Diagenetic Features Of Glutenite And Their Implications For Reservior In Mahu Sag In Junggar Basin

Mahu Sag in Junggar Basin is the largest conglomerate-type oil reservoir in the world with complex rock structure and pore characteristics.Therefore,this paper selects the oil-rich slope area in the sag as the target.The petrological and pore characteristics,diagentic activities and their effects on the porosity of the glutenite in the key stratum Triassic Baikouquan Formation and Permian Wuerhe Formation glutenite in the study area have been well reserched from both macroscopic and microscopic aspects,inculding core and thin section observation using microscope,scanning electron microscopy(SEM),X-ray diffraction,cathodoluminescence,electron probe and so on.Main conclusions are as follows.(1)The Baikouquan Formation and the Wuerhe Formation are dominated fan-delta deposits and mainly developed glutenites,sandstones and mudstones.The glutenite is the most important rock facies with content above 70%.They are composed of less than 35% of quartz and feldspar grains,and more than 65% of rock fragments.The interstitial materials are mainly matrices of clay minerals,cements of carbonate,zeolite,and a little of silica,etc.(2)Both Baikouquan and Wuerhe formations develop alternate diagenetic processes of acidic andalkaline.Main evidences of acidic diagenesis are dissolution of feldspar and rock fragment grains,and cements of zeolite and carbonate crystals,etc.Evidences of alkaline diagenesis are(1)dissolution of quartz,(2)albitization,(3)appearance of clay minerals such as montmorillonite,illite,chlorite,(4)precipitation of zeolite and carbonate cement,and(5)the development of multiple secondary porosity zones,etc.The diagenetic stage of Baikouquan Formation is mainly middle diagenetic stage of A-B.The Wuerhe Formation is mainly middle diagenetic stage of A,and some deep-buried sections are in the middle diagenetic stage of B or even late diagenetic stage of A.(3)The pore types of Baikouquan and Wuerhe formations are primary pores,intragranular dissolved pores,intergranular dissolved pores,argillaceous shrinkage pores,intercrystalline pores and microcracks,etc.However,the pore types and reservoir physical properties of different areas and different oil-bearing intervals are diverse with uneven distribution.The heterogeneity of the reservoir is very strong,probably due to the development of fractures.(4)The influence of diagenesis on pores is mainly compaction,cementation and dissolution.The study area develops multiple secondary pore zones due to the influence of acidic and alkaline diagenesis,which has a good effect on the reservoir performance.The acidic and alkaline diagenesis of the study area has an important impact on reservoir porosity,which promotes the further understanding of the reservoir-forming mechanism and influencing factors of glutenite reservoirs,and provides ideas and references for the study of diagenesis of other conglomerate reservoirs.