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The AAPG/Datapages Combined Publications Database

Oklahoma City Geological Society

Abstract


The Shale Shaker
Vol. 53 (2002), No. 3. (November/December), Pages 71-78

Surface Fracture Characterization of Jackfork Group Turbidite Sandstones in the Ouachita Mountains, Oklahoma

Kimberly D. Combs-Scott, Kevin J. Smart

ABSTRACT

The Ouachita Mountains in Oklahoma and Arkansas represent an exposed part of a >2,000 km long orogenic system that stretches from the Marathon thrust belt in southwestern Texas to the southern Appalachians. The central Ouachitas in southeastern Oklahoma are dominated by Mississippian and Pennsylvanian clastic rocks that includes the Pennsylvanian Jackfork Group, a predominantly deepwater turbidite succession. In addition to extensive surface exposure, the Jackfork represents a significant natural gas reservoir in the subsurface in the Ouachita region.

A preliminary outcrop fracture characterization was undertaken as part of a larger study that focuses on the interaction of stratigraphic and structural controls on fracturing in the Jackfork. Standard scan-line techniques were used to collect data on fracture orientations and intensities on Jackfork bedding surfaces in the north limb of the Lynn Mountain syncline south of Big Cedar, OK. Estimates of stratigraphic and mechanical bed thickness were also recorded at each sampling locality. Observations reveal two dominant fracture orientations, both of which are approximately bed-normal. The primary set is strike-parallel with an average orientation of 105°/61°N. The secondary set, which generally terminates against the primary set, is dip-parallel with an average orientation of 017°/89°W. The primary set shows a moderately-strong positive correlation of fracture spacing with bed thickness (i.e., thicker beds have larger spacings). The fracture spacings for the secondary set show little correlation with bed thickness. Interestingly, secondary-set spacings are strongly correlated to primary-fracture spacing. We interpret this to indicate that the secondary-fracture spacing was controlled by the pre-existing primary fractures in addition to bed thickness.

Our analyses also suggest that estimates of subsurface-fracture spacing in Jackfork sandstones are possible given measurement of bed thickness. Generally, primary-fracture spacing is 30-50% that of bed thickness, whereas the spacing of the secondary fractures is roughly twice the bed thickness. Work in progress will focus on the areal fracture density, outcrop-scale connectivity, as well as the relationship between lithology and fracture characteristics, with the goal of better defining the controls of fracture development and providing a better refinement of our predictions. If successful, this research will aid current and future exploration efforts of subsurface Jackfork intervals in the Ouachita region, as well as similar stratigraphic systems elsewhere, and should offer important new data on the structural/tectonic development of the Ouachita orogenic system.


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