GENERATION AND ACCUMULATION OF QUATERNARY SHALLOW GAS IN EASTERN QAIDAM BASIN, NW CHINA

This study presents an overview on the geological setting and geochemical characteristics of Pleistocene shallow gas accumulations in eastern Qaidam Basin, NW China. Five largest gas accumulations discovered in this region have a combined enclosure area of about 87 km2 and {7.9} trillion cubic feet (tcf) of proven plus controlled gas reserves. The dominance of methane ({>99.9%}) and the {δ{}{13}C} and δD values of methane ({-68.51‰} to {-65.00‰} and {-227.55‰} to {-221.94‰}, respectively) suggest that these gases are biogenic, derived from the degradation of sedimentary organic matter by methanogens under relatively low temperatures (<75℃). A sufficient supply and adequate preservation of organic matter in the Pleistocene sediments is made possible by the lake basin’s high altitude (2600-3000 m), high water salinity (>15%) and strong stratification. The deposition and extensive lateral occurrence of shore and shallow lake sands/silts in beach sand sheets and small sand bars provided excellent reservoirs for the biogenic gas generated from adjacent rocks. Effective but dynamic gas seals were provided by such factors as intermittent vertical variations in the sediment lithologies, hydraulic trapping due to mudstone water saturation, the hydrocarbon gradient created as a result of gas generation from potential caprocks, and the presence of a regional caprock consisting of 400-800-m-thick mudstones and evaporites. It appears that the most favorable traps for large gas accumulations occur on structural slopes near the major gas kitchen, and the prolific gas pools are often those large gentle anticlines with little faulting complication.

This study presents an overview on the geological setting and geochemical characteristics of Pleistocene shallow gas accumulations in eastern Qaidam Basin, NW China. Five largest gas accumulations discovered in this region have a combined enclosure area of about 87 km： and 7.9 trillion cubic feet （tcf） of proven plus controlled gas reserves. The dominance of methane （ 〉 99.9% ） and the δ^13C and 5D values of methane （ - 68.51%o to - 65.00%0 and - 227.55%o to - 221.94%v, respectively） suggest that these gases are biogenic, derived from the degradation of sedimentary organic matter by methanogens under relatively low temperatures （ 〈 75℃）. A sufficient supply and adequate preservation of organic matter in the Pleistocene sediments is made possible by the lake basin＇ s high altitude （2600 -3000 m）, high water salinity （ 〉 15% ） and strong stratification. The deposition and extensive lateral occurrence of shore and shallow lake sands/silts in beach sand sheets and small sand bars provided excellent reservoirs for the biogenic gas generated from adjacent rocks. Effective but dynamic gas seals were provided by such factors as intermittent vertical variations in the sediment lithologies, hydraulic trapping due to mudstone water saturation, the hydrocarbon gradient created as a result of gas generation from potential caprocks, and the presence of a regional caprock consisting of 400 - 800-m-thick mudstones and evaporites. It appears that the most favorable traps for large gas accumulations occur on structural slopes near the major gas kitchen, and the prolific gas pools are often those large gentle anticlines with little faulting complication.