The special sealing mechanism of caprock for Quaternary biogenetic gas in Sanhu area, Qaidam Basin, China

The Quaternary biogenetic gas reservoirs in the east of Qaidam Basin have many characteristics such as late forming time, shallow burial depth, low diagenetic grade, high porosity and high permeability and so on. It cannot be considered as caprock according to the traditional evaluation criterion. However, the large scale and high efficient biogenetic gas reservoirs of the Gaidam Basin are really formed under these kinds of caprocks, so it does have some specialty in its sealing mechanism. Aiming at the special sealing mechanism, some simulating experiments have been done. The research results show that the sealing ability of biogenetic gas caprock is related with water saturation, the caprock that is saturated with salt water can effectively block seepage and diffusion. Furthermore, the multiple reservoir-caprock groups have accumulated sealing effect, causing the formation of big gas fields. The evaluation method with traditional caprock parameters cannot be adopted in evaluating the study area.