THE ROLE OF OVERPRESSURE IN THE RETARDATION OF ORGANIC-MATTER MATURATION

Abnormal profiles of organic matter maturation in overpressured sequen ces, which express the retardation of maturation, are a worldwide phen omenon; however, the cause of this retardation has not been explained satisfactorily. Clearly, the different types of pressure in an overpre ssured regime must be identified in order to determine their role in t he retardation of organic matter maturation. Differential pressure is shown to have a marked influence on the maturation of organic matter t hrough its effects on the thermal properties of the source-rock system . Vitrinite reflectance data from several wells in basins offshore NW Europe have been used to calibrate source-rock maturation against diff erential pressure and ''effective temperature '' (i. e. observed tempe rature in an overpressured sequence corrected to the temperature at th e same depth in a normally-pressured sequence). High correlation coeff icients with vitrinite reflectance have been measured for both relatio nships. Differential pressure is a determining factor in porosity redu ction. Temperature and porosity affect the thermal conductivity of a r ock in both overpressured and normally-pressured sequences according t o established formulae. Cumulative heat flow, the product of observed temperature and thermal conductivity, is an important value which is a ffected indirectly by differential pressure. A close linear correlatio n between cumulative heatflow and vitrinite reflectance is proved. Cum ulative heat flow can be related to thermodynamic properties of source -rock systems, and it can be compared to the enthalpy of a system or t he internal energy of a reaction. In overpressured sequences, conditio ns resemble those of constant pressure and constant volume reactions. As a result, cumulative heatflow can be taken as a measure of the fina l state of a reaction, corresponding in this discussion to the level o f vitrinite reflectance. So it can be used as a guide to the state of source-rock maturation in both normally-pressured and overpressured se quences.