Origin of Gases from Gas Hydrate in the Qilian Mountain Permafrost, Qinghai: Comparison of Gas Composition and Carbon Isotopes

摘要

Thermal simulation experiments are conducted with cores (mudstone,rnoil shale and coal) at gas hydrate-bearing intervals in gas hydraterndrilling holes of DK-2 and DK-3 in the Qilian Mountain permafrost.rnGas composition and carbon isotopes of hydrocarbon gases from gasrnhydrate are compared with simulated gases from samples. The possiblernrelationship between these potential gas source rocks and gas source ofrngas hydrate is studied. The results indicate that thermally simulatedrngases are mainly composed of CO_2 with a small amount of hydrocarbonrngases on the low temperature conditions (lower than 300℃); yields ofrnhydrocarbon gases from mudstone, oil shale and coal graduallyrnincrease, displaying differences between adsorbing gases with differentrnrocks. With increasing of thermal simulation temperatures, yields ofrnhydrocarbon gases from mudstone, oil shale and coal obviouslyrnincrease and reach maximum values at 500 ℃; conversely, yields ofrnCO_2 vary little. With increasing of thermal simulation temperatures,rncarbon isotope values of hydrocarbon gas first become lighter and thenrnheavier and are characteristic of normal carbon isotopic series of δ~(13)C_1rn<δ~(13)C_2<δ~(13)C_3. The consequences suggest that hydrocarbon gases fromrnmudstone at 350 to 400 ℃ or oil shale at 380 to 400 ℃ have similarrnfeatures of gas composition and carbon isotope with gas hydrate,rnindicating that the gas source of gas hydrate has geochemically geneticrnrelation with deep mudstone or oil shale. In contrast, gas compositionrnof hydrocarbon gases from coal and gas hydrate are similar, but bothrncarbon isotope values are incomparable, suggesting that the relationshiprnbetween coal and gas source of gas hydrate is not the same.