Sedimentology and lithofacies of organic-rich Namurian Shale, Namur Synclinorium and Campine Basin (Belgium and S-Netherlands)
In: Marine and Petroleum Geology. Elsevier: Guildford. ISSN 0264-8172; e-ISSN 1873-4073, meer
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| Author keywords |
Namurian; Mudstone; Sedimentology; Lithofacies; Depositional processes; Organic matter |
| Abstract |
Organic-rich Namurian A mudstone is the main potential shale gas play in Belgium and neighboring countries. In order to reveal the regional depositional setting and its influence on lithofacies and origin of organic matter, this study investigates lithofacies, depositional processes and mechanisms controlling organic matter accumulation based on analysis of sedimentary structures, grain size, mineralogy, optical petrography, pyrite framboid diameter, total organic carbon and gamma log in five wells from Namurian A mudstone in Belgium and the southern part of the Netherlands. These mudstones, located in the Namur Synclinorium and Campine Basin, consist of an organic-rich mudstone succession that was deposited in the epicontinental basin within the Variscan foredeep within a moderate deep water environment below storm wave base in a distal shelf to basin floor setting under mainly euxinic and anoxic conditions with occasional dysoxic and oxic conditions. These sediments were deposited through various processes, including turbidity currents, debris flows, pelagic suspension settling with minor contribution of bottom currents, slumping processes and storm events. Lithofacies stacks generate repeated regressive and transgressive units. In the Campine Basin and the western Namur Synclinorium, 3 regressive units are characterized by basal lithofacies derived from pelagic suspension settling changing upward to lithofacies derived from mainly turbidity currents and debris flows while 2 transgressive units are characterized by basal lithofacies derived from mainly turbidity currents and debris flows changing up to lithofacies from pelagic suspension settling. In contrast in the eastern Namur Synclinorium which reflects a more proximal depositional setting, only 2 regressive units were identified. They are characterized by basal lithofacies derived from turbidity currents and debris flows changing upward to lithofacies from debris flows, bottom currents and slumping processes. 2 transgressive units were also identified. The first unit is characterized by basal breccia from karstification while in the second unit, lithofacies evolve from debris flows and slumping processes changing upward to lithofacies deposited from turbidity currents, debris flows and bottom currents. Overall, carbonate and silica dominated lithofacies from pelagic suspension settling with minor input from debris flows were deposited during high relative sea-level under euxinic to anoxic conditions. Quartz-rich lithofacies from debris flows and turbidity currents with minor input from pelagic suspension settling and bottom currents were deposited during intermediate relative sea-level under anoxic conditions. In contrast, clay, silt and carbonate-rich lithofacies from turbidity currents, debris flows, bottom currents, slumping processes and storm events were deposited during low relative sea-level under anoxic to dysoxic conditions with occasional oxic conditions near the basin margin. The highest total organic carbon observed in lithofacies from intermediate relative sea-level is caused by relative high primary marine productivity with dominant marine organic matter, low siliciclastic dilution and excellent preservation under anoxic conditions. Organic matter accumulation is low in lithofacies deposited during high and low relative sea-level. During high relative sea-level, it was controlled by clay mineral content, organic matter consumption and carbonate dilution, while during low relative sea-level, it was controlled by water redox conditions and siliciclastic dilution. |
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