Abstract Lava flows form important fluid reservoirs and have been extensively exploited for water aquifers, geothermal energy, hydrocarbon production and, more recently, for carbon storage. Effusive subaerial mafic to intermediate lava flows account for vast rock volumes globally, and form reservoirs with properties dictated by well-known lava flow facies ranging from pāhoehoe through several transitional forms to ‘a’ā lava. These variations in flow type lead to critical differences in the pore structure, distribution, connectivity, strength and fracturing of individual lava flows, which, alongside lava flow package architectures, determine primary reservoir potential. Lava flow margins with vesicular, fracture and often autobreccia-hosted pore structures can have porosities commonly exceeding 40% and matrix permeabilities over 10 −11 m 2 (>10 D) separated by much lower porosity and permeability flow interiors. Secondary post-emplacement physicochemical changes related to fracturing, meteoric, diagenetic and hydrothermal alteration can significantly modify reservoir potential through a complex interplay of mineral transformation, pore-clogging secondary minerals and dissolution, which must be carefully characterized and assessed during exploration and appraisal. Within this contribution, a review of selected global lava flow-hosted reservoir occurrences is presented, followed by a discussion of the factors that influence lava flow reservoir potential.

Abstract Offshore CO 2 sequestration in basaltic formations of the North Atlantic Igneous Province may allow permanent storage of large volumes of CO 2 through rapid carbonate mineralization. Characterizing the internal architecture of such reservoirs is key to assessing the storage potential. In this study, six photogrammetry models and three boreholes on the Faroe Islands have been used to characterize the internal lava sequence architectures as a direct analogue to potential offshore North Atlantic Igneous Province storage sites. The studied formations are dominated by c. 5 to 50 m thick simple and compound lava flows, with drill core observations documenting a transition from pāhoehoe moving towards ‘a’ā lava flow types interbedded with thin (<5 m thick) volcaniclastic rock units. The identification of flow margin breccias is potentially important as these units form excellent reservoirs in several other localities globally. Stacked, thick simple flows may present sealing units associated with dense flow interiors. Connected porous and permeable lava flow crusts present potential reservoirs; however, the degree of secondary mineralization and alteration can alter initially good reservoir units to impermeable barriers for fluid flow. Large-scale reservoir volumes may be present mainly within both vesicular, fractured pāhoehoe and brecciated flow margins of transitional simple lava flows.

Abstract Understanding the formation of volcanic and epiclastic reservoirs is pivotal for exploring geoenergy resources such as geothermal energy, hydrocarbons, and new CO 2 sequestration and hydrogen storage opportunities. This paper examines the processes controlling the quality of pyroclastic and epiclastic reservoirs of the Kora volcano, an extinct stratocone presently buried in the offshore Taranaki Basin, New Zealand. We conduct detailed seismic reflection interpretation, drillcore lithofacies and wireline-log description, petrographic analysis, and analytical tests to generate a unified framework that explains the formation of volcaniclastic reservoirs from basin to pore-scale. Each stage of construction and degradation of the Kora volcano is associated with particular processes that increase or reduce reservoir quality. Primary processes include quench fragmentation, deuteric mineral dissolution, and epiclastic sedimentation. Secondary processes comprise mineral alteration (mainly meteoric; minor hydrothermal and diagenetic), mechanical stress fracturing (mainly tectonic; minor magmatic and burial deformation), and pervasive biogenic cementation. Epiclastic conglomerates present the highest reservoir quality (average 23% porosity and up to 997 mD permeability), followed by lapilli-tuffs and tuff-breccias. In contrast, bioclastic epiclastic sandstones are typically cemented by carbonates and pyrite. Our models and interpretations will increase understanding of the formation of volcaniclastic reservoirs and aid exploration of geoenergy resources in volcanic terrains.

ABSTRACT Field studies of tephra-fall deposits traditionally use the density of juvenile pyroclasts to determine vesicularity of the host magma at the point of fragmentation. A range of pyroclast sizes between 16 and 32 mm has commonly been chosen for this purpose. Larger pyroclasts outside this range may undergo postfragmentation vesiculation due to slow cooling of the interior of the clasts, while smaller pyroclasts may be too small to represent accurately the distribution of the largest vesicles. The assumption of this method, of course, is that the 16–32 mm size range is representative of the fragmented magma. We explore, in detail, variations in density over a size range of 4–128 mm from Unit 2 pyroclasts of the 1.8 ka Taupo eruption and make inferences about the roles of postfragmentation vesiculation and secondary breakage of pyroclasts. We find (1) there is a clear threshold for onset of postfragmentation vesiculation at >32 mm, and (2) there are broken small pieces of the largest pyroclasts in the sample that artificially skew the density distribution for smaller size fractions. We constrain uncertainty associated with vesicularity measurements and offer best-practice recommendations in the hope of improving consistency of field sampling and laboratory processing of pyroclast populations for vesicularity studies.