Application of microbial oil surveying to exploration of lithologic gas reservoirs: A case from the Sanhu Depression, Qaidam Basin, NW China

To effectively detect lithologic biogas reservoirs in the Sanhu Depression, an experimental investigation of Microbial Oil Surveying Technique (MOST) was conducted in the Tainan-Taidong area. Samples were from 156 sampling stations located 150 m apart in a near straight line of 24 km and extending from the west to the east of the investigated area, 200 g of soil from each sample site was taken at depth of 20 cm, and GMT Inc. of U.S.A. completed the analysis of soil methanotrophs. The result shows that moisture of soil at depth of 20 cm can maintain the growth of methanotrophs; The average of soil pH is 8.20, which does not inhibit the growth of methanotrophs; The salinity of many samples are more than 20%, the high salinity decreases methanotrophs concentrations, but the difference of methanotrophs concentrations between gas reservoirs and background areas is obvious, and the samples above gas reservoirs whose salinity are more than 50% still have microbial anomaly; Gas well testing production shows a significant positive correlation with methanotrophs concentrations of soil above the well, indicating the MOST is applicable to the Sanhu Depression. According to the microbial anomaly, two favorable exploration targets were identified in the eastern side of Well Tainan 9 and the western side near the limits of the Tainan gas field, their gas bearing abundance is better than the Well Tainan 9.