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The Colmar-Plymouth oil field is the only significant field found so far in the northwesternmost Illinois basin. Since its discovery in 1914, it has produced more than 5 million bbl, and still produced almost 28,000 bbl in 1977. Intermittent, but persistent search for additional production since 1914, has been largely unsuccessful. Why have no other significant discoveries been made? Does reexamination of the region considering current concepts and conditions hold any hope for additional hydrocarbon production?

The reservoir at Colmar-Plymouth is the Hoing Sandstone, an isolated lens of well sorted, mature, Devonian shoreline sand. The shoreline borders the northern flank of the contemporaneous Sangamon arch. Oil in the field apparently originated in shale of the Upper Ordovician Maquoketa Group, which is exposed on the pre-Devonian unconformity beneath the reservoir. The field also lies almost on the crest of one of a series of broad northwest-trending structural noses crossing the arch. Consideration of these conditions provides our only geologic key to the Colmar-Plymouth conundrum.

One legacy of the dismal and largely unguided exploration history of the past 70 years in west-central Illinois, is the accumulation of a large mass of sociologically interesting data of unpredictable scientific value. Perceptive analysis and interpretation of these data might well lead to additional discoveries and have a certain entertainment value in its own right.

The shallow depth at which oil occurs in the area, the demonstrated longevity of production, and current market conditions, encourage exploration.

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