Deformation of Ductile Strata: Implications for
Reservoir Compartmentalization in Foreland Structures

Chris Zahm
Colorado School of Mines

Reservoir compartmentalization by faulting that is oriented approximately perpendicular to the fold axis is recognized in foreland structures, particularly in areas where strata are partitioned into both brittle and ductile units. However, the kinematic mechanisms that cause this type of faulting are not well understood. This study quantifies compartmentalization features mapped in outcrop and describe the mechanisms responsible for their development. Better knowledge of the 3D geometry and kinematics of this type of faulting may reveal unexplored reservoir compartments as well as improve production and development schemes in existing fields.

Thermopolis anticline is located in the southern Bighorn basin and provides an excellent exposure of Permian Phosphoria through Late Cretaceous Mesaverde Formations. Within this stratigraphic package are numerous interlayered competent and incompetent units. The units that have been of particular focus for this study are the Triassic Chugwater, including the Alcova Limestone Member, and the Jurassic Gypsum Springs Formation. Within these units, 47 faults with greater that one meter of offset have been mapped on the forelimb of Thermopolis anticline using differential GPS. The faults have an average strike and dip of 255, 49NW while the fold axis orientation is 320. Nearby subsurface fields have distinct directional high permeability anisotropy, which is likely caused by faults with similar orientation to those that outcrop along the forelimb of Thermopolis anticline.

Three mechanisms are responsible for fault compartmentalization on the forelimb of Thermopolis anticline. In addition, each mechanism has important implications for reservoir and seal development. The first mechanism is 3D stretching parallel to the fold axis during folding. This causes an overall tension within the fold that may be significant for creating open fractures. The second mechanism causing compartmentalization is faulting in competent strata caused by differential 3D attenuation within incompetent strata. As ductile strata deforms, preexisting fractures become reactivated during increased folding. Correlation between areas of high differential attenuation and fault offset is observed. This has important reservoir implications because the faults juxtapose reservoir and sealing strata. Finally, bed-parallel back thrusting cause large reservoir or seal strata to be structurally thickened. In one observed example, the Phosphoria Formation, a local hydrocarbon producer, was nearly doubled in thickness. The prediction of these types of reservoir compartmentalization features in subsurface reservoirs is necessary if best development techniques are to be developed.


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