Temporal and spatial differences between predicted and measured organic carbon in South Atlantic sediments: constraints to organic facies modelling
The evaluation of total organic carbon (TOC) content from wells is a routine practice for assessing source rock potential, and because of this, empirical relationships to estimate TOC content have been proposed and used in organic facies modeling. However, no critical evaluations address how well the available TOC-predictive equations estimate the TOC content in recent and past sediments of the South Atlantic. In order to investigate the performance of the available TOC-predictive equations in this study, we compiled a large South Atlantic dataset composed of new and published modern TOC, dry bulk density, and sedimentation rate with the associated satellite-derived net primary productivity. We repeat this exercise for another time interval using the same input parameters but estimating net primary productivity using transfer functions. Our results indicate that all equations tend to overestimate the modern TOC distribution, mainly at the continental margins and at the edges of the subtropical gyre. In addition, the equations perform better in the eastern compared to the western South Atlantic. The reason for that may rely on the fact that these equations were built under the consideration that an increase in primary productivity must generate an increase in TOC, which is not always the case. The tendency to overestimate the TOC content was also apparent for the downcore data, although with reduced magnitude. The reasons behind the different performances of equations are explained by the way they incorporate the input parameters into their formulations, which is especially relevant for primary productivity, sedimentation rates, and the inclusion of processes such as the carbon flux. Our compilation indicates that the use of a single equation is not adequate to resolve the broad spatial TOC distribution in marine environments and that organic facies modelling software should consider regional aspects in order to produce adequate quantitative estimations of TOC content. Additionally, we used our dataset to investigate the optimal environmental conditions linked to organic-rich sediments in the South Atlantic. Our results show that high productivity, shallow depths, and ideal sedimentation conditions are required for the occurrence of organic-rich sediments and can be viewed as important factors preconditioning the formation of black shales in the South Atlantic.