ABSTRACT

The organic geochemistry of the shale source rocks has been a subject for extensive research during the past two decades. Many useful interpretive techniques have been developed for the assessment of hydrocarbon potential of sedimentary basins in which shales are the principal and logical source for petroleum generation. In contrast, the present understanding of carbonate and evaporite source rocks remains superficial. The criteria generally employed to assess shale source rocks are inadequate and misleading when applied to carbonate-evaporite basins.

Most misconceptions regarding the hydrocarbon potential of carbonate and evaporite rocks stem from a simplistic notion that organic matter associated with the sediments on well-aerated carbonate shelves and evaporite-depositing environments is not likely to be preserved. Recent data on organic geochemistry of Holocene carbonates from shallow shelves suggest that:

1. Organic matter can be preserved in certain environments.
2. The kerogens produced from degradation of organic matter in carbonate sediments are predominantly sapropelic and therefore much more efficient sources for hydrocarbons than the mixed humic-sapropelic kerogens of shales.

The preservation of organic matter in carbonates and evaporites is controlled both by environments of deposition and diagenetic overprints. Sabkha, lagoonal, and basinal environments, for example, are excellent for preservation of organic matter. Vadose and fresh-water phreatic diagenetic environments are not favorable for organic preservation. The marine phreatic diagenetic environment, however, is favorable for preservation of organic matter.

Source rocks can be formed in transgressive carbonate-evaporite facies. The regressive facies, however, are favorably situated for development of leached secondary porosity. The transgressive-regressive couplets should be ideal sequences for petroleum generation and accumulation.

Geochemical data on ancient rocks strongly suggests that sabkha evaporites should be seriously considered as a possible source rock for petroleum. The late structural movements could produce the micro-fracture systems which would form the avenues for petroleum migration from source to reservoir rocks.