Effects of weathering on biological marker and aromatic hydrocarbon composition of organic matter in Phosphoria shale outcrop

GC-MS analyses were performed on core samples collected from a shale outcrop of the Permian Phosphoria Formation in Utah, U.S.A., to study effects of weathering on selected biological marker and aromatic (phenanthrene) hydrocarbon compounds. Among the biological markers, the most important weathering effects are a decrease in the 20S20R">20S20R diastereomer ratio of the C₂₉ steranes and loss of low molecular weight triaromatic steroids. A decrease in the C₁₉ through C₂₂ tricylcic terpanes occurs relative to the total C₁₉-C₂₆ tricyclic fraction. Pronounced loss of methyl-substituted phenanthrenes occurs relative to phenanthrene. No major effect on the overall distribution of pentacyclic terpanes is evident.