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The seismic characteristics of focused fluid-related features such as pockmarks and carbonate build-ups on the Nigerian continental slope have been investigated using complementary seismic data sets (reprocessed 3D exploration data and 2D Very High Resolution hull-mounted and near-bottom seismic data) coupled with the results of previous studies (sedimentological, geotechnical and geochemical analyses). The results show different types of fluid-related features within the hemipelagic phase of long duration (∼1 Myr) Turbidite/Hemipelagic cycles. They are the product of the disturbance of the hosting sediments following two main distinct processes: (1) no or low sediment deposition caused by fluid escape associated with seafloor settlement (benthic fauna, carbonate precipitation, gas hydrate formation) and (2) post-depositional erosion, caused either by fluid release (dewatering from underlying features such as channels and upward migration from a deep source) or by collapse (destabilisation of gas hydrates). The temporal evolution reveals a decrease in fluid flow during the hemipelagic deposition as witnessed by the decrease in methanogenic activity and by gas hydrate destabilisation. The spatial distribution of fluid-related features seems to be controlled by the type of fault (small fault or structural lineament). [ABSTRACT FROM AUTHOR]

Abstract: A novel variable-volume type high-pressure apparatus has been designed, constructed and used for gas hydrate investigations. The apparatus has an operating temperature ranging from 253 K to 473 K and pressure ranging from 0.1 MPa to 60 MPa. Its central component consists of a viewing windows cell to which several sensors or analytical instruments can be connected. At its present configuration, a Raman spectrometer and a gas chromatograph are connected for the study of the liquid (or solid) and the gas phases respectively. The apparatus was used for the study of two different systems. The first system was composed of carbon dioxide (CO2) and water for which the hydrate formation and dissolution has been investigated by injecting water into liquid CO2. The evolution of the system was monitored by means of visual observation in combination with Raman spectroscopy. The second system consists of thermogenic-like gases (i.e. synthetic natural gas) for which the hydrate formation and dissociation have additionally been investigated by monitoring the change of the vapour phase composition. The consequences of the destruction of the hydrate skin formed at the water–gas interface have been studied. Thus, an attempt has been made to study the importance of the interfacial contact layer between the gas phase and the aqueous phase for the hydrate growth process. In this paper, we describe in detail the apparatus, followed by the presentation of the results on both systems investigated. [Copyright &y& Elsevier]

Natural Gas Hydrates (NGH) collected during the Ghass cruise 2015 in the Western Black Sea onboard the R/V Pourquoi pas? are characterized by a suite of techniques. Gas Chromatography and Raman spectroscopy are used for the identification of the nature of the gas source, the hydrate structure and spatial variability of cage occupancies. The nature and source of hydrate forming gases primarily reveal a high methane content (99.6 mol%) and small amount of nitrogen (>0.29 mol%) and CO 2 (0.056 mol%). Isotopic analyses from the hydrate-bound methane and recently published results from Pape et al. (2020) clearly indicate a microbial source of gas supplying the hydrate deposit generated by the reduction of carbon. For the first time, Raman imaging spectroscopy was applied on NGH recovered in the Western Black Sea. The results show a heterogeneous distribution of the encapsulated guest molecules (CH 4 , N 2 and H 2 S), which is associated with a spatial variability of the guest-gas composition at the micron-scale. Some portions of the 2D-Raman images clearly exhibit a relative N 2 -enrichment (with a concentration exceeding 6 mol% N 2 at some positions), while H 2 S shows a rather minor contribution on all the spectral maps investigated. A correlation is then established between the composition of the gas in the NGH and its impact on the CH 4 cage occupancy, with a ratio of θ LC /θ SC (large cage/small cage) between ~ 0.5 and 1.26 depending on the positions analyzed. The departure from the expected ratio in pure methane hydrate is attributed to the preferential encasement of N 2 in the large cage of the NGH structure. In addition, the occurrence of carotenoids identified in sediment-rich zones show a minor impact on the CH 4 cage occupancies. The results are discussed within the context of natural gas resource estimates in NGH to emphasize how the measured cage occupancies may impact the volumetric conversion factor commonly used with other geologic parameters to determine the resource endowment and global volume of methane. The small-scale heterogeneities revealed by the 2D-Raman images point out the importance to better understand stages of hydrates formation in methane-rich seafloor environment. • NGH from Black Sea are investigated by gas chromatography and Raman spectroscopy. • The hydrates are structure sI and contain mainