What's The Matter With The Ericson? Gas Shows, Calculated Pay, and Water!
Shanley, Keith W.1, Robert M. Cluff1, and John W. Robinson2
1The Discovery Group, Denver, CO
2North Ranch Resources, Littleton, CO
The Ericson Formation is a clean, quartzose, sheet-like
sandstone that underlies the gas productive Almond Formation
throughout the Washakie basin. The majority of wells that penetrate
the Ericson have indications of gas pay including 6-11% porosity,
resistivity in excess of 50 ohm-m, neutron-density gas cross-over, and
mudlog shows. Saturation calculations using appropriate electrical
properties and Rw values indicate “gas pay” with saturations similar
to producing intervals in the Almond. Nearly all Ericson completions,
however, result in high water volumes and minor gas.. Commercial
production from the Ericson is largely restricted to structural closures.
We suggest the Ericson is at, or near, residual gas saturation with high relative permeability to water and low relative permeability to gas. Calculated water saturations (40-60%) are approximately correct as corroborated by gas shows during drilling; there is gas in the porespace, however, the gas does not form a connected column and lacks capillary pressure. Basin history suggests the Ericson was either gascharged as part of a large accumulation, or served as a migration pathway during the Late Cretaceous to Early Tertiary. Late Tertiary uplift resulted in structural re-adjustment, gas re-migration, and imbibition leaving the Ericson at residual saturation across large portions of the Washakie Basin. Imbibition and scanning capillary pressure data suggests residual gas saturation in tight-gas reservoirs range from 25% to 80% water saturation (Sw).
The high net/gross and widespread nature of the Ericson, reduce the opportunities for significant stratigraphic and fault-related traps and explain the structural control of virtually all significant Ericson accumulations. Minor accumulations will continue to be found, however, they will be difficult to distinguish from residual gas saturation. These difficulties in recognizing residual gas saturation exist in other sheet-like sandstones such as the Castlegate Sandstone, in the Uinta Basin, and the Rollins, Corcoran, and Cozzette Sandstones in the Piceance Basin.
AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah