The Shale Reservoir Evaluation Of Shimengou Formation Of Middle Jurassic In Yuqia Sag, North Qaidam Basin, NW China

The breakthrough of marine shale gas exploration in the south of China has not only widened the space for natural gas exploration,but also accumulated rich experience for the development of shale gas in complex areas.Compared with marine shale gas,the exploration of continental shale gas in China is lagging behind.The preliminary comparison with the geological conditions in the southern marine shale gas reservoirs and the continental shale gas reservoirs in Yanchang formation of the Ordos Basin holds that the shale in Shimengou formation,the Middle Jurassic in the northern margin of the Qaidam Basin,has a certain potential for shale gas reservoir,but at the present stage,the characteristics of the shale reservoir distribution and the evaluation parameters and standards of the shale reservoir in the study area are not determined,and further research is needed.Qaidam Basin is located in the northeastern part of the Tibet Plateau.It is an important coal bearing basin in the western part of China.The Yuqia Sag is located in the central part of the northern margin of the Qaidam Basin.The lower and upper Shimengou formation of the Middle Jurassic is a set of braided river delta and lacustrine facies respectively.The existing exploration shows that there are two sets of shale with abundant organic matter in the Shimengou formation.Through the analysis of core observation,organic geochemistry test,physical property test,XRD,scanning electron microscope and nitrogen adsorption experimental analysis,combined with the previous research data,the shale reservoir of the Shimengou formation in the Middle Jurassic of the Yuqia Sag was evaluated.Through the study,the following conclusions are drawn:?1?the average thickness of shale in the lower and upper Shimengou formation is30m and 50m respectively.The mineral components are mainly clay minerals and quartz,and the reservoir space types are mainly intergranular and intragranular pores,with a average porosity of 15.063%and 10.964%and a average permeability of 0.062x 10^-3?m² and 0.016 x 10^-3?m² respectively,which have good reservoir conditions.?2?the average TOC in the lower Shimengou formation is 3.02%,and the organic matter type is dominated by II₂^III,which is in the immature to low mature stage;the average TOC in the upper is 5.49%and the organic matter type is dominated by II1,which is in the immature to low mature stage.?3?the comprehensive study confirmed the evaluation criteria of continental shale in the study area:total organic carbon content?TOC?is more than 2.0%,maturity?Ro?is more than 0.7%,shale thickness is more than 30m,brittle mineral content is more than 40%,clay mineral content is less than 45%,and montmorillonite content is low,porosity is more than 2.0%,gas content is more than 0.5 m³/t,buried depth is in 500³500m,And the degree of later transformation is low.?4?considering the evaluation parameters of the shale reservoir in the study area,the favorable shale gas exploration area is predicted,and two favorable areas in the lower part and one in the upper part of the Shimengou formation are determined.