Architectural elements of buried volcanic systems and their impact on geoenergy resources.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This PhD investigates the architecture of volcanic systems buried in New Zealand sedimentary basins. These “fossil” volcanoes occur in great numbers around the globe, typically comprising complex magmatic-sedimentary systems that produce large impacts in the evolution of the host sedimentary basin. The interaction between magmatism and sedimentation creates a range of geological conditions that can favour the occurrence of geoenergy resources, such as hydrocarbons and geothermal energy. Interpretation of volcanoes in the subsurface requires a multidisciplinary approach that combines insights from complementary disciplines, such as sedimentology, stratigraphy, and volcanology into a unified model. Over the last two decades, knowledge of volcanic systems in sedimentary basins has increased significantly, largely due to improvements in the quality and availability of seismic reflection data. This PhD research uses data from 2D and 3D seismic reflection surveys to characterise the spatio-temporal distribution of the fundamental building blocks (i.e. architectural elements) of buried volcanic systems, aiming to provide insights for exploration of geoenergy resources. Here, we divide the stratigraphic record of these “volcanic basins” into three first-order magmatic sequences (i.e. pre-, syn- and post-magmatic), which can be sub-divided into second-order magmatic stages related to the emplacement, construction, degradation and burial of the volcanoes.
Two case-study areas have been utilised to determine how the architectural elements vary systematically in buried monogenetic and polygenetic volcanic systems. These study areas are the Kora Volcanic System (KVS) and Maahunui Volcanic System (MVS), located in the Taranaki and Canterbury basins respectively. Both volcanic systems formed in marine environments during the Miocene, and show systematic spatio-temporal distributions of architectural elements. In both cases, each one of the magmatic sequences and stages are characterised by a network of genetically related architectural elements, formed by interactions between intrusions, eruptions and sedimentation. Syn-intrusive architectural elements are formed by magma emplaced into the host basin strata (emplacement stage), and include hypabyssal intrusions such as sills, dikes and small plutonic bodies, together with associated deformed strata. Syn-eruptive and inter-eruptive architectural elements are formed during the constructional stage, and include all primary eruptive, epiclastic, and associated sedimentary deposits formed during active and quiescent volcanism. Post-magmatic architectural elements are formed during the passive degradational and burial stages of volcanism, comprising sedimentary deposits impacted by the presence of volcanic structures, which can influence sedimentation millions of years after complete burial of volcanic edifices.
In detail, the architectural elements of each volcanic system also display differences. Dikes and sills of the KVS plumbing system typically formed along, or branching from, simultaneous Miocene rift faults. Explosive submarine volcanism in Kora formed a large-volume (ca 95 km³) basaltic-andesitic compound volcano erupted from a fixed central conduit, which dominated seafloor topography and created localized debris deposits that interfinger with hemipalagic sediments and deep-water channel deposits. In contrast, the monogenetic MVS plumbing system distributed magma to dispersed eruptive centres, which formed small-volume (< 6 km³) basaltic submarine volcanoes equivalent of maar-diatreme and tuff cones erupted at ca 1000 m depth. Degradation of volcanoes in the MVS was differential, with strong erosion on the top of shallower and higher edifices that were emergent above sea-level during a late Miocene base-level fall, while volcanoes that remain below the sea-level were not subject to significant degradation and are now well preserved beneath bathyal sediments.
Analysis of the architectural elements of the KVS and MVS provide insights into the exploration of geoenergy resources in buried and active volcanic systems elsewhere. Intrusions and magmatic deformation, including large saucer-sills and intrusion swarms, have the potential to produce four-way closures and reservoirs that can host significant oil and gas accumulations. These intrusions can also produce high-temperature intrusion-related geothermal systems. Both petroleum and geothermal systems can be enriched in CO₂, CH₄, and H₂S if the intrusions were emplaced in carbonate or organic-rich host rocks. Eruptive and sedimentary architectural elements can form substantial hydrocarbon fields with reservoirs in paleogeomorphic structures formed by changes in lithologies and by the presence of stratigraphic discontinuities between the volcano and enclosing sedimentary strata. Syn-eruptive reservoirs can be sealed, if volcanic structures are buried by fine-grained marine sediments or evaporitic rocks. Progressive burial of the volcanoes can create ideal conditions for deposition of high-quality carbonate reservoirs located on topographic seabed highs above buried volcanic structures. Due to differential compaction between the volcanoes and enclosing sedimentary rocks, these carbonate reservoirs can be entrapped in large four-way closures, with potential to host world-class hydrocarbon fields. Therefore, analysis of volcano-stratigraphic architecture of buried volcanoes can be used to build models for the exploration of geoenergy resources such as hydrocarbons and geothermal electricity.