Geochemistry and origin of the giant Quaternary shallow gas accumulations in the eastern Qaidam Basin, NW China

This study provided an overview of the geological setting and geochemical characteristics of the Pleistocene shallow gas accumulations in the eastern Qaidam Basin, NW China. The five largest gas accumulations discovered in this region have a combined enclosure area of about 87 km(2) and 7.9 trillion cubic feet (tcf) of proven gas reserves. The dominance of methane (mostly more than 99.9%) and the delta C-13 and delta D values of methane (-68.51 to -65.00 parts per thousand and -227.55 to -221.94 parts per thousand, respectively) suggest that these gases are biogenic, derived from the degradation of sedimentary organic matter by methanogens under relatively low temperatures (<75 degrees C). A sufficient supply and adequate preservation of organic matter in the Pleistocene sediments was made possible by the lake basin's high altitude (2600-3000 m), high water salinity (> 15% TDS) and strong stratification. The deposition and extensive lateral occurrence of lacustrine - shoreline sands/silts in beach sand sheets and sand bars provided excellent reservoirs for the biogenic gas generated from adjacent rocks. Effective but dynamic gas seals are provided by a combination of factors, such as the intermittent vertical variation in the sediment lithologies, hydraulic trapping due to the mudstone water saturation, the hydrocarbon gradient created as the result of gas generation from potential caprocks, and the presence of a regional caprock consisting of 400-800 m of muds 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. (c) 2005 Elsevier Ltd. All rights reserved.