Eastern Section Meeting

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Comparative Optical Properties of Vitrinite and Other Macerals From New Albany Shale (Devonian and Mississippian): Implications for Thermal Maturity


Vitrinite reflectance (% Ro) routinely quantifies thermal maturity in sedimentary basins. Reflectance and fluorescence of other macerals such as solid bitumen, amorphous organic matter (AOM), and liptinite provide additional data for evaluating thermal conditions. Available vitrinite Ro values do not provide adequate basin-wide coverage across the New Albany Shale and in some cases may not be reliable due to suppression of Ro. We selected 23 samples from all members of New Albany Shale from a recently drilled core hole in Daviess County, Indiana for comparative reflectance measurements.

Vitrinite reflectance Ro values range from 0.59 to 0.70% and express no trend with depth. In contrast, reflectance values of solid bitumen and AOM are lower and range from 0.45 to 0.57% and 0.28 to 0.33%, respectively. Samples with higher vitrinite Ro also feature elevated solid bitumen and AOM reflectance values. With increasing maturity, samples change alginite fluorescence color from greenish-yellow to dull yellow. Our new vitrinite reflectance Ro values are higher than previously compiled data centering at ~0.58% in this area. This updated vitrinite Ro range places source rocks into the early mature zone close to the mid-mature transition, and thus assigns a higher oil and gas potential than traditionally expected. Vitrinite reflectance equivalent values calculated from the reflectance of solid bitumen are higher than the measured vitrinite reflectance values. Differences may be caused by erroneous differentiation between solid bitumen and vitrinite, or by the presence of vitrinite-like material. Fourier transform infrared spectroscopy was applied to add constraints on maturity and maceral identification.