Total Organic Carbon Trends in 3-D Space using Data Analytic Modeling Approaches for Prediction of Organic Matter Variability in the New Albany Shale Group of Central and Southern Illinois
The amount of retained and expulsed hydrocarbon from organic-rich shale horizons is strongly linked to the total organic carbon (TOC) contained in the interval. Total organic carbon is measured either from site and depth-specific lab analyses or from petrophysical log data. Accurate quantification of TOC content and spatial distribution of these intervals are crucial parameters in the evaluation of organic-rich unconventional reservoirs. This research applies geostatistical modeling methods for the purpose of prediction and distribution of organic carbon in shale intervals between well locations. The organic-rich New Albany Shale Group (NASG) (Mississippian-Devonian) within the Illinois Basin is used as a test case in this study as over 2,000 wells have penetrated the formation. To estimate TOC content of shale intervals, the petrophysical data of over 1,000 wells were assessed. Wireline logs were utilized to estimate TOC by the ΔlogR method, which relies on the density difference between organic matter and mineral matrix. The results of the petrophysical assessment were used to estimate the spatial distribution of TOC’s in a 3-D model. Prior to modeling, variogram data analyses were conducted to extract the continuity of the data in terms of horizontal and vertical variogram ranges. A comparison of deep resistivity log data against computed TOC data from ΔlogR technique indicates a logarithmic relationship with a correlation coefficient of over 0.8. The deep resistivity log data was geostatistically propagated using sequential Gaussian simulation (SGS). The TOC model was built by applying the SGS algorithm to the derived TOC data. The co-simulated deep resistivity data was used as a secondary variable to aid in predicting the spatial distribution of TOC data. Utilization of the geostatistical model provides continuity of TOC estimates in both vertical and lateral space across the Illinois Basin. The modeling simulations were constrained using core-derived TOC values determined from site-specific locations within the basin. Results computed from the geostatistical 3-D model range between 2.0 to 9.0% TOC in lateral space between wells and in the vertical depth direction. The generated 3-D model indicates that the uppermost shale intervals (NASG) contain elevated amounts of organic matter relative to the middle and lower intervals. The model delineates significantly higher amounts of TOC incorporated in the NASG in southern Illinois. The derived values from the model compare reasonably well with site-specific data, with a correlation coefficient of over 0.75. This technique can improve and constrain calculated resource parameters in basin evaluations by the inclusion of detailed variations in organic matter content.
AAPG Datapages/Search and Discovery Article #90373 © 2019 AAPG Eastern Section Meeting, Energy from the Heartland, Columbus, Ohio, October 12-16, 2019