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

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.