Abstract: The Mid-Meramecian Unconformity--Relevance to Development, Destruction, and Compartmentalization of Madison Reservoir: Elk Basin Field and Sheep Mountain Anticline, Big Horn Basin, WY

Mark D. Sonnenfeld

The Elk Basin Madison Unit is often cited as a "karst-controlled" reservoir; however, Madison reservoir development should be viewed in three stages: (1) pre-unconformity diagenesis, principally fabric-selective dolomitization of presumed reflux origin that is best evaluated within the stratigraphic context of six 3rd-order sequences within the Madison Formation; (2) the direct consequences of meteoric diagenesis associated with a 2nd-order mid-Meramecian unconformity; (3) post-unconformity diagenetic and tectonic overprints. At Elk basin and Sheep Mountain Anticline (a breached reservoir comparable in size to Elk basin) unconformity development at the mid-Meramecian unconformity was incidental to ultimate porosity creation yet central to porosity destruction and reservoir compartment lization.

Five sequence boundaries within the Madison are selected at accommodation "turnarounds" representing the culmination of thinning-upward trends among high-frequency cycles. Early meteoric lithification and subtle Mississippian tectonics resulted in a fracture-controlled karst on top of the Madison that channeled water into regionally widespread solution collapse zones and cave systems within the Madison that were stratigraphically controlled by soluble evaporitic zones and/or argillaceous aquitards overlying sequence boundaries II-V, rather than occurring at various unconfined water-table stillstands. Each of these argillaceous, formerly evaporitic intervals accumulated before re-establishment of normal marine conditions and is interpreted as a lowstand/transgressive phase when sedimen ation rates kept up with increasing accommodation. Evaporite solution collapse breccias presently form partial to complete barriers to vertical fluid flow depending on thickness and degree of associated argillaceous influx.

By the end of 2nd-order karsting nearly all porosity within Madison limestones had been occluded by calcite spar; however, fracture, breccia, and cavernous porosity, together with intercrystalline and skeletal moldic porosity within dolomites remained. Karst-related(?) fractures provided pathways for later dolomitization resulting in erratic, non fabric-selective, yet often highly permeable pay within open marine skeletal packstones and grainstones. Although almost all depositional anhydrites within the Madison were dissolved during mid-Meramecian karsting, the northern Big Horn Basin experienced a "late" phase of anhydrite-plugging that occludes most fracture, breccia, cavernous, and moldic macroporosity while partially plugging intercrystalline porosity within dolomitic paleoaquifer .

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994