Total organic carbon calculation using geophysical logs for 31 wells across the Palaeozoic of the Central North Sea

This report details the source rock total organic carbon (TOC) content calculated from geophysical logs for 31 wells across the Palaeozoic rocks of the UK Central North Sea for the 21CXRM Palaeozoic project. A companion report examines the reservoir evaluation of 12 wells based on petrophysical evaluation of digital wireline log curve data (Hannis, this study). The TOC weight % was calculated for source rock intervals using geophysical well logs calibrated to measured TOC values from core and/or cuttings in the petrophysics software Interactive Petrophysics (IP™), which uses the TOC estimations equations defined by Passey et al. (1990). The Passey et al. method is most reliable in thick shale intervals and cannot calculate TOC in coals and reservoir intervals, which were removed from the analyses. Where possible the geographical distribution of calculated TOC was commented on, however, in general the spatial distribution of the assessed wells was too great to make any confident extrapolations of TOC on a regional scale. Petrophysical log analysis has been used as a regional screening tool to highlight potential TOC rich source rock intervals (shales), over larger depth ranges than is available for core/cuttings sample data. Given time constraints, data availability and the variable nature of the Carboniferous sedimentation, kerogen types have not been taken into consideration. To further the work presented in this report, investigation of the kerogen type in conjunction with the calculated TOC would give a more complete understanding of the hydrocarbon source rocks. Outputs of this part of the project include continuous (along borehole) interpretations of total organic content and clay volume. These interpreted curves were used to calculate net shale to gross formation thickness, ‘pay’ TOC rich (>1 wt%) shale thickness to gross formation thickness (known as Pay to Gross in this report (P/G)) and calculated TOC over the net shale thickness. The source rock formations (according to the reinterpreted stratigraphic formations defined and correlated for this project) assessed for TOC calculation and a summary of the results are given in Table 1. In summary, the results indicate significant thicknesses of organic rich shale through the Carboniferous succession. The Lower Devonian had very low calculated TOC of 0.7 wt% and P/G of 0.07, however, the cutting measured TOC suggest even these figures may be an overestimation.