--> ABSTRACT: Petroleum Potential of Precambrian Nonesuch Formation, Mid-Continent Rift System, by G. B. Hieshima, D. A. Zaback, and L. M. Pratt; #91022 (1989)

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Petroleum Potential of Precambrian Nonesuch Formation, Mid-Continent Rift System

G. B. Hieshima, D. A. Zaback, L. M. Pratt

The Precambrian Nonesuch Formation crops out in northern Wisconsin and Michigan and is recognized as a possible petroleum source rock based on active oil seeps in copper mines of the White Pine area. Sedimentological and organic geochemical analyses of about 200 samples from outcrops, cores, and mine excavation were undertaken to evaluate the petroleum potential of the Nonesuch Formation.

Organic carbon (Corg) contents above 0.3% are restricted to thin intervals of finely laminated calcareous and noncalcareous silty shale. Corg for finely laminated samples ranges from 0.25% to 2.8% and averages 0.6%. Total sulfur (TS) values range from 0.4% to 2.7%. Using samples containing only framboidal pyrite, a positive covariance between Corg and TS is recognized, with an approximate CorgTS ratio of 0.5. This value is significantly higher than the Goldhaber/Kaplan, Gautier, or Tuttle ratios for Phanerozoic shales.

Nonesuch bitumens are alkane rich and depleted in asphaltenes. Gas chromatography of extracts shows a decreasing distribution of alkanes from C10 to C30 with a slight enrichment of n-C17 and n-C19. Pristane is abundant in several samples, with pristane/n-C17 ratios approaching 1.0. Pristane/phytane ratios range from 2.5 to 6.0 and average 4.1. Biomarkers suggest that the unit is within the middle part of the oil window. If thicker sections of the organic-rich facies are present in the southern extension of the rift or downdip of the outcrop belt under Lake Michigan, the petroleum source potential of the Nonesuch should be good.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.