Blaine C. Cecil, Ronald W. Stanton, Eleanora I. Robbins
Effects of pressure on the diagenesis of organic material generally are assumed to be minimal. Current literature suggests that coal rank and hydrocarbon maturity are related primarily to maximum paleotemperatures and time. Such assumptions with respect to pressure do not recognize that temperature and pressure are dependent on thermodynamic variables whereas time is a factor in chemical kinetics.
During diagenesis of organic matter, low molecular-weight hydrocarbons are generated. Low hydrostatic pressure during diagenesis (an open thermodynamic system) allows volatile matter to escape; this enhances the probability of continued maturation of organic macromolecules. However, when hydrostatic pressure approaches or is equal to lithostatic pressure (a closed thermodynamic system), the escape of lightweight hydrocarbons is retarded; thus organic maturation is inhibited.
The effect of high hydrostatic pressure explains both the presence of oil at great depths in "overpressured" zones as well as anomalies in coal rank with depth and/or intensity of deformation.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC