Mohammed Saeed, Opuwari Mimonitu, Titinchi Salam
Department of Earth Sciences, Faculty of Natural Sciences, University of the Western Cape, Western Cape, Bellville, South Africa.
Department of Chemistry, Faculty of Natural Sciences, University of the Western Cape, Western Cape, Bellville, South Africa.
Sci Rep. 2020 Jul 21;10(1):12036. doi: 10.1038/s41598-020-68918-y.
The Cretaceous Afowo Formation in the Eastern Dohamey Basin is characterized by an admixture of lithofacies ranging from sandstones, claystones, shales, clays, sand/shale, and sand/clay intercalations. The sandy facies, a mix of sandstone, clay, shale, and intercalations, contain biodegraded hydrocarbons while the shales and claystones that underlie it are rich in organic matter. The hydrocarbon-bearing interval is commonly referred to as the oil sand or tar sand. In this study, Afowo clay type underlying an outcrop of the oil sand was appraised for its hydrocarbon potential with loss on ignition, thermogravimetry, and rock evaluation pyrolysis. Results obtained from loss on ignition showed that total organic matter content, a proxy to total organic carbon, for the Afowo clay type ranged from 9.410 to 38.750 wt%. The organic maturation temperature (Tmax) was determined using both thermogravimetry and rock evaluation pyrolysis (Rock-Eval). Thermogravimetric analysis produced reliable Tmax within the range of 417-424 °C for all the samples. The results from rock evaluation pyrolysis on the same samples showed that total organic carbon ranged from 0.81 to 18.46 wt% with Tmax ranging from 417 to 424 °C. It was not possible to determine Tmax for one of the samples with Rock-Eval due to a small S2 value (0.22 mg Hc/g). The variations in organic matter contents from loss on ignition agree with total organic carbon computed from rock evaluation pyrolysis; samples with high organic matter contents have corresponding high TOC values. This study demonstrates that loss on ignition and thermogravimetry could complement and augment rock evaluation pyrolysis data for petroleum source rock characterization.
多哈梅伊盆地东部的白垩纪阿福沃组以砂岩、泥岩、页岩、黏土、砂/页岩和砂/黏土夹层等多种岩相混合为特征。砂质相由砂岩、黏土、页岩和夹层组成，含有生物降解烃，其下伏的页岩和泥岩富含有机质。含烃层段通常被称为油砂或焦油砂。在本研究中，对油砂露头下伏的阿福沃黏土类型进行了研究，通过灼烧减量、热重分析和岩石评价热解来评估其含烃潜力。灼烧减量结果表明，阿福沃黏土类型的总有机质含量（作为总有机碳的替代指标）范围为9.410至38.750 wt%。使用热重分析和岩石评价热解（Rock-Eval）测定了有机成熟温度（Tmax）。热重分析对所有样品得出的可靠Tmax范围为417 - 424°C。对相同样品进行岩石评价热解的结果表明，总有机碳范围为0.81至18.46 wt%，Tmax范围为417至424°C。由于一个样品的S2值较小（0.22 mg Hc/g），无法用Rock-Eval测定其Tmax。灼烧减量得出的有机质含量变化与岩石评价热解计算的总有机碳一致；有机质含量高的样品具有相应较高的TOC值。本研究表明，灼烧减量和热重分析可补充并增强用于石油源岩表征的岩石评价热解数据。
