Oil shales: Compaction, Petroleum Generation and Expulsion Von der Fakultät für Georessourcen und Materialtechnik der Rheinisch-Westfälischen Technischen Hochschule Aachen zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften genehmigte Dissertation vorgelegt von M.Sc. Applied Geology Emmanuel Eseme aus Kumba Berichter: Univ.-Prof. Dr. rer. nat. Ralf Littke Univ.-Prof. Dr. Janos L. Urai Tag der mündlichen Prüfung: 22. August 2006 Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfügbar ii Acknowledgements Sincere thanks to all I came across in the course of this research for their kind assistance. Gratitude to Prof. Dr. Ralf Littke, Head of the Institute of Geology and Geochemistry of Petroleum and Coal and my principal Supervisor whose gratuity enabled me to conduct this research. The concern attached to my work throughout the research duration was a continuous source of motivation. Prof. Dr. Janos Urai provided the much-needed support and persuaded me to pay a little more attention to concepts of rock mechanics. As my co-supervisor, it was a great honour, to get him dedicate his time to introduce basic concepts to me from first principles. Dr Bernhard Krooss and Priv. Doz. Dr. Jan Schwarzbauer provided invaluable theoretical and laboratory support that enabled various stages of this research to be completed successfully. Their perennial availability for discussion, suggestions and direction was an exceptional privilege. Encouragement and advice from Prof. Dr. D. Leythaeuser (Retired) constituted part of the motivation to complete this research. Sincere gratitude to Prof. Peter Kukla and Priv. Doz. Dr. Harald Stolhofen of the Institute of Geology and Palaeontology, who, made recommendations that were crucial for the onset of this research. My research contemporaries were always ready to share their knowledge and experience and I did benefit so much from them especially Drs. Meier. R., Amijaya. H, Schwarzer. D, Senglaub. Y, Rodon. S, Heim. S, Kronimus. A, Prinz. D, Busch. A, Tcherny. R, Mrs. Blumenstein. I, Mr. Koester. J and Mr. Wenniger. P. The laboratory technicians always laid the foundation for every stage of this research and their contribution is no less valuable especially Mrs. Pohl. Y, Mr. Mindenberger. R., Mr. Gensterblum. Y and Mr. Alles. S. Much thanks also to Dr Rehbach. W, of the Central Facility for electron Microscopy for technical assistance during electron microscopy, and Dr. Witzke. T, of the Institute of Mineralogy and Economic Geology for X-ray diffraction. Special thanks to Mrs. D. Kanellis and R. Wuropulos for their enormous assistance on administrative and extra- curricullar issues that facilitated my stay in the Institute as well as in Germany. Friends and relatives were always so nice, understanding and supportive. Having the kids, Kuve Isaac Jackai, Bessong Cheyenne, Seke Anyokon Rovin Armstong, Sona Lombrosso Mukete, Eseme Edith Endale, Sona Olivienne Bande and Moukoutou Kanthyla Kuna-Ngose in mind was a strong driving force during the research period. iii Abstract Permian to Miocene oil shales (Torbanite, Posidonia, Messel, Himmetoglu and Condor) from six basins in Australia, Germany and Turkey were studied using a variety of techniques that incorporate petrophysics, geochemistry and petrology. The objectives of this project were to improve understanding of compaction, petroleum generation and expulsion in nature as well as provide insights that may be exploited by technology for oil shale exploitation. The physical properties of the oil shales were compared to those of other oil shales from previous studies. Similar to other oil shales, grain densities ranged from 1.1g/cm³ to 2.4 g/cm³ with a strong correlation to organic matter content. Organic matter content is related to the oil shale grade used for economic assessment. The organic matter content strongly controls the behaviour of the oil shales including their mechanical properties. Existing data shows that mechanical properties are very unpredictable at high temperature especially for high-grade oil shales. The relevance of the evolution of mechanical properties at high temperature with repect to exploitation and basin modelling was investigated. Only strength can be discussed with some certainty at the moment and indicates the need for more tests to be conducted at high temperature. First indications were found demonstrating how temperature can enhance microfracturing during petroleum generation. Compaction behaviour of the six oil shales was studied under different thermo-mechanical conditions. Strength determined by compressive loading to failure at room temperature showed that the unconfined compressive strengths of the oil shales ranged from very weak to medium (5.3 to 70 MPa). Strength considered