Characteristics and Accumulation Mechanism of Biogenic Methane Reservoirs in the Quaternary Mudstone of the Qaidam Basin in China

Abstract

Three Quaternary biogenic methane fields in the Sanhu depression of the Qaidam Basin in northwest China have yielded industrial gas flows from mudstones in recent years. These discoveries confirm the exploration potential of these mudstones, and considerable reserves likely remain to be discovered in the mudstones in the three gas fields. This paper uses core sampling, thin section identification, physical property testing, and measurements of pore diameter, displacement pressure, and diffusion coefficients to show that the mudstones in this area are neither tight rock nor shale but rather can form “conventional gas reservoirs” under appropriate conditions. A geological model of thin sand–mudstone interbedding is established to clarify the biogenic methane accumulation process, further confirming the process of reservoir formation by digital simulation. This simulation clarifies that with persistent tectonic activity, free-phase methane in the Sanhu depression likely first accumulated in the sandstone reservoir due to the buoyancy effect, encountering minimum capillary resistance. When the sandstone reservoir was saturated with natural gas, the silty dolomite, silty mudstone, and mudstone likely took on natural gas from subsequent gas migration, albeit with gas saturation lower than that of the sandstone reservoir. As it is difficult to form mudstone reservoirs in slope and sag areas, the formation of mudstone reservoirs in this area requires a structural trap. Therefore, natural gas exploration in this area may benefit from exploration concepts other than those developed for shale gas and tight gas. The next step in the exploration of mudstone gas reservoirs focuses mainly on exploiting potentials in the three gas fields, while the exploration of the conventional sandstone gas reservoirs focuses mainly on continuously searching for low-amplitude structural traps and lithologic traps in slope or sag areas.