IMPROVING OUR UNDERSTANDING OF PALEOKARST RESERVOIRS: EVALUATING EPIGENE AND HYPOGENE CAVE COLLAPSE

Paleokarst reservoirs in carbonate rocks develop from preserved epikarst, or as preserved subsurface caves. Epikarst and epigene caves requires post-depositional subaerial exposure of the carbonates to allow karst development; hypogene caves may form in subaerial carbonates but it is not a requirement if speleolegenesis is a result of fluid mixing at depth. Both epikarst and epigene caves require special circumstances to achieve burial and preservation; hypogene caves form in a preservational setting. Flank margin caves, a subset of hypogene caves, commonly form in carbonate depositional environments and preservation by burial is likely.

The paleokarst reservoir potential of caves is expanded by void collapse at depth, which expands the paleokarst footprint. Modeling of such collapse has been to date applied only to buried epigene caves, which have dispersed cave passage patterns commonly of a dendritic or branch-work form. These branch-work forms when collapsed only integrate together if the passages are proximal, and thus the collapse footprint is reduced. Hypogene caves have a ramiform or spongework pattern that modeling reveals yield a different and larger footprint following collapse given the more globular and proximal nature of the cave passages. Collapse occurs when burial creates stresses that lead to void roof failure; voids at depth are fluid filled which creates buoyant conditions that suppress collapse. Replacement of dense connate brines by light crude oil may be a collapse initiator; reservoir charging may enhance the paleokarst reservoir.

Successful characterization of paleokarst reservoirs requires understanding epigene versus hypogene speleogenesis; epigene paleokarst reservoirs should be rare and have a different footprint (linear and restricted) than the more abundant hypogene paleokarst reservoirs (globular and extensive). The role of fluid migration in reservoir development cannot be ignored.