Structural and stratigraphic controls on fracture distribution within sand bodies of the Upper Iles and Lower Williams Fork formations, Mamm Creek and Divide Creek fields, Piceance Basin, Colorado

Abstract

The Mesaverde Group in the Piceance Basin of northwestern Colorado is a continuous tight gas accumulation that has established itself as one of the more prolific producers of natural gas in the Rocky Mountain Region. In the Piceance Basin, the most significant producing intervals within the Mesaverde Group are the Iles and Williams Fork Formations, which provide key reservoir and source aspects of the Mesaverde total petroleum system. The Williams Fork and Iles Formations were deposited by sediment shed from the Sevier fold and thrust belt and transported eastward into the Cretaceous Interior Seaway. As a result of depositional processes and grain composition, the sandstones in these formations have very low permeability, typically less than 0.1 mD; so low that successful production of hydrocarbons requires the presence of fractures, either natural fractures or induced fractures from hydraulic stimulation. Natural fractures have been documented to significantly enhance reservoir permeability. This thesis examines the stratigraphic and structural controls on fracture distribution within the Mamm Creek field through the use of Borehole Image Logs, electrical logs, and outcrop. Sand bodies were divided into 5 different facies associations for fracture analysis, in order to investigate the stratigraphic controls on fracture properties. Positive linear relationships were identified between bed thickness and fracture aperture and spacing; thicker beds support larger fractures with larger apertures. Open and healed-resistive fractures were observed in the study area. The overall mean strike of all fractures in the study area is 315°, with the majority of dips ranging between 81°-90°. Open fractures contain a mean strike of 319°. Healed-resistive fractures contain a mean strike of 122°. The present day stress orientation (SHmax) determined from the orientation of drilling induced tensile fractures in Mamm Creek field is 130°. This orientation is parallel with naturally occurring fractures and creates limitations in connecting natural fractures during hydraulic fracturing. Fracture counts per well and fracture densities increase significantly based on their proximity to the hinge line of the Divide Creek Anticline and the Rifle Syncline. This suggests a strain increase along the hingelines of these features, where the highest rate of dip change and maximum curvature occur. The increase in fracture counts suggests reservoir rocks found in these areas will contain higher overall fracture induced reservoir permeabilities.