Fold Related Fractures in Woodford Shale Outcrops Along the Arbuckle Overturned Anticline, Arbcuckle Mountains, Oklahoma

Ibrahim Cemen1 , Onur Attaman2, James Puckette3, Darwin Boardman4

¹Oklahoma State University,   ²Oklahoma State University,  ³Oklahoma State University,  ⁴Oklahoma State University

Detailed fracture analysis of the Woodford shale was conducted 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.

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

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 in length and spacing by mechanical stratigraphy. Therefore, folded Woodford made mostly of siliceous shale to chert beds may serve as a better reservoir in the subsurface.

AAPG Search and Discover Article #90097 © 2009 Tulsa Geological Society, Tulsa, Oklahoma

 

 

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