| dc.description.abstract | Stable nitrogen isotopic compositions of asphaltene and NSO hydrocarbon fractions from sixty-two crude oils derived from siliciclastic, carbonate/marl, hypersaline and lacustrine source rocks of known geologic age ranging from the Precambrian/ Cambrian to the Neogene were analyzed to determine whether the stable nitrogen isotope composition of crude oils record the secular change for δ15N that have been observed for kerogen and sedimentary organic matter (e.g. Stüeken et al., 2015 and references therein). The stable nitrogen isotope compositions of the sixty-two crude oils appear to correlate well with the kerogen and sedimentary organic matter from the literature. In addition, two trends in enrichments in 15N are observed for the Carboniferous/Permian and in the Miocene, resulting from incomplete denitrification. Three shifts in depletion in 15N are also observed in the Cambrian/Ordovician, Silurian/Devonian, and the Triassic through the Paleogene. The Cambrian/Ordovician depletion is thought to result from more oxic ocean waters driving nitrification and assimilation. The Silurian/Devonian, and the Triassic through the Paleogene depletions are thought to derive from anoxic ocean waters driving nitrogen fixation. Enrichments and depletions in 15N were independent of source rock type. Within the sixty-two crude oils, another study was conducted on 5 marine carbonate oils from Mexico and 4 marine carbonate oils from Middle East to determine the effects of thermal maturity on the stable nitrogen isotope composition of these oils. There was no systematic increase in δ15N values with increasing thermal maturity observed. | en_US |
