Effects of Sub-Aerial Weathering on Hydrocarbon Distributions in Oil Sands (Athabasca Oil Sand, Canada and Parana Basin, Brazil)

Sub-aerial weathering plays a significant role in the reduction of low molecular weight hydrocarbons in surface exposed oils. In order to investigate the detailed effects of sub-aerial weathering as opposed to sub-surface biodegradation, a suite of ten (10) oil sand samples were solvent extracted and analyzed in the laboratory using liquid column chromatography separation, gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). The gas chromatograms showed total loss of n-alkanes and isoprenoids while the ion mass chromatograms showed alternation in steranes, little loss of hopanes and no pronounced loss of higher molecular weight of triaromatic steroids; thus, they were at between levels 5-8 on the Peters and Moldowan (PM) scale of biodegradation. GC-MS analysis was performed to analyze biological markers. Among the biological markers, the most important and noticeable effects were a decrease in the 20S/20R diastereomer ratio of the C29 steranes. There were also evident effects on the overall distributions and abundance of pentacyclic terpanes thus, leading to alteration of biomarker parameters which could result in erroneous interpretation of source depositional environments, source correlation and maturity.