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Iowa Department of Natural Resources, Geological Survey Bureau, Iowa City, Iowa

Abstract: Petroleum Potential of the Middle Proterozoic Midcontinent Rift System (MRS) in Iowa

The MRS, a billion year old failed rift that extends from Lake Superior to Kansas, is characterized in Iowa by a central horst, dominated by mafic volcanics, and deep, clastic-filled flanking basins. These basins, which reach a maximum model depth in excess of 12,000 m, include a basal clastic stratigraphic sequence that is roughly correlative with the Oronto Group of northern Wisconsin. Unit C, the middle unit of this basal clastic sequence, Is a dark shale equivalent to the Nonesuch Shale, and 586 m of this unit were penetrated by the Amoco M.G. Eischied #1 well at a depth of 5,400 m. Similar dark shales were also encountered in cores from the Manson Impact Structure, and, although an intact sequence was not penetrated, the abundance of this litholog suggests that a relatively thick sequence of Unit C rocks originally existed at that location. The presence of thick Unit C shales in the Eischeid well, 7 km outside of the original axial rift graben, and a similar thick sequence at the Manson Structure, 25 km from the graben, implies a widespread original distribution of this unit, deposited in a water body at least 150 km wide and probably extending along most of the 1450 km length of the feature.

Unit C in the Eischeid well is now over-mature and is only a marginal source rock (Tmax 497° - 508°C, TOC avg. 0.13 - 0.6 % genetic potential avg. 0.22 mg HC/g rock) that reached peak oil generation about 800 Ma. However, the Eischeid rocks originally lay in an area of high heat flow from the voluminous volcanic and plutonic rocks in the active rift graben. Rocks in more distal locations would not have been subjected to these high heat conditions and would likely have released their petroleum much later. The most likely place to recover MRS petroleum would be in the Keweenawan fluvioclastics that overlie Unit C and at some distance from the over mature rock near axis of the rift.

AAPG Search and Discovery Article #90921©1999 AAPG Mid-Continent Section Meeting, Wichita, Kansas