as maximum effective stress attained during burial and initial porosity (7.6 to 20.1%) showed that none could be used for burial depth estimation as suggested for organic matter-poor mudstones. Vitrinite reflectance (0.19-0.52 %) limited the maximum burial to between zero and 2 km for the different oil shales. Axial strain at room temperature (1.9-23 %) compared to that at 310 ºC (12-79 %) and 350 ºC (1.38-40%) showed that temperature superceedes effective stress as the principal factor controlling mudstone deformation when rocks are rich in organic matter. Only dehydration of smectite (94-150 ºC) showed a distinct contribution of mineralogy to compaction and was corroborated by X-ray diffraction. High organic matter content favours creep that is very important with increasing temperature. Transformation of organic matter characterised by the petroleum generation index was found to be a source of porosity during compaction (0.7 to 51.4 %). Volume balance (2.6- 12.5 % solid to liquid conversion) supported the increase in porosity experienced (1.5-6.4 %) by samples after compaction. The limitation of the effective stress approach to predict porosity increase during compaction was iv highlighted and experiments that incorporate thermo-mechanical conditions recommended for studying compaction based on axial strain rather than porosity and void ratio change. Characterization of the fluid transport potential from steady-state flow tests of two samples revealed absolute permeabilities from 0.72·10-21 m² to 2.63·10-21 m² which is within the range given for other mudstones (10-18 to 10-24 m²). Measurement of porosity and specific surface area from nitrogen gas sorption before and after high temperature compaction experiments gave permeability values based on an empirical relation from 6.97·10-24 m² to 5.22·10-21 m² for pre-deformation and from 0.2·10-21 m² to 0.6·10-21 m² for post-deformation samples all within the permeability range for mudstones. Flow rates based on permeability from steady-state tests suggested that several million years were required for primary migration from thick source rocks in nature and hence fractures were required for rapid expulsion. Petroleum expulsion efficiencies from samples were high varying from 38.6 to 96.2 % consistent with those from other organic matter-rich rocks. The expulsion efficiencies showed weak correlation to compaction, porosity and average pore diameter. The principal factor controlling expulsion was found to be the petroleum generation index. Calculation of pore volume saturations based on oil generated during experiments relative to pore volume demonstrated that pore volume saturation determined whether expulsion occurred through intergranular or fracture permeability. Petroleum expulsion occurred using both pathways during experiments with 20 % as threshold pore volume saturation above which microfracturing occurred with evidence from electron microscopy. Pore volume saturations also showed that not all pore volume is required for expulsion. Consideration of capillary displacement pressure, organic matter expansion, existence of transport porosity and optical evidence suggests that fracture generation constitutes the principal pathway for primary migration. Detailed molecular investigation of aliphatic, aromatic and polar compounds revealed aspects that complemented bulk data. No fractionation based on molecular weight was observed in all functional groups consistent with the lack of lithologic controls during primary migration. The direction of compositional fractionation between residual and expelled products was also observed to be in accordance with the concomitant relation between generation and expulsion. Unlike in other studies, no preferential expulsion of n-alkanes relative to acyclic isoprenoids was observed. However, cyclics were retained to a greater extent than straight and branched chain compounds. v For oil shale exploitation, several interesting aspects can be deduced from these experiments. Primarily of relevance to exploitation as a whole is the limitation of maximum retorting temperature. Using slow rate heating to attain final retorting temperature is suggested. In situ exploitation would eliminate most environmental problems and remains the best option. The experiments showed that artificial fracturing by explosives to raise permeability is not necessary because the driving force for migration is related to the generation process itself. Based on the retorting process, timing of generation, expulsion and prediction of product composition can all be achieved. As shown for the n-alkanes, products would reflect recovery efficiency after expulsion and adequate location of recovery wells remains a challenge. vi Zusammenfassung