Sequence stratigraphy of the Upper Cretaceous middle Williams Fork formation, Piceance Basin, northwestern Colorado: Implications for reservoir sandstones

The Upper Cretaceous middle Williams Fork is a significant gas reservoir in the Piceance Basin in northwestern Colorado. Regional stratigraphic correlation of more than 3000 well logs identified three informal units within the Williams Fork Formation, the lower, middle and upper.;The middle Williams Fork was further divided into three additional units (1-3), based on architecture of channel-fill deposits. The middle Williams Fork 1 is a shale-rich zone of single-story and multistory channel-fill sandstones ranging in thickness from 15 to 80 feet (4.6 to 24.4 meters). This unit gradationally overlies the shale-rich lower Williams Fork Formation. The middle Williams Fork 2 is a sandstone-rich unit composed of multistory and amalgamated channel-fill sandstones deposited within an incised valley disconformably overlying the middle Williams Fork 1.;The middle Williams Fork 3 consists of multi-story channel-fill belts overlain by the upper Williams Fork Shale 1. There is a decrease in net sandstone deposition of the middle Williams Fork moving northward from the Colorado River Valley and an increase in net sandstone westward in the Piceance Basin.;The middle Williams Fork includes portions of the late highstand of a preceding sequence to the lowstand and transgressive deposits above the 71.5 Ma 3rd order sequence boundary. The middle Williams Fork 2 sandstone deposits show evidence of deposition within an incised valley in the west of the Parachute Field along the Colorado River Valley of the Piceance Basin. To the east of the Parachute Field, the middle Williams Fork 2 loses the characteristic incised valley depositional morphology, and there is less amalgamation of channel-fill deposits. This change in depositional character of the middle Williams Fork 2 across the Parachute Field can be correlated to a syndepositional folding or faulting event that trends northwest-southeast through the field.