CO 2 metasomatism in a basalt-hosted petroleum reservoir, Nuussuaq, West Greenland

Abstract

Extensive subaerial exposures of a basalt-hosted petroleum reservoir around Marraat Killiit on Nuussuaq, West Greenland, provide an unparalleled opportunity to investigate water–rock–hydrocarbon interactions in an unconventional petroleum system. Exposed sections in this locality include picritic and olivine-phyric basalts, with both subaerial and subaqueous textures that initially had significant primary porosity in the form of gas vesicles, scorias, and interclast voids within hyaloclastites. Alteration of these low-silica, high-magnesium basalts formed regionally-extensive, silica-deficient zeolite-facies mineral assemblages that dominantly consist of mixed-layer chlorite/smectite clays, thomsonite, natrolite, gonnardite, analcime and chabazite. In the study area, faulting and fracturing allowed petroleum migration from sediments into the overlying basalts. Migration of petroleum and the associated brine led to the pseudomorphic replacement of zeolite and clay mineral assemblages by carbonates (siderite, magnesite, dolomite and calcite) and quartz. Carbonates associated with petroleum migration are rich in Mg and Fe, reflecting the interaction of a CO 2-rich fluid with a picritic basalt and associated Fe- and Mg-rich smectites. Only late stage carbonates are calcium-rich, the result of depletion of aqueous magnesium and iron following precipitation of magnesite–siderite solid solutions. Values of δ 13C vPDB and δ 18O vSMOW of carbonates at Marraat form two distinct groups: (1) paragenetically early dolomite veins and magnesite–siderite replacement of low-grade metamorphic minerals that have δ 18O vSMOW between 12.6 and 20‰ and δ 13C vPDB in the approximate range of 0 ± 5‰, and (2) late stage calcite veins that are lighter in 18O ( δ 18O vSMOW between 6.4 and 8.7‰) and have a restricted range of δ 13C vPDB that is near zero. Comparison with isotopic properties of carbonates from petroleum reservoirs worldwide and carbonates formed from meteoric water in basalts from the North Atlantic Igneous Province suggests that the first group of carbonates are formed by reaction of oil field brines with basalts, but the latter group formed by reactions involving meteoric waters. Reservoir properties and quality are a sensitive function of the sequence of hydrothermal and deformation events affecting this region. Regional metamorphism resulted in extensive occlusion of primary porosity by mafic phyllosilicates and zeolites. Subsequent brittle deformation resulted in the formation of fracture sets that served as migration pathways for hydrocarbons and carbonate-rich brines. Interaction of these brines with the zeolite facies metabasalts resulted in extensive alteration reactions that replaced metamorphic mineral assemblages with carbonate + quartz, and generated significant secondary porosity that hosted the altering fluids and live hydrocarbons. Mineral paragenesis at Marraat indicates that zeolite and clay stability is a sensitive function of the oxidation state of the system and that this is controlled by water–hydrocarbon–basalt interactions.