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Overview of Paleocave Systems
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
Although paleocave systems
are an important class, they are a complex type of carbonate reservoir.
Systems
may range from isolated, single, cave passages to coalesced, collapsed-paleocave
systems
composed of numerous passages and associated deformation. Most major collapsed-paleocave reservoirs, such as the Lower Ordovician Ellenburger reservoirs in West Texas, are associated with large-scale coalesced
systems
.
Large-cave systems
commonly form at composite unconformities in carbonates. The general pattern of cave
systems
may reflect previously established regional fault, fracture, and bedding-plane patterns. Cave
systems
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 pore
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 systems
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
systems
are major aquifers in a background of generally low-porosity carbonate strata.
Above the coalesced, collapsed-paleocave system, several thousand feet of strata can be crackle-brecciated, folded, and faulted (suprastratal deformation;
Figure 3). The result is a 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 systems
, therefore, affects not only the host-rock strata, but also, in many cases, as much as several thousand feet of overlying strata. The
pore
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
pore
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
systems
are prime exploration targets. Modified from Loucks (1999).