A Mississippian black shale record of redox oscillation in the Craven Basin, UK

Early diagenetic redox oscillation processes have been rarely recognised in the ancient rock record but potentially exert an important control on mineral authigenesis, hydrocarbon prospectivity and supply of metals and/or reduced S as part of associated mineral systems. The upper unit of the Mississippian Bowland Shale Formation is a candidate record of diagenetic redox oscillation processes because it was deposited under a relatively high sediment accumulation rate linked to a large delta system, and under dominantly anoxic and intermittently sulphidic bottom-water conditions. In order to characterise the syngenetic and early diagenetic processes, sedimentological and geochemical data were integrated through the Upper Bowland Shale at three sites in the Craven Basin (Lancashire, UK). Organic matter (OM) comprises a mixture of Type II, II-S, II/III and III OM. ‘Redox zones’ are defined by patterns of Fe-speciation and redox-sensitive trace element enrichment and split into two groups. ‘Sulphidic’ zones (EUX, AN-III, AN-I and AN-IT) represent sediments deposited under conditions of at least intermittently active sulphate-reduction in bottom-waters. ‘Non-sulphidic’ zones (OX-RX, OX-F and OX) represent sediments deposited under non-sulphidic (oxic to ferruginous anoxic) bottom-waters. Operation of a shelf-to-basin ‘reactive Fe’ (FeHR) shuttle, moderated by sea level fluctuation and delta proximity, controlled the position and stability of redoxclines between zones of Fe and sulphate reduction, and methanogenesis. Early diagenetic redoxclines were capable of migration through the shallow sediment column relatively quickly, in response to sea level fluctuation. Preservation of syngenetic and early diagenetic geochemical signals shows redoxclines between Fe and sulphate reduction, and the upper boundary of sulphate-methane transition zone, were positioned within decimetres (i.e., 10 s cm) of seabed. Falling sea level and increasing FeHR supply is recognised as a switch from zones EUX (high sea level), AN-III and ultimately AN-I and AN-IT (low sea level). Zone AN-I defines the operation of ‘redox oscillation’, between zones of Fe and sulphate reduction in shallow porewaters, associated with enhanced degradation of OM and complete dissolution of primary carbonate. Preservation of OM and carbonate, in this system, was a function of changing bottom and pore water redox processes. Redox oscillation operated in a siliciclastic, prodeltaic environment associated with a relatively high sediment accumulation rate and high loadings of labile organic matter and metal oxides. These findings are important for understanding Late Palaeozoic black shales in the context of hydrocarbon and mineral systems

La pyrolyse Rock-Eval et ses applications. Troisième partie. Rock-Eval Pyrolysis and Its Applications (Part Three)

Conçue pour répondre aux besoins de l'exploration pétrolière, la méthode de pyrolyse Rock-Eval est maintenant largement utilisée. Elle fournit, en effet, et d'une façon rapide, différentes informations sur le contenu organique des roches, telles que le potentiel pétrolier des séries rencontrées, la nature des kérogènes, leur état de maturation. En ce qui concerne le matériel, deux nouvelles versions ont été mises au point à l'Institut Français du Pétrole (IFP) depuis l'apparition en 1977 des premiers appareils Rock-Eval : un Rock-Eval II complètement automatisé grâce à un microprocesseur et doté, en option, d'un module de dosage du carbone organique; un Rock-Eval III (Oil Show Analyzer) qui se distingue du précédent par le fait qu'il analyse séparément le gaz et l'huile et qu'il effectue le dosage du carbone organique à la place du pic S3. En ce qui concerne l'interprétation de la méthode, l'expérience acquise tant par les applications aux bassins sédimentaires que par les études expérimentales menées en laboratoire a permis de mieux connaître les paramètres utilisés (pics S1, S2, S3, température de pyrolyse Tmax) à travers leurs variations et, de là, de mieux discerner les limites d'application de la méthode. En ce qui concerne l'application de la méthode, la représentation verticale des résultats sous forme de logs géochimiques conduit à une interprétation à la fois efficace et pratique. Des abaques et des diagrammes de référence permettent de caractériser les roches mères (potentiels pétroliers, types de matière organique, degré d'évolution, altérations . . . ) ainsi que les phénomènes de migration. L'établissement de cartes géochimiques à l'échelle du bassin devient alors possible. Enfin la méthode est sortie du domaine de l'exploration pétrolière proprement dite pour trouver des applications à l'étude des charbons, des roches bitumineuses, des sédiments récents et même aux techniques du raffinage et de la récupération secondaire des bruts. <br> The Rock-Eval pyrolysis method was designed to meet the needs of petroleum prospection and has now gained widespread use. It quickly provides different data on the organic content of rocks, such as the petroleum potential of series encountered, the nature of kerogens and their state of maturity. Two new versions of this equipment have been developed by Institut Français du Pétrole (IFP) since the appearance of the first Rock-Eval devices in 1977. One is a Rock-Eval II that is completely automated thanks to its microprocessor and is also equipped with a module for determining the organic-carbon content. The other is a Rock-Eval III (Oil Show Analyzer) which is different from the preceding one in that it makes a separate analysis of gas and oil as well as determining the organic-carbon content in place of peak S3. Concerning the interpretation of the method, experience gained both by applications to sedimentary basins and by experimental investigations in the laboratory has provided a better understanding of the parameters used (peaks S1, S2 and S3, pyrolysis temperature Tmax) through their variations and, from this, better insight into the application limits of the method. Concerning the application of the method, the vertical representation of results in the form of geochemical logs leads to an interpretation that is both effective and practical. Reference charts and diagrams are used to characterize source rocks (petroleum potentials, types of organic matter, degree of evolution, weathering, etc. ) as well as migration phenomena. The compiling of geochemical maps on a basin-wide scale then becomes possible. Likewise, the method has left the field of petroleum prospection per se and found applications in the analyzing of coals, bituminous rocks, recent sediments and even techniques of the refining and secondary recovery of crude oils

