Emplacement and evolution of sills into shallow magma chambers and hydrocarbon reservoirs

Volcanism occurred 56Ma along the Vøring Margin, offshore Norway, during continental break-up. The focus of this thesis is on sills along this margin, particularly (1) on sill emplacement, (2) how they may evolve into a shallow magma chambers or (3) act as fractured reservoirs for hydrocarbons. Numerical models show that because of mechanical layering, dykes are commonly deflected into sills due to debonding, stress barriers, and elastic mismatch. Once emplaced a sill can take on a variety of geometries and begin to expand via elastic-plastic deformation of the strata. In order for a sill to evolve into a shallow magma chamber, a high magma injection rate is needed so that the sill remains at least partially molten. The molten sill creates a stress barrier causing subsequent dyke injections to be absorbed into the initial sill.
The majority of sills, however, do not evolve into shallow magma chambers, but may act as fractured hydrocarbon reservoirs, depending on (1) sill geometry, (2) sill thickness, and (3) sill margins. For sills to act as fractured reservoirs their lower margins must be ruptured, while the upper margins remain intact and form a seal, allowing the accumulation of hydrocarbons within the sills. By contrast, if the lower margins remain intact, forms a seal, the sill may trap hydrocarbons, particularly when in conjunction with sealing normal faults and dykes. While sill propagation may reactivate faults, and temporarily increase their permeability, subsequent geothermal fluid circulation (due to the sill) may contribute to ‘healing’ and ‘sealing’ of the fault, thereby reducing its permeability. Fluid transport in sills is primarily through fracture networks, most of the fractures being columnar joints, which favour transport particularly if (1) they have large apertures (through the cubic law and flow channelling) and (2) favourably orientated in relation to the local stress field.