ALBERT

Description: This dataset includes multi-channel seismic reflection data from three surveys: https://doi.pangaea.de/10.1594/PANGAEA.925803. Post-stack time migrated seismic sections that were collected in 2018, during Research Voyage TAN1808 aboard RV Tangaroa. Multi-channel seismic reflection data from the APB13 survey, collected by Anadarko Petroleum Company, in 2013. We have re-processed data from Line APB13-25, and have displayed industry processing of Line APB13-32. Multi-channel seismic reflection data from Voyage SO214 aboard RV Sonne in 2011 This datasets also includes bathymetry data, seafloor backscatter data and water column backscatter data. These data were all collected in 2018 during Research Voyage TAN1808 aboard RV Tangaroa: https://doi.pangaea.de/10.1594/PANGAEA.925804 This dataset also includes gridded horizons generated from seismic interpretation: https://doi.pangaea.de/10.1594/PANGAEA.925896

Description: This data set includes bathymetry data, seafloor backscatter data and water column backscatter data displayed in Figure 1c-e of Crutchley et al. (submitted for review to JGR Solid Earth, 2020). These data were all collected in 2018 during Research Voyage TAN1808 aboard RV Tangaroa, east of New Zealand. The voyage report describing data collection is located here: https://www.gns.cri.nz/Home/Our-Science/Energy-Futures/Gas-Hydrates/Recent-Expeditions/HYDEE-I-TAN1808/TAN-1808-report-2018

Description: The Hikurangi Margin off the east coast of the North Island (Te Ika-a-Māui) is a tectonically active subduction zone and the location of New Zealand’s largest gas hydrate province. Faults are internally complex volumetric zones that may play a significant role in the migration of fluids beneath the seafloor. The combined processes of deformation and fluid migration result in the formation of concentrated hydrate accumulations along accretionary ridges. It is not fully understood to what extent faults control fluid migration along the Hikurangi Margin, and whether deep-seated thrust faults provide a pathway for thermogenic gas to migrate up from sources at depth. Using 2D models based on seismic data from the region we investigated the role of thrust faults in facilitating fluid migration and contributing to the formation of concentrated gas hydrates. By altering permeability properties of the fault zones in these transient state models we can determine whether faults are required to act as fluid flow pathways. In this study we focus on two study sites offshore southern Wairarapa, using realistic yet simplified fault geometries derived from 2D seismic lines. The results of these models allow us to start to disentangle the complex relationship between fault zone structure, permeability, geometry, fluid migration and gas hydrate formation. Based on the model outputs we propose that faults act as primary pathways facilitating fluid migration and are critical in the formation of concentrated gas hydrate deposits.

Description: Highlights • There is direct and indirect evidence for hydrate occurrence in several areas around Europe. • Hydrate is particularly widespread offshore Norway and Svalbard and in the Black Sea. • Hydrate occurrence often coincides with conventional thermogenic hydrocarbon provinces. • The regional abundance of hydrate in Europe is poorly known. Abstract Large national programs in the United States and several Asian countries have defined and characterised their marine methane hydrate occurrences in some detail, but European hydrate occurrence has received less attention. The European Union-funded project “Marine gas hydrate – an indigenous resource of natural gas for Europe” (MIGRATE) aimed to determine the European potential inventory of exploitable gas hydrate, to assess current technologies for their production, and to evaluate the associated risks. We present a synthesis of results from a MIGRATE working group that focused on the definition and assessment of hydrate in Europe. Our review includes the western and eastern margins of Greenland, the Barents Sea and onshore and offshore Svalbard, the Atlantic margin of Europe, extending south to the northwestern margin of Morocco, the Mediterranean Sea, the Sea of Marmara, and the western and southern margins of the Black Sea. We have not attempted to cover the high Arctic, the Russian, Ukrainian and Georgian sectors of the Black Sea, or overseas territories of European nations. Following a formalised process, we defined a range of indicators of hydrate presence based on geophysical, geochemical and geological data. Our study was framed by the constraint of the hydrate stability field in European seas. Direct hydrate indicators included sampling of hydrate; the presence of bottom simulating reflectors in seismic reflection profiles; gas seepage into the ocean; and chlorinity anomalies in sediment cores. Indirect indicators included geophysical survey evidence for seismic velocity and/or resistivity anomalies, seismic reflectivity anomalies or subsurface gas escape structures; various seabed features associated with gas escape, and the presence of an underlying conventional petroleum system. We used these indicators to develop a database of hydrate occurrence across Europe. We identified a series of regions where there is substantial evidence for hydrate occurrence (some areas offshore Greenland, offshore west Svalbard, the Barents Sea, the mid-Norwegian margin, the Gulf of Cadiz, parts of the eastern Mediterranean, the Sea of Marmara and the Black Sea) and regions where the evidence is more tenuous (other areas offshore Greenland and of the eastern Mediterranean, onshore Svalbard, offshore Ireland and offshore northwest Iberia). We provide an overview of the evidence for hydrate occurrence in each of these regions. We conclude that around Europe, areas with strong evidence for the presence of hydrate commonly coincide with conventional thermogenic hydrocarbon provinces.

