Topic 7 Source Rocks
We turn our attention next to the source rocks where petroleum generation has occurred. It is important to be able to recognize these rocks, particularly in the early stages of exploration in a new venture area because if more than one source rock is present the area is much more attractive. An estimate of how prolific the source has been and some indication of the nature of the products (oil or gas) is also valuable.
The source rocks available for examination contain the hydrocarbon residue left after some of the organic material has been removed (by a migration mechanism which is not fully understood). Since only a few percent of the hydrocarbons generated in the source rock migrate presumably there is only a small change in the chemical composition of the source rock extract. If migration occurs in aqueous solution, for example, the source rock should be enriched in the least soluble components. However, an enrichment can only be determined relative to the original composition and this is not normally known. In practice the methods currently in use for identifying source rocks are methods for detecting generation. It is then assumed that migration has occurred.
Use of the term “source rock” is often ambiguous and may be applied to rocks which are in very different stages of generation and expulsion. Dow (1977) has provided a list of definitions “to alleviate the confusion which exists in the geochemical literature” and a slightly modified version of this is given here