ABSTRACT
The elevated organic maturity observed in shallowly buried units from the Michigan basin implies that higher temperatures and thicker overburdens once existed in the basin. Evidence from sediment-accumulation rates, regional dips, and maturity of Pennsylvanian-age coals suggests that up to 1,000 m (3,280 ft) of sediment were removed by erosion prior to the Late Jurassic, when the basin became stable. Geothermal gradients during Paleozoic basin subsidence probably ranged from 35° to 45°C/km (19° to 25°F/1,000 ft), in contrast to the average present value of 25°C/km (14°F/1,000 ft). Depths to the top of the oil window ranged from 1,900 to 2,300 m (6,230 to 7,540 ft) during the Paleozoic. Post-Pennsylvanian erosional uplift and further thermal maturation of the basin have combined to raise the top of the oil window to its present level of 500 m (1,640 ft).
