VOID COLLAPSE AS RELATED TO DISSOLUTIONAL MEGAPOROSITY

Collapse of karstic voids at depth results in the formation of highly fractured regions that are commonly breccia filled. These collapse features have high porosity and permeability. Previous research assumes paleokarst breccias are the result of the collapse of epigene caves. This research differs from previous research in that it examines epigene, hypogene and flank margin caves as potential sources of paleokarst breccias and hence paleokarst reservoirs. Hypogene caves are likely to be preserved in the rock record because they form at depth, within the mesogenetic environment. Flank margin caves are also likely to be preserved because they develop in carbonate depositional environments where continued deposition and isostasy lead to burial and preservation. Preservation of epigene caves as paleokarst requires cratonic adjustments of a major scale, a less likely occurrence.

The previous research conducted on modeling collapse with epigene cave morphologies, assumed hemispherical collapse patterns. This research presents new models and equations to better represent the morphologies of epigene, hypogene and flank margin caves.

Hypogene caves are modeled using a sphere. As the sphere collapses, its collapse footprint expands along three radii, creating a large brecciated footprint. Epigene caves, modeled with a horizontal cylinder, will collapse along a hemispherical radius and will integrate throughout the cave passage. Flank margin caves, modeled with a vertical disc shape, will only collapse in the vertical direction. The end result of each is a function of the accommodation space, which is based on the nature of the cave (cave volume); the more accommodation space, the larger the initial cave volume, and the larger the collapse footprint. For example, previous collapse equations result in a 347% increase in hemispherical radius at a 5% collapse porosity. The new epigene collapse equation results in a 535% radius increase, under the same conditions. Replacement of dense oil field brines by lighter hydrocarbons during reservoir maturation creates a buoyancy change that may trigger cave collapse and reservoir expansion.