Spatial Properties of Natural Fractures in the Mancos Shale, Eastern Utah

John M. Bartley, Ziqiang Yuan, Lauren Birgenheier, and Andrew D. McCauley
University of Utah

The response of rocks to hydraulic fracturing is affected by the presence and geometrical properties of pre-existing natural fractures. Owing to poor outcrop, the presence and spatial properties of fractures in mudstones can be difficult to assess at the surface. This is particularly true for the Mancos Shale, which is a potential target for hydrocarbon production via hydraulic fracturing. We therefore examined the properties and spatial distribution of fractures imaged in formation image (FMI) logs through the Mancos Shale from six wells in the east-central part of the Uinta Basin. The locations and orientations of natural fractures were compared to depth, gamma-ray intensity, and borehole breakout orientation to investigate the spatial properties of the fractures and possible controls on their distribution related to lithologic and stress field characteristics. Imaged fractures strike 270° - 280° and 90% dip more than 60°. This orientation is approximately perpendicular to the in situ least horizontal stress trajectory indicated by borehole breakouts, which are rare within the Mancos Shale but are more common in overlying sandstone strata. This relationship predicts that induced hydraulic fracturing of the Mancos in this area will mainly result in propagation of existing fractures rather than the initiation of new ones. Fine laminations of siltstone and mudstone within the Mancos Shale results in a fairly uniform gamma ray character throughout the formation, which averages about 93 API. The highest and lowest API values in the Mancos, about 115 and 65 respectively, are only rarely present, with insufficient frequency for robust statistical evaluation. Fractures tend to be concentrated in portions of the shale with higher API values, although this trend is inconsistent across the study area. The abundance of fractures that dip < 80° generally increases with depth, suggesting a gradual transition from primarily tensile to shear fractures. However, the lower portions of the Mancos Shale are generally characterized by more mudstone rich facies, and some concentrations of shallowly dipping fractures are contained in particular, high API stratigraphic intervals. Both increasing confining pressure and lithologic changes could be controlling the character of these observed fractures.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013