Generation and accumulation of Quaternary shallow gas in eastern Qaidam Basin, NW China

• Published in: Acta geochimica Vol. 25; no. 1; pp. 43 - 55
• Main Author: 苏爱国 赵文智 党玉琪 张水昌 黎茂稳 庞雄奇 周瑞年 管自强 徐子远 张道伟 谭彦虎 陈践发 李素梅
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Nature B.V 2006; China University of Mining & Technology (Beijing) , Beijing 100083, China; PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China%PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China%PetroChina Qinghai Oilfield Company, Denghuang 736202, China%Geological Survey of Canada, Calgary, T2L 2A7, Canada%China University of Petroleum (Beijing), Beijing 102249, China
• Subjects: Anticlines; Basins; Evaporites; Gas seals; Gases; Geochemistry; High altitude; Lake basins; Lakes; Low temperature; Methane; Methanogenic bacteria; Mudstone; Natural gas; Organic matter; Petrology; Pleistocene; Quaternary; Saturation; Sediment; Sedimentary geology; Sediments; Stratification; Studies; Water salinity; 地球化学; 地质条件; 天然气; 更新世; 第四纪; Qaidam Basin; China; generation; biogenic gas; Qaidam Basin; accumulation; Quaternary
• ISSN: 1000-9426; 2096-0956; 1993-0364; 2365-7499
• DOI: 10.1007/BF02894795

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.