The Impact of Igneous Intrusions on Sedimentary Host Rocks: Insights from Field Outcrop and Subsurface Data

Pervasive igneous intrusive complexes have been identified in many sedimentary basins which are prospective for petroleum exploration and production. Seismic reflection and well data from these basins has characterised many of these igneous intrusions as forming networks of interconnected sills and dykes, with distinctive morphologies and typically cross-cutting sedimentary host rocks. Intrusions have also been identified in close proximity to many oil & gas fields and exploration targets (e.g. Laggan-Tormore fields, Faroe Shetland Basin). It is therefore important to understand how igneous intrusions interact with sedimentary host rocks, specifically reservoir and source rock intervals, to determine the geological risk for petroleum exploration and production. The risks for petroleum exploration include low porosity and permeability within reservoirs, and overmaturity of source rocks, which are intruded. Additionally, reservoirs may be compartmentalised by low permeability igneous intrusions, inhibiting lateral and vertical migration of fluids. Based on a range of field studies and subsurface data, we demonstrate that sandstone porosity can be reduced by up to 20% (relative to background porosity) and the thermal maturity of organic rich claystones can be increased. The extent of host rock alteration away from igneous intrusions is highly variable and is commonly accompanied by mechanical compaction and fracturing of the host rock within the initial 10 to 20 cm of altered host rock. Reservoir quality and source rock maturity are key elements of the petroleum system and detrimental alteration of these intervals by igneous intrusions increases geological risk and should therefore be incorporated into any risk assessment of an exploration prospect or field development.