Genetic mechanism and logging evaluation of low-resistivity gas reservoirs in the Tainan Gas Field, eastern Qaidam Basin

The gas layers in the Quaternary Lower Pleistocene Sebei Fm in the Tainan Gas Field, eastern Qaidam Basin, are commonly low in resistivity and are difficult to differentiate from medium- and high-resistivity gas layers with high salinity formation water and complex geologic conditions. In combination with the logging responses of low-resistivity gas layers, we systematically studied the genetic mechanisms of the low-resistivity gas layers by integrating various data such as core, logging, structural and gas testing data. Geologically, the major genesis of the low-resistivity gas layers in the study area are the fine grain size and high clay content of reservoir rocks deposited in shallow lakes with relatively low energy and the high irreducible water saturation formed in a low-amplitude structural setting. In addition, the high salinity of formation water, complex pore structure, strong heterogeneity and highly-developed microfracture further lower the resistivity of gas layers. From the engineering point of view, drilling mud invasion and soaking time have significant influences on resistivity logging. The variable-pore structure index (m) and saturation index (n) proposed based on the analysis can effectively improve the calculation accuracy of water saturation. The cross plots of invasion index" vs. "mirror image" and resistivity vs. porosity vs. gas saturation can be effectively used to identify gas and water layers."