Pore-throat sizes in sandstones, tight sandstones, and shales: Discussion

Abstract

Wayne K. Camp Nelson (2009) prepared a nice compilation describing the wide range of pore-throat sizes that span a continuum from conventional sandstone reservoirs to unconventional tight-gas siliciclastic reservoirs and shale seals. In this article, Nelson (2009) proposes a new and rather unique definition for unconventional reservoirs (including tight-gas sands) that lacks sufficient supporting evidence or references to support this definition. Nelson (2009, p. 329) defines a conventional reservoir as, “…one in which evidence that buoyant force has formed and maintained the disposition of oil and gas is present,” and thus for unconventional reservoirs, it follows that Nelson concludes (p. 330) that, “In these systems, evidence for buoyancy as a dominant force in the disposition of oil and gas is lacking.” It appears that the sole argument for nonbuoyant or buoyancy-subordinate unconventional reservoir systems is the perceived lack of evidence of buoyancy, which is generally not good science (absence of evidence is not evidence of absence). It would be much better to present a clear and concise hypothesis followed by objective observations that support the conclusions. I appreciate that such a thesis may be beyond the scope of a Geologic Note article, but until such evidence is presented, it is difficult to have intelligent discussions and design experiments to test the hypothesis of nonbuoyant gas systems. Although it is not clear from the definition of Nelson (2009), the reader is left to conclude that the natural laws of buoyancy are not dominant factors during the evolution of tight-gas sand accumulations from hydrocarbon migration to the present-day observed distribution of hydrocarbons and water. This conclusion is misleading and ignores a significant body of recent published research (Shanley et al., 2004; Cluff et al., 2005; Fassett and Boyce, 2005; Camp, 2008). Nelson (2009, p. 339) further concludes …