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The organic geochemistry of shale source rocks has been a subject for extensive research during the past 2 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. Nevertheless, 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 and 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 in evaporite-depositing environments is not likely to be preserved. Recent data on organic geochemistry of Holocene carbonate sediments from shallow shelves suggest that (1) organic matter can be preserved in certain environments, and (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 carbonate and
evaporite units is controlled primarily by environments of deposition and the diagenetic overprints. Sabkha, lagoonal, and basinal environments, for example, are excellent for organic-matter preservation. Vadose and freshwater phreatic diagenetic environments are not favorable for organic preservation. The marine-phreatic diagenetic environment, however, is favorable for preservation of organic matter.
The upward-shoaling cycles, which are the buildup blocks of the carbonate-evaporite sequences, provide for source-reservoir couplets. The base of a cycle generally includes the potential source rocks. The top of a cycle contains the leached and/or primary porosities which provide the reservoir potential.
Synchronous and post-sedimentary tectonic events also seem to have a positive influence on the source-rock potential of carbonate and evaporite rocks. Rapidly subsiding shelves would place the organic-bearing carbonate units below the destructive influence of the freshwater phreatic zone. Late structural movements could produce the microfracture systems which would form the avenues for petroleum migration from source to reservoir rocks.
Geochemical data on ancient rocks strongly suggest that sabkha evaporites should be seriously considered as possible source rocks for petroleum.
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