Introduction

A large number of the future petroleum exploration areas worldwide are related to volcanic margins. Successful exploration on a volcanic margin requires good understanding of its tectonic development and heat-flow history. Present standard models for heat flow, crustal stretching, and subsidence at volcanic margins have to be refined and improved by including the details of the magmatic processes, particularly sill and dyke emplacement, in models.

One volcanic margin with a particular importance to the petroleum systems of Norway is the Vøring area, which covers a large part of the mid-Norwegian continental margin. Exploration in this large area has been going on since the early 1990's. However, the geological/tectonic development and the corresponding temperature history in the area are still not completely understood (Fjeldskaar et al., 2009; Grunnaleite et al., 2009; Wangen et al., 2009).  

Figure 2.  Sill in hyaloclastite (sedimentary volcanic rocks) in Southwest Iceland. The sill is about 30 m thick.

 The Vøring Basin is a large sedimentary basin province with grabens, sub-basins and structural highs (Fig. 3). The formation of Vøring Marginal High during early Tertiary break-up was associated with massive extrusive and intrusive magmatic activity (Skogseid, 1994).  Sills and dykes that intrude Mesozoic sediments are observed over a wide area (100-200 km) landward of the continent-ocean boundary.

Reasonably thick sills are easily detected on seismic images using conventional seismic interpretation methods. Sills are normally layer-parallel, and thus sub-horizontal or gently dipping, and with higher densities and seismic velocities than the surrounding sedimentary layers. Dykes are mostly sub-vertical and thus more difficult to detect using standard seismic reflection techniques. However, recent technological developments make it now easier to image dykes. Furthermore, since most sills worldwide are supplied with magma through dykes (dykes acting as feeders), and since dykes are one of the main structural elements associated with volcanotectonic rifting, it is reasonable to assume that there are numerous dykes associated with sill complexes in sedimentary basins.

Theoretically, rifting of a homogeneous, isotropic crust would normally not generate any sills but rather normal faults and dykes. Sedimentary basins, however, are heterogeneous and, in particular, anisotropic. Their layering results in local stresses in individual layers or at contacts that are commonly unfavourable to dyke emplacement, but favourable to sill emplacement, thereby encouraging the deflection of dykes into sills (Gudmundsson, 1990,  2010).

Figure 3.  Depth converted profile over the Vøring area (Fjeldskaar et al., 2009).

Magmatic intrusions in the form of sills and dykes affect the sediments they intrude, both through the stresses acting on the sediments as well as through the heat carried by the hot magmatic material.

Dykes and sills are hydrofractures, that is, they are initiated when the fluid pressure exceeds the minimum compressive principal stress, sigma₃, plus the tensile strength of the rock. Hydrofractures thus form in a direction perpendicular to sigma_3. The tips of propagating sills and dykes are associated with very high tensile stresses. These stresses may greatly affect the surrounding rock, and previous studies have shown that propagation of sills and dykes may lead to fracture generation and fault reactivation (Gudmundsson, 2010; Gudmundsson and Lotveit, 2005; Gudmundsson et al., 2010). This may lead to enhanced permeability of the associated petroleum reservoir, but may also cause leakage reactivation of nearby faults and remigration of trapped hydrocarbons.

In addition to the tip-induced stress effects, there are also stress effects on the rock units and layers on either side of the intrusions. In particular, as we will show, numerical models indicate that, in layered rocks, intrusion-induced stresses concentrate in the stiff layers which, thereby, may develop and reactivate fractures. This is likely to be a major effect of sill emplacement in sedimentary rocks, where the stiff (high-Young's modulus) rocks (often far) above and below the sill may fracture and offer potential sites for fractured reservoirs.

Thus, timing and style of magmatic activity in relation to hydrocarbon maturation and migration is of great important for assessing the exploration risk in volcanic rifted basins. Special research topics important for the petroleum system include the study of hydrothermal vents as possible migration path ways, and to investigate if sills (and, partly, dykes) can act as an efficient top seal of reservoirs or, alternatively, generate (through induced stresses and fracture formation) potential reservoirs in stiff layers far away from the sills themselves.