A Plankton-Residue Model to Explain Trace-Element Enrichments in Oil-Source Rocks

D. Z. Piper and C. M. Isaacs

Sedimentary deposits enriched in organic matter commonly have high concentrations of many trace elements. In the past, trace-element enrichment was attributed to accumulation by precipitation/adsorption reactions under conditions of bottom-water sulfate reduction, or from a seawater that itself had an unusually high concentration of trace elements. Examination of the ancient and modern sediment record shows, however, that many trace elements in these deposits accumulated within an organic fraction whose composition closely approached that of modern plankton; their accumulation further required only a moderate rate of primary productivity. Specific examples are represented by the accumulation of Cu, Cd, Mo, Ni, and Zn in the California Borderland today and by their abundan e in Quaternary sediment from the Japan Sea, Cretaceous sediment from the Atlantic Ocean, the Miocene Monterey Formation, and the Permian Phosphoria Formation.

Accordingly, we propose that the elevated trace-element concentration of many oil-source rocks, above that contributed by the detrital fraction, is a residue from the diagenetic degradation of marine plankton. Recent studies have shown that the burial rate (accumulation rate) of organic matter can represent less than 5% of its rain rate (depositional rate) onto the sea floor and as little as 1 % of primary productivity. By contrast, several of the trace elements, once deposited on the sea floor, can be largely retained. In the Japan Sea sediment, for example, Cu: and Zn:organic-matter ratios in the marine fraction of sediment alone average 10 times their ratios in plankton, suggesting a 90% loss of the organic matter that rained onto the sea floor, but Zn:Cu ratios and other trace-ele ent:Cu ratios in this and other deposits closely approach modern plankton values. Thus, primary productivity in the photic zone and rain rate of organic matter onto the sea floor are possibly better represented in this type of deposit by the abundance of the marine fraction of selected trace elements than by the abundance of organic matter itself.

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