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Facies Variability, Fracture Heterogeneity, and Reservoir Properties of the Marble Falls Limestone, Jack County Texas

William A. Ambrose¹, Robert G. Loucks¹, John Hooker¹, Edmund L. Frost¹, Craig Reynolds², A. Dax McDavid³, Douglas Dawson4, and Bill Walker, Jr.4
¹Bureau of Economic Geology, The University of Texas at Austin, Austin, TX 78713
²Cobra Oil & Gas Corporation, 2201 Kell Blvd., Wichita Falls, Texas 76308
³Formerly with Stalker Energy L. P., currently with Brigham Oil and Gas, L.P.
4Stalker Energy L. P., 1717 W. 6th Street, Suite 230, Austin, Texas 78703

The Marble Falls Limestone in Jack County, although composed partly of low-porosity and low-permeability ramp spiculites and carbonate debris-flows of slope origin, has a potential for production owing to locally, well-developed natural fractures and total organic carbon (TOC) values that approach 2% within flooding surfaces. The dominant ramp facies consist of spiculitic wackestones and mudstones with porosity ranging from 1 to 4% and permeability from 0.001 to almost 0.1 md. Stratification in the spiculite facies, where not disrupted by pervasive burrows, is dominantly low-angle scour surfaces and low-amplitude undulose laminae, suggesting periodic scouring of the distal ramp by traction currents. Carbonate debris-flow deposits consist of poorly sorted, heterolithic assemblages of transported crinoid, mollusk, and mudstone fragments, whereas other Marble Falls lithofacies in the lower part of the section are composed of featureless, dark-gray carbonate mudstones of deepwater origin. The dominant fracture type is defined by vertical to subvertical, postdepositional and tectonic fractures variably filled with calcite. These fractures are not particularly stratabound, although they are more common within light-grey spiculite zones than in thin (commonly <3-in) dark mudstone beds. Although many fractures are closed, many zones of continuous, open fractures up to 1.5 ft in length are observed in core. Another fracture population consists of stratabound, sediment-filled, reticulate fractures interpreted to be of syndepositional origin.


AAPG Search and Discovery Article #90164©2013 AAPG Southwest Section Meeting, Fredericksburg, Texas, April 6-10, 2013