Alfred. H. Bell, 1934. "Origin of the Oil and Gas Reservoirs of the Eastern Interior Coal Basin in Relation to the Accumulation of Oil and Gas", Problems of Petroleum Geology, W. E. Wrather, F. H. Lahee
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The oil and gas reservoirs of the Eastern Interior Coal basin have been formed chiefly in four ways: (a) by the doming or arching of porous strata which are overlain by impervious strata, (b) by the deposition of lenticular bodies of porous sand surrounded at the edges and above by impervious beds, (c) by the beveling of interbedded porous and non-porous strata by an erosional unconformity above which lies an impervious stratum, and (d) by the local development of secondary porosity by subaerial erosion in limestone beds which were subsequently covered by impervious beds.
The Southeastern Illinois oil field which has produced by far the greatest quantity of oil in the region, namely, 400 million barrels from 92,000 acres, is selected for special discussion. This field has reservoirs of types a and b and possibly one or both of c and d.
Of the various hypotheses regarding the mode of origin and accumulation of the oil and gas in the reservoirs, that which best seems to fit the known facts is as follows:
Organic material deposited with the finer sediments which now surround the reservoirs was chemically altered to oil which entered the reservoirs, either before lithification or later through joints, fissures, or faults. Accumulation in the structurally high parts of the reservoirs took place by gravitational readjustment.
The hypothesis of Clark that oil pools result from exceptionally rich concentrations of organic material deposited with the reservoir sediments, with little or no subsequent lateral migration of the oil, does not hold for the Lawrence County (Illinois) field unless the original concentration of organic material was greatly in excess of any found by Trask in modern sediments of similar type.