RESULTS

The gross composition of the Mizzen oil sample is as follows:

% hydrocarbons 81.9

% saturated hydrocarbons 48.3

% aromatic hydrocarbons 33.6

% resins plus asphaltenes 14.8

The predominance of n-alkanes and lack of a hump of unresolved compounds in the gas chromatograms indicate that the oil is not biodegraded. It generally shows saturate hydrocarbon and gasoline range characteristics similar to those observed for Jeanne d’Arc Basin oils (Fowler and Obermajer, 2001). The pristane/phytane ratio of 1.53 is on the high side of the range observed previously for Jeanne d’Arc Basin oils and is in the range of values observed for those oils believed to have a contribution from a source other than the Egret Member with a greater terrestrial contribution to its organic matter (Fowler et al., 1989; Fowler and McAlpine, 1995).

As indicated by the m/z 191 fragmentogram, the terpane distributions are very similar to those observed for Jeanne d’Arc Basin oils that are sourced from the Egret Member (e.g. Fowler and McAlpine, 1995); for example, the relatively high amounts of the rearranged hopanes including C29 Ts and the C30 diahopane and the low abundance of tricyclic and tetracyclic terpanes. The sterane carbon number (C27-C29) distribution is similar to Egret Member sourced oils. However, the sterane isomer distribution appears to indicate a very low maturity, lower than would be expected from the hopane isomers and closer to that expected for an immature source rock rather than a conventional oil. This is suggested by the predominance of the 5α(H),14α(H),17α(H) 20R over the 5α(H),14α(H),17α(H) 20S and 5α(H),14β(H),17β(H) isomers and low abundance of diasteranes. Dinosteranes are abundant but not as high as generally found in Egret Member sourced oils.

Aromatic maturity parameters do not indicate a consistent maturity for the Mizzen oil. For example, the vitrinite reflectance calculated based the distribution of methylphenanthrenes ranges from 0.72% using the equation of Boreham et al. (1988) to 0.83% using the equation of Radke (1988). However, it is the experience of this laboratory that the Methylphenanthrene Index is not a reliable indicator of maturity for Jeanne d’Arc Basin oils. The distribution of methyldibenzothiophenes has been found to work better for these oils. Using the equation of Radke (1988) to link the distribution of these compounds with a vitrinite reflectance gives a vitrinite reflectance equivalent of 0.67% indicating a relatively low maturity oil but not as low as suggested by the sterane distributions. Additional evidence for the low maturity of the Mizzen sample is provided by the very low triaromatic cracking ratio of 0.06.

DISCUSSION

The variation in the maturity of the Mizzen L-11 oil sample indicated by different parameters suggests it likely has a complex origin. Some characteristics are those expected for an oil sourced from mature Egret Member or equivalent. Others, especially the steroidal compounds, suggest a very low maturity for an oil. A possible explanation is that this sample contains hydrocarbons from multiple sources. Previous reports have suggested the occurrence of multiple source rock intervals within the Upper Jurassic in Flemish Pass wells such as Panther P-52 (e.g. Fowler and McAlpine, 1995). Thus this sample could have a contribution from a deeper, more mature source interval such as those in the Rankin Formation (including the Egret Member) and from a lower maturity interval in close proximity to where the oil sample originated (perhaps the Tithonian source rock interval). A “Rankin Equivalent” was identified in this well at a depth of 3770 m (Canada Newfoundland Labrador Offshore Petroleum Board, 2005) and the sample would appear to come from the Fortune Bay-Jeanne d’Arc Formation interval where the Tithonian source interval is usually recognised (Fowler and McAlpine, 1995). It may also be significant that it is the sterane distributions that appear to be anomalous as these compounds are in significantly lower concentrations than the terpanes and therefore more likely to be affected by any mixing of hydrocarbons from a lower and higher maturity source. However, at present in the absence of additional data from this well, this must remain conjecture.

While the sample shows some characteristics similar to the Egret Member sourced oils from the Jeanne d’Arc Basin, there are also some differences. These could reflect changes in the Egret Member organic facies on moving into the Flemish Pass or the mixing of hydrocarbons from multiple sources as suggested above to explain the variations in thermal maturity indicated by different classes of compounds.

REFERENCES

Boreham, C., Crick, I.H., Powell, T.G. (1988) Alternative calibration of the Methylphenanthrene Index against vitrinite reflectance; Application to maturity measurements on oils and sediments. Organic Geochemistry vol. 12, 289-294.

Canada Newfoundland Labrador Offshore Petroleum Board (2005) Schedule of Wells, Well #234, Petro-Canada et al. Mizzen L-11.

Fowler, M.G., Brooks, P.W. (1990) Organic geochemistry as an aid in the interpretation of the history of oil migration into different reservoirs at the Hibernia K-18 and Ben Nevis I-45 wells, Jeanne d'Arc Basin, offshore eastern Canada. Organic Geochemistry vol. 16, 461-475.

Fowler, M.G., McAlpine, K.D. (1995) The Egret Member, a prolific Kimmeridgian source rock from offshore eastern Canada. In Source Rock Case Studies (Ed. Katz, B.), Springer-Velag. p.111-130.

Fowler, M.G. and Obermajer, M. (2001) Gasoline Range and Saturate Fraction Gas Chromatograms of Jeanne d’Arc Basin Crude Oils. Geological Survey of Canada Open File Report #3945.


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