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Mid-Continent Rift: New Frontier in an Old Area

Donald M. Davidson, Jr. Michael G. Mudrey, Jr.

The Mid-Continent rift (MCR) is a 2,000-km-long intracontinental feature of middle Proterozoic age (1.1 Ga) that extends from Kansas northeastward through the Lake Superior basin and then southeastward through the lower peninsula of Michigan. The MCR has a pronounced positive gravity and magnetic expression, except over the Lake Superior basin. We believe that rift-related marginal basins overlying axial basins and other structures associated with this feature may locally be prospective within four geographically identifiable rift segments: Kansas, Iowa, Minnesota, and Michigan.

Source is the most problematic aspect of exploration potential along this structure. Calculations of source rock availability lead us to predict ultimate recoverable reserves of less than 5 billion bbl of oil or 15 tcf of gas along the MCR. Source beds are probably restricted in areal extent, and they are old (Proterozoic-lower Paleozoic). Both organic and inorganic sources are possible.

Adequate volumes of reservoir rock have been identified in all rift segments. The most promising hosts are rift-related clastics of Proterozoic age, or more extensive basal Paleozoic sands that locally occur in marginal basins along and in sheets adjacent to the MCR. Migration is assumed to have occurred under normal hydrodynamic conditions (load) with close spatial proximity between source and reservoir units. Permeability problems related to diagenesis are not anticipated.

We postulate that both structural and stratigraphic traps probably occur. Fault-related structures vary in style within the various rift segments: rotated, half-graben in Kansas; overthrust and normal in Iowa; and high-angle reverse in Minnesota and Michigan. Unconformities involving Proterozoic and possibly Paleozoic units may occur in all segments. Seals along faulted structures are assumed to be gouge-related and their effectiveness will depend on the abundance and types of clay within individual faults. Post-rift fault displacements are generally believed insufficient to affect reservoir geometries considerably. Seals along unconformable structures are lithologic in nature resulting from differential capillary pore pressures between units.

Geochemical and vitrinite reflectance studies of available oil and natural gas samples from identified accumulations in the Minnesota part of the rift yield values in the mature to immature range. The degree of preservation will vary with sedimentational, erosional, and structural factors within the specific rift segments.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.