Bedrock gas reservoirs in Dongping area of Qaidam Basin, NW China

The geological characteristics of bedrock gas reservoirs and the reason for the enrichment and high production of gas in the Dongping area of the Qaidam Basin are studied based on logging data, image log data, core observation, thin section analysis, reservoir microscopic study and cap rock condition evaluation. The main lithology of the bedrock reservoirs in the Dongping area is granite and granite gneiss. The reservoir space mainly consists of fractures, dissolution pores and micro-pores, among which massive matrix micro-pores and dissolution pores are the key factors for the high and stable gas production in the study area. Due to the Tertiary salty environment, the fractures and pores 0 to 18 meters from the bedrock top are filled with gypsum and calcite, forming good “top-sealing” cap rock, this special reservoir-cap rock combination in wide distribution results in the high production of these gas reservoirs. There are two types of gas reservoirs: one is fracture-pore reservoirs at the top of the bedrock, mainly distributed 20−50 m below the “top sealing” cap rock, strongly controlled by tectonic background, and high and stable in gas production; the other is fractured reservoirs inside the bedrock, large in gas-bearing depth, great gas-bearing differences, abrupt change in lateral direction, and high but not stable in production.