OVERVIEW OF PETROLEUM POTENTIAL OF PROTEROZOIC CLASTIC ROCKS OF THE MIDCONTINENT RIFT SYSTEM IN IOWA
R.R. Anderson and R.M. McKay
The Geological Society of America
27th Annual North-Central Section
Rolla, Missouri, March 29-30, 1993
1993 Abstracts with Program, v.25, no.3, p.3
Recent modeling suggests the presence of about 160,000 km3 of Proterozoic clastic sediments flanking the central horst of the Midcontinent Rift System (MRS) in Iowa. The Amoco M.G. Eischeid #1 deep petroleum test well, drilled in 1987, penetrated 4470 m of these clastic rocks, affording the opportunity to study the thickest penetration of MRS clastics. Among the seven units identified in the Eischeid well was a formation informally named Unit C. Unit C, encountered at a depth of 4497 m, is 441 m thick and is dominated by medium gray to black shales and mudstones and closely resembles the Nonesuch Formation of the MRS in the Lake Superior region. As such, Unit C represents the best potential source-rock in the Eischeid clastic sequence. The Eischeid well is located about 7 km west of the fault zone that bounds the MRS central horst. Since this same fault zone apparently also bounded the axial graben in the early stages of rift development, the Eischeid well penetrates rocks that were deposited outside of the central graben. Michigan exposures of the Nonesuch Formation were deposited within the axial graben. The presence of Unit C rocks outside of the central graben indicates that the body of water in which Unit C was deposited was widespread. Interpretation from petroleum industry seismic data suggests that the water body may have extended as much as 24 km beyond the limits of the central graben in Iowa, and may have been as wide as 100 km, running the length of the MRS. These interpretations are supported by the recent discovery of Proterozoic black shale (presumably Unit C rocks) associated with the crystalline basement rocks in cores on the Central Peak of the Manson Impact Structure. These black shales, observed in cores more than 25 km west of the horst-bounding faults, are the only MRS clastic units associated with crystalline rock of the central uplift. They have apparently overstepped all underlying MRS clastic rocks on the rift-basin margins. Although source-rock analysis of Unit C by Palacas and others (1990) indicated a maximum TOC of only 0.6% and indicated that the unit was overmature, the proximity of the Eischeid well to the inferred high heat flow associated with the igneous rocks in the central graben and the effects of hydrothermal alteration reported by Barker (1990) may have led to an early release of hydrocarbons. Similar source rocks at more graben-distal locations probably experienced subdued thermal regimes and may have released hydrocarbons at a much later time. Stratigraphic and structural traps above these more distal occurrences of Unit C may present appealing targets for petroleum exploration in future years.
Barker, C.E., 1990, Reconnaissance fluid inclusion study, Amoco Eischeid #1 well, Defiance basin, Midcontinent Rift System, Iowa, in Anderson, R.R., (ed.), The Amoco M.G. Eischeid #1 deep petroleum test, Carroll County, Iowa: preliminary investigations: Iowa Department of Natural Resources, Geological Survey Bureau, Special Report Series #2, p. 169-174.
Palacas, J.G., Schmoker, J.W., Pawleicz, M.J., Davis, T.A., and Anderson, R.R., 1990, Petroleum source-rock assessment of Middle Proterzoic (Keweenawan) sedimentary rocks, Eischeid #1 well, Carroll County, Iowa, in Anderson, R.R. (ed.), The Amoco M.G. Eischeid #1 deep petroleum test, Carroll County, Iowa: preliminary investigations: Iowa Department of Natural Resources, Geological Survey Bureau, Special Report Series #2, p. 119-134.