2019 AAPG Annual Convention and Exhibition:

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Sedimentation Rate Variations and Trace Metal Elements as Paleoenvironmental Proxies: What Are We Doing Wrong?

Abstract

Trace metal elements are commonly used in the characterization of organic-rich rocks. Primary productivity or redox conditions in sedimentary basin are commonly characterized using proxies established from trace metal. With the development of self-sourced reservoirs (shale gas – shale oil) there is increasing interest in these proxies, especially to characterize the large-scale heterogeneities of organic-rich rocks as a record of the syn-sedimentary paleoenvironmental conditions. Most of these proxies are defined using present-day case studies across short-time spans and supposed constant sedimentation rates. The work presented here is based on a high-resolution stratigraphic framework, geochemical analyses and numerical simulations. The results show that sedimentation rate variations are paramount to understanding basin paleoenvironmental evolution when using geochemical proxies at basin scale over long-time periods.

Stratigraphic modelling of the Montney and Doig Formations provides an alternative interpretation compared to the geochemical proxy analysis, to explain the change of organic richness across the boundary between these two depositional sequences. Higher concentrations of authigenic phosphorus, nickel and copper in the organic-rich upper sequence (TOC > 10 wt%) suggest an increase in primary productivity. However, our numerical simulations show that the organic enrichment is primarily controlled by a drop of sedimentation rate in an anoxic environment and cannot be explain by an increase of organic productivity alone. These results suggest that the dilution effect must be considered when using trace metal elements as paleoenvironmental proxies.

To tackle this challenge, we introduce the Corrected Ratio for Element enrichment in Basins (CREB) and present its applications on two datasets. This new index integrates spatio-temporal sedimentation rate variations and normalises the elementary authigenic fraction on data distributed across large areas and over long-time period. Variations of this ratio are therefore more representative of changes in authigenic element concentrations related to paleoenvironmental conditions. Using this ratio rather than conventional proxies will help better characterize the spatial and temporal variations of primary productivity and anoxia in sedimentary basin. This has direct implications on the prediction of potential hydrocarbon generating areas in sedimentary basins.