Kathleen Counter Benison¹

(1) Central Michigan University, Mt. Pleasant, MI

ABSTRACT: The Influence of Acidic, Saline Groundwaters on Petroleum Reservoirs in the Permian Minnelusa and Opeche Rocks of the Northern Midcontinent

The Permian Opeche Shale of the northern midcontinent formed under extremely acidic conditions and serves as a caprock for petroleum aquifers in the underlying Permian Minnelusa Formation. The Opeche Shale consists of red shales and sandstones with some thin evaporite beds, as well as some displacive evaporites, representing a continental saline pan and distal alluvial system. The Minnelusa Formation, containing aeolian, oil-stained sandstones, siliciclastic and carbonate mudstones, and bedded and displacive evaporites, reflects more diverse depositional environments. Did the acidic, saline Opeche lake and ground waters occlude pores effectively enough to form an aquitard? Did these acid waters also play a role in secondary porosity formation in the Minnelusa Formation? Petrographic descriptions of ten cores of the Minnelusa Formation and Opeche Shale from western North Dakota and eastern Wyoming, Colorado, and Montana indicate localized versus regional acid influence on porosity formation and occlusion. This is among the first studies linking ancient acidic, saline waters to petroleum migration and entrapment.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado