STRATIGRAPHY
midwest stratigraphy image
(after Ojakangas et al., 2001)

EXPLORE FOR MINNESOTA GAS

The State of Minnesota is encouraging the growth of economically significant and environmentally responsible resource industries in the State. The Minnesota Minerals Coordinating Committee therefore is promoting awareness of the potential for natural gas production from the Midcontinent Rift in Minnesota. Recent world-class natural gas discoveries in Siberia have demonstrated the potential in a similar setting.

Overview: The U.S. Geological Survey National Oil and Gas Assessment (Palacas, 1995) indicated that the hypothetical Precambrian Midcontinent Rift System Play consists of possible oil and gas accumulations in structural and stratigraphic traps within the 800-mile long Midcontinent Rift System. This 1.1 billion year-old rift extends from Kansas to Michigan.

image of midcontinent rift(after Ojakangas et al., 2001)

Potential reservoirs: Primary targets would be fluvial, deltaic, and shoreline sandstones of the Nonesuch Formation and the underlying upper Copper Harbor Conglomerate, both with porosities up to 13%. Fractured shales also occur in the Nonesuch Formation, and sandstones with porosities up to 18% occur in the overlying Freda Sandstone.

Source rocks: Nonesuch Formation shales up to 700 feet thick contain up to 3% total organic carbon by weight, and live oil seeps in Michigan confirm that liquid hydrocarbons have been generated in this formation. Kerogens are type II and type I, and moderately mature Tmax values of 435 to 440C have been determined. Although the organic matter is oil prone, gas is the more likely target due to the typical degree of thermal maturity.

Timing and migration of hydrocarbons: During extensional tectonism, the Nonesuch Formation and equivalents may have generated oil and gas, especially in the deeper portions of the basins, prior to compressional tectonism. In the shallower portions of flanking basins, a second phase of oil and gas generation probably occurred following deposition of Paleozoic sediments. In addition, hydrocarbons that might have accumulated during initial rifting may have re-migrated into structures formed during compression.

Traps: Dual stages of tectonism would have produced many trapping conditions, with varying styles of fault-related structures. Tectonic inversion may also have created structural features of varying scale that could contain large accumulations of hydrocarbons. Drag folds against reverse faulting offer multiple reservoir possibilities. It is likely that stratigraphic traps also occur.

Seals: Probable seals include shales of the Nonesuch Formation, as well as tight horizons in the overlying Freda Sandstone and Bayfield Group. Fault gouge may also account for some seals.

Exploration status: Very few wells have penetrated the lower Keweenawan Supergroup rocks that have the highest potential for hydrocarbon reserves. Drilling at sites from Kansas to Michigan was stimulated by increasing awareness of source rock potential and oil seeps, and also improved knowledge of large reserves in other rift basins such as the North Sea, Gulf of Suez, and Pripyat Basin, as well as other Precambrian terranes such as the Lena-Tunguska Petroleum Province of eastern Siberia, the Sichuan Basin of southern China, and the Huqf Group of Oman.

AVAILABLE AEROMAGNETIC AND GRAVITY DATA
(data and maps from the Minnesota Geological Survey)

magnetic anomaly map gravity map of mn

Resource potential: The rift is regarded as a high-risk play by the U.S. Geological Survey because few wells have been drilled, potential source rocks may be overmature, and reservoir porosities in some regions may be unfavorable. However, it is reasonable to speculate that source rocks may have more favorable levels of thermal maturity if present at shallower depths of burial along the basin flanks. Drilling depths would vary from 3,000 feet to as much as 25,000 feet. Broad, transverse-faulted medial volcanic horsts are bounded by high-angle faults and flanked by asymmetric sedimentary basins up to 30,000 feet thick, with basins up to 6,500 feet thick atop the horsts.

AVAILABLE SEISMIC PROFILING AND INTERPRETATION

seismic refraction image

rift geologic xsection image
(after Chandler et al., 1989)

References:

Chandler, V.W., McSwiggen, P.L., Morey, G.B., Hinze, W.J., and Anderson, R.R., 1989, Interpretation of seismic reflection, gravity, and magnetic data across Middle Proterozoic Mid-Continent Rift System, northwestern Wisconsin, eastern Minnesota, and central Iowa: American Association of Petroleum Geologists Bulletin, v. 73, no. 3, p. 261-275.

Ojakangas, R.W., Morey, G.B., and Green, J.C., 2001, The Mesoproterozoic Midcontinent Rift System, Lake Superior region, USA: Sedimentary Geology, v. 141-142, p. 319-341.

Palacas, J.G., 1995, Superior Province: U.S. Geological Survey National Oil and Gas Assessment, http://energy.cr.usgs.gov/oilgas/noga/index.htm.

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