Restoration of Source Rock TOC in 3-D Basin Modeling: A New Experimentally-Constrained Methodology

Lorant, François¹, Luiz Felipe Carvalho Coutihno² (1) Institut Français du Pétrole, Rueil-Malmaison, France (2) Petrobras Research Center, Rio de Janeiro, Brazil

The determination of organic carbon (TOC) distribution in source rocks before thermal maturation occurred is still a key problem for the estimation of generated hydrocarbons vol­umes via 3D-basin simulation. Geoscientists usually attempt to evaluate initial carbon con­tents by linearly correlating TOC variations with source rocks transformation ratios, obtained from hydrogen indexes (HI). With such an approach, one has to assume that the rate of car­bon conversion equals that of hydrocarbon generation. Unfortunately, this hypothesis is inaccurate. Experiment data (e.g., weight loss measurements upon artificial maturation of kerogens) show that the relationships between the loss of carbon mass in source rocks and transformation ratios is strongly non-linear, and variable from a source rock to another. In order to compute original TOC maps, taking specific non-linear TOC versus TR variations, a methodology has been developed, based on both pyrolysis mass balances and thermal modeling pre-runs. Laboratory experiments provide hydrocarbons generation rates (Rock Eval kinetics) and carbon weight loss (thermogravimetry analysis) measurements. These data are then used together to calibrate specific analytical functions that represent carbon weight losses against TR. Given such functions, TR calculated by the basin model for each source rock layer can directly be converted into original carbon masses, and therefore orig­inal TOC distribution. This approach has successfully been applied in a 3D study of the Reconcavo Basin (Brazil), showing that the ‘linear’ assumption approach leads to a signifi­cant underestimation of initial TOC values. Consequences on predicted hydrocarbons vol­umes will be discussed.