A biomarker-derived paleo-redox indicator

ROCHER, DON L., TRADER, ROGER K., and STARLING, ROGER A., Core Laboratories, Inc., Carrollton, Texas, USA 75006

As the biomarker distributions found in the extracts of thermally immature rocks do not resemble those found in mature petroleum, many explorationists believe that the study of these extracts provides little meaningful geochemical information. However, it is often necessary to speculate on the nature of deeply buried source rocks based upon data derived from immature, but accessible well or outcrop samples. A technique has been devised that takes advantage of the relatively unaltered state of the immature rocks to study the relationship between biomarker groups that are normally obscured by maturity effects.
A new biomarker parameter is introduced that monitors the interplay between primary production in the photic zone and the forces of consumption and decomposition in the underlying water column and sediment. Steroid-derived biomarker ratios from GC-MS analysis of rock extracts are used to provide a value for productivity (algal biomass modified by consumption) and a value for decomposition capacity. Subtracting decomposition capacity from the productivity parameter results in a depositional redox value (depV) that differentiates ancient sedimentation based upon the dominant mode of bacterial respiration -- i.e., methanic, sulfidic or oxic. The elucidation as to which mode is dominant has useful geochemical applications as this technique allows an analyst to rank related and non-related sedimentation in terms of redox potential.

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