Source rocks of western Newfoundland

Abstract

Studies on the organic geochemistry of natural oil seeps, although not always associated with major economic deposits, offer opportunity to compare chemical characteristics of an oil to a sedimentary depositional environment. One such seep occurs in the geologically well-documented Parsons Pond region of Western Newfoundland. A number of potential sources for the oil, which has been observed for 170 years, have been proposed including shales from the Shallow Bay and Green Point Formations of the Cow Head Group, the Table Head Group and similar sediments from the Curling Group, all which represent deposition along a continental margin in the Cambro-Ordovician. Total organic carbon in the shales ranges from quite low (0.6%) to moderate (5.2%). Gas chromatography-mass spectrometry shows evidence that suggest weathering. Odd predominance prior to C 21 is seen in these oils and is characteristic of most Ordovician oils. The oils contain a slight even predominance in the C 25+ alkanes. Pristane and phytane were both observed to be present yielding a ratio of 1.6. Isotopic compositions from the Green Point Formation extracts are closest to that of the oils (−28.5%). Similar variations in isotope values from one fraction to another are observed in both the oils and Green Point extracts. Nitrogen isotope studies show a large range in the extracts with the best comparison between the Broom Point (0.5%ℵ) and seep oils (0.6%ℵ). The composition of the organic matter from the Broom Point and Martin Point Members of the Cow Head Group reflect the hydrocarbon compositions of the oil The effect of thermal maturation eastward across the Parsons Pond region is observed in increased bitumen contents and heavier carbon isotope compositions.