ALBERT

Description: We present a comprehensive new pyrolysis data set for a maturity series (0.55–2.86% vitrinite reflectance) of Pennsylvanian coals (Westphalian, Ruhr Basin). The broad similarity of the organofacies was established by elemental and petrographic data. Comparison of maximum temperatures reached by the coals was based on commonly applied algorithms (Easy%Ro, Easy%RoDL, Basin%Ro). These temperatures were finally compared to predicted temperatures of petroleum generation from the same coals in order to test the reliability of kinetic data for numerical petroleum generation reconstruction from coal. Kinetic parameters were determined based on data from open-system pyrolysis performed on two instruments (Rock-Eval™ and Source Rock Analyzer™) and basin modelling was performed using PetroMod™ software and a simple, rapid burial and heating history (about 20 °C/million years), which is similar to the rapid heating which caused natural maturation of these coals. With increasing maturity, pyrolysis curves shift towards higher temperatures, no matter which heating rate is used. However, pyrolysis curves of higher maturity samples are not embedded within the more extended and larger pyrolysis curves of low maturity coals. HI values and H/C ratios do not significantly decrease over the maturity range from 0.55 to 0.8–0.9% vitrinite reflectance. These observations indicate significant restructuring of organic matter in coals and only limited petroleum generation and expulsion within this maturity range. The kinetic parameters were derived from single (5 °C/min) and multiple (0.7, 2, 5, 15 °C/min) heating rate pyrolysis tests. After temperature correction, kinetic parameters obtained by the two instruments differ only slightly. Kinetics based on multiple heating rates using a variable frequency factor and variable activation energies provide the best mathematical fit to the laboratory data but geologically inconsistent predictions for natural petroleum generation, i.e. no increase in predicted generation temperatures with increasing sample maturity. In contrast, kinetics based on a constant, physically meaningful frequency factor and a calculated range of activation energies provide more consistent results for the maturity sequence. Interestingly, single-ramp kinetics (5 °C/min) show similar results, i.e. increasing calculated temperatures for petroleum generation with increasing maturity. For the lowest maturity sample, petroleum generation temperatures (10–75% conversion) are higher than calculated maximum rock temperatures, while this is not the case for all higher maturity samples, for which much of the predicted petroleum generation occurs at “too low” temperatures, i.e. temperatures clearly lower than calculated maximum rock temperatures. Maximum temperatures predicted by the Easy%RoDL approach are about 10 °C closer to the lower limit of petroleum generation than those calculated by Basin%Ro. Thus, kinetic data can act as a useful tool for calculating petroleum generation in petroleum system modelling providing clues towards the thermal stability of kerogen. However, such data are far from describing petroleum generation in nature exactly and quantitatively, especially for coals.

Description: This study investigates the hydrocarbon generation potential, kerogen quality, thermal maturity and depositional environment of Middle – Upper Jurassic sedimentary rocks in the Blue Nile Basin, Ethiopia, using organic petrography, Rock‐Eval pyrolysis and molecular organic geochemistry. Thirty‐seven outcrop samples were analysed for their total organic carbon (TOC) and inorganic carbon (TIC) contents. The samples came from a Toarcian – Bathonian transitional glauconitic shale‐mudstone unit, the overlying Upper Bathonian Gohatsion Formation, and the Lower Callovian – Upper Tithonian Antalo Limestone Formation. Thirteen samples with sufficient TOC contents for further analysis of the organic matter, eight from the Antalo Limestone Formation and five from the glauconitic shale‐mudstone unit, were selected and analysed using Rock‐Eval pyrolysis. Vitrinite reflectance (VRr) was measured on random particles, and qualitative maceral analysis was performed under normal incident and UV light. Nine samples were selected for molecular organic‐geochemical analyses. All the samples originating from the Gohatsion Formation showed TOC values which were too low for further analyses of the organic matter. The TOC contents of shales and limestones from the Antalo Limestone Formation and and of shales from the glauconitic shale‐mudstone unit were 3.43‐6.43% (average 4.85%) and 0.76‐3.15% (average 1.72%), respectively, and two coaly shale samples from the latter unit have average TOC values of 18.48%. HI values are very high for shales in the Antalo Limestone Formation (average 575 mg HC/g TOC) but lower for the shales in the glauconitic shale‐mudstone unit. The vitrinite reflectance of shales from the Antalo Limestone Formation ranged between 0.21% and 0.47%; coaly shales from the glauconitic shale‐mudstone unit have VRr% of between 0.29% and 0.35%. Pr/Ph ratios for samples of the Antalo Limestone Formation shales ranged from 0.8 to 1.1, indicating anoxic to suboxic depositional conditions; while shales in the glauconitic shale‐mudstone unit show higher values of up to 4.9. In terms of organic petrography, the Antalo Limestone Formation samples are dominated by finely dispersed liptinite particles and alginite; the organic material in the glauconitic shale‐mudstone unit is of higher land plant origin, with abundant vitrinite and inertinite. Sterane and hopane biomarker ratios suggest an anoxic/suboxic depositional environment for the Antalo Limestone Formation shales and limestones. These values together with Rock‐Eval Tmax (average 414 °C), the high ratio of pristane and phytane over the n‐alkanes C17 and C18, and hopane biomarker ratios indicate that the Middle – Upper Jurassic succession is of low thermal maturity in the central parts of the Blue Nile Basin. The Antalo Limestone Formation shales have a high petroleum generation potential, making them a viable target for future exploration activities.