An analysis of formation conditions of biogenic gas in Gushi depression, Weihe basin, Shaanxi Province

Researches on geological environment of Neogene Zhangjiapo Formation in Gushi depression of Weihe basin show that the conditions of high deposition rate，thick deposition，average formation temperature between 48.09℃ to 78.24℃, proper salinity and optimum Ph values of water media are favorable for the generation of biogenic gas. The analytical result of water-soluble gas samples from geothermal wells show that the content of CH4 is extremely high, heavy hydrocarbon is rare, and δ13C1 values are less than -60‰. It is thus concluded that the water-soluble gas in this area is biogenic gas. The analysis of Zhangjiapo Formation source rock shows that the average values of organic carbon and chloroform bitumen "A" are 0.46% and 0.041%, respectively. The type of organic matter is Ⅱ1 and the mean value of Ro is 0.56%, belonging to biochemical to thermal catalytic transitional stage. The biogenic gas formation condition of Gushi depression is identical to that of Sanhu area which is the famous biogenic gas field. Calculation shows that there exists certain exploration prospect of biological gas in this area.