Recognition of abnormal thermal histories of Jurassic source rocks from integrated analyses of vitrinite reflectance and organic matter colour / fluorescence

Abstract

By investigating three sedimentary horizons through the aureole of a Tertiary dyke intruding a Middle Jurassic sedimentary section in NE Skye, the thermal response rates of the various organic maturity indicators to short-term high temperature heating events were evaluated. A detailed assessment was conducted, of the reaction of vitrinite reflectance (VR), kerogen colour and fluorescence, to the short-lived high temperatures associated with the dyke. Image analysis (lA) techniques were applied to develop a new system for vitrinite reflectance measurement at Trinity College Dublin. Vitrinite reflectance shows a marked reaction to the high temperatures close to the dyke. Within a zone equivalent to ca. 50% dyke thickness (d.t.), rank changes from the high-volatile bituminous (0.72-0.79% Rr) through medium volatile bituminous, low volatile bituminous, semi-anthracite to anthracite (3.1-3.6% Rr) are observed. Reflectance reversals, previously reported adjacent to igneous intrusions, were not observed in any of the sample horizons. Peak palaeotemperatures in excess of 300°C, computed from the measured VR data, are indicated in the host rocks adjacent to the dyke contact.