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The AAPG/Datapages Combined Publications Database

AAPG Special Volumes


Pub. Id: A030 (1984)

First Page: 45

Last Page: 61

Book Title: SG 18: Petroleum Geochemistry and Previous HitSourceNext Hit Rock Potential of Carbonate Rocks

Article/Chapter: Some Aspects of the Hydrocarbon Geochemistry of a Middle Devonian Barrier-Reef Complex, Western Canada

Subject Group: Geochemistry, Generation, Migration

Spec. Pub. Type: Studies in Geology

Pub. Year: 1984

Author(s): T. G. Powell (1)


Organic geochemical data have been obtained on potential Previous HitsourceNext Hit 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 le els of maturity are sufficiently high for these carbonates to be Previous HitsourceNext Hit 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 inherit d from a Previous HitsourceTop rock deposited under highly reducing conditions. These oils are more mature than heavy oils encountered in the low-maturity area.

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