Geological Conditions of Tight Sandstone Gas Accumulation by Diffusion in Sulige Gasfield, Ordos Basin, China

Tight sandstone gas reservoirs have become the focused field in the global unconventional natural gas exploration and tight sandstone gas output accounts for 75% of the unconventional gas output (Zou et al., 2013). In China, tight sandstone gas reserves and output account for a third of the whole country natural gas reserves and output (Zou et al., 2013). The Sulige gasfield is a typical gasfield with a large amount of tight sandstone gas reservoirs in China and the studies of its Upper Paleozoic tight sandstone gas reservoirs have made a great deal of progresses in the identification and evaluation of source rocks and reservoir rocks, and the analysis of gas reservoirs characteristics, etc. (Liu and Hao, 1996; Fu, 2001; Ma, 2004; Zhang et al., 2009; Liu et al., 2013). However, the reservoir-forming mechanism in the Upper Paleozoic tight sandstone gas reservoirs is not clear because pressure-driven volume flow, as the most important way of forming gas reservoirs, is difficult for gas migration in tight sandstones due to its high capillary force and low driving forces (Wang, 2002; You et al., 2007; Gong et al., 2008; Zhu et al., 2009; Liu et al., 2013). Based on the study of geological conditions and gas reservoir characteristics, we suggest that diffusion could be an important method of gas migration in tight sandstone reservoirs. Gas diffusion can occur spontaneously so long as there is concentration difference even in the tight sandstones because the diameter of gas molecule is much smaller than that of pore and throat in the tight sandstones. Furthermore, diffusion has still been taken as one of important mechanism of gas dissipation (Antonov, 1968; Stklyanin et al., 1968; Pandey et al., 1974; Thompson, 1979; Thomas, 1989; Krooss et al., 1992; Krooss and Leythaeuser, 1996; Fang et al., 2001; Shuai et al., 2004; Liu et al., 2008). Gas diffusion in reservoirs has been mainly studied in the measurement of diffusion coefficients, establishment of numerical diffusive models, calculation of diffusive amount, etc. (Antonov, 1954, 1964; Stklyanin and Litvinova, 1971; Leythaeuser et al., 1982; Krooss and Leythauser, 1988; Hao et al., 1993; Nelson and Simmons, 1995; Fang et al., 2001; Wang and Gao, 2005; Liu et al., 2012). However, there are still many aspects needed to be improved. At least we should know under what geological conditions gas diffusion can become an alternative mechanism of gas accumulation in tight sandstone reservoirs. This paper provides some favourable geological conditions for gas diffusion in tight sandstone reservoirs of Sulige gasfield, Ordos Basin.