Modelling TOC and anoxia from elemental data in the Wolfcamp Formation: a reality check
Milly Wright, Eliza Mathia and Ken Ratcliffe – Chemostrat Inc
The one of the keys to the successful evaluation and exploitation of shale reservoirs is the establishing reliable constraints on the present day total organic content (TOC) and the quality of the organic matter before the onset of thermal degradation. For a long time, the relationship between high gamma and high TOC has been observed, with "hot-shales" being recognised as organic rich and therefore potentially good source rocks.. The high gamma values are typically the result of high Uranium (U) concentrations, which are caused by authigenic U precipitation in anoxic sediments.
As shale resource plays have proliferated and the acquisition of elemental data on core and cutting has become routine on shale formations, the use of U concentrations as a proxy for TOC has become almost accepted. Several papers effectively demonstrate that a U vs. TOC cross-plot often yields a high correlation coefficient. Similarly, workers have also successfully used Mo or one of the chalcophile elements (Nickel (Ni), Copper (Cu), Zinc (Zn)) as a proxy for TOC. Furthermore, the use of enrichment factors of elements such as Vanadium (V) are routinely used to highlight sediments that were deposited under anoxic / euxinic conditions..
In reality, the concentrations of the redox sensitive elements are controlled by a complex interplay of the terrigenous input, biogenic sediment deposition, bottom water oxygenation and diagenesis. Consequently, in certain shale formations while a high level of covariation with TOC is typical for certain elements, other elements show a high degree of scatter. Furthermore, the absolute elemental concentrations and enrichment factors vary greatly not only between shale basins, but also between formations regardless of the organic content. Therefore, it is hardly surprising that no single element provides a global panacea to all anoxia and TOC modelling requirements in shale plays.
In this paper we will discuss the controls on key element concentrations that are used for modelling TOC and anoxia in the Wolfcamp Formation (Delaware Basin), proving an indication of when and where certain relationships can be expected to hold-up and also reason why certain relationships will breakdown. We will focus on the vast heterogeneity of shales to highlight the impact of the siliciclastic, biogenic or calcareous deposition on the specific TOC - trace element associations. The intention of this study is to provide insights to shale workers as to when, where and how to use elemental geochemistry as proxies for anoxia and TOC.