Charging and Dissipating of the Biogas and Its Accumulation in the Sebei Gasfield, Qaidam Basin, China

Qiang Jin¹, Fuqi Cheng¹, and Guangyou Zhu^2
1China University of Petroleum, Dongying, Shandong, China
²Research, Petroleum Exploration & Production Institute, Petrochina, Beijing, China

The Sebei Gasfield is the largest biogas accumulation found in China and a syndepositional anticline in Quaternary. Major biogas producers are incompacted sands or even sandy muds in 900-1500m depth, but poor gas producers or gas shows are represented in sands shallower than 900m. The authors believe that the biogas is diffusing from deep to shallow sands, as they find that the shallower the gas producer, the more methane in the biogas, and the lighter stable carbon isotope composition of methane. A diffusion model for the biogas is built up and diffused gas quantities are calculated for different sand layers. The gas charging quantities for the sand layers are estimated by the diffused gas quantity plus gas reserve in-place. Meanwhile, the biogas source is also investigated and recognized at 1000-1800m depth southern to the Sebei gasfield. Therefore, a biogas accumulation model is proposed as lateral charging direct from gas source for the sands deeper than 900m and indirect charging from lower gas-bearing sands by diffusion at depth shallower than 900m. With this model and charging & diffusion quantitative method, the authors conducted re-evaluation on each wildcat in the central area of the Qaidam basin, and found many commercial biogas layers.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas