Natural Surface Fractures in the Woodford Gas-Shale, Southern Oklahoma
Production from unconventional gas reservoirs depends on natural and hydraulic fractures. The Upper Devonian/Lower Mississippian Woodford Shale is an unconventional reservoir and produces gas in Southern Oklahoma. We conducted detailed fracture analysis of the Woodford Shale in isolated surface outcrops in the Arbuckle Mountains of Southern Oklahoma to determine the effect of structural control, mechanical stratigraphy and lithology on the origin, geometry and density of natural fractures.
The Woodford Shale contains three well fracture sets when it is exposed at flanks of an anticline or a syncline. The first set is parallel to the trend of the fold axis and is also roughly parallel to the strike of bedding plane. The second set is perpendicular to the fold axis and, in general, is parallel to dip direction of bedding. The third set is oblique both to the trend of the fold axis and to the strike of bedding plane. There is also an outcrop with a fourth fracture set which is parallel to the strike of kink bending along minor fault-bend folds superimposed on the major structural feature, the Arbuckle overturned anticline. The minor folds are probably drape folds related to faults at depth.
Mechanical stratigraphy and thin section analyses suggest that siliceous and cherty shale beds are more fracture populated than the organic-rich shale beds. Silica content directly effects brittleness of the Woodford Shale beds. Natural fractures are more closely spaced in brittle siliceous to cherty shale beds than the ductile organic rich shale beds. Microfractures are more noticeable in thin sections of silica-rich beds than organic-rich ones. Also, there is a correlation between bed thickness and natural fracture frequency; thin shale beds have more fractures per square foot than thick shale beds.
In summary, the geometry and density of natural fractures of the Woodford Shale outcrops are directly related to their proximity to local structures. Mechanical stratigraphy and diagenetic constituents also have a significant role in natural fracture abundance and intensity. In the subsurface, cherty and siliceous Woodford Shale in close proximity to the anticlinal fold axial surfaces may serve as a better reservoir and give better results to hydraulic fracture treatments.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009