Evaporite Paleokarst on a Tectonically-Enhanced Unconformity, Mississippian Madison Formation, Wyoming, USA

Abstract

Exploitation of hydrocarbons from strata with evaporite paleokarst development can be challenging due to tremendous permeability variability as a result of disrupted bedding, irregular pore types, allochthonous and autochthonous sediment fills and the development of persistent fractures throughout. Porosity networks and permeability barriers linked to evaporite paleokarst are critical elements of major hydrocarbon accumulations such as the Madison Formation of the Bighorn Basin. The extensive suprastratal deformation can create substantial permeability heterogeneity, directly juxtaposed to the dissolution zones themselves that commonly form low flow baffles or barriers. Despite these important and widespread characteristics, no systematic treatment of this style of carbonate reservoir heterogeneity exists and as a result of the “vanished” nature of the key controlling lithofacies, these systems are commonly controversial and poorly understood. The Upper Mississippian Madison Group offers a superb, if not spectacular, exposure of laterally continuous evaporite paleokarst zones. Many studies have described solution enhanced zones within the Madison. The focus of this study is the reservoir heterogeneity scale issues associated with this evaporite removal system. Thus we build upon the impressive regional syntheses available for this Mississippian platform and treat a limited number of key localities in Wyoming and Montana in some detail. Observations from these localities have led to a list of criteria for recognition of evaporite paleokarst that, while based on the Madison, are also present in other evaporite paleokarst systems. We interpret the timing of the paleokarst system, including the two distinct styles of paleokarst: end Madison subaerial exposure and the intrastratal solution collapse. We relate this to overall paleogeography and tectonic elements within the late Mississippian. Finally, we highlight important observations regarding reservoir architectural elements that have significant implications for hydrocarbon development, especially in the form of highly fractured strata above the evaporite removal zone.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015