Research On Characteristics And Controlling Factors Of Chang10-Chang9 Reservoir Of The Triassic Yanchang Formation In Shanbei Area,Ordos Basin,China

There is a huge exploration potential of low-permeability reservoir of the Upper Triassic Yanchang Formation in the Shanbei area,Ordos Basin,but the research on the genesis of low-permeability of reservoirs and the control factors of relatively high-permeability areas is weak.In this research,based on the geological background data of the area,through core observation,casting thin section identification,X-ray diffraction,physical properties,mercury intrusion analysis,combined with well logging data,characteristics,genesis and control factors of low-permeability reservoirs in the study area are analyzed to provide theoretical support for reservoir exploration and development with similar geological conditions.The Chang 10-Chang 9 reservoir in the Shanbei area,Ordos Basin is in the early stage of lake evolution,mainly developing delta-lacustrine sedimentary environment.The Chang 10 reservoir is dominated by the delta plain environment,and the reservoir sand body is dominated by the distributary channel sand body and the distributary channel flank sand body.The Chang 9 reservoir is dominated by the delta front and the shallow lake subfacies.The sand bodies are mainly subaqueous distributary channel sand bodies.The type of sandstones of Chang 10-Chang 9 reservoir are dominated by fine-medium sandstone arkose and lithic arkose.The full maturity is medioum-poor and the structure maturity is medium.The types of reservoir space are mainly composed of primary intergranular pores and intergranular residual pores,followed by feldspar dissolved pores and laumontite dissolved pores.The average porosity of the Chang 10 reservoir is 10.69%,the average permeability is 2.88 mD,the average porosity of the Chang 9 reservoir is 9.34%,and the average permeability is 1.17 mD,which is a low-permeability reservoir,and there is a high-permeability zone partly.It is also a “sweet point” of oil distribution.There are mainly three types of diagenesis,which are compaction,cementation and dissolution.The reservoir sandstones are in the stage A of medium diagenesis and undergo a complex diagenetic evolution process.The main controlling factors of the Chang10-Chang 9 relatively high-permeability reservoir in the study area are:(1)the sedimentary environment controls the macroscopic distribution of sand bodies and the primary quality of the reservoirs.Distributary channel sand bodies and subaqueous distributary channel sand bodies are the most favorable reservoir,which have good connectivity in both vertical and horizontal directions,and characterised by medium-grained feldspar sandstone with high compositional maturity and good quality.(2)Diagenesis controls the micro-heterogeneity of the reservoir and strongly reconstruction the reservoir quality.Compaction and cementation destroy the primary pores,resulting in reservoir densification.The authigenic chlorite film retains the primary pores and the early laumontite cementation preserves pore space for dissolution.Dissolution increasing reservoir pores,especially the dissolution of feldspar and laumontite cements forms secondary pores,greatly improving reservoir quality.Therefore,multi-stage high-energy distributary channel sand bodies,with weaker compaction and more chlorite films and laumontite cements dissolution,are most favorable area for oil accumulation in the low permeability reservoirs.