Fracture and Non-Matrix Pore Development Related to Evaporite Paleokarst Collapse, Lower Cretaceous Comanche Shelf, Texas

Loucks, Robert G.¹; Zahm, Christopher¹
(1) Bureau of Economic Geology, The University of Texas at Austin, Austin, TX, United States.

Laterally continuous, intrastratal breccia zones and the resulting non-matrix pore structures and fractures are significant reservoir features that modify the permeability in subsurface reservoirs. The associated non-matrix pores occur within the brecciated zone and within suprastratal beds overlying the collapsed zones. Despite the chaotic appearance of the brecciated zones, the deformation styles and associated fracture intensity can be defined and characterized for improved reservoir characterization. Significant layer-bound faulting, folding and brecciation occur within regions that are devoid of tectonic activity, but preexisting structures may influence the overall geometry of the deformation zones.

Using outcrops that extend over 80 miles across the Comanche Shelf of central Texas, we have mapped evaporite paleokarst brecciation and associated deformation within the Lower Cretaceous Edwards Formation, specifically the Kirschberg Evaporite Member. Within this expansive outcrop we have analyzed three major intervals: (1) a substratal interval beneath the evaporite zone; (2) an intrastratal interval containing the evaporites or breccias in which the former evaporites were dissolved; and (3) a suprastratal interval overlying the evaporites that commonly shows deformation generated by collapse into the caverns created by evaporite dissolution. Deformation within the substratal interval is characterized by low-intensity opening-mode fractures with orientations that parallel preexisting Paleozoic structures that unconformably subcrop below the zone of interest. Intrastratal deformation is characterized by both extensional and compressional structures, including normal and reverse faults and folding. Faults and fold axes within this zone also parallel preexisting structural trends. In addition, zones of extensive chaotic brecciation occur in localized areas. Finally, suprastratal deformation including extensional and compressional faulting and zones of high intensity opening-mode and shear fracture zones.

We use a model of coalesced, isolated collapse to explain deformation zones observed in outcrop which result in predictable belts of deformation styles. Many factors can influence the style of deformation within evaporite paleokarst, but comparison of this work to other areas of evaporite paleokarst suggest that potentially reactivated preexisting structures and sediment input may be an important factors in the style of deformation observed.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.