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The distribution of oil fields in the Illinois basin overlaps but extends considerably northward from the area where the New Albany Group, the basin's most prolific source rock, has generated oil. Oils from reservoirs in Silurian strata from central and western Illinois can be correlated geochemically and isotopically to bitumen from the New Albany near the southern tip of Illinois. We believe that these oils migrated long distances, perhaps 100 km (62 mi) or more from the south or southeast. Migration probably occurred late in the basin's history, after basin sediments were fully compacted, in response to a regional northward flow system set up by tectonic uplift in the south. The hydrodynamic drive for migration during this period was comparable in magnitude to the comp nent of buoyant force acting along migration pathways; both hydrodynamic and buoyant forces contributed to migration. Migration efficiency and velocity are greatest when oil saturates a small part of the carrier bed's porosity. Capillary forces can aid migration by segregating oil into heterogeneities such as especially porous laminae, fractures, or karst channels in carrier beds. We suggest that the oil migrated through Devonian and Silurian carbonates along the weathered and karstified surface of a regional unconformity that formed during Kaskaskia deposition.
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