Contact metamorphic devolatilization of shales in the Karoo Basin, South Africa, and the effects of multiple sill intrusions

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 (~ 15 m thick) sills have a gas production potential of several gigatons of methane (CH4) if emplaced over a > 1000 km2 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.

Research Highlights
► Intrusive heating leads to loss of organic carbon and metamorphic mineral reactions.
► One single, thin (15.5 m) sill intruded can generate several gigatons of CH4.
► Multiple sills can increase the relative total gas generation by up to 35%.
► Multiple sills can induce organic maturation throughout entire sedimentary sections.