Natural Fracture Analysis Related to Facies and Strain Variability in the Middle and Upper Williams Fork Formations, Piceance Basin, Colorado
Lee, Edward C.; Trudgill, Bruce D.
Given the industry's continuous evolution towards exploiting more unconventional reservoir rock, it is essential to characterize the properties and understand the underlying driving mechanisms behind zones of increased production. In the Williams Fork Formation tight rock reservoirs, located in Colorado's Piceance Basin, higher gas production seems to correlate with an increasing number of intersected fracture networks along a well path. The aim of this study is to characterize fracture networks based off of their relationships with facies and strain variability within the basin. In order to develop the understanding of these relationships, the study will utilize a series of scan lines, Rorhbaugh circles, core data, and outcrop descriptions to create rose diagrams summarizing the fracture characteristics relating to the independent variables. Previous studies have examined fracture characteristics in Williams Fork Formation outcrops, but have stopped short of relating these fracture properties to the localized strain environment or the associated facies. Initial results from this study mirror the observations made by earlier researchers who noted multiple structurally related fracture networks, as well as the presence of a fairly regional set.
High strain environments along the eastern edge of the basin show an increased fracture density, while medium strain environments along the western edge show a lower density with a higher relative contribution of fractures from the regional set. Seismic imaging and core data suggest that the low strain basin center, where production is centralized, is fractured purely by the regional set. Therefore it is imperative to isolate in outcrop and characterize the regional set in order to to relate the study to basin production.
A facies influence on the fracture intensity, density, and spacing seems to be a function of the calcite cementation present in the facies. Swarming in fracture sets commonly appear in facies with higher cementation. Swarming appears to significantly amplify the permeability of the reservoir.
Examining the initial findings of this ongoing study suggests that the strong production derived from increased permeability, could be associated with wells intersecting regional fracture sets, which appear to swarm more often in highly calcified facies.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013