The influence of source depositional conditions on the hydrocarbon and nitrogen compounds in petroleum from central Montana, USA

Abstract

The hydrocarbon and pyrrolic nitrogen compound compositions of 21 crude oils from reservoirs in the Heath–Tyler–Amsden and Swift-Morrison formations of central Montana have been investigated by combined gas chromatography–mass spectrometry. The biomarker and aromatic hydrocarbon data enabled recognition of three composition types within the Heath–Tyler–Amsden oils. The Swift-Morrison oils displayed different hydrocarbon characteristics and are classified as a separate oil group. A number of geochemical parameters, e.g. dibenzothiophene/phenanthrene, C 20-triaromatic steroid hydrocarbon/(C 20 + C 28 triaromatic steroid hydrocarbons) indicate strong variations in petroleum composition from north to south across the region. Oils from the central Montana uplift with high relative C 27-diasteranes/C 29 ααα steranes and Ts/(Ts + Tm) suggests increasing clay contribution to the source, while abundant gammacerane indicates the development of hypersaline conditions with restricted circulation. Oils from the Bull Mountain Basin, represented by the Amsden oils, show relatively low C 27-diasteranes/C 29 ααα steranes and Ts/(Ts + Tm) ratios indicating increasing carbonate contribution to the source, while the decrease of a gammacerane contribution suggests decreasing salinity levels, which may explain the dominance of tricyclic terpane and hence their precursors to the source organic matter. While the changes recorded by the biomarker and aromatic hydrocarbon compositions show strong regional variations, the nitrogen compounds do not respond so strongly. However, weak trends coinciding with source carbonate content were apparent in the following ratios: 1,8-dimethylcarbazole/(1,8-dimethylcarbazole + 1-ethylcarbazole) and carbazole/sum of C 2-carbazoles. The second parameter may be responding to variations in salinity since it co-varies with the behaviour exhibited by the tricyclic terpanes. The source information that can be extracted from biomarkers and aromatic hydrocarbons suggests that primary migration and compositional fractionation during expulsion has exerted little influence on this fraction of petroleum. Meanwhile, primary migration and compositional fractionation during expulsion appear to effect the carbazole and benzocarbazole distributions thereby limiting their application as indicators for source rock facies composition.