NATURAL FRACTURES IN WOODFORD SHALE, ARBUCKLE MOUNTAINS, OKLAHOMA

The Upper Devonian /Lower Mississippian Woodford shale produces gas at isolated locations across the southern Mid-continent region. We conducted detailed fracture analysis of the Woodford shale along the northern (overturned) and southern (gentle) limbs of the Arbuckle anticline to determine the control of structure and mechanical stratigraphy on the origin and geometry of natural fractures.

In both outcrops, the Woodford contains well developed three fracture sets. The first set is parallel to the trend the fold axis of Arbuckle anticline and is also roughly parallel to the strike of the bedding plane. The second set is perpendicular to the fold axis and, in general, is parallel to dip direction of the bedding. The third set is oblique both to the trend of the fold axis and to the strike of the bedding plane. The outcrop along the northern limb also contains a fourth fracture set parallel to the strike of kink banding along the minor fault-bend folds superimposed on the Arbuckle anticline. The minor folds are probably related to blind faults at depth. In both outcrops, fractures tend to be more concentrated and closely spaced in siliceous shale to chert beds (stiff layers). Fractures are well developed in siliceous shale to chert beds. However, they die out in the organic rich shale beds overlying siliceous shale to chert beds. There is also a correlation between fracture spacing and bedding thickness; thinner the siliceous shale to chert beds lesser the fracture spacing.

These observations suggest to us that fractures in the Woodford shale are a) directly related to the Arbuckle anticline in their origin, and b) controlled by the mechanical stratigraphy in length and spacing. Therefore, folded Woodford made mostly of siliceous shale to chert beds may serve as a better reservoir in the subsurface.