Three-dimensional Seismic Analysis and Modelling of Marine Hydrate Systems Offshore of Mauritania

Marine hydrates, which lock-up vast quantities of methane, are considered to be a prospective alternative energy source, a slow tipping point in the global carbon cycle and a probable trigger for submarine failures. In this thesis marine hydrate systems offshore of Mauritania and associated structural and sedimentary features are investigated by utilising two surveys of high-quality three-dimensional (3-D) seismic data. Interpreting them provides new insights into marine hydrate systems and how they respond to changes in ambient conditions. In one region of one of the 3-D seismic surveys, a shear zone covering 50 km2 is identified immediately above the hydrate bottom simulating reflector (BSR). It is considered to be the initial stages of a failure that did not result in widescale downslope transport of the succession. Due to this failure not going to completion, some free gas remains trapped at the level of the BSR. At this level the presence of free gases is supported by the continuous high-amplitude reflections. It is proposed that buoyancy built up by the inter-connected gas accumulation increases the pore pressure of the overlying hydrate-bearing to the level such that its base was critically stressed. In this research there is no seismic evidence for failures triggered by hydrate dissociation but the role of free gas in priming submarine failures is examined. Whether marine hydrates can release significant amounts of methane into the atmosphere is inconclusive. In this research a proposed model indicates that methane was re-captured in the hydrate stability zone after being liberated. Ocean warming since the last glacial maximum (LGM) gave rise to the shoaling of the base of the hydrate stability zone (HSZ). Gases released from hydrate accumulating at the base entered the HSZ, driven by buoyancy built up in the gas accumulation. The hydrate seal was breached and this is manifested by 15 gas chimneys in seismic data. Hydrates then re-formed at a specific level within the HSZ. This study implies that not all of methane would enter the ocean after released from hydrates and therefore the contribution of marine hydrates to the atmospheric methane budget may be not that much as it was predicted before. Gas venting is an effective way to transport methane at depth vertically to the ocean and an example of it is found in the feather edge of marine hydrate. This venting was possible due to the presence of faults above a salt diapir and is manifested by a series of pockmarks and mounds at the seabed. The BSR at this site is convex upwards and hence formed a trapping geometry for underlying free gases. Numerical model shows that this up-convex geometry is caused by the salt diapir having a higher thermal conductivity. Permeable migration conduits along the faults and excess pore pressure at the top of the trap allow for the happening of the venting. Compared with the neighbouring area where the BSR can be well observed, the region affected by diapirism has a limited scale of the observable BSR. This absence is proposed to result from the formed trap intercepting methane-rich pore fluid that would migrate landwards along the level of the base of the HSZ

Soil-Water Conservation, Erosion, and Landslide

The predicted climate change is likely to cause extreme storm events and, subsequently, catastrophic disasters, including soil erosion, debris and landslide formation, loss of life, etc. In the decade from 1976, natural disasters affected less than a billion lives. These numbers have surged in the last decade alone. It is said that natural disasters have affected over 3 billion lives, killed on average 750,000 people, and cost more than 600 billion US dollars. Of these numbers, a greater proportion are due to sediment-related disasters, and these numbers are an indication of the amount of work still to be done in the field of soil erosion, conservation, and landslides. Scientists, engineers, and planners are all under immense pressure to develop and improve existing scientific tools to model erosion and landslides and, in the process, better conserve the soil. Therefore, the purpose of this Special Issue is to improve our knowledge on the processes and mechanics of soil erosion and landslides. In turn, these will be crucial in developing the right tools and models for soil and water conservation, disaster mitigation, and early warning systems

Multi-scale 3D Imaging for Characterization of Microstructural Properties of Gas Shales

