Changes

Karstified landscapes and paleocave systems form an important class of carbonate reservoirs. Caves present within a limestone bedrock are liable to collapse on compaction, creating a collapse breccia and with associated fracturing of the roof rock. Not all caves fall in with increasing burial; some can survive. When these are penetrated during drilling, the bit can suddenly drop several meters and large losses of drilling mud into the cave system can ensue.

Karstified landscapes and paleocave systems form an important class of carbonate reservoirs. Caves present within a limestone bedrock are liable to collapse on compaction, creating a collapse breccia and with associated fracturing of the roof rock. Not all caves fall in with increasing burial; some can survive. When these are penetrated during drilling, the bit can suddenly drop several meters and large losses of drilling mud into the cave system can ensue.

Numerous cycles of cave formation and subsequent collapse can result in coalescing collapsed cave systems of considerable size, typically hundreds to several thousands of meters across. These systems may be mappable on 3-D seismic data. Collapse and sag structures form circular karst features that may be discernable from amplitude displays (Loucks, 1999).

Numerous cycles of cave formation and subsequent collapse can result in coalescing collapsed cave systems of considerable size, typically hundreds to several thousands of meters across. These systems may be mappable on 3-D seismic data. Collapse and sag structures form circular karst features that may be discernable from amplitude displays.<ref>Loucks, R. G., 1999, [http://archives.datapages.com/data/bulletns/1999/11nov/1795/1795.htm Paleocave carbonate reservoirs: Origins, burial-depth modifications, spatial complexity and reservoir implications]: AAPG Bulletin, v. 83, no. 11, p. 1795–1834.</ref>

Paleocave systems contain some very large hydrocarbon accumulations, such as the Lower Ordovician Puckett field in west Texas (Loucks and Anderson, 1980), in the Permian Yates field in west Texas (Craig, 1988), and in the Lower Cretaceous Golden Lane fields of eastern Mexico (<ref name=V-O />; Coogan et al., 1972).

Paleocave systems contain some very large hydrocarbon accumulations, such as the Lower Ordovician Puckett field in west Texas,<ref>Loucks, R. G., and J. H. Anderson, 1980, Depositional facies and porosity development in Lower Ordovician Ellenburger dolomite, Puckett field, Pecos County Texas, in R. B. Halley and R. G. Loucks, eds., Carbonate reservoir rocks: SEPM Core Workshop 1, p. 1–31.</ref> in the Permian Yates field in west Texas,<ref>Craig, D. H., 1988, Caves and other features of Permian karst in San Andres dolomite, Yates field reservoir, west Texas, in N. P. James and P. W. Choquette, eds., Paleokarst: Berlin, Springer-Verlag, p. 342–363.</ref> and in the Lower Cretaceous Golden Lane fields of eastern Mexico.<ref name=V-O /><ref>Coogan, A. H., D. G. Bebout, and C. Maggio, 1972, [http://archives.datapages.com/data/bulletns/1971-73/data/pg/0056/0008/1400/1419.htm Depositional environments and geologic history of Golden Lane and Poza Rica trend, Mexico, an alternative view]: AAPG Bulletin, v. 56, no. 8, p. 1419–1447.</ref>

Karst and paleocave reservoirs can show poor recoveries. Fracture production is common, and the recovery is sensitive to the nature of the fracture framework. The better reservoirs have a fracture system that connects to an aquifer with a water drive operating. However, overproduction of these systems is detrimental to recovery because this will result in rapid water breakthrough and an early production decline.<ref name=S&S />

Karst and paleocave reservoirs can show poor recoveries. Fracture production is common, and the recovery is sensitive to the nature of the fracture framework. The better reservoirs have a fracture system that connects to an aquifer with a water drive operating. However, overproduction of these systems is detrimental to recovery because this will result in rapid water breakthrough and an early production decline.<ref name=S&S />