Subsurface stratigraphy of the Upper Cretaceous upper Williams Fork Formation, Piceance Basin, northwest Colorado

Mike Leibovitz, Paul Weimer, Stephen Cumella, Renaud Bouroullec, and Edmund (Gus) Gustason

The Upper Cretaceous upper Williams Fork Formation of the Piceance Basin in northwestern Colorado is an important stratigraphic unit in the basin-centered gas accumulation. A regional sequence stratigraphic analysis of more than 3000 well logs defined seven informal lithologic units in the upper Williams Fork Formation based on their wireline-log facies and regional distribution. The upper Williams Fork Shale 1 and 2 units are present in the southern and central portion of the basin; they consist of thick overbank shales with thin interbedded sandstone and shales deposited in a coastal plain setting. The upper Williams Fork Sandstone unit is a northwest trending, valley fill unit present in the central portion of the basin. The valley eroded into the upper Williams Fork Shale 1 and underlying middle Williams Fork sandstones, and underlies the upper Williams Fork Shale 2. These three units change laterally to a northeast-thickening interval that pinches out in the middle Piceance Basin. This wedge includes (a) the Goff Canyon and Lion Canyon coal units, which consist of interbedded thin coals, carbonaceous shales, and thin sandstones, separated by (b) the Lion Canyon Sandstone unit, a marginal-marine sandstone deposited as part of a regional transgression of the seaway. The Lion Canyon Sandstone unit can be traced laterally updip to the southwest, to the (c) Price Coal, which is a thin widespread unit throughout the northern and central portion of the basin. The updip pinchout of the Price Coal is constrained by a regional, northwest-striking syn-depositional fault. The upper Williams Fork Formation is overlain unconformably by the Ohio Creek Conglomerate, a Paleocene unit associated with the Laramide orogeny. Throughout much of the Piceance Basin, the commercial gas accumulations are controlled by source rock maturation, burial depth, as well as lithofacies in the case of the upper Williams Fork Formation. In the southern portion of the basin, the upper Williams Fork is the seal for the basin-centered gas accumulation in the lower and middle Williams Fork Formation reservoirs. However, in the northern Piceance Basin, Laramide faulting allowed gas to migrate into the upper Williams Fork and overlying Wasatch section, forming a hybrid basin-centered gas / conventional petroleum system.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012