Maturity of kerogen, petroleum generation and the application of fossils and organic matter for paleotemperature measurements

The origin, composition and maturity of kerogen, and its conversion to petroleum is reviewed. Various methods for estimating the maximum temperature to which kerogen and potential source rocks were subjected are also reviewed. Several kerogen maturation parameters can be distinguished and grouped into two categories: chemical paleothermometres and biological paleothermometres. Biological paleothermometres are largely based on the colour and optical properties of kerogen and fossils with an organic composition or with an organic component in their mineralized skeleton. The consistent changes in colour and reflectance through thermal gradients (burial depth) have made vitrinite, conodonts, graptolites, scolecodonts, acritarchs, spores and pollen, invaluable for maturation studies and prediction of hydrocarbon windows. Spores, pollen and vitrinite are frequently used as thermal indicators in lacustrine and near-shore marine strata ranging in age from the Devonian to Recent. Spores and pollen can be used to assess the thermal alteration at low temperatures, whereas vitrinite can be used to estimate the degree of maturation of organic matter at low to high temperatures, even to the graphite-grade metamorphic facies. Conodonts are most common in carbonate facies and used as thermal indicators of marine strata ranging in age from the Cambrian to the uppermost Triassic They can provide information about thermal alteration at temperatures as high as 500 °C, a temperature far above the destruction of unicellular palynomorphs. Graptolites are most common in fine-grained clastics of Ordovician and Silurian age, i.e. in sedimentary rocks in which pollen, spores and vitrinite are virtually absent. Scolecodonts are common in Ordovician–Devonian shallow marine carbonates and show optical properties similar to those of graptolites and vitrinite.