Description: Highlights • Recently acquired high-resolution seismic data and existing low-resolution industry data are presented. • Two large concentrated hydrate deposits are identified beneath Glendhu and Honeycomb ridges. • A novel method involving analysis of seismic velocity and reflectivity is used to obtain estimates of hydrate saturations. • Hydrate saturations peaks of 〉80% are estimated locally. • The main driving mechanism for hydrate accumulations is inferred to be along-strata gas migration. Abstract In the southern Hikurangi subduction margin, widespread gas hydrate accumulations are inferred based on the presence of bottom simulating reflections and recovered gas hydrate samples, mainly associated with thrust ridges. We present a detailed analysis of high- and medium-resolution seismic reflection data across Glendhu and Honeycomb ridges, two elongated four-way closure systems at the toe of the deformation wedge. High-amplitude reflections within the gas hydrate stability zone, coincident with high seismic velocities, suggest the presence of highly concentrated gas hydrate accumulations in the core regions of the anticlinal ridges. A novel method involving combined seismic velocity and reflectivity analysis and rock physics modelling is used to estimate hydrate saturations in localised areas. The effective medium model consistently predicts gas hydrate saturations of ~30% of the pore space at Glendhu Ridge and 〉60% at Honeycomb Ridge, whereas the empirical three-phases weighted equation likely underestimates the amount of gas hydrate present. We note that our estimates are dependent on the vertical resolution of the seismic data (5–14 m), and that the existence of thin layers hosting gas hydrate at higher concentrations is likely based on observations made elsewhere in similar depositional environments. A comparison between the two ridges provides insights into the evolution of thrust related anticlines at the toe of the accretionary wedge. We propose that the main driving mechanism for concentrated hydrate accumulation in the study area is along-strata gas migration. The vertical extent of these accumulations is a function of the steepness of the strata crossing the base of gas hydrate stability, and of the volume of sediments from which fluid flows into each structure. According to our interpretation, older structures situated further landward ofthe deformation front are more likely to host more extensive concentrated hydrate deposits than younger ridges situated at the deformation front and characterised by more gentle folding. The method introduced in this work is useful to retrieve quantitative estimates of gas hydrate saturations based on multi-channel seismic data.

Description: The highest concentration of cold seep sites worldwide has been observed along convergent margins, where fluid migration through sedimentary sequences is enhanced by tectonic deformation and dewatering of marine sediments. In these regions, gas seeps support thriving chemosynthetic ecosystems increasing productivity and biodiversity along the margin. In this paper, we combine seismic reflection, multibeam and split-beam hydroacoustic data to identify, map and characterize five known sites of active gas seepage. The study area, on the southern Hikurangi Margin off the North Island of Aotearoa/New Zealand, is a well-established gas hydrate province and has widespread evidence for methane seepage. The combination of seismic and hydroacoustic data enable us to investigate the geological structures underlying the seep sites, the origin of the gas in the subsurface and the associated distribution of gas flares emanating from the seabed. Using multi-frequency split-beam echosounder (EK60) data we constrain the volume of gas released at the targeted seep sites that lie between 1,110 and 2,060 m deep. We estimate the total deep-water seeps in the study area emission between 8.66 and 27.21 × 10 6 kg of methane gas per year. Moreover, we extrpolate methane fluxes for the whole Hikurangi Margin based on an existing gas seep database, that range between 2.77 × 10 8 and 9.32 × 10 8 kg of methane released each year. These estimates can result in a potential decrease of regional pH of 0.015–0.166 relative to the background value of 7.962. This study provides the most quantitative assessment to date of total methane release on the Hikurangi Margin. The results have implications for understanding what drives variation in seafloor biological communities and ocean biogeochemistry in subduction margin cold seep sites.

Description: Sub-seabed fluid flow, gas hydrate accumulation and seafloor methane seepage are tightly interwoven processes with implications for marine biodiversity, ocean chemistry and seafloor stability. We combine long-offset seismic reflection data with high-resolution seismic data to investigate shallow structural deformation and its relationship to focused gas migration and hydrate accumulation in the southern Hikurangi subduction wedge. Anticlines, effective traps for focusing free gas, are characterized by both normal faults and vertical zones of hydraulic fracturing within the hydrate stability zone. The normal faults form as a result of sediment layer folding and gravitational collapse of ridges during uplift. We document both longitudinal (ridge-parallel) and transverse (ridge-perpendicular) extensional structures (normal faults and elongated hydraulic fracture zones) in the sub-seafloor of anticlinal ridges. Intriguingly, gas flow through ridges close to the deformation front of the wedge exploits longitudinal structures, while ridges further inboard are characterized by gas flow along transverse structures. This highlights pronounced changes in the shallow deformation of ridges in different parts of the wedge, associated with a switching of the least and intermediate principal stress directions. It is critical to understand these shallow stress fields because they control fluid flow patterns and methane seepage out of the seafloor. Key Points Gas migration through ridges occurs along both longitudinal (ridge-parallel) and transverse (ridge-perpendicular) zones of fracturing Shallow stress fields differ significantly between ridges, reflecting differences in ridge evolution and deformation Seismic reflection images of the base of gas hydrate stability and gas-water contacts are strongly affected by seismic frequency content