Characteristics and significance of the carbonate cements in the lower member of the Palaeogene Xiaganchaigou Formation in the Pingtai region, northern Qaidam Basin

In the light of petrological, mineralogical and geochemical data, the present paper deals, in detail, with diagenetic temperature, genetic environment and origin of the carbonate cements in the lower member of the Palaeogene Xiaganchaigou Formation in the Pingtai region, northern Qaidam Basin. The reservoir sandstones comprise the carbonate cements calcite and dolomite, and authigenic clay minerals illite, chlorite and illitemontmorillonite. The carbonate cements have low carbon isotope (δ¹³C) values ranging from -7.5‰ to -1.4‰ with an average of -4.0‰, oxygen isotope (δ¹⁸O) values from -14.4‰ to -10.1‰ with an average of -11.6‰, and palaeosalinity (Z) values from 106.38 wt% to 119.23 wt% with an average of 113.24 wt%. All these values indicate the involvement of minor amount of organic carbon during the early diagenetic stage of the reservoir sandstones from the lower member of the Palaeogene Xiaganchaigou Formation in the Pingtai region. The carbonate cements have the formation temperatures from 71.49℃ to 101.01℃, with an average of 81.32℃. The abovementioned results show that the diagenetic stage of the reservoir sandstones from the lower member of the Palaeogene Xiaganchaigou Formation in the Pingtai region belongs to the late stage of the early diagenesis.