The Evolution of Cave Systems from the Surface to Subsurface

Robert G. Loucks and C. R. Handford

Many 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. Cave sediment is 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 breccias. These piles can be tens of meters thick and contain large voids and variable amounts of matrix. Cave-roof crackle breccia forms from str ss- 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 decreases. Large cavities collapse forming burial chaotic breakdown breccias. Differentially compacted strata over the collapsed chamber fracture and form burial cave-roof crackle breccias. Continued burial leads to more extensive mechanical compaction causing previously formed clasts to fracture and pack closer together. The resulting product is a rebrecciated 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 episodes of fracturing and recciation. The resulting collapsed-paleocave reservoir targets are not single collapsed passages of tens of feet across, but are homogenized collapsed-cave systems hundreds to several thousand feet across.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California