Reservoir Characteristics And Genesis Of Low Resistivity Reservoirs In The Second Member Of Sangonghe Formation In Mahu Sag,the Junggar Basin

The discovery of the Jurassic Sangonghe Formation oil and gas reservoirs in Mahu sag shows that the shallow strata of Mahu sag in the Junggar Basin also have great exploration potential.The reservoir pore structure of the Sangonghe Formation in Mahu sag is complex,and low resistance oil layers are widely developed.However,the basic characteristics of the low resistance oil layers in this area are unclear,the genetic mechanism is complex,and accurate identification standards are lacking,which brings difficulties to the subsequent exploration and development of the oilfield.This paper takes J₁s₂⁴ of the Jurassic Sangonghe Formation in the southern slope of Mahu sag as the research object.Through the analysis of logging,logging,and dynamic and static data of development and production,using testing and analysis methods such as core,thin section,and mercury intrusion,the reservoir characteristics are comprehensively analyzed from the aspects of petrological characteristics,physical properties,pore structure,four property relationships,and heterogeneity.The research shows that the J₁s₂⁴ oil layer of the Sangonghe Formation generally exhibits a low resistance characteristic.This paper systematically studies the genetic mechanism of this set of low resistivity reservoirs,discusses the distribution rules of low resistivity reservoirs,and establishes the identification criteria for low resistivity reservoirs in the study area.It has certain reference significance for the exploration and development of low resistivity oil and gas reservoirs in Sangonghe Formation in Mahu sag.The main understanding is as follows.（1）The south slope of Mahu sag in the Junggar Basin is a monoclinic structure with high northwest and low southeast,and the structural amplitude is low.There are many faults developed in the study area,including the Great Jurassic Gully strike slip fault,the South Ke 81 Well strike slip fault,and a series of associated secondary faults.（2）The J₁s₂⁴ sand layer group of the Sangonghe Formation in the study area is subdivided into two sub layers,J₁s₂^4-1 and J₁s₂^4-2,which are sedimentary subfacies of the discernible river delta front.Five sedimentary microfacies are developed,including composite underwater distributary channel,underwater distributary channel,sheet sand,estuarine bar,and interdistributary bay.The reservoir rock types are mainly feldspathic lithic sandstone and lithic sandstone.Due to the complex pore structure,the overall performance of the reservoir is characterized by medium porosity and low permeability.（3）The J₁s₂⁴ sand formation of the Sangonghe Formation generally develops low resistivity oil layers with good continuity.The core oil content of low resistivity reservoirs is mainly oil impregnated,with low oil saturation.The genesis of low resistivity in oil reservoirs is mainly related to the high salinity of formation water,high irreducible water saturation,complex pore structure,and reservoir hydrophilicity,while the secondary genesis is the fine rock particles and clay mineral interaction.（4）The high salinity of formation water may be related to the widespread development of fractures in the region,while the high irreducible water saturation is closely related to complex pore structures and reservoir hydrophilicity.The mineral composition of the reservoir rocks in the study area is mainly volcanic rock debris such as tuff.Due to strong compaction and dissolution,the pore structure of the reservoir is complex and micro pores are developed,resulting in high irreducible water saturation and hydrophilic characteristics of the reservoir.（5）The lower limit of porosity for low resistivity oil layers in the study area is19.0%,the lower limit of density is 2.35 g/cm³,the lower limit of resistivity is 4.4Ω·m,and the lower limit of oil saturation is 40%.The continuity of low resistivity oil layers in the study area is good,mainly developed in the small layer J₁s₂^4-1.