ABSTRACT: Porosity and Organic Geochemistry of Precambrian Midcontinent Rift System, Iowa--Petroleum Potential of One-Billion-Year-Old Rocks

James W. Schmoker, James G. Palacas

The Amoco Eischeid #1 well was drilled in 1987 approximately 75 mi (120 km) west-northwest of Des Moines in Carroll County, Iowa. The well penetrated about 3600 ft (1100 m) of Cambrian and younger strata, 14,100 ft (4300 m) of Precambrian (Middle Proterozoic) unmetamorphosed shales, siltstones, and sandstones of the Midcontinent Rift System, and over 100 ft (30 m) of gabbro before reaching total depth of 17,851 ft (5441 m).

Most of the Precambrian sedimentary section of the Eischeid well is oxidized and has no hydocarbon source-rock potential. However, gray to black shales, possibly equivalent to the Nonesuch Formation of northern Michigan, are common between 15,000 and 16,400 ft (4570-5000 m). In these shales, TOC averages 0.6% and is as high as 1.4%, the hydrogen index ranges from 20 to 80 mg HC/g TOC, and genetic potential (S₁ + S₂) varies from 0.1 to 0.4 mg HC/g rock. These shales are overmature and have little remaining capacity for hydrocarbon generation but could have generated petroleum in the geologic past.

Arkosic sandstone is common in the Eischeid well. Sandstone porosity, as determined from density and neutron logs, reaches 10% at about 5000 ft (1520 m) in the upper part of the Precambrian section,

but ranges between 1 and 6% and averages only 2.3% below 12,000 ft (3660 m). Most porosity visible in thin section is secondary. Some sandstones contain abundant feldspar and have relatively high volumes of replacement carbonate and intergranular carbonate cement. Such lithologies have the potential to develop substantial secondary porosity, but this has not happened at the location of the Eischeid well.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990