Porphyrin Distributions in Lacustrine and Marine Shales as Indicators of Paleodepositional Environments

Janina K. Rafalska and Geoffrey Eglinton

Porphyrins, organic molecules derived from chlorophyll, were assessed as potential paleoenvironmental indicators (biomarkers) of oil shales. The differences between the porphyrins from shales and coals are well established. However, the more subtle differentiation between the porphyrin distribution patterns of the lacustrine and marine oil shales is not documented. The lacustrine shales studied here are from Tertiary fault basins in China. The Mesozoic marine oil shales came from a variety of geographic and stratigraphic settings.

The bulk of porphyrins occurs in the carbon number range from C₃₀ to C₃₃. The C₃₂ porphyrin is the most abundant irrespective of shale origin. The lacustrine shales contain porphyrins in the C₂₈- C₃₅ range whereas the marine shales usually have porphyrins in the C₂₅-C₃₆ range. The DPEP, i.e., Class A-2 porphyrins are dominant both in lacustrine and marine shales. However, the di- DPEP, i.e., Class A-4 porphyrins are absent in lacustrine shales. Moreover, marine shales contain vanadyl porphyrins whereas lacustrine shales contain nickel porphyrins. Finally, the absolute amounts of porphyrins in lake sediments are much lower than in marine sediments.

Differences amongst porphyrin characteristics of thermally immature organic-rich shales are only in part attributed to the great variety of their precursor molecules, i.e., chlorophylls. Most likely, the dominant control was exerted by the intensity of anoxia during early diagenesis.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California