The Nature of Seals in a Large Underpressured Gas System, San Juan Basin, New Mexico
Nelson, Philip H. 1, Steven M. Condon1 (1) U.S. Geological Survey, Denver, CO
Characteristics of large gas accumulations in Cretaceous rocks of the San Juan Basin include gentle dips, lack of structural complexity, low permeability reservoir rock, and underpressured gas. Most gas pressures cluster around two distinct gas-density gradients on a pressure-elevation graph. Each cluster lies below its hydrostatic gradient, showing that the gas is not in buoyant equilibrium with an underlying aquifer. The gas gradient through Mesaverde pressure data intersects the hydrostatic gradient at a higher elevation than the gas gradient through Dakota data. Each intersection establishes the updip boundary of its respective gas accumulation.
Throughout most of the basin, the bottom of the gas accumulation is sealed by the upper member of the underlying Jurassic Morrison Formation, which separates Dakota gas from underlying aquifers. Along the southwestern flank of the gas accumulation, the updip water-saturated sandstones are more porous and permeable than the gas-saturated rocks to the northeast. Due to the low dip angle of the southwestern flank of the basin, the water forms a lateral boundary and pressure reference to the gas rather than an “updip/ downdip” relation. Thick shales isolate the Mesaverde from the Dakota accumulations and also isolate the Mesaverde from aquifers at the top of the system.
What is trapping the gas? Our model is based upon a laboratory experiment in which an underpressured air column was formed in a fine-grained sand pack overlain by a water-saturated coarse-grained sand. The experiment helps explain the pressure regimes and the stability of the gas accumulations in the San Juan Basin: the gas is stable because there is no buoyancy effect and because the basin-centered gas and water at the periphery are in pressure equilibrium. Gas pressure is determined by the elevation of the water-gas transition zone, whereas variations in gas producibility throughout the field are attributed to stratigraphic controls.
AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana