Some Aspects of the Hydrocarbon Geochemistry of a Middle Devonian Barrier-Reef Complex, Western Canada
T. G. Powell, 1984. "Some Aspects of the Hydrocarbon Geochemistry of a Middle Devonian Barrier-Reef Complex, Western Canada", Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks, James G. Palacas
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Organic geochemical data have been obtained on potential source rocks, bitumens, and oils from within and in the vicinity of a Middle Devonian carbonate-barrier complex in western Canada. Organic-carbon values in carbonate rocks vary widely (0.11 to 30.1%) according to facies type. Extract yields from immature samples from the basinal facies can be extremely high and appear to be a consequence of the highly reducing conditions in which the organic matter was deposited. Characteristic features of these high-yield extracts are a low proportion of hydrocarbon, low saturate-to-aromatic ratios, pristane-to-phytane ratios less than 1.0, a high content of acyclic isoprenoids relative to n-alkanes, and an even-to-odd predominance in the n-alkanes. The hydrocarbon yields at low levels of maturity are sufficiently high for these carbonates to be source rocks. The main migration product is heavy oil/bitumen, which shows all the characteristics of the parent-rock extracts. Locally these bitumens have been altered by heat during dolomitization to form an insoluble hydrogen-rich pyrobitumen classified as epi-impsonite. This pyrobitumen probably was formed by polymerization of the NSO/asphaltenes fraction. With increasing depth the normal rock extracts show a decline in NSO (nitrogen, sulfur, and oxygen) compounds and asphaltenes yield owing to polymerization under normal conditions of maturation but with essentially no change in hydrocarbon yield. The oils in pinnacle-reef reservoirs in the Rainbow and Zama fields south of the barrier complex also show features inherited from a source rock deposited under highly reducing conditions. These oils are more mature than heavy oils encountered in the low-maturity area.