Lysaker, Norway
Lysaker, Norway

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Aarnes I.,University of Oslo | Aarnes I.,Roxar ASA | Fristad K.,University of Oslo | Planke S.,University of Oslo | And 2 more authors.
Geochemistry, Geophysics, Geosystems | Year: 2011

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 H2O with limited CH4 and CO 2 production on a time-scale of 600-3000 years. Cracking of organic matter and CO2 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 CH4 and CH3Cl (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 km2 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 CH4, and 2) >700 Gt of CH 3Cl, 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. Copyright 2011 by the American Geophysical Union.

Aarnes I.,University of Oslo | Aarnes I.,Roxar ASA | Svensen H.,University of Oslo | Polteau S.,University of Oslo | And 3 more authors.
Chemical Geology | Year: 2011

Quantification of fluid generation during contact metamorphism of shale is important for the understanding of metamorphic processes, fluid flow in sedimentary basins and perturbations of the global carbon cycle. In this study we provide geochemical and numerical analyses from the organic-rich Ecca Group in the Karoo Basin, South Africa, which was affected by contact metamorphism from multiple sill intrusions in the Early Jurassic. Organic matter was efficiently converted to hydrocarbons during contact metamorphism, and complete loss of organic carbon in the innermost aureole is common. Mineral dehydration reactions are evident from the occurrence of metamorphic minerals like biotite and loss of the clay fraction towards the intrusive contact. We have developed a numerical model in order to quantify fluid production from both inorganic and organic reactions during contact metamorphism. The modelling results are constrained by data from two case studies in the Karoo Basin in order to obtain reliable estimates of the carbon loss from metamorphism of shale. We show that single, thin (~15m thick) sills have a gas production potential of several gigatons of methane (CH4) if emplaced over a >1000km2 area. Furthermore, the vertical spacing between simultaneously emplaced sills has an important influence on the gas generation potential. When two sills are emplaced with a vertical spacing of ~7 times the intrusion thickness, the total CH4 generation is up to ~35% more than for two separate sills. Data and modelling from five sills emplaced within the Ecca Group show hydrocarbon generation throughout the organic-rich section, with total carbon loss next to the sills. This has implications for the fluid production and metamorphism in volcanic basins where multiple sills are common. © 2010 Elsevier B.V.

Aarnes I.,Roxar ASA | Aarnes I.,University of Oslo | Podladchikov Y.,University of Lausanne | Svensen H.,Roxar ASA
Geofluids | Year: 2012

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. © 2012 Blackwell Publishing Ltd.

Escalona A.,University of Stavanger | Yang W.,IRIS - International Research Institute of Stavanger | Yang W.,Roxar AS
AAPG Bulletin | Year: 2013

We reviewed the tectonostratigraphic evolution of the Jurassic-Cenozoic collision between the North American and the Caribbean plate using more than 30,000 km (18,641 mi) of regional two-dimensional (2-D) academic seismic lines and Deep Sea Drilling Project wells of Leg 77. The main objective is to perform one-dimensional subsidence analysis and 2-D flexural modeling to better understand how the Caribbean collision may have controlled the stratigraphic evolution of the offshore Cuba region. Five main tectonic phases previously proposed were recognized: (1) Late Triassic-Jurassic rifting between South and North America that led to the formation of the proto-Caribbean plate; this event is interpreted as half grabens controlled by fault family 1 as the east-northeast-south-southwest-striking faults; (2) Middle-Late Jurassic anticlockwise rotation of the Yucatan block and formation of the Gulf of Mexico; this event resulted in north-north west-soudi-southeast-striking faults of fault family 2 controlling half-graben structures; (3) Early Cretaceous passive margin development characterized by carbonate sedimentation; sedimentation was controlled by normal subsidence and eustatic changes, and because of high eustatic seas during the Late Cretaceous, the carbonate platform drowned; (4) Late Cretaceous-Paleogene collision between the Caribbean plate, resulting in the Cuban fold and thrust belt province, the foreland basin province, and the platform margin province; the platform margin province represents the submerged paleoforebulge, which was formed as a flexural response to the tectonic load of the Great Arc of the Caribbean during initial Late Cretaceous-Paleocene collision and foreland basin development that was subsequently submerged during the Eocene to the present water depths as the arc tectonic load reached the maximum collision; and (5) Late Cenozoic large deep-sea erosional features and constructional sediment drifts related to the formation of the Oligocene-Holocene Loop Current-Gulf Stream that flows from the northern Caribbean into the Straits of Florida and to the north Atlantic. Copyright © 2013.

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