ALBERT

Description: Highlights • Identify new fine-grained hydrate filled fracture units in the Terrebonne Basin. • Identify new hydrate bearing thin sands, mostly within fractured muds. • Present detailed seismic amplitude maps of the new hydrate bearing units. • Discuss methane migration mechanisms and hydrate formation in thin sands. • Identify and discuss source-reservoir relationships between thick muds and thin sands. Abstract The interactions of microbial methane generation in fine-grained clay-rich sediments, methane migration, and gas hydrate accumulation in coarse-grained, sand-rich sediments are not yet fully understood. The Terrebonne Basin in the northern Gulf of Mexico provides an ideal setting to investigate the migration of methane resulting in the formation of hydrate in thin sand units interbedded with fractured muds. Using 3D seismic and well log data, we have identified several previously unidentified hydrate bearing units in the Terrebonne Basin. Two units are 〉100 m-thick fine-grained clay-rich units where gas hydrate occurs in near-vertical fractures. In some locations, these fine-grained units lack fracture features, and they contain 1–4-m thick hydrate bearing-sands. In addition, several other thin sand units were identified that contain gas hydrate, including one sand that was intersected by a well at the location of a discontinuous bottom-simulating reflector. Using correlation of well log data to seismic data, we have mapped and described these new units in detail across the extent of the available data, allowing us to determine the variation of seismic amplitudes and investigate the distribution of free gas and/or hydrate. We present several potential source-reservoir scenarios between the thick fractured mud units and thin hydrate bearing sands. We observe that hydrate preferentially forms within thin sand layers rather than fractures when sands are present in larger marine mud units. Based on regional mapping showing the patchy lateral extent of the thin sand layers, we propose that diffusive methane migration or short-migration of microbially generated methane from the marine mud units led to the formation of hydrate in these thin sands, as discontinuous sands would not be conducive to long-range migration of methane from deeper reservoirs.

Description: Submarine currents are a principal factor in controlling seafloor geomorphology. Herein, we investigate the role of dynamic current systems associated with the Subtropical Front in the formation and modification of seafloor depressions off the east coast of New Zealand’s South Island. Seafloor depressions are widespread in this region, with a diverse range of morphologies and sizes. We focus on two ‘end-member’ classes of depressions; densely spaced decametre-scale structures and more isolated ‘giant’ depressions of up to 12 km in diameter. Our results reveal a direct correlation between the dominant current flow direction, and the modification and alignment of depressions. We present a model to illustrate the role of submarine currents in shaping the morphology of these enigmatic seafloor depressions. This model demonstrates how contour currents, and potentially eddy currents, have extensively modified seafloor structures, resulting in elongate, asymmetrical depressions, partially infilled by sediment drift deposits.

Description: Highlights: • Identify 3 groups of gas migration structures in seismic data from the Danube Fan. • Migration structures related to shallow gas migration and flares at the seafloor. • Gas migration is controlled by lithological heterogeneity and sediment deformation. • Mass transport deposits play a role in controlling vertical migration occurrence. Abstract: A large geophysical dataset, including bathymetry, and 2D and 3D P-cable seismic data, revealed evidence of numerous gas flares near the S2 Canyon in the Danube Fan, northwestern Black Sea. This dataset allows us to investigate potential relationships between gas migration pathways, gas vents observed at the seafloor and submarine slope failures. Vertical gas migration structures as revealed in the seismics appear to be concentrated near submarine slope failure structures. Where these seismically defined features extend upwards to the seafloor, they correlate with the location of gas flares. However, not all these structures reach the seafloor, in some cases because they are capped by overlying sediments. A strong correlation is inferred between gas migration pathways, heterogeneous mass transport deposits and contacts between adjacent units of contrasting lithology. Although missing age constrains prevent a final judgement, we discuss the potential relationship between submarine slope failures and gas migration in order to determine if gas migration is a precursor to failure, or if the presence of slope failures and associated mass transport deposits facilitates the migration of gas. Our observations indicate that lithological heterogeneity, mass transport deposits and minor sediment deformation control gas migration pathways and the formation of gas chimney-like features. Gas migration is focused and gradual, resulting in gas flares where the chimney-like features extend to the seafloor, with no evidence of erosive features such as pockmarks.

Description: The goal of this study is to computationally determine the potential distribution patterns of diffusion-driven methane hydrate accumulations in coarse-grained marine sediments. Diffusion of dissolved methane in marine gas hydrate systems has been proposed as a potential transport mechanism through which large concentrations of hydrate can preferentially accumulate in coarse-grained sediments over geologic time. Using one-dimensional compositional reservoir simulations, we examine hydrate distribution patterns at the scale of individual sand layers (1-20 m thick) that are deposited between microbially active fine-grained material buried through the gas hydrate stability zone (GHSZ). We then extrapolate to two-dimensional and basin-scale three-dimensional simulations, where we model dipping sands and multilayered systems. We find that properties of a sand layer including pore size distribution, layer thickness, dip, and proximity to other layers in multilayered systems all exert control on diffusive methane fluxes toward and within a sand, which in turn impact the distribution of hydrate throughout a sand unit. In all of these simulations, we incorporate data on physical properties and sand layer geometries from the Terrebonne Basin gas hydrate system in the Gulf of Mexico. We demonstrate that diffusion can generate high hydrate saturations (upward of 90%) at the edges of thin sands at shallow depths within the GHSZ, but that it is ineffective at producing high hydrate saturations throughout thick (greater than 10 m) sands buried deep within the GHSZ. Furthermore, we find that hydrate in fine-grained material can preserve high hydrate saturations in nearby thin sands with burial.

Description: All around the world, beneath the seafloor, there are huge volumes of natural gas. But these are not the normal gas reservoirs that we collect to use for cooking, heating our homes, and making electricity in power stations. This gas is locked up in what we call gas hydrates. Gas hydrates are a solid form of water, rather like ice, that contains gas molecules locked up in a “cage” of water molecules. Gas hydrates are found on continental shelves around the world and in permafrost in the arctic. We are interested in gas hydrates because they could be used as a future source of natural gas. They are also important because they can cause large landslides on the seafloor, damaging offshore pipelines and cables and contributing to the formation of tsunami waves.

Description: These findings are described in the article entitled Investigating a gas hydrate system in apparent disequilibrium in the Danube Fan, Black Sea, recently published in the journal Earth and Planetary Science Letters (Earth and Planetary Science Letters 502 (2018) 1-11). This work was conducted by Jess I.T. Hillman, Ewa Burwicz, Timo Zander, Joerg Bialas, and Ingo Klaucke from GEOMAR Helmholtz Centre for Ocean Research, and Howard Feldman, Tina Drexler, and David Awwiller from the ExxonMobil Upstream Research Company.

Description: Did you know that we have better maps of the moon, Mars, and Venus than we do of the seafloor on Earth? Since oceans cover 71% of the Earth’s surface, understanding what the seafloor looks like, and where different processes, such as ocean currents are active, is hugely important. Mapping the seafloor helps us to work out things like where different types of fish live, where we might find resources, such as rare metals and fossil fuels, and whether there is a risk of underwater landslides happening that might cause a tsunami. Mapping the seafloor is very challenging, because we cannot use the same techniques that we would use on land. To map the deep ocean, we use a tool called a multibeam echo-sounder, which is attached to a ship or a submarine vessel.