Geochemical study of Mississippian to Pennsylvanian Namurian Shale in the Namur Synclinorium and Campine Basin (Belgium and S-Netherlands): Implication for paleo-redox reconstruction and organic matter characteristics

Abstract

The Variscan foreland basin in Belgium and the Netherlands (Namur Synclinorium and Campine Basin) possesses potential targets for shale gas exploration based on the presence of Mississippian to Pennsylvanian Namurian black shales. Organic carbon and nitrogen contents and isotope data, major and trace element analysis, petrographic analysis, Rock-Eval pyrolysis, and basin modeling were carried out on a large sample set from 5 wells in the above-mentioned study area. Carbonate-dominated, silica-dominated, and quartz-rich mudstones with dominant type II kerogen were deposited within distal positions close to the basin floor while clay-rich, carbonate-rich, and silt-rich mudstones with mixed type II and III kerogens reflect depositional conditions at more proximal positions close to the basin margin. Sedimentary rocks were deposited in redox-stratified marine environments. Oxygen depleted environments occurred during the early Namurian A age, especially at distal depositional positions in contrast to elevated oxygenated environments during the late Namurian A age, especially at more proximal positions. From distal positions to proximal positions, redox conditions changed from mainly euxinic to anoxic conditions with some dysoxic episodes to mainly anoxic to dysoxic conditions with a few oxic episodes. The variation in δ15Nbulk values in the Namur Synclinorium suggests that N2-fixation dominated in anoxic environments during the early Namurian A age while nitrification, denitrification, and anammox in the water column dominated in anoxic to dysoxic environments with oxic episodes during the late Namurian A age. In the Campine Basin, denitrification and anammox dominated in anoxic to dysoxic environments during the early Namurian A age while N2-fixation dominated in anoxic environments at distal positions and nitrification dominated in dysoxic to oxic environments at proximal positions during the late Namurian A age. Namurian A mudstones are overmature with higher maturity in the Campine Basin than in the Namur Synclinorium. They display low potential for petroleum and gas generation based on Rock-Eval results. Basin modeling results indicate that the thermal maturity of Namurian A mudstones already reached its main phase of dry gas generation with overmature conditions during the Pennsylvanian in the Namur Synclinorium and the southern Campine Basin. In the northern Campine Basin corresponding to a depositional setting further away from the Variscan mountains, however, these sedimentary rocks became overmature during the Jurassic. Conversion of all type II kerogen to petroleum and dry gas occurred during increasing burial depth and thermal maturity. This left only overmature, hydrogen-poor kerogen remaining in the mudstones. The western Namur Synclinorium is regarded as the most prospective exploration area with high gas generation and storage potential as well as good hydraulic fracturing potential in terms of very high total organic matter content, large shale thickness, and high amount of brittle mineral contents. The Campine Basin is a secondary target with more limited gas generation and storage capacity, based on the moderate to high total organic matter content, the limited thickness of the mudstone interval, and limited brittle mineral contents.