The Geology of the Gibson Gulch Gas Unit, Piceance Basin,

Garfield County, Colorado

By

Jonathan R. Cantwell

Calpine Natural Gas, Denver, CO

Gibson Gulch Gas Unit produces gas primarily from stacked, compartmentalized, pointbar sandstone reservoirs within the Williams Fork Formation of the Mesaverde Group. Gibson Gulch Unit is stratigraphically similar to many other fields, including Mamm Creek, Grand Valley, Parachute and Rulison, which are part of a basin-centered gas cell accumulation in the Piceance basin. However, Gibson Gulch Unit has certain geologic and reservoir characteristics that differ considerably from these other fields.

The lower part of the Williams Fork Formation in this area contains several marine shoreface sands that are absent in the fields to the west. These sands were deposited during eastward progradations following westward transgressions of the shoreline. These marine sands produce poorly, or not at all. They dominate hundreds of feet of the lower section of the Williams Fork, resulting in the interval of gas productive fluvial sands in certain areas to thin significantly.

Another highly variable condition is the thickness of the gas-saturated interval. In this area the basin-center cell exhibits considerable thickness change, from 800 feet to 2000 feet thick. The interplay of these factors, as well as others, present a unique challenge in the development of Gibson Gulch Unit.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming