Basin-centered Gas or Subtle Conventional Traps?

Abstract

Basin-centered gas models have been proposed to characterize various low-permeability (tight) sandstone gas plays that are an important gas resource found in many Rocky Mountain basins. Recent drilling and three-dimensional seismic results indicate that modifications are required of the currently accepted basin-centered gas models that were first introduced more than 25 yr ago. Current models of basin-centered gas accumulations depict gas trapped below a relatively uniform, enigmatic pressure seal defined by a given structural elevation or thermal-maturation depth that cuts across stratigraphic boundaries. In the prevailing model, this surface separates normally pressured conventional traps from unconventional traps, which are characterized by anomalous reservoir pressure and lack of associated water production. These principles have led to a commonly held misconception that basinwide, commercial gas deposits may exist below this pressure boundary and, further, to predictions of overstated reserves and overly optimistic drilling success rates. New studies of several gas plays in tight sandstones from the Greater Green River Basin in southern Wyoming, reviewed in this chapter, suggest that the primary controls for the occurrence of these fields are better explained as conventional, although subtle, stratigraphic and structural traps. Subsurface data are described, which illustrate additional inconsistencies with the prevalent basin-centered gas models, including the presence of downdip water, and natural fracture and stratigraphic variations that influence productivity. These subtle controls have previously been accepted as poorly understood areas of enhanced production or sweet spots. With a revised understanding, geologic methods can be applied to identify areas of improved reservoir quality, thereby increasing the probability of favorable economic development. More importantly, the key to future exploration success in the Rocky Mountain region and other tight-gas sandstone provinces is recognizing that subtle, conventional stratigraphic and structural traps provide controls on commercial gas deposits in these plays.