Abstract

Clay minerals provide information on the burial and thermal history of sedimentary rocks that is useful in the exploration, evaluation, and production of hydrocarbons. In particular, changes in clay minerals may be used as geothermometers that record temperatures corresponding to various stages of hydrocarbon generation.

The most important diagenetic clay mineral reaction in sedimentary rocks is the progressive transformation of smectite to illite via mixed-layer illite/smectite (I/S). Changes in the expandability and ordering types of I/S correspond to changes in temperature due to burial. Although the smectite-to-illite reaction may be influenced by several factors, it appears to be most strongly controlled by temperature in burial diagenetic settings from about 2 to 300 Ma.

Two simple temperature models for the I/S geothermometer may be applied here to basin history and petroleum studies. One model is referred to as the “short-life geothermal” model and is applied to studies where relatively young basins are heated for short periods (commonly <2 m.y.). These basins typically contain rocks younger than Miocene in age. In these young rocks, the temperature indicated for the change from random to short-range ordering of I/S is about 135-140°C. The othermodel, referred to as the Hoffman and Howermodel, is more commonly utilized in basin and petroleum studies because it is applied to long-term burial diagenetic settings and includes rocks and basins from about the Miocene through the Paleozoic. In the Hoffman and Hower model, the temperature for the change from random to short-range I/S is about 100-110 °C which corresponds to the onset of oil generation in rocks Tertiary through Cretaceous age. Analysis of I/S from surface and well samples, thus, provides a tool for evaluating both thermal history and maturity of sedimentary basins and potential hydrocarbon source rocks.