La pyrolyse Rock-Eval et ses applications. Deuxième partie. Rock-Eval Pyrolysis and Its Applications (Part Two)

Conçue pour répondre aux besoins de l'exploration pétrolière, la méthode de pyrolyse Rock-Eval est maintenant largement utilisée. Elle fournit, en effet, et d'une façon rapide, différentes informations sur le contenu organique des roches, telles que le potentiel pétrolier des séries rencontrées, la nature des kérogènes, leur état de maturation. En ce qui concerne le matériel, deux nouvelles versions ont été mises au point à l'Institut Français du Pétrole (IFP) depuis l'apparition en 1977 des premiers appareils Rock-Eval : un Rock-Eval II complètement automatisé grâce à un microprocesseur et doté, en option, d'un module de dosage du carbone organique; un Rock-Eval III (Oil Show Analyzer) qui se distingue du précédent par le fait qu'il analyse séparément le gaz et l'huile et qu'il effectue le dosage du carbone organique à la place du pic S3. En ce qui concerne l'interprétation de la méthode, l'expérience acquise tant par les applications aux bassins sédimentaires que par les études expérimentales menées en laboratoire a permis de mieux connaître les paramètres utilisés (pics S1, S2, S3, température de pyrolyse Tmax) à travers leurs variations et, de là, de mieux discerner les limites d'application de la méthode. En ce qui concerne l'application de la méthode, la représentation verticale des résultats sous forme de logs géochimiques conduit à une interprétation à la fois efficace et pratique. Des abaques et des diagrammes de référence permettent de caractériser les roches mères (potentiels pétroliers, types de matière organique, degré d'évolution, altérations. . . ) ainsi que les phénomènes de migration. L'établissement de cartes géochimiques à l'échelle du bassin devient alors possible. Enfin la méthode est sortie du domaine de l'exploration pétrolière proprement dite pour trouver des applications à l'étude des charbons, des roches bitumineuses, des sédiments récents et même aux techniques du raffinage et de la récupération secondaire des bruts. <br> The Rock-Eval pyrolysis method was designed to meet the needs of petroleum prospection and has now gained widespread use. It quickly provides different data on the organic content of rocks, such as the petroleum potential of series encountered, the nature of kerogens and their state of maturity. Two new versions of this equipment have been developed by Institut Français du Pétrole (IFP) since the appearance of the first Rock-Eval devices in 1977. One is a Rock-Eval II that is completely automated thanks to its microprocessor and is also equipped with a module for determining the organic-carbon content. The other is a Rock-Eval III (Oil Show Analyzer) which is different from the preceding one in that it makes a separate analysis of gas and oil as well as determining the organic-carbon content in place of peak S3. Concerning the interpretation of the method, experience gained both by applications to sedimentary basins and by experimental investigations in the laboratory has provided a better understanding of the parameters used (peaks S1, S2 and S3, pyrolysis temperature Tmax) through their variations and, from this, better insight into the application limits of the method. Concerning the application of the method, the vertical representation of results in the form of geochemical logs leads to an interpretation that is both effective and practical. Reference charts and diagrams are used to characterize source rocks (petroleum potentials, types of organic matter, degree of evolution, weathering, etc. ) as well as migration phenomena. The compiling of geochemical maps on a basin-wide scale then becomes possible. Likewise, the method has left the field of petroleum prospection per se and found applications in the analyzing of coals, bituminous rocks, recent sediments and even techniques of the refining and secondary recovery of crude oils