Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
AAPG Annual Convention, Houston, Texas, April 9–12, 2006
Reservoir-Model Analog and Pore-Network Summary for Ellenburger Coalesced Collapsed Paleocave Systems
Paleocave systems are not easy to describe, and there is no simple description that can be applied to these reservoir types. The edges of the fields are generally structurally controlled, the extent and magnitude of porosity are difficult to define, and the pore networks evolve with depth.
Collapsed paleocave reservoirs commonly display an internal rectilinear pattern where trends of porous breccias (chaotic and crackle breccias) are separated by tighter rock. The pattern probably reflects penecontemporaneous karst regional fracture patterns. The breccias may be several thousand meters across, kilometers long, and 100+ meters thick. These scales are larger than individual caves, indicating the collapse and coalescing of cave systems that formed at composite unconformities.
A detailed description of a paleocave system in central Texas can be used as an analog for understanding breccia (reservoir) distribution and reservoir heterogeneity. The three main facies are undisturbed host rock, disturbed host rock (crackle brecciated), and collapse cave passages (chaotic breccias). The brecciated reservoir zones are separated by tighter, nonbrecciated zones.
A complication in understanding paleocave reservoirs is that the pore network evolves from a megapore system near the surface to a crackle-breccia-dominated pore system with deep burial. Delineation of reservoir burial history, therefore, helps us comprehend the pore network present. The reservoir may be fairly well connected because of the large amount of fracturing, and strong heterogeneity of reservoir quality should be expected. Fortunately, many Ellenburger reservoirs are dolomitized, and the dolomite promotes preservation of pores into the very deep subsurface (>7,000+ m).