Natural fracture analysis related to depositional environment and strain variability in the middle and upper Williams Fork Formation, Piceance Basin, Colorado

Abstract

As Americans continue to focus on the use of cleaner, and more efficient forms of energy, tight gas reservoirs like those in the Piceance Basin in Colorado are becoming increasingly important. The Williams Fork Formation is potentially capable of 1.2 BCF/day of natural gas production. With continued tight gas development in the Piceance Basin, and a better understanding on the controls behind zones of enhanced production, that contribution can be expected to grow. The understanding of natural fracture networks, and the drive to exploit their characteristics as migration pathways, will allow for more efficient gas production. Laramide east-west compression resulted in the formation of regional sub parallel fracture networks within the Williams Fork Formation. Major and minor localized deformation resulted in the development of structurally derived fracture networks. At least three separate sets of fractures exist at outcrop, but the extent of these networks across different preserved depositional environments is poorly constrained. Regional compressive stress directions were the primary control on fracture orientation. However, localized structures related to the Laramide deformation developed structural networks of extensional fractures that are orientated parallel to the axial trace of folds, and strike of normal faults. These locally derived structurally related networks in addition to the regional networks form overall tighter fracture spacing than bedding in lesser deformed portions of the Piceance Basin. Increased local deformation is associated with both normal surface faulting in the western Piceance Basin, and subsurface intrabasin imbricated thrust faults in the southern Grand Hogback area. Additionally, the depositional environment of a single bed controlled bed thickness and brittleness. Tighter average fracture spacing is associated with thinner, more brittle beds.