Temperature effects on kerogen and on molecular and isotopic composition of organic matter in Pierre Shale near an igneous dike

Abstract

A suite of siltstone samples from the Upper Cretaceous Pierre Shale from the contact zone of a 130-cm thick igneous dike near Wolcott, Colorado, U.S.A., was taken from the contact to 170 cm from the dike to study the effects of temperature on the organic matter. The sampled bedding interval was about 10 cm thick, so variation in lithology and type of organic matter is minimal. Vitrinite reflectance values ( R 0) increase from 0.4 far from the dike, to 3.3% near the dike contact. Geochemical measurements show systematic thermal effects analogous to those often observed for catagenesis and metagenesis in the depth range of 1–4 km within a sedimentary basin. The H/C ratio of kerogen and the hydrogen index (Rock-Eval) decrease most rapidly in the 0.6–1.7% R 0 range, in which the transformation ratio (Rock-Eval) increases from 0.1 to 0.3. Based on extraction of C 15+ compounds, the main increase of hydrocarbons and total extractable organic matter occurs between 0.6 and 1.0% reflectance. The saturated/aromatic hydrocarbon ratio increases almost twofold in this range of maturity. However, the pristane/phytane ratio is essentially constant through the hydrocarbon generation zone but decreases slightly at high levels of thermal alteration ( R 0 > 1.2%). The δ 13 C values for aromatic and saturated hydrocarbons are about −27 and −29‰, respectively, and are constant to about 1.0% R 0, then both become heavier by about 2‰ at higher R 0 values.