ENVIRONMENTAL CONTROLS ON THE DEPOSITION OF ORGANIC-RICH ROCKS: THE PERMIAN IRATI OIL SHALES

The Irati Formation constitutes important oil source rocks, and also bears compelling geologic and environmental questions. The one we attempted to address was what were the main controls in the accumulation of organic-rich sediments. The approach taken was the analysis of redox-sensitive elements, Total Organic Carbon (TOC) and sulfur content data, coupled with sedimentological and paleontological information.

Redox-sensitive elements, e.g. V, Cr, Ni, Cu, Zn, Mo, Cd and U, are deposited in sediments under anoxic conditions, as opposed to Mn and Fe, which are precipitated under oxic conditions. The precipitation of these metals under anoxic conditions causes large enrichments above detrital values in the resulting mudrocks.

Irati shales were analyzed for C/S, Degree of Pyritization (DOP) and redox-sensitive elements. We found that the Irati black shales are actually comprised of two groups of rocks. The dominant group is organic-poor, with redox-sensitive element concentrations similar to the average shale, and constant, low DOP values (< 0.75). The second group contains organic-rich rocks, which are enriched in U and Mo, have very low Mn concentrations, and low C/S (between 0.3 and 3.7).

These data suggest dysoxic conditions during the deposition of the sediments of the Irati Formation, rather than the persistently anoxic conditions inferred from sedimentology and paleontology alone. The DOP data also suggest that the environment was Fe-limited. The C/S ratios are indicative of marine settings.

The following scenario is proposed: the Irati shales were deposited in a stratified basin with predominantly dysoxic conditions. Occasionally, a higher influx of fresh water lowered the salinity and brought nutrients in, increasing primary productivity, which ultimately led to anoxia and accumulation of organic-rich sediments. Under the circumstances, it seems that the main control in the accumulation of the organic-rich rocks of the Irati Formation was the increased delivery of organic matter to the sediment rather than geographic or hydrographic factors.