Diagenesis And Favorable Reservoir Prediction Of The Upper Fourth Member Of Shahejie Formation In Bonan Sag

With the development of oil and gas exploration,deep oil and gas exploration is becoming more and more important.The reservoir of the upper fourth member of Shahejie Formation in Bonan Sag belongs to typical deep clastic reservoir with abundant petroleum resources and great exploration potential,which has become the major target area of deep research in the Jiyang Depression.However,because of deep burial,the reservoir develops complicated diagenesis which has great influence on reservoir physical property.Furthermore,the distribution law of favorable reservoir is not clear yet.Focusing on the problems,by means of drill core physical property analysis,cast thin slice,cathodoluminescence,scanning electron microscopy and X-ray diffraction of clay minerals,the paper takes reservoir of the upper fourth member of Shahejie Formation in Bonan Sag as the main research object and studies the reservoir characteristics,diagenesis,types and distribution characteristics of diagenetic facies and favorable reservoir prediction by combining logging information and well testing data.Following is the main results:(1)The lithology of reservoir in the upper fourth member of Shahejie Formation of Bonan Sag is mainly medium and fine sandstone.The type of rock is mainly lithic feldspar sandstone.The reservoir is with low compositional maturity and textural maturity as a whole and belongs to very low porosity and extra low permeability reservoir.With the increase of buried depth,the reservoir physical property is getting worse and worse,but there are three abnormally high porosity zones vertically in the study area.The pore types are mainly primary pores,followed by secondary dissolution pores.(2)Compaction,carbonate cementation and feldspar dissolution are the main diagenetic types in the study area.The reservoir of the upper fourth member of Shahejie Formation in Bonan Sag mainly belongs to stage A and B of middle diagenesis phase.With multi-period of acid-alkaline fluid evolution,the reservoir went through a complex evolution of multiple cementation,dissolution and metasomatism.(3)Based on types and intensity of main diagenesis,six types of diagenetic facies are identified as follows,strong feldspar dissolution facies,medium chlorite cementation facies,medium compaction-weak cementation facies,strong calcite cementation facies and matrix recrystallization facies.The spatial distribution characteristics of diagenetic facies are determined based on logging recognition of diagenetic facies.(4)The reservoir physical property can be influenced by many factors.Sedimentation is the most fundamental factor affecting the reservoir physical property.Diagenesis has reconstructive effect on reservoir quality in burial process.The abnormally high pressure and hydrocarbon filling can preserve primary pores of reservoir by inhibiting compaction and cementation.Based on reservoir characteristics and influential factors on reservoir physical property,three types of reservoir are recognized.Finally,it is determined that the favorable reservoir in the upper fourth member of Shahejie Formation of Bonan Sag is mainly distributed in subaqueous distributary channel of fan delta front where strong feldspar dissolution facies and medium compaction-weak cementation facies develop by combining plane distribution characteristics of sedimentary facies,diagenetic facies and pressure coefficient.The paper determines diagenetic characteristics,reveals spatial distribution characteristics of diagenetic facies,analyzes influencing factors of reservoir physical property and finally predictes the favorable reservoir,thus providing guidance for deep oil-gas exploration and exploitation of Bonan Sag.