Abstract:

The distribution of gas hydrate is very complex in Qilian area, therefore it is urgent to study the controlling factors and the formation pattern of gas hydrate in this area. The geological data and analytical results of various samples collected from the series of gas hydrate drilling holes are emphatically studied in Sanlutian. Results show that gas source for gas hydrate in the study area is mainly composed of oil-typed thermo-genic gases and this kind of gas source is partially mixed with some biogenic gas or coal gas in the shallow. And the oil-typed thermo-genic gases are mainly derived from lower or deeper Upper Triassic or Permian. When gas source migrated upward and arrived at the shallow formation, gases were directly or indirectly blocked by fracture and other seals such as mudstone, oil shale, thus causing shallow gas accumulation. Shallow gas was locally added by microbial gas or coal gas. They together with water were formed into gas hydrate in gas hydrate stability zone after these gases were encountered into frozen formation which happened no later than early Middle Pleistocene. Parts of them still exist as abnormally high-pressured gas reservoir or free (adsorbed) gases in shallower strata when they are outside of the gas hydrate stable zone. Because of variation of gas source types and supply conditions, migration and accumulation conditions, and range of gas hydrate stability zone, these factors’matching relation is various at different location, which affects occurrence and distribution of gas hydrate in a horizontal plane and vertical profile in this area.

Key words: permafrost, gas hydrate, accumulation pattern, Sanlutian of Muli in Qinghai