Characteristics And Formation Mechanism Of Low Porosity And Permeability Reservoirs Of The Lower Yanchang Formation In Jiyuan Oilfield

This study focuses on the low porosity and low permeability reservoirs in Chang 10-Chang 9 Member of Jiyuan Oilfield,Ordos Basin.Based on the regional geological data,logging data,core and thin section observation,as well as analysis of X-ray diffraction(XRD)and mercury intrusion,reservoir characteristics and its formation mechanism were investigated.In further,favorable reservoir in the study area were predicted.Major conclusions are listed as follow:The periods of Chang 10 and Chang 9 is the early stage of basin evolution and saw a development of deltaic and lacustrine sediments.The succession of Chang 10 member is composed by braided river delta plain and sand bodies dominantly developed in distributary channels.Whereas in Chang 9 period,depositional environments include braided river delta plain,braided river delta front and shallow lake.Sand bodies mainly developed in distributary channels and subaqueous distributary channels.The reservoir lithology in Chang 10-Chang 9 member includes lithic feldspar sandstone,feldspar sandstone and feldspar lithic sandstone.The sandstone is featured by medium sort and angular grains which are mainly fine sand,subordinated by medium sand.The principle reserving space in sandstone is intergranular pores,as well as some pores due to secondary dissolution.The average porosity of the reservoir in Chang 10 member is 12.74%,and is 12.33% in Chang 9 member.The average permeability of the reservoirs in Chang 10-Chang 9 member is 38.73 mD and 27.01 mD respectively.Therefore,according to classification of reservoirs,the reservoirs in Chang 10-Chang 9 member are generally characterized by low porosity and low permeability with high porosity and high permeability in specific zone.The sandstone in Chang 10-Chang 9 reservoirs in the middle diagenetic A stage.These reservoirs have experienced several phases of diagenesis,including compaction,cementation,dissolution.The diagenetic evolution saw sequentially compaction ?chlorite cementation I ? early calcite cementation ? quartz secondary growth I ? laumontite cementation ? Secondary pore dissolution ?chlorite cementation II ? quartz secondary increase II ? ferrocalcite cementation in late period and growth of autogenic quartz eventually.The reservoir formation mechanism in Chang 10-Chang 9 member is mainly under control by sedimentary and diagenetic factors.The sedimentary factors include sedimentary microfacies,content of clastic grains,structure maturity,matrix content.In terms of microfacies,reservoir properties are the best in distributary channels and subaqueous distributary channels,while are worst in interdistributary bays.Reservoir properties are correlated positively with content of quartz and feldspar,and negatively with content of litharenite and matrix.The sorting,roundness and packing of grains also make a certain contribution to physical properties.Subsequently,diagenetic factors modify reservoir properties.Firstly compaction are blamed for decline in porosity and permeability.Intense carbonate cementation makes porosity and permeability decline in some degree.On the other hand,chlorite film effectively reduces impact of compaction.Dissolution can not only enlarge original pores,but also can stimulate formation of various kinds of secondary pores.Those are both favorable for improving reservoirs properties.