Xu, Tianguang1, Chenxia Xie1
(1) IHS Energy, Houston, TX
ABSTRACT: Generation and Accumulation of Quaternary Biogenic Gas in the Eastern Qaidam Basin, NW China
The Qaidam Basin, a composite Jurassic foreland basin and Cenozoic rift basin, is located in northwest China. In the eastern part of the basin, the largest biogenic gas accumulation in China has been discovered in Quaternary in the Sanhu Sag, with proven reserve of about 7.9 Tcf gas. The Sanhu Sag covers an area of about 37,000 km2, with a maximum thickness of Quaternary sediments of 3,400 m. The gases are methane-rich (95.17-99.41%), with δ13C1 values ranging from -65 to -68.54‰. Source rocks consist of Quaternary shallow lacustrine organic-rich carbonaceous mudstones and shales with an average TOC value of 9.06% and relatively deep saline lacustrine dark mudstones with an average TOC value of only 0.3%. Methanogen bacteria have been tested at depths in the range of 0.05 to 1,705 m. Microbial modeling indicates that gases are generated at temperatures of 35-55 ˚C, and generated methane has δ13C1 values ranging from -66.05 to -56.64‰. Reservoirs consist of Lower Pleistocene siltstones and muddy siltstones with porosity ranging from 25 to 41% and permeability from 10 to 1,800 mD. Regional seals are Quaternary gypsiferous mudstones with a thickness of 400-1,000 m. Traps are typically syn-depositional anticlines with gently dipping limbs (< 5˚). Geochemical and geological data indicate that the generation and accumulation of the biogenic gas in the basin are attributed to: (1) thick source rocks; (2) low surface temperature; (3) high depositional rates; (4) saline lacustrine environment; (5) thick regional seals; and (6) the formation of the syn-depositional anticlines.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.