Concentrations of liquid hydrocarbons in modern sediments are only a few parts per million, but hydrocarbons in subsurface shales reach several thousand parts per million. The increase in hydrocarbons is the result of the alteration of insoluble organics called kerogens. The primary energy for this reaction is heat associated with burial. Significant hydrocarbon generation begins at 150°F, but liquid hydrocarbon destruction dominates at temperatures greater than 300°F. Thus, there is a liquid window between 150 and 300°F which encompasses the zone of oil occurrence.

Present geothermal gradients can be used only to predict organic maturity in young downwarping basins. Paleotemperatures can be estimated from kerogen. Electron spin resonance (ESR) of kerogen has been used to estimate paleotemperatures in the Mesozoic-Tertiary trends of the Gulf Coast. The data indicate that some rocks have been 50-100°F hotter than their present temperatures and that some present geothermally cool areas were "hot spots" in the past.

End_of_Article - Last_Page 1836------------