SURFACE FRACTURES IN WOODFORD SHALE IN ARBUCKLE MOUNTAINS, SOUTHERN OKLAHOMA

The Upper Devonian /Lower Mississippian Woodford shale is well exposed in four outcrops in the Arbuckle Mountains of Southern Oklahoma. Two of these outcrops are located along the northern (overturned) limb of the Arbuckle anticline in the northern part of the Arbuckle Mountains. The third outcrop is located along the southern (gentle) limb of the Arbuckle anticline in the southern part of the Arbuckle Mountains. The fourth outcrop is located in the Criner Hills area of the Arbuckle Mountains.

We conducted detailed fracture analysis of the Woodford shale along these four outcrops to determine the control of the structure, the mechanical stratigraphy and petrography. The three outcrops along the Arbuckle anticline contain three well developed fracture sets. The first set is parallel to the strike of the bedding plane and also parallel to the fold axis of the Arbuckle anticline. The second set is perpendicular to the strike of the bedding plane and the fold axis of the Arbuckle anticline. The third set is approximately oblique with minor deviations to the strike of the bedding plane and to the fold axis of the Arbuckle anticline. In the fourth outcrop in the Criner Hills area which is located northeast to the Rock Crossing anticline, two fracture sets strike to N40E and N70E along the Woodford shale. The third fracture set can be measured only in the middle section where the bedding planes of the Woodford Shale beds are well exposed. These observations indicate a strong structural control in the fracture patterns developed in the Woodford outcrops of the Arbuckle Mountains. In all of the four outcrops, our fracture analysis work indicates that fracture system of the Woodford Shale is influenced by local structural events.

Our field examinations thin sections analyses indicate that siliceous and cherty shale beds are more fracture populated than the organic rich shale beds since silica content directly effects the brittleness of the shale beds. Natural fractures are more closely spaced in brittle siliceous to cherty shale beds than the ductile organic rich shale beds. Microfractures are relatively more noticeable in thin sections of silica rich beds to organic rich ones. Also, there is a correlation between bed thickness and natural fracture frequency that thinner shale beds are more densely fractured than the thicker shale beds.