Abstract: Stratigraphic Hierarchy as a Key to a Genetic Classification of Paleokarst Heterogeneity Styles

KERANS, CHARLES, M.D. SONNENFELD, F. JERRY LUCIA

Paleokarst reservoirs present a unique set of challenges for both exploration and reservoir management. Extreme lateral heterogeneities in reservoir quality, including both total porosity and permeability, result in high levels of uncertainty in estimating reserves, recovery factors, and the drainage area and relative success of infill wells. Classifying paleokarst reservoirs in terms of (1) unconformity rank (2nd, 3rd, or 4th order), (2) icehouse-greenhouse setting, and (3) tectonic setting provide the basis of a genetic classification of karst systems that will aid in categorizing existing data on paleokarst reservoirs and assist in prediction of reservoir heterogeneity styles.

Paleokarst reservoirs developed at 2nd order supersequence boundaries are characterized by low matrix porosity, complex, commonly multitiered collapse breccia deposits, and a poor correlation between porosity profile and the bounding unconformity. In addition, paleokarst distribution at 2nd-order boundaries typically shows little relationship to paleogeography of the host carbonate. End-member styles within this category include the Lower Ordovician Ellenburger Group and lower Carboniferous Madison Formation.

The Madison's principal collapse-brecciated reservoir interval (24% of OOIP at Elk Basin field) resulted indirectly from dissolution of a formerly evaporite-rich aggradational interval (TST) overlying a 3rd order sequence boundary. Preferential dissolution of evaporites coupled with the role of underlying argillaceous dolomudstones (HST) acting as a basal aquitard generated regionally extensive collapse breccias that follow stratigraphy rather than unconfined paleowatertables. Nearly complete dissolution of the evaporites left few if any cave pillars. Lateral heterogeneity is a product of the undulatory thickness of collapse and roof breccia facies, coupled with vertically oriented breccias which have stoped upwards to 80 m probably at intersections of Mississippian fractures. Ellenburger karst reservoirs occur in cave-roof breccias and in cave-floor breccias. Vertical segmentation created by tight cave-fill facies is a key heterogeneity in these reservoirs. Locally stacked paleokarst sequences extending up to 300 m below the unconformity surface can be interpreted as either lithology-controlled or multiple watertable positions rather than a product of a constant hydrologic regime being deflected by internal aquitards. Both these 2nd-order paleokarst systems show only indirect relationships between the karst development and the master unconformity surface.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria