AAPG Bulletin, V. 83, No. 11 (November 1999), P. 1795-1834.

Paleocave Carbonate Reservoirs: Origins, Burial-Depth Modifications, Spatial Complexity, and Reservoir Implications¹

Robert G. Loucks²
 

©Copyright 1999.  The American Association of Petroleum Geologists.  All Rights Reserved

¹Manuscript received December 5, 1997; revised manuscript received October 27, 1998; final acceptance November 5, 1998.
²ARCO Exploration and Production Technology, 2300 West Plano Parkway, Plano, Texas 75075; e-mail: bloucks@flash.net

ARCO Exploration and Production Technology supported this research. I appreciate Art Palmer, Peggy Palmer, and Robert Handford for introducing me to modern cave systems and to Ray Slay who supervised the cave photography. Julio Aguilar-Chang, Joachim Amthor, Alton Brown, Bill Belfield, Dave Entzminger, Ursula Hammes, Robert Handford, Charles Kerans, Julie Kupecz, Jerry Lucia, Paul Mescher, Art Palmer, Peggy Palmer, Bill Purves, and John Troschinetz contributed to many discussions on paleocave systems. Special thanks go to Alton Brown, Bill Belfield, Philip Choquette, Susan Longacre, Paul Mescher, Art Palmer, Peggy Palmer, Kim Patty, Mark Scheihing, Scott Tinker, and John Troschinetz for reviewing and making important suggestions to this paper. Julio Aguilar-Chang produced the three-dimensional seismic map and cross section. Art Palmer kindly supplied data on sizes of modern cave passages. 

ABSTRACT

Paleocave systems form an important class of carbonate reservoirs that are products of near-surface karst processes and later burial compaction and diagenesis. Features and origins of fractures, breccias, and sediment fills associated with paleocave reservoirs have been studied in modern and ancient cave systems. Information about such cave systems is used in this paper to reconstruct the general evolution of paleocave reservoirs and their associated scale, pore networks, and spatial complexities.

Spatial complexities in paleocave reservoirs result from near-surface and burial processes. Near-surface processes include dissolutional excavation, clastic sedimentation, chemical precipitation, and localized fracturing, brecciation, and collapse of cave walls and ceilings. Burial processes begin as cave systems subside into the subsurface. Remaining cave passages commonly collapse and early-formed breccia clasts are rebrecciated. Differential compaction of strata around and over collapsed passages produces fractures, crackle breccias, and mosaic breccias. Near-surface and burial processes combine to produce typically complex reservoirs with several scales of heterogeneity.

Hydrocarbon reservoirs of paleocave origin are commonly the product of coalesced collapsed-paleocave systems. The coalescing of passages in a cave system into larger, connected porosity zones results from a combination of multiple, cave-forming episodes at composite unconformities and from the collapse of cave systems during burial where surrounding host strata are brecciated and fractured. This combination of processes creates spatially complex reservoirs that can be hundreds to several thousands of meters across, commonly forming large exploration targets. Final size, pore-network types, and spatial complexities of coalesced collapsed-paleocave systems are products of their evolution from near-surface development through burial into the deeper subsurface. The coalesced collapsed-paleocave reservoir hypothesis explains the scale of reservoirs observed and the spatial complexities involved.