Model for prediction of organic carbon content in possible source rocks

Abstract

A quantitative model has been developed to improve the prediction of the occurrence of marine siliciclastic source rocks and their likely organic carbon content. The input parameters of this quantitative model are: organic matter supply in the surface layer of the ocean (photic zone); water depth (calculation of the carbon flux); sedimentation rate; and preservation conditions (presence or absence of oxygen at the sediment-water interface). To handle the difficulties in the quantification of input parameters a probabilistic approach is applied. Depending on the available data or the interpretation of the kind of environment, different risk distribution functions are used to describe the possible range of each input parameter. The calculation is then carried out using the Monte Carlo simulation technique, resulting in a probability distribution of the total organic carbon content which forms the basis of the analysis. Different scenarios currently under debate, e.g. high primary productivity or the possible effects of enhanced preservation under anoxic bottom water conditions and the influence of varying water depth and sedimentation rate, can be tested very quickly. Using examples of recent (Peru continental margin) and ancient (Lower Toarcian Shale, Germany) organic-rich, marine shales, the potential of this tool is demonstrated, firstly to quantify souce rock quality (in this case total organic carbon content only), secondly to assess quantitatively the uncertainties in source rock prediction (e.g. evaluation of organic matter supply, water depth and sedimentation rates) and thirdly to determine source quality risk values to be used in the appraisal of sedimentary basins and prospects.