Mapping Rocky Mountain Fractures: GIS Methods for Resource Plays
Laura Kennedy, Eric Erslev, and Karen Aydinian
Fracture orientations in rocks hosting unconventional resource plays (e.g., the Niobrara Formation in eastern Colorado and Wyoming) provide important controls on the permeability and economic success of tight reservoirs. As part of the University of Wyoming School of Energy Resources NioFracture initiative, a GIS database integrating surface and subsurface fracture data is being assembled to reveal regional and local fracture trends. Minor fault and joint data in ArcGIS were plotted over satellite imagery and geologic bedrock maps as rose diagrams of fracture strike, using color to represent fracture mode: green for joints, yellow for faults, and red for ideal compression orientations as determined from fault geometries and slip patterns. At a local (outcrop) scale, rose diagrams are overlain by a point at their actual geographic location. The point size is scaled to the number of fractures for each locality. Local fracture data were combined at subregional and regional scales to provide larger perspectives. Data attributes for each rose diagram include fracture characteristics (orientation, clustering, etc.), geology, inferred stress directions, and timing. Stereonets of 3D data, available field photos, and diagrams from the published data source were also linked to each rose plot. This compilation makes public domain data available in one comprehensive database, allowing potential users to make informed models for reservoir fracture permeability. Initial results show a remarkable uniformity of Laramide east-northeast compressive stress orientations despite major local variations in structural trends. Joint patterns can be considerably more diverse, with east-northeast-striking, northwest-striking, and west-northwest-striking joints found regionally. East-northeast-striking joints are typically the earliest joints (J1) in Mesozoic and Paleozoic strata and commonly parallel Laramide compression directions calculated from minor faults. Northwest-striking joints (J2) are found in pre-, syn-, and post-Laramide rocks, indicating regional northeast-southwest extension after Laramide tectonics ceased. Based on these regional patterns, horizontal drilling legs should be NW-SE if Laramide joints and minor faults are crucial for reservoir permeability. If most reservoir permeability is created by NW-SE post-Laramide jointing, NE-SW horizontal legs may be more ideal.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012