The Wettability Of Fengcheng Formation Shale In Mahu Sag And Its Controlling Effect On Oil-bearing Properties

Shale wettability significantly affects relative permeability,capillary force,and finally impacts the hydrocarbon accumulation process in shale as well as the ultimate shale oil and gas recovery.The evaluation of shale oil-bearing properties is one of the core issues of shale oil exploration and development,and the wettability of shale reservoir has an important effect on oil content.Therefore,it is of great significance to clarify the characteristics of shale wettability and its control on oil content for the efficient development of shale oil in Mahu Sag.This study takes Permian Fengcheng Formation shale in Mahu sag of Junggar Basin as the research object,including Rock-Eval pyrolysis,Field emission scanning electron microscopy,N₂ adsorption,high-pressure mercury injection（MIP）,contact angle,spontaneous imbibition,micro-CT and other technical means,The characteristics of shale reservoir,wettability and oil-bearing Properties are characterized.We analyzed main factors of shale reservoir wettability,and clarified the effect of wettability on shale Oil-bearing PropertiesThe results show that:（1）The Fengcheng Formation shale in Mahu Sag is generally more oil-wet,and the affinity of different lithofacies to water is as follows:felsic shale>limestone-bearing felsic shale>dolomite-bearing felsic shale>felsic-bearing dolomitic shale>siltstone.（2）The shale wettability is mainly controlled by organic matter abundance,mineral composition,pore structure:The hydrophobicity of shale is positively correlated with TOC,dolomite content,negatively correlated with quartz content,and in a pieceway correlation with calcite content.The larger the pore volume occupied by macropores,the more oil-wet is the shale sample;The connectivity of oil-wet pores becomes better along the direction of cracks and lamina;The dolomite mass can form a clustered pore system with certain connectivity,which can form the unique shale oil storage space and migration pathway together with the connected small pores in the shale matrix;（3）The pores less than 1μm have better connectivity and provide the main migration pathway for shale oil,while the pores larger than 1μm with poor connectivity provide the main storage space for shale oil;（4）Shale wettability controls oil-bearing properties in stages.When the contact angle is less than 80°,the influence of wettability on oil-bearing properties is not obvious,but when the contact angle is greater than 80°,the wettability has a strong control on oil-bearing properties.The more oil-wet of shale,the better the oil-bearing.The outcomes of this study have important theoretical significance and application value for clarifying the occurrence mechanism of shale oil and predicting the high-quality shale reservoirs of Fengcheng Formation in Mahu Sag.