ABSTRACT

Sedimentary organic-matter constituents, including thermal-alteration products, are useful in the evaluation of petroleum potential in western Canada. It is an accepted hypothesis that organic matter in sediments is the source or the complement, or both, of the hydrocarbons in sedimentary reservoirs. Through visual observation and comparison of the organic matter, the hydrocarbon potential of a sedimentary body and the types of hydrocarbon likely to be found are evaluated.

Hydrogen-rich material that has been modified under subaqueous low-oxygen sapropelic conditions lends itself to the formation of liquid and "wet" (C₂ plus) gaseous hydrocarbons through compaction and temperature elevation. This material can be recognized by the presence of amorphous translucent debris. Deposits containing only structured plant cuticles or opaque fusainized (high-carbon) material produce mainly dry gas; those containing a mixture of the amorphous sapropelic debris and recognizable plant debris generally have "wet" hydrocarbon potential.

Organic metamorphism induced by heat (maximum paleotemperature) and pressure modifies the organic matter locked in the sedimentary matrix as well as the mobile products, leading to paraffinic oils and condensates, then to dry gas and pyrobitumens, and finally to graphitization under severe conditions. For mapping, a thermal index of five progressive degrees of thermal alteration is distinguished. Where the two highest degrees prevail, prospects for "wet" gas and liquid hydrocarbons are negligible unless there has been post-thermal migration from outside the thermally affected area. The highest degree reduces prospects even for dry gas, and the Devonian dry-gas area of northern British Columbia is interpreted on this basis. Many areas with large dry gas reserves have been exposed to low-grade thermal metamorphism.

Bitterli (1963a, 1963b) Philippi (1957, 1965) and Gutjahr (1966) have reported on organic-matter studies carried out for Shell Petroleum Corporation; Correia (1967) and others have reported on work in France and the Sahara; and Vassoevich et al. (1967) have summarized certain studies carried out in the U.S.S.R. This paper summarizes some of the work carried out by Imperial Oil Limited.