Total Organic Carbon (TOC) Estimation and Generation Potential of the Ordovician Maquoketa Shale, Illinois Basin – Using a Modification of the _logR Technique

Since the 1990’s, petrophysical logs have been utilized to screen, quantify, and assess the range of TOC in potential source intervals and unconventional reservoirs. The Illinois Basin can be considered a mature basin, with over 2500 well penetrations of Silurian and Ordovician strata. The vast majority of petrophysical data is from pre-2000 logging suites. Utilization of these wireline logs to estimate TOC requires the use of the _logR technique which relies on the density difference of organic carbon (0.9-1.05_o) and mineral matrix (2.65-2.71_m) that sonic, density, or neutron logs measure in conjunction with resistivity data. While the actual TOC content can vary significantly in a lateral and vertical sense, if the organic macerals and kerogen types are fairly consistent throughout a potential source interval – a relationship can be developed between TOC content and resistivity log values. This allows a rapid screening technique to assess source horizons utilizing older (in some cases, 1965 – present) resistivity data. In addition to using core-derived TOC values to constrain the algorithm, the standard _logR technique was determined in selected wells for comparison to ensure reliable TOC estimates. Once a range of values was calculated for the Maquoketa Shale, geochemical and maturity modeling techniques were applied to specific basinal areas to derive generation potential and calculate the transformation ratio (TR) in portions of the Maquoketa Shale. Calculations of the TOC content using the modified _logR technique in the Maquoketa Shale range from 0.5 to 4.8% with values increasing in specific basinal areas. Importantly, lower TOC values calculated in the Maquoketa Shale in southern and eastern parts of Illinois correspond to TR values in excess of 0.7 – 0.8. This relation implies that significant hydrocarbon generation from the Maquoketa in these areas may have occurred. Maceral and kerogen studies on the Maquoketa in eastern Kentucky and Indiana indicate abundant reworked and oxidized liptinite. Organic matter (OM) identification in central and western Illinois indicate abundant amorphous OM along with concentrated layers of alginite. In summary, the low TOC content in portions of the Maquoketa Shale in Illinois may be primarily the result of hydrocarbon generation rather than hydrogen-poor organic matter.