Geometries and Kinematics of Laramide Basement-Involved Anticlines

By

Amanda B. Fisher and Eric Erslev

Colorado State University, Fort Collins, CO

Mechanical and kinematic understanding of anticlinal folds is essential to describing reservoir characteristics, and hence efficient exploitation, of anticlinal hydrocarbon traps. This research details the geometry and deformation of three Laramide basement-involved anticlines southeast of the Bighorn Mountains. Defined by the Tensleep Formation, a major hydrocarbon reservoir, the folds provide an excellent surface analog of similar, structurally trapped oil fields.

EK, Gardner, and Fraker Mountains are asymmetric anticlines with uniform dips up to 5-10° in the backlimb and 55° in the forelimb. Fold axis trends vary counterclockwise from N-S at EK Mountain (east) to NW-SE at Fraker Mountain (west). Fault measurements (n=667) from the anticlines were collected to help describe the kinematic development of the folds.

Deformation in the forelimbs and hinges is dominated by strike-slip faults resulting from ENE-WSW shortening and compression. In the hinge of EK Mountain, a doubly plunging anticline, normal faults form perpendicular to the fold axis, where vertical fold axis curvature is large. Curiously, oblique normal faults strike parallel to fold axes in the anticlinal hinges of thrust-related fault-propagation folds. This is observed at Fraker Mountain, where the fold axis orientation is oblique to the direction of shortening. Fractures are rarely encountered in the backlimbs of the anticlines.

Current 2D and 3D geometric analyses will be integrated with fracture measurements to constrain fold evolution. This information can be applied to subsurface structures to infer deformation occurring within the fold and predict reservoir heterogeneity.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming