FRACTURE CHARACTERIZATION OF THE PENNSYLVANIAN JACKFORK GROUP, OUACHITA MOUNTAINS, SOUTHEASTERN OKLAHOMA

The Pennsylvanian Jackfork Group in southeastern Oklahoma contains approximately 1,500 meters of heavily-fractured clastic rocks identified as deepwater turbidite deposits. Although overall structural evolution is established, controls on fracture development in the Jackfork Group are still poorly understood. Recent hydrocarbon exploration in the Ouachita Mountains has demanded greater understanding of the structural complexity in the Jackfork Group and how it affects reservoir characteristics. Immediate application exists in southeastern Oklahoma, where the fractured Jackfork sandstones serve as a producing natural gas reservoir.

Fractured bedding surfaces at the Lynn Mountain and Rich Mountain synclines were analyzed to assess the relationship between stratigraphic and structural fracture controls. Linear scanlines were used to collect orientation and spacing data, and stratigraphic and mechanical bed thicknesses were recorded. Two dominant fracture sets were identified oriented orthogonal to one another and normal to bedding. The primary set is parallel to bedding strike at approximately 100°/60°N. The second set is parallel to bedding dip at 010°/89°W and generally terminates against the primary set. The alignment of both sets to bedding as opposed to the syncline axis suggests that fractures formed prior to folding. An approximately linear correlation exists between fracture spacing and bed thickness, meaning that thicker beds typically exhibit larger spacings. Primary set fractures closely correlate to bed thickness with a spacing to layer thickness ratios (S/T) ranging from 0.3 to 0.9. Secondary set fractures exhibit a weaker correlation to bed thickness, and S/T ranges from 0.5 to 4.3. We interpret that fractures in the primary set developed due to bed thickness, rock strength, and interaction with adjacent layers, until the fracture saturation limit (S/T»1) was achieved. Continued stress transfer between adjoining fractures controlled further fracture development, possibly including the formation of secondary set fractures. In general, secondary set fractures do not saturate the rock mass, indicating subsequent fracture development under lithologic constraints and influenced by preexisting primary set fractures.