Abstract:

The mudstone caprocks of biogenetic gas reservoir of Quaternary in Qaidam Basin is in the early stage of diagenesis, with the characteristics of poor consolidation, relatively high porosity, big pore diameter and relatively lower breakthrough pressure, which results in the weak sealing of biogenetic gas reservoirs. There must be special sealing mechanism to form large scale gasbearing province. Through analysis of petrology and microstructure characteristics of mudstone, it is indicated in this paper that the cap rock quality is the main reason that forms weak sealing state if there are sufficient gas resource and no fault destruction. In this kind of geologic condition, it can be concluded that the physical properties between mudstone and sandstone are the fundamental sealing condition to prevent and alleviate biogenetic gas loss, the mudstone saturated with underground water with high salinity and mudstone pore filled with retained water improve the difficulty of biogenetic gas permeation, the cap rock , as source rock at the same time, forms hydrocarbon density sealing to prevent biogenetic gas from migrating vertically； the compensation of cap rock thickness inhibits the biogenetic gas loss further. All of the above factors accelerate the formation of large scale biogenetic gasbearing province.

Key words: Quaternary, biogenetic gas reservoir, mudstone cap rock, sealing, Qaidam Basin