Clay Minerals as Geothermometers and Indicators of Thermal Maturity--Application to Basin History and Hydrocarbon Generation: ABSTRACT

Abstract

The conversion of smectite to illite is the major clay-mineral reaction that occurs in sedimentary rocks during both long-term progressive burial and much shorter lived geothermal systems. Changes in composition and degree of ordering of mixed-layer illite/smectite (I/S) clay minerals have been used as geothermometers and as indices of thermal maturity. Whether these changes are interpreted by the mixed-layer or the fundamental particle concept, the reaction results in a progressive and irreversible increase in illite and corresponding decrease in expandability. These changes in illite and expandability are commonly defined qualitatively and quantitatively by x-ray powder diffraction profiles. Although clay diagenesis in well profiles appears to be a function of depth, reactions are controlled primarily by temperature. Because most measures of organic-matter maturation and hydrocarbon generation models assume time-temperature calculations, time at temperature should also be considered when clay diagenesis models are applied to hydrocarbon exploration.