Integrated characterization of igneous intrusions in Central Spitsbergen

Abstract

Igneous intrusions have a strong influence on petroleum systems, groundwater aquifers, and CO2 storage reservoirs, particularly because they can act as either barriers to, or carriers for fluid flow. This depends on several syn- and post emplacement processes, including fracture development in the intrusion and the host rocks, contact metamorphism of the host rocks, and hydrothermal activity. Especially a highly fractured intrusion-host rock interface can channelize fluids along the intrusion contact. This study integrates outcrop and core data from central Spitsbergen, Arctic Norway, in order to constrain the impact of Lower Cretaceous igneous intrusions on subsurface fluid flow within a siliciclastic reservoir (i.e. the Triassic Kapp Toscana Group), targeted for CO2 storage. The applied methods include fracture analyses of intrusions and host rocks, partly using digital geology and constructed 3D virtual outcrop models, and porosity analyses of the intrusion-proximal host rocks. At several localities, fracture sets both within the dolerites and the nearby host rocks, reflect the orientation of the intrusions. Fracture frequencies in general increase towards the intrusion contacts. Porosity analyses of host rocks positioned up to 4 m away from the intrusion contact of a 2.28 m thick sill in the DH4 borehole show generally low matrix porosities (on average 0.3%), with calcite cement observed to be the most abundant secondary mineral. In addition, calcite cement is frequently observed in fractures and veins within most intrusions and along the intrusion contacts, but also in intrusion-proximal host rock fractures and in faults that cross-cut the intrusions. In relation to the 2.28 m thick sill, a decrease in the occurrence of precipitated calcite, and an increase in host rock porosity is documented to occur somewhere between 4.82 m and 17.64 m from the intrusion contacts. This is largely attributed to hydrothermal activity occurring both during and after magma cooling. Based on the results from this study, a time transgressive conceptual model for rock evolution and fracture development in and around the igneous intrusions is presented, strongly benefited from 7993 fracture orientation measurements from virtual outcrop analyses. The results suggest that the igneous intrusions in central Spitsbergen primarily act as impermeable barriers to fluid flow. However, they can also provide high-permeability pathways along the highly fractured intrusion-host rock interface. Due to the presence of exceedingly fractured chilled margin of sills, a lack of calcite cement along sill-host rock contacts, no observed chilled margins of dykes, and the observation of thick (up to 10 cm) calcite veins along the dyke contacts, fluid flow is inferred to be higher along the margins of sills than along the margins of dykes. The combination of sills, dykes and other sealing features, can therefore explain why the targeted aquifer has been previously shown to be compartmentalized into several pressure units.