In recent years, gas shale has attracted renewed attention as an unconventional energy resource, with massive, fast-growing, and largely untapped reserves. Shale is a fine-grained sedimentary rock containing a high content of organic matter (kerogen) from which gas can be extracted. The identification of the pore structure and quantification of the geometry, sizes, volume, connectivity, and distribution of extremely fine-grain pores, kerogen, and minerals are all extremely significant for the characterization of gas shale reservoirs. These features determine fluid flow and ultimate hydrocarbon recovery, however, they are also highly challenging to determine accurately. X-ray micro and nano-computed tomography (μ-CT and Nano-CT) combined with 3D focused ion beam scanning electron microscopy (FIB-SEM) are used in this thesis to address this challenge and to provide more information for understanding the complex microstructures in 3D from multiple scales within shale samples. In this thesis, state-of-the-art multi-scale imaging with multi-dimensional potential was applied to the image and quantified the microstructures' properties of gas shale. Samples were first imaged with X-ray micro-and nano-tomography (μ-CT and Nano-CT), and then with Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) measurements. Results of image analysis using SEM (2D), μ-CT (3D), ultrahigh-resolution Nan-CT (3D), and FIBSEM (3D) under backscattered electron (BSE) images reveal a complex fine-grained structure at specified phases such as pores, kerogen, and minerals within samples. The results show low connectivity of pores and high connectivity of kerogen which suggests that porous gas flow through samples used in this study cannot be the main transport through the pores. This implies that the gas transport through the pores is unlikely to be important, but cannot be ignored, as it is very important and constitutes the basis for understanding permeability in the rock. However, the high connectivity of kerogen provides the potential pathways for gas flow throughout the whole sample. The combination of multiscale 3D X-ray CT techniques (micro-nano) with 3D FIB-SEM provides a powerful combination of tools for quantifying microstructural information including pore volume, size, pore aspect ratio and surface area to volume distributions, porosity, permeability in shales, and also allowing the visualization of pores, kerogen and minerals phases over a range of scales. Other methods, such as physical measurements Gas Research Institute (GRI), mercury injection (MIP), and nitrogen adsorption (N2) are also presented in this study. The combination of these data sets has allowed the examination of the microstructure of the shale in unprecedented depth across a wide range of scales (from about 20 nm to 0.5 mm). Overall, the shale samples from Bowland shale formation shows a porosity of 0.10 ± 0.01%, 0.52 ± 0.05%, and 0.94 ± 0.09% from three FIB-SEM measurements, 0.67 ± 0.009% from the nano-CT data and 0.06 ± 0.008% from one μ-CT measurement, which compare with 0.0235 ± 0.003% from nitrogen adsorption, and 0.60 ± 0.07% from MIP. The porosity was also observed to be 0.43± 0.009% and 0.7% ± 0.007% for FIB-SEM and Nano-CT methods, respectively in different shale reservoirs from a Sweden formation. The data vary due to the different scales at which each technique interrogates the rock and whether the pores are openly accessible (especially in the case of nitrogen adsorption). The measured kerogen fraction is 32.4 ± 1.45% from nano-CT compared with 34.8 ± 1.74%, 38.2 ± 1.91%, 41.4 ± 2.07%, and 44.5 ± 2.22% for three FIB-SEM and one μ-CT measurement. The pore size imaged by nano-CT ranged between 100 and 5000 nm, while the corresponding ranges were between 3 and 2000 nm for MIP analysis and between 2 and 90 nm for N2 adsorption. The distribution of pore aspect ratio and scale-invariant pore surface area to volume ratio (σ) as well as the calculated permeability shows the shale sample in this study to have a high shale gas potential. Aspect ratios indicate that most of the pores that contribute significantly to pore volume are oblate, which is confirmed by the range of σ (3−30). Oblate pores have greater potential for interacting with other pores compared to needle-shaped prolate pores as well as optimizing surface area for the gas to desorb from the kerogen into the pores. Permeability has also been calculated and values of 2.61 ± 0.42 nD were obtained from the nano-CT data, 2.65 ± 0.45 nD from MIP, 13.85 ± 3.45 nD, 4.16 ± 1.04 nD, and 150 ± 37.5 nD from three FIB-SEM measurements and 2.98 ± 0.75 nD from one μ-CT measurement, which are consistent with expectations for generic gas shales (i.e., tens of nD). The quantitative results of 2D and 3D imaging datasets across nm-μm-mm length scales provided a view of understanding the heterogeneous rock types, as well as great value to better understand, predict and model the pore structure, hydrocarbon transport, and production from gas shale reservoirs