Description: Fluids are an important agent in nearly all geologic processes that shape the planet Earth. Fluid abundance and composition are governed by flow along permeable beds or natural and anthropogenic structures in the subsurface including faults, wells, and chimneys/pipes. Spatial and temporal variations in fluid flow activity modify total fluxes between geosphere, cryosphere, hydrosphere, and atmosphere. These fluxes have broad implications for geological processes including the formation of natural resources or the occurrence of geohazards including landslides, earthquakes and blowouts. They further play a crucial role for the global carbon cycles and the climate system. A qualitative and quantitative understanding of fluid flow in the subsurface is therefore important to assess the role of fluids in the Earth system and to quantify fluxes from the geosphere into the hydro- and atmosphere. In this Ph.D. thesis I use an integrated, interdisciplinary approach to study natural and anthropogenic fluid migration pathways in marine sediments in the North Sea, the convergent Hikurangi margin, and a section of the ancient Tethys margin which is now exposed near Varna, Bulgaria. The applied methods include conventional 3D seismic, high-resolution 3D seismic, and 2D seismic data as well as hydroacoustic, sedimentological, unmanned aerial vehicle-based photogrammetric and geochemical data. In each of the studied systems, natural and/or anthropogenic fluid migration pathways allow the transport of significant amounts of fluids through marine sediments towards the seafloor. Often the co-existence of multiple pathways enables the fluids to bypass permeability barriers within the Earth’s crust resulting in the formation of structurally complex flow systems. Focused fluid flow along normal faults in the Hikurangi margin likely plays an active role in the subduction drainage system, influences the slope stability and the morphotectonic evolution of the margin. Results from the Eocene Tethys margin show that focused fluid flow in marine sediments is possible in unconsolidated sands if seepage is focused at the top of faulted units and the flux rate is high enough. This stands in contrast to the general assumption that focused fluid flow in marine sediments is limited to low-permeable sediments. In the marine environment the term fluid flow is often used to exclusively refer to the flow of hydrocarbons. However, geochemical data from the North Sea and the Tethys margin indicate that the involved fluids are of different origin including compaction-related dehydration and submarine groundwater discharge. In each of the investigated cases, the temporal and spatial evolution of fluid flow is not fully addressed yet, especially with regard to vertical fluid conduits or the safety of subsurface drilling and storage operations. The results of my thesis highlight that the investigation of fluid migration pathways requires an interdisciplinary approach which may indicate the origin of the fluids, help understand the fluxes of fluids from the geosphere into the hydrosphere and atmosphere of the past, present and future and reveal the resulting consequences for the global carbon cycles and the climate system.

Description: Highlights • Gas release from wells may counteract efforts to mitigate greenhouse gas emissions. • An approach for assessing methane release from marine decommissioned wells. • This gas release largely depends on the presence of shallow gas accumulations. • Methane release from hydrocarbon wells represents a major source in the North Sea. Abstract Hydrocarbon gas emissions from with decommissioned wells are an underreported source of greenhouse gas emissions in oil and gas provinces. The associated emissions may partly counteract efforts to mitigate greenhouse gas emissions from fossil fuel infrastructure. We have developed an approach for assessing methane leakage from marine decommissioned wells based on a combination of existing regional industrial seismic and newly acquired hydroacoustic water column imaging data from the Central North Sea. Here, we present hydroacoustic data which show that 28 out of 43 investigated wells release gas from the seafloor into the water column. This gas release largely depends on the presence of shallow gas accumulations and their distance to the wells. The released gas is likely primarily biogenic methane from shallow sources. In the upper 1,000 m below the seabed, gas migration is likely focused along drilling-induced fractures around the borehole or through non-sealing barriers. Combining available direct measurements for methane release from marine decommissioned wells with our leakage analysis suggests that gas release from investigated decommissioned hydrocarbon wells is a major source of methane in the North Sea (0.9-3.7 [95% confidence interval = 0.7-4.2] kt yr−1 of CH4 for 1,792 wells in the UK sector of the Central North Sea). This means hydrocarbon gas emissions associated with marine hydrocarbon wells are not significant for the global greenhouse gas budget, but have to be considered when compiling regional methane budgets.

Description: Submarine gravity flows are ubiquitous across the seafloor, occurring in all sizes and are the primary mechanism for transporting sediment from the shelf to the deep ocean. Giant flows are an enigmatic phenomenon because they often do not originate from an appropriately large landslide. Theoretical arguments propose that giant events can ignite from much smaller flows. However, quantifying how much a flow can enlarge is problematic due to their extreme size. Here, we reconstruct the properties and evolution of a giant gravity flow by mapping its traces from source to sink. The initial failure (~ 0.8 km 3 ) entrained ~ 200 times its starting volume: catastrophically evolving into a giant flow with a total volume of ~ 162 km 3 with estimated flow speeds between 15–30 m/s, and a run-out of ~ 2000 km. The entrainment of mud was the critical fuel for ignition, which promoted run-away flow growth and extreme levels of erosion.

