Distribution of Biogenic Gas Reservoirs and Optimization of Favorable Zones in the Sanhu Area, Qaidam Basin, China

Based on the drilling results of the Taishen-1 well in the Sanhu area in the Qaidam Basin, the study proposed new findings on the conditions of source rock, reservoir, and overlaying strata, as well as accumulation characteristics for biogenic gas. Further, the controlling factors of biogenic gas accumulation as well as two favorable zones for exploration were identified based on the comparison of drilling results and geological findings in adjacent areas. The result of the study showed that: (1) the biogenic gas source rock in the Sanhu area is lacustrine dark mudstone, and the LLTOC (lower limit of total organic carbon) in Q 1+2 (the Qigequan Formation) and N (the Neogene) is 0.25% and 0.40%; (2) the ‘self-generation and self-storage’ source-reservoir combination developed across all the formations due to the absence of faults and the development of mudstone; (3) the controlling factors of gas accumulation in the studied area include the limit of burial depth, the methane yield, and climate conditions. The failure of the Taishen-1 well owes to the fact that the gas generated failed to support the demand for gas accumulation; (4) influenced by the southward movement of gas-containing groundwater from the high potential area in the north, the biogenic gas reservoirs are distributed in the northern slope and the central sag of the Sanhu depression, and the lower limit of exploration depth of the biogenic gas reservoirs is about 2119 m; (5) two favorable zones were selected for natural gas exploration: the Upper Tertiary thermogenic gas-bearing Yahu-Sebei area and the biogenic gas-bearing Tainan-Sebei area.