Pressure retardation of vitrinite diagenesis, offshore north-west Europe

Abstract

PHYSICO-CHEMICAL processes that increase coal rank also increase vitrinite reflectance, or the level of vitrinite diagenesis. The principal physical factors controlling these processes have been identified as geothermal gradient, thermal conductivity of lithological section, duration of heating, and pressure^1,2. Interaction of these factors on coalification has not been demonstrated conclusively in nature or by laboratory experiments, but temperature and time have been generally regarded³ as most influential. Although the retarding effect of pressure has been widely recognised, the role of pressure is not clearly understood and has been considered³ negligible. Here the same values of mean vitrinite reflectance in oil (R_mo%) have been related to present temperature, (T), and to a function of the present temperature and the ratio of the gradients of normal hydrostatic pressure (P_NH) and observed pore pressure (P_P), T(γP_NH/γP_p), in Fig. 1a and b respectively. Comparison of correlation coefficients of Fig. 1a and b demonstrates that, for values of R_mo less than 1.50%, retardation of vitrinite diagenesis by abnormally high formation pressure, is both significant and predictable. As a corollary, the time factor seems to be of limited significance on a geological scale.