AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Sequence Stratigraphy of the Upper Cretaceous Upper Williams Fork Formation, Piceance Basin, Northwest Colorado, and Its Relationship to the Basin-Centered Gas Accumulation
(1) University of Colorado, Boulder, CO.
(2) Bill Barrett Company, Denver, CO.
The Upper Cretaceous upper Williams Fork Formation of the Piceance Basin in northwest Colorado is an important stratigraphic unit in the basin-centered gas accumulation (BCGA). 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 facies are present in varying thickness across 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 fluvial channel system that was initially incised into the Williams Fork shale 1 and middle Williams Fork sandstones. The upper part of the upper Williams Fork sandstone was deposited after the entrenchment of a major river system that did not migrate far from the confines of the original incised valley-fill.
These three units change laterally in a northeast-thickening stratal wedge. This wedge includes (a) the Goff Canyon and (b) Lion Canyon coal units, which consist of interbedded thin coals, carbonaceous shales, and thin sandstones, separated by (c) the Lion Canyon Sandstone, a marginal-marine sandstone deposited as part of a regional transgression of the Lewis seaway. The Lion Canyon Sandstone can be traced up depositional dip to the southwest to the (d) Price coal, which is a thin widespread coal present throughout the northern and central portion of the basin. The upper Williams Fork Formation is overlain unconformably by the Ohio Creek Conglomerate, a Paleocene unit associated with the Laramide Orogeny.
Five kinds of gas accumulations are present in the Williams Fork Formation: (1) upper Williams Fork shale is the seal of a BGGA in the lower and middle Williams Fork, (2) source rock maturation controlled BCGA, (3) hybrid BCGA and structural closure, (4) tectonically influenced BCGSAs, and (5) hybrid BCGA and stratigraphic traps. Throughout much of the Piceance Basin, the commercial gas accumulations are controlled by source rock maturation, burial depth, as well as the lithofacies of the upper Williams Fork Formation. However, tectonics and stratigraphy in the northern Piceance Basin allowed gas to migrate shallower in the section, forming a hybrid basin-centered gas/ conventional petroleum system.