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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
AAPG Bulletin, V.
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
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.
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