Description: Focused fluid flow shapes the evolution of marine sedimentary basins by transferring fluids and pressure across geological formations. Vertical fluid conduits may form where localized overpressure breaches a cap rock (permeability barrier) and thereby transports overpressured fluids towards shallower reservoirs or the surface. Field outcrops of an Eocene fluid flow system at Pobiti Kamani and Beloslav Quarry (ca 15 km west of Varna, Bulgaria) reveal large carbonate‐cemented conduits, which formed in highly permeable, unconsolidated, marine sands of the northern Tethys Margin. An uncrewed aerial vehicle with an RGB sensor camera produces ortho‐rectified image mosaics, digital elevation models and point clouds of the two kilometre‐scale outcrop areas. Based on these data, geological field observations and petrological analysis of rock/core samples; fractures and vertical fluid conduits were mapped and analyzed with centimetre accuracy. The results show that both outcrops comprise several hundred carbonate‐cemented fluid conduits (pipes), oriented perpendicular to bedding, and at least seven bedding‐parallel calcite cemented interbeds which differ from the hosting sand formation only by their increased amount of cementation. The observations show that carbonate precipitation likely initiated around areas of focused fluid flow, where methane entered the formation from the underlying fractured subsurface. These first carbonates formed the outer walls of the pipes and continued to grow inward, leading to self‐sustaining and self‐reinforcing focused fluid flow. The results, supported by literature‐based carbon and oxygen isotope analyses of the carbonates, indicate that ambient seawater and advected fresh/brackish water were involved in the carbonate precipitation by microbial methane oxidation. Similar structures may also form in modern settings where focused fluid flow advects fluids into overlying sand‐dominated formations, which has wide implications for the understanding of how focusing of fluids works in sedimentary basins with broad consequences for the migration of water, oil and gas.

Description: Highlights • Unprecedented dense coverage of ocean-bottom seismometer data reveals seismic velocity variations within a vertical fluid pathway. • There are zones of both positive (faster) and negative (slower) velocity within the fluid pathway compared to the background formation velocities. • Velocity reductions are related to free gas in the fluid pathway, while the reason for velocity increases is unclear but potentially caused by cementation. Abstract Subsurface CO2 storage is a key strategy to reduce greenhouse gas emission, but leakage of CO2 along natural fluid pathways may affect storage formation integrity. However, the internal structure and the physical properties of these focused fluid conduits are poorly understood. Here, we present a three-dimensional seismic velocity model of an active fluid conduit beneath the Scanner Pockmark in the Central North Sea, derived from ocean-bottom seismometer data. We show that the conduit, which manifests as a pipe structure in seismic data, is separated into two parts. The upper part, extending to 260 m depth, i.e. 110 m below the seafloor, is characterised by seismic velocities up to 100 m/s slower than the surrounding strata. The deeper part is characterized by a 50 m/s seismic velocity increase compared to background velocity. We suggest that the upper part of the pipe structure represents a network of open fractures, partly filled with free gas, while the reason for the velocity increase in the lower part remains speculative. These observations suggest that active pipes can be internally heterogeneous with some intervals probably being open fluid pathways and other intervals being closed. This study highlights the complexity in evaluating focused fluid conduits and the necessity of their detailed assessment when selecting CO2 storage sites.

Description: Seafloor heat flow provides information about the thermal evolution of the lithosphere, the magnitude and timing of volcanic activity, and hydrothermal circulation patterns. In the central Gulf of California, the Guaymas Basin is part of a young marginal spreading rift system that experiences high sedimentation (1–5 km/Myr) and widespread magmatic intrusions in the axial troughs and the off-axis regions. Heat flow variations record magmatic and sedimentary processes affecting the thermal evolution of the basin. Here, we present new seismic evidence of a widespread bottom-simulating reflection (BSR) in the northwestern Guaymas Basin. Using the BSR depths and thermal conductivity measurements, we determine geothermal gradient and surface heat flow variations. The BSR-derived heat flow values are less than the conductive lithospheric heat flow predictions for mid-oceanic ridges. They suggest that high sedimentation (0.3–1 km/Myr) suppresses the lithospheric heat flow. In the central and southeastern regions of the basin, the BSR-derived geothermal gradient increases as the intruded magmatic units reach shallower subsurface depths. Thermal modeling shows that recent (〈5000 years) igneous intrusions (〈500 m below the seafloor) and associated fluid flow elevate the surface heat flow up to five times. BSR-derived geothermal gradients correlate little with the depth of the shallowest magmatic emplacements to the north, where the intrusions have already cooled for some time, and the associated hydrothermal activity is about to shut down. Key Points - A regional bottom-simulating reflection (BSR) in the Guaymas Basin indicates a widespread occurrence of gas hydrates - The BSR derived thermal gradients show wavy patterns farther away from the spreading centre, indicating strong lateral heat flow variations - High sedimentation suppresses heat flow, while recent magmatic intrusion and fluid advection increase heat flow

