Triaromatic dinosteroids – Isomeric distributions and their geochemical significance

The distribution of triaromatic dinosteroids in a range of oils and some source rock bitumens was determined by GC–MS selected ion monitoring and GC–MS-MS in order to investigate possible ambiguity in the observed distribution of isomers, including those related to maturation and alteration processes commonly affecting oil accumulations. The results confirm that the eight possible isomers for the triaromatic dinosteroids and their 28-nor equivalents (triaromatic 23,24-dimethylcholesteroids) in post-Paleozoic samples are consistently represented by six peaks on commonly used GC stationary phases, although the fifth eluting is broadened and probably represents a pair of isomers. The presence of a seventh, early eluting, triaromatic dinosteroid isomer occasionally encountered in m/z 245 mass chromatograms in the literature was not substantiated. Closely co-eluting compounds can give the seemingly misleading impression that one or two triaromatic dinosteroid isomers (including the last eluting) are present at low levels in pre-Mesozoic samples and in younger oils from dominantly higher plant sources, so care is required in inferring source age for such samples. The relative abundance and retention time pattern observed for the triaromatic dinosteroids is duplicated by their 28-nor analogues. Triaromatic dinosteroid isomer distributions vary little throughout the oil window, based on data from Norwegian North Sea oils representing a range of mean maturities. Overall abundance of the series seems to decline in a similar way to other biomarkers towards the end of the oil window. In source rock bitumen corresponding to maturities of ≤0.5% vitrinite reflectance, when diasterenes are still abundant, the second eluting triaromatic dinosteroid peak was enhanced and the fifth depleted in the limited set of samples studied (Late Jurassic marine shales from the Norwegian continental shelf). Both dinosteroid series appear particularly resistant towards biodegradation and metamorphism, with no obvious change in isomeric distributions compared to unaltered oil. As a consequence, their relative abundances, as represented by age parameters that ratio each series to their 24-ethyl counterparts, increase when alteration is severe. Lacustrine oils can be difficult to date because post-Paleozoic samples do not always contain detectable dinosteroids and, when they are present, they can vary significantly in abundance. Unambiguous age resolution at the Period level was not possible for the Triassic-Tertiary oils studied, neither could depositional environments be distinguished with confidence.