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ABSTRACT: The Lake Superior Oronto Group, a Middle Proterozoic Exploration Model for the Late Proterozoic Chuar Group of the Grand Canyon

DICKAS, ALBERT B., University of Wisconsin-Superior, Superior, WI, and M. G. MUDREY, JR., Wisconsin Geological and Natural History Survey, Madison, WI

The Lake Superior Oronto Group and the Grand Canyon Chuar Group are the most significant Precambrian hydrocarbon targets within the conterminous United States. These frontier terrains share common Proterozoic age, comparable total organic carbon source rock values, association with Indian-interest properties, plus similarities in reservoir, trap, and maturation characteristics. Extensively studied since 1980, the exploration philosophy applied to the Oronto Group is presented as a model for Chuar Group hydrocarbon evaluation.

Hydrocarbon shows have been reported since 1852 from middle Proterozoic rocks of the Lake Superior basin. Occurrences include stains within stromatolitic facies of the Copper Harbor Conglomerate, live subsurface seeps within Nonesuch units in the White Pine copper mine of Michigan, and impsonite-like inclusions within calcite veins of the Freda Formation. These formations compose the Oronto Group, a synrift package infilling the Lake Superior basin of the mid-continent rift system. Seep analyses identify a low sulfur (0.02%), paraffinic (67%), 34 degree API crude indirectly dated (Rb/Sr) at a minimum of 1047 (+/-) 35 Ma.

Nonesuch Formation source shales are present within both central horst structures and flank half-grabens. Reservoir-quality criteria are associated with adjacent Copper Harbor and overlying Freda Formation units. Seismically identified traps range from anticlinal and drag folding to onlap, stratigraphic, and unconformity closures. The Lake Superior segment of the mid-continent rift system is subdivided into four structural units (I-IV). Association of oil seeps with stratiform copper deposits (unit III) suggests evidence of geochemical symbiosis. This crude entered oil-window status circa 1.0 Ga due to migrating cupriferous thermal systems.


AAPG Search and Discovery Article #91018©1992 AAPG Southwest Section Meeting, Midland, Texas, April 21-24, 1992 (2009)