Chapter 17

A New Paradigm for Gas Exploration in Anomalously Pressured "Tight Gas Sands" in the Rocky Mountain Laramide Basins

Ronald C. Surdam

Institute for Energy Research, University of Wyoming
Laramie, Wyoming, U.S.A.

A significant portion of the Cretaceous shales in the Rocky Mountain Laramide Basins (RMLB) are overpressured on a basinwide scale. The change of pressure regime from normally pressured to overpressured coincides with marked changes in the geochemical and geophysical properties of the Cretaceous rock/fluid system. Sandstone bodies within the overpressured shale section are subdivided stratigraphically and diagenetically into relatively small, isolated, gas-saturated, anomalously pressured compartments. The driving mechanism of the pressure compartmentalization is the generation and storage of liquid hydrocarbons that subsequently react to gas, converting the fluid-flow system to a multiphase regime in which capillarity controls permeability.

A new exploration paradigm and an exploitation strategy have been created that significantly reduce exploration risk in the RMLB. Two elements crucial to the development of prospects in the deep, gas-saturated portions of the RMLB are (1) the determination and, if possible, three-dimensional evaluation of the pressure boundary between normal and anomalous pressure regimes and (2) the detection and delineation of porosity/permeability "sweet spots" (i.e., areas of enhanced storage capacity and deliverability) in potential reservoir targets below this boundary. Certainly there are other critical aspects, but completion of these two tasks is essential to the successful exploration for the unconventional gas resources present in anomalously pressured rock/fluid systems in the RMLB.