Study On The Development Patterns Of Effective Reservoirs Of Glutenite In The Upper Part Of The Fourth Member Of The Shahejie Formation In The Lijin Area

The lithofacies of the nearshore submarine fan glutenite reservoirs in the upper part of the fourth member of the Paleogene Shahejie Formation(Es4s)in the Lijin area are complex.The reservoirs in the Es4 s have experienced complicated diagenesis during the burial process,which leads to complex formation and distribution of effective reservoirs.Complexity of the reservoir distribution increases the risk of hydrocarbon exploration.Core observation,casting thin section identification,SEM observation and EDS analysis,electron probe composition analysis,and fluid inclusion petrography and homogenization temperature measurement were used in combination investigate reservoir characteristics,genetic mechanism of high-quality reservoirs and distribution for effective reservoirs.The following achievements have been obtained.Eight types of lithofacies,such as conglomerate facies,conglomerate sandstone facies,gravel sandstone facies,coarse sandstone facies,medium sandstone facies,fine sandstone facies,siltstone facies and mudstone facies,have been identified in the nearshore submarine fan deposits in the Es4 s in the Lijin area.Diagenesis is mainly compaction,cementation,replacement and dissolution.The cementation is mainly carbonate cementation such as calcite and iron calcite,the two-stage quartz overgrowth and kaolinite cementation.Dissolution is generally weak.Feldspar dissolution is relatively common and dissolution of a small amount of limestone particles is also observed.There are 5 diagenetic facies like strong compaction diagenetic facies,strong cementation diagenetic facies,strong compaction-medium cementation diagenetic facies,medium compaction-medium cementation diagenetic facies and medium compaction diagenetic facies.The reservoir space is mainly composed of primary pores and secondary pores and fractures.The primary pores are the most important and secondary pores are dissolution pores of feldspar grains.The nearshore submarine fan glutenite reservoirs have experienced complex diagenesisaccumulation evolution sequence in the Es4 s in the Lijin area.From the beginning of the deposition to 43 Ma,the diagenesis was mainly compaction and early strong calcite cementation occurred in some areas.From the 43-34 Ma,feldspar dissolution and the first period of quartz overgrowth and kaolinite development occurred.From the 34-5 Ma,late carbonate cementation occurred.From the 34-29 Ma,the first oil and gas were charged.A small amount of quartz overgrowth was developed during 5-4 Ma.The second stage of oil and gas emplacement occurred during 5-0 Ma.The effective reservoirs are controlled by factors such as lithofacies,diagenetic facies,early oil and gas charging and fluid overpressure in the Es4 s in the Lijin area.Coarse sandstones,medium sandstones and fine sandstones are the most favorable lithofacies.Compaction and carbonate cementation determine the development and distribution of effective reservoirs.The amount of feldspar dissolution to increase porosity is limited.The diagenetic facies of medium compaction and medium compaction-medium cementation are favorable for the development of effective reservoirs.Early oil and gas charging causing overpressure effectively inhibited carbonate cementation and compaction,and preserved a large number of primary pores.There are 4 types of reservoirs developed in the nearshore subaqueous fan glutenite in the Es4 s in the Lijin area.Type I reservoirs and type II reservoirs are effective reservoirs.Type Ⅰreservoirs with the best reservoir properties and abundant primary pores coarse sandstones,medium sandstones,and fine sandstones,the diagenetic facies of which is moderate compaction diagenetic facies.Type Ⅱ reservoirs with some primary and secondary pores are coarse sandstones,gravel-bearing sandstones and medium sandstones,the diagenetic facies of which are medium compaction or medium compaction-medium cementation.Type III reservoirs with almost no pores are gravel sandstones and fine sandstones and experienced strong compaction.Type IV reservoirs with almost no pores are medium sandstones,matrix-supported conglomerates,grain-supported conglomerates,the diagenetic facies of which are strong cementation,strong compaction,and strong compaction-moderate cementation.