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Integrated Fracture Analysis: An Important Tool for Deciphering Complex Fracture Patterns*
Search and Discovery Article #40456 (2009)
Posted October 15, 2009
*Adapted from oral presentation at AAPG Convention, Denver, Colorado, June 7-10, 2009
1Department of Geosciences, Colorado State University, Ft. Collins, CO (mailto: [email protected])
Understanding fracture timing and genesis is critical to efficient hydrocarbon exploration and production. Current hypotheses for fracture formation in the Rocky Mountains include regional tectonic compression and extension during pre-, syn- and post-Laramide times as well as more localized deformation due to folding and gravitational collapse of contractile structures.
The integrated analysis of both shear (minor faults) and extensional (joints) fractures in rocks of a variety of ages provides a key tool for differentiating fracture mechanisms. Kinematic data from conjugate minor faults (n = 21,129) within pre-Laramide units give similar average slip (N67E-01) and compressive stress (N67E-02) directions for the Laramide Orogeny. Average Laramide fold (N24W) and arch (N23W) axes are consistently perpendicular to these directions, indicating genesis by thrust-related folding. The largely unimodal shortening and compression directions vary slightly in space, with more E-W directions in the southern and eastern Rockies and more NE-SW directions in the Colorado Plateau.
Whereas some open, potentially hydrocarbon-transmitting joints strike ENE and are thus consistent with syn-Laramide formation; many are highly oblique, commonly with NW-SE strikes. In some areas, hypotheses invoking pre-Laramide jointing can be falsified by the fact that these joints cut Paleogene rocks. Locally, NW-SE-striking joints also cut Miocene strata, making them post-Laramide in age. Multiple mechanisms for post-Laramide joint formation make generalizations inadvisable, with evidence at different localities for fracturing during (1) regional Rio Grande extension, (2) localized gravity detachment into basin lows, and (3) localized back-sliding on Laramide thrust faults. These mechanisms predict very different fracture and hydrocarbon production patterns, indicating the value of integrated fracture analyses.
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