LOUCKS, R. G., and C. R. HANDFORD, ARCO Exploration and Production Technology, Plano, TX

ABSTRACT: Paleocave-Reservoir Networks; Their Origin and Recognition

Many pore networks and permeability barriers in carbonate reservoirs are the result of cave-forming processes. The origin and recognition of fractures, breccias, and sediment fills associated with paleocaves were determined through the study of modern and paleocave systems. Cave formation and destruction are the products of near-surface and burial processes. Near-surface processes include solutional excavation, clastic and chemical sedimentation, and collapse of cave walls and ceilings. Initial fracture, breccia, and vug porosity develops during this stage. Cave sediment may either be derived from inside and/or outside the system. Depositional mechanisms include suspension, tractional, mass-flow, and rock-fall. Collapse of ceilings and walls form chaotic breakdown breccia. These piles can be tens of meters thick and contain large voids and variable amounts of matrix. Breakdown clasts may fracture upon impact forming crackled breakdown breccia. Subterranean fluvial and mass-flow processes can rework breakdown to form transported breakdown breccia deposits. Cave-roof crackle breccia forms from stress- and tension-related fractures in cave-roof strata.

As the cave-bearing strata subside into the subsurface, mechanical compaction increases and restructures the existing breccias and remaining cavities. Fracture porosity increases and breccia and vug porosity decrease. Large cavities collapse forming burial chaotic breakdown breccia. Differentially compacted strata over the collapsed chamber fracture and form burial cave-roof crackle breccia. Continued burial leads to more extensive mechanical compaction of the previously formed breakdown, thus causing clasts to fracture and pack closer together. The resulting product is are brecciated chaotic breakdown breccia composed predominantly of small clasts. Rebrecciated blocks are often overprinted by crackling. Subsurface paleocave systems commonly have a complex history with several episode of fracturing and brecciation.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.