Maturity and hydrocarbon potential of the New Albany Shale and Maquoketa Shale in the Illinois Basin, USA

A total of 51 samples were selected from the New Albany Shale and Maquoketa Shale from Pike, Gibson and Washington counties in Indiana. Both organic petrographic (i.e. visual maceral examination and vitrinite reflectance measurements) and geochemical methods (Rock-Eval pyrolysis and biomarkers) were used to evaluate their hydrocarbon potentials and to determine possible suppression of vitrinite reflectance (Ro), particularly for New Albany Shale samples. The New Albany Shale is an effective source rock in the Illinois Basin due to its organic matter content, type of organic matter (Type I and Type II) and correlation with other hydrocarbon-producing shales, but also the Maquoketa Shale has drawn attention as a possible source rock in the Illinois Basin especially because of considerable thermal maturity. Our results show that although the Maquoketa Shale (435 to 445 °C T max from Rock-Eval pyrolysis) is more mature than the New Albany Shale (427 to 442 °C Tmax), the relatively higher organic matter content in the New Albany Shale with up to 13.80 wt. % total organic carbon (TOC) support a higher hydrocarbon potential than the Maquoketa Shale's TOC values between 0.09 to 1.32 wt. %. Interestingly, the New Albany Shale's notable differences between directly measured vitrinite reflectance values and calculated Ro values both from Tmax and biomarkers demonstrate a suppression of vitrinite reflectance by as much as 0.21% in Pike County and 0.20% in Gibson County. This vitrinite reflectance suppression is probably caused by (i) the presence of higher amounts of liptinite macerals, and (ii) deposition under anoxic paleoenvironmental conditions. Correction for the suppression of Ro values would place the New Albany Shale more favorably into the oil window.