Description: We present porosity and free gas estimations and their uncertainties at anactive methane venting site in the UK sector of the North Sea. In the Scan-ner Pockmark area in about 150m water depth, we performed a multi-disciplinary experiment to investigate the physical properties of fluid flowstructures within unconsolidated glaciomarine sediments. Here we focus onthe towed controlled source electromagnetic (CSEM) data analysis with con-straints from seismic reflection and core logging data. Inferred backgroundresistivity values vary between 0.6–1 Ωm at the surface and 1.9–2.4 Ωm at150 mbsf. We calibrate Archie’s parameters with measurements on cores, andestimate porosities of about 50±10% at the seafloor decreasing to 25±3% at 150 mbsf which matches variations expected for mechanical compaction ofclay rich sediments. High reflectivity in seismic reflection data is consistentwith the existence of a gas pocket. A synthetic study of varying gas contentin this gas pocket shows that at least 33±8% of free gas are required to causea distinct CSEM data anomaly. Real data inversions with seismic constraintssupport the presence of up to 34±14% free gas in a 30–40 m thick gas pocketunderneath the pockmark within the stratigraphic highs of a till layer abovethe glacial unconformity in the Aberdeen Ground Formation.

Description: Abrupt fluid emissions from shallow marine sediments pose a threat to seafloor installations like wind farms and offshore cables. Quantifying such fluid emissions and linking pockmarks, the seafloor manifestations of fluid escape, to flow in the sub-seafloor remains notoriously difficult due to an incomplete understanding of the underlying physical processes. Here, using a compositional multi-phase flow model, we test plausible gas sources for pockmarks in the south-eastern North Sea, which recent observations suggest have formed in response to major storms. We find that the mobilization of pre-existing gas pockets is unlikely because free gas, due to its high compressibility, damps the propagation of storm-induced pressure changes deeper into the subsurface. Rather, our results point to spontaneous appearance of a free gas phase via storm-induced gas exsolution from pore fluids. This mechanism is primarily driven by the pressure-sensitivity of gas solubility, and the appearance of free gas is largely confined to sediments in the vicinity of the seafloor. We show that in highly permeable sediments containing gas-rich pore fluids, wave-induced pressure changes result in the appearance of a persistent gas phase. This suggests that seafloor fluid escape structures are not always proxies for overpressured shallow gas and that periodic seafloor pressure changes can induce persistent free gas phase to spontaneously appear. Key Points - Storm-induced pressure changes can lead to spontaneous appearance of free gas phase near the seafloor - This process is driven by pressure-sensitive phase instabilities - This mechanism could help explain elusive gas sources in recently observed pockmarks in the North Sea

Description: Highlights • An artificial CO2 release demonstrated MMV techniques for offshore CCS. • Detection of leakage was demonstrated using acoustic, chemical and physical approaches. • Attribution of leakage was proved possible using artificial and natural tracer compounds. • Leakage quantification was possible using approaches not previously applied to CCS studies. • Non-catastrophic leaks were detected at levels below those that would cause environmental harm. Carbon capture and storage is a key mitigation strategy proposed for keeping the global temperature rise below 1.5 °C. Offshore storage can provide up to 13% of the global CO2 reduction required to achieve the Intergovernmental Panel on Climate Change goals. The public must be assured that potential leakages from storage reservoirs can be detected and that therefore the CO2 is safely contained. We conducted a controlled release of 675 kg CO2 within sediments at 120 m water depth, to simulate a leak and test novel detection, quantification and attribution approaches. We show that even at a very low release rate (6 kg day−1), CO2 can be detected within sediments and in the water column. Alongside detection we show the fluxes of both dissolved and gaseous CO2 can be quantified. The CO2 source was verified using natural and added tracers. The experiment demonstrates that existing technologies and techniques can detect, attribute and quantify any escape of CO2 from sub-seabed reservoirs as required for public assurance, regulatory oversight and emissions trading schemes.

Description: In 1964, exploration drilling in the German Sector of the North Sea hit a gas pocket at ∼2900 m depth below the seafloor and triggered a blowout, which formed a 550 m-wide and up to 38 m deep seafloor crater now known as Figge Maar. Although seafloor craters formed by fluid flow are very common structures, little is known about their formation dynamics. Here, we present 2D reflection seismic, sediment echosounder, and multibeam echosounder data from three geoscientific surveys of the Figge Maar blowout crater, which are used to reconstruct its formation. Reflection seismic data support a scenario in which overpressured gas ascended first through the lower part of the borehole and then migrated along steeply inclined strata and faults towards the seafloor. The focused discharge of gas at the seafloor removed up to 4.8 Mt of sediments in the following weeks of vigorous venting. Eyewitness accounts document that the initial phase of crater formation was characterized by the eruptive expulsion of fluids and sediments cutting deep into the substrate. This was followed by a prolonged phase of sediment fluidization and redistribution widening the crater. After fluid discharge ceased, the Figge Maar acted as a sediment trap reducing the crater depth to ∼12 m relative to the surrounding seafloor in 2018, which corresponds to an average sedimentation rate of ∼22,000 m 3 /yr between 1995 and 2018. Hydroacoustic and geochemical data indicate that the Figge Maar nowadays emits primarily biogenic methane, predominantly during low tide. The formation of Figge Maar illustrates hazards related to the formation of secondary fluid pathways, which can bypass safety measures at the wellhead and are thus difficult to control.