facies,weak-moderate compaction-weak dissolution-strong?mud-crystal?carbonate cementation facies,moderate compaction-weak dissolution-strong carbonate and sulfate cementation facies.In general,compaction contributes to 6%²7%porosity reduction and cementation leads to 4%²3%porosity reduction while dissolution causes 1%¹3%porosity increase.Based on the diagenetic facies,reservoir property and sandstone layer thickness,the Qianjiang sandstone reservoir can be classified into 3 types,which is mainly distributed in the northwest of study area.The target reservoir,sand group Eq3₁⁴,mainly distributes in the eastern slope,with a part in northern slope;while sand group Eq4₂³,is situated in the northern slope,Banghu syncline,Zhouji syncline and earthern slope.4.Depending on the characteristic of typical biomarkers,the crude oil in study area can be divided into 4 types.Type?is mainly in the first member of Qianjiang Formation in Wangchang area while type?is dominantly in the second member.Type?mainly distributes in the third member of Qianjiang Formation,partially distributed in first member in Wangchang and Gaochang area,and in second member of Qianjiang Formation to Jinghezhen Formation in Zhongshi area.Type?is mainly in the fourth member of Qianjiang Formation.Oil-source rock correlation shows that Type?is from the fourth member of Qianjiang source rocks,type?and type?derive from the second and the third member of Qianjiang source rocks respectively,Type?originate from both the third and the fourth member of Qianjiang source rocks.Additionally,analysis of nitrogen compounds and paleo-oil potential simulation indicates that oils in Wangchang,Guanghua,Zhongshi,Tankou,Haokou and Xijiakou areas are mainly from Banghu subdepression,oils in Zhouji and Wangchang areas migrate from Zhouji subdepression,and oils in Huangchang and Zhanggang areas derive from Wangchang subdepression.The hydrocarbon migration in study area is characterized by lateral short distance migration.However,the large scale faults?e.g.,Qianbei fault and Chedang fault?could provide vertical migration path.5.The traps and hydrocarbon sources are well configured over time in the northern Qianjiang Depression.But the favorable reservoir is mainly distributed in the northwest,far away from the hydrocarbon sources,and the hydrocarbon migration is characterized by short lateral migration,the effective traps therefore have a limited distribution.Depending on the distribution of effective source rocks and favorable reservoirs and analysis of hydrocarbon migration,the favorable hydrocarbon accumulation areas are concentrated near three hydrocarbon generating subdepression.And because of the effects of reservoir distribution and migration path,they have a limited distribution.Additionally,the distribution area of favorable accumulation in the third member of Qianjiang Formation is relatively small while that of the four member is relatively large.