Abstract: Paleocave Systems: Origins, Burial-Depth Modifications, Spatial Complexity, and Reservoir Implications

LOUCKS, ROBERT, ARCO Exploration and Production Technology, Plano, TX

Paleocave systems form an important class of carbonate reservoirs that are a product of near-surface karst processes and  later burial compaction and diagenesis. The origin and recognition of fractures, breccias, and sediment fills associated with paleocave reservoirs have been studied in both modern and ancient cave systems. This information was used to determine the general evolution of paleocave systems and their associated pore networks and spatial heterogeneities.

Heterogeneity within paleocave systems results from near-surface and burial processes. Near-surface processes include dissolutional excavation, clastic and chemical sedimentation, and collapse of cave walls and ceilings. Burial processes begin as the cave system subsides into the subsurface. During burial, remaining passages collapse and early formed breccia clasts are rebrecciated. Differential compaction of strata around and over collapsed passages produces fractures, crackle breccias, and mosaic breccias. The combination of near-surface and burial processes creates a complex reservoir with several scales of heterogeneity.

Economic paleocave reservoirs are commonly the product of coalesced collapsed-paleocave systems. The coalescing of several cave systems into larger, connected porosity zones results from a combination of multiple cave-forming episodes at composite unconformities and by collapsing and coalescing of cave systems during burial where surrounding host strata are brecciated and fractured. These processes create a spatially-complex reservoir hundreds to several thousand meters across and thousands of meters long, forming a much larger exploration target than would result from collapse of a single cave passage or system. The final size, pore-network types, and spatial heterogeneities are a product of the evolution of a cave system from its near-surface origin through burial.

This research on paleocave systems may be applied wherever carbonate deposits were exposed at composite unconformities. Identification of subsurface paleocave systems can be accomplished through the use of seismic, wireline image logs, and cores.

AAPG Search and Discovery Article #90912©1999-2000 AAPG International Distinguished Lectures