Large volumes of greenhouse gases such as CH 4 and CO 2 form by contact metamorphism of organic-rich sediments in aureoles around sill intrusions in sedimentary basins. Thermogenic gas generation and dehydration reactions in shale are treated numerically in order to quantify basin-scale devolatilization. We show that aureole thicknesses, defined as the zone of elevated metamorphism relative to the background level, vary within 30-250% of the sill thickness, depending on the temperature of the host-rock and intrusion, besides the sill thickness. In shales with total organic carbon content of >5 wt.%, CH 4 is the dominant volatile (85-135 kg/m 3 ) generated through organic cracking, relative to H 2 O-generation from dehydration reactions (30-110 kg/m 3 ). Even using conservative estimates of melt volumes, extrapolation of our results to the scale of sill complexes in a sedimentary basin indicates that devolatilization can have generated $2700-16200 Gt CH 4 in the Karoo Basin (South Africa), and $600-3500 Gt CH 4 in the Vøring and Møre basins (offshore Norway). The generation of volatiles is occurring on a time-scale of 10-1000 years within an aureole of a single sill, which makes the rate of sill emplacement the time-constraining factor on a basin-scale. This study demonstrates that thousands of gigatons of potent greenhouse gases like methane can be generated during emplacement of Large Igneous Provinces in sedimentary basins.

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Voluminous volcanic intrusive activity took place in the Vøring and Møre basins at the Paleocene–Eocene boundary at about 56 Ma. This event caused thermal maturation of Cretaceous sedimentary rocks in the basins. We have estimated the resulting thermogenic gas generation potential from contact metamorphism using numerical simulations calibrated using borehole data. The borehole 6607/5-2 from the Utgard sill complex in the Vøring Basin contains two c . 100 m thick sills and is used as a case study. We present both new and compiled data showing that (1) the bulk organic content is reduced towards the sill intrusions, (2) a c . 1 km thick stratigraphic interval is thermally affected, based on vitrinite reflectance data, (3) relative emplacement timing can affect the gas yield by up to 25%, and (4) some of the thermogenic methane is still present in the aureoles. The numerical model is calibrated using data from 11 wells. We estimate that the total gas generation potential for the two Utgard sills equals that of the Troll field ( c . 10 Gt CH 4 ), the largest producing gas field offshore Norway. We show that in the Vøring and Møre basins, the total gas generation potential is up to 1500 Gt CH 4 ( c . 1100 Gt C), even from the relatively organic-poor Cretaceous source rocks with c . 1 wt% organic carbon, with implications for the carbon cycle at the Paleocene–Eocene boundary.

[1] Sedimentary rocks represent vast reservoirs for hydrous and carbonaceous fluids (liquid or gas) that can be generated and released during contact metamorphism following the emplacement of igneous sill intrusions. A massive release of these fluids may impose perturbations in the global climate. In this study we assess the influence of varying host-rock compositions on the magnitude and type of fluids generated from thermal devolatilization, with particular emphasis on carbon and halogens released from heated limestone, coal and rock salt, and the different timescales of metamorphism. In limestones the generated fluids are dominated by H 2 O with limited CH 4 and CO 2 production on a time-scale of 600-3000 years. Cracking of organic matter and CO 2 production (8000-28,000 years) dominates the fluid products from a coal sequence. In the case of evaporites, the presence of reactive organic matter or petroleum results in the generation of CH 4 and CH 3 Cl (260-1000 years). In order to compare the basin scale impacts of the differing host-rocks, two plausible scenarios are explored in which a 100 m thick and 50 000 km 2 large sill is emplaced into 1) organic-rich shale and coal, and 2) limestones and rock salt. The results show the formation of 1) >1600 Gt CH 4 , and 2) >700 Gt of CH 3 Cl, demonstrating that the sill emplacement environment (i.e., the composition of the host rocks) is of major importance for understanding both gas generation in sedimentary basins and the environmental impact of a Large Igneous Province. By evaluation of the isotopic signature of carbonaceous fluids from shales and coals, we show that intrusions into coal-rich sediments are potentially of much less importance for perturbing the atmospheric carbon isotope values than shales.

Generation of fluids during metamorphism can significantly influence the fluid overpressure, and thus the fluid flow in metamorphic terrains. There is currently a large focus on developing numerical reactive transport models, and with it follows the need for analytical solutions to ensure correct numerical implementation. In this study, we derive both analytical and numerical solutions to reaction‐induced fluid overpressure, coupled to temperature and fluid flow out of the reacting front. All equations are derived from basic principles of conservation of mass, energy and momentum. We focus on contact metamorphism, where devolatilization reactions are particularly important owing to high thermal fluxes allowing large volumes of fluids to be rapidly generated. The analytical solutions reveal three key factors involved in the pressure build‐up: (i) The efficiency of the devolatilizing reaction front (pressure build‐up) relative to fluid flow (pressure relaxation), (ii) the reaction temperature relative to the available heat in the system and (iii) the feedback of overpressure on the reaction temperature as a function of the Clapeyron slope. Finally, we apply the model to two geological case scenarios. In the first case, we investigate the influence of fluid overpressure on the movement of the reaction front and show that it can slow down significantly and may even be terminated owing to increased effective reaction temperature. In the second case, the model is applied to constrain the conditions for fracturing and inferred breccia pipe formation in organic‐rich shales owing to methane generation in the contact aureole.