J.G. Palacas, J.W. Schmoker, T.A. Daws, M.J. Pawlewicz, and R.R. Anderson

Iowa Department of Natural Resources, Geological Survey Bureau
Special Paper Series No. 2, 1990, p. 119-134
The Amoco M.G. Eischeid #1 Deep Petroleum Test, Carroll County, Iowa
R.R. Anderson, ed.


A petroleum source-rock assessment has been made of the thickest section (14,100 ft; 4,298 m) of Precambrian sedimentary rocks sampled anywhere along the Midcontinent Rift System. Assessment is based on analysis of 40 core and cutting samples from the 17,851-foot (5,441 m) deep Amoco M.G. Eischeid #1 well, which was drilled in 1987 in a half-graben-like basin northwest of the medial horst, near the town of Halbur, Carroll County, Iowa. Underlying a 3,600-foot (1,100 m) thick Phanerozoic section of sedimentary rocks, the Precambrian rocks encountered consist of a 6,900-foot (2,103 m) thick Upper "Red Clastic" Sequence that resembles rocks of the Middle Proterozoic Bayfield Group in Wisconsin and a 7,200-foot (2,195 m) thick Lower "Red Clastic" Sequence broadly correlative to the Middle Proterozoic Oronto Group of Wisconsin. The Oronto and Bayfield groups are part of the Keweenawan Supergroup.

All of the rocks in the Upper "Red Clastic" Sequence and 80 percent of the rocks in the Lower "Red Clastic" Sequence are extremely lean in hydrocarbon-generating organic matter [total organic carbon (TOC) contents less than 0.1 percent and genetic potentials (S1 + S2 ) less than 0.1 mg HC/g rock], strongly implying no petroleum source-rock potential now or in the geologic past. In sharp contrast to the above predominantly red, reddish brown, and green clastic rocks, a darker colored section, continuing a cumulative thickness of 200 to 300 feet (60-90 m) of thin interbeds of medium- to dark-gray, and partly black, pyrite-bearing, laminated shale, occurs in the Lower "Red Clastic" Sequence at depths between 15,000 and 16,425 feet (4,570-5,006 m). This darker colored section is believed to be stratigraphically equivalent to the Nonesuch Shale of the Oronto Group of northern Michigan and Wisconsin.

These laminated shale beds are thermally overmature with respect to oil generation and are characterized by TOC contents as much as 1.4 percent (average 0.6), genetic potentials (S1 + S2) range from about 0.1 to 0.4 mg HC/g rock, hydrogen indices range from about 20 to 80 mg HC/g TOC, and extractable bitumen contents range from about 10 to 60 ppm. At present, these shale beds have no potential of generating significant amounts of hydrocarbons in the geologic past. Furthermore, we speculate that equivalent shale facies might have fair to good petroleum source-rock potential if present at shallower depths of burial, and subject to lower levels of thermal maturity. The area along the basin flanks, away from the frontal fault zone of the medial horst offers a more favorable situation for petroleum potential.