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Large-Scale Fracture and Breccia Development Associated with Paleocave Previous HitSystemsNext Hit and Associated Suprastratal Deformation

Robert G. Loucks
Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX

Collapsed paleocave Previous HitsystemsNext Hit and associated suprastratal deformation produce large scale brecciated and fractured bodies of rock. Previous HitSystemsNext Hit may range from isolated, single, cave passages to coalesced, collapsed-paleocave Previous HitsystemsNext Hit composed of numerous passages and associated deformation covering hundreds of square kilometers. Most major collapsed-paleocave reservoirs, such as the Lower Ordovician Ellenburger reservoirs in West Texas, are associated with large-scale coalesced paleocave Previous HitsystemsNext Hit.

Large-cave Previous HitsystemsNext Hit commonly form at composite unconformities in carbonates. The general pattern of cave Previous HitsystemsNext Hit may reflect previously established regional fault, fracture, and bedding-plane patterns. Cave Previous HitsystemsNext Hit begin producing cave-related breccias and fractures early in their history and continue to produce more breccias and fractures with burial (Figure 1). 

The paleocave system and associated Previous HitporeNext Hit network evolve with burial (Figure 2). Larger vugs and caverns are more common in the shallower subsurface, and fine interclast pores and crackle- and mosaic-breccia fractures are more common in the deeper subsurface. 

A striking feature of these buried, coalesced, collapsed-paleocave Previous HitsystemsNext Hit is the large-scale, intensely brecciated and fractured carbonate bodies that are formed by coalescing of the collapsed-paleocave system (Figure 3). These collapsed-paleocave Previous HitsystemsNext Hit are major aquifers in a background of generally low-porosity carbonate strata.

Above the coalesced, collapsed-paleocave system, hundreds of meters of strata can be crackle-brecciated, folded, and faulted (suprastratal deformation; Figure 3). The result is megasag, which is a response to the collapse and compaction of the paleocave system. These features can be mapped from seismic and wireline-log data. The collapse of cave Previous HitsystemsNext Hit, therefore, affects not only the host-rock strata, but also, in many cases, as much as several hundred meters overlying strata. The Previous HitporeNext Hit network associated with suprastratal deformation might form potential reservoirs under proper trapping and sealing settings.

Figure 1. Evolution of a cave passage with burial. Modified from Loucks (1999).

Figure 2. Evolution of Previous HitporeNext Hit types with burial. Modified from Loucks (1999).

Figure 3. Schematic diagram showing stages of development of a coalesced, collapsed-paleocave system. Multiple cave-system development at a composite unconformity may be necessary in order to produce a high density of passages. As the multiple-episode cave system subsides into the deeper subsurface, wall and ceiling rocks adjoining open passages collapse and form breccias that radiate out from the passage and intersect with fractures from other collapsed passages and older breccias within the system. The collapsed-paleocave Previous HitsystemsTop are prime exploration targets. Modified from Loucks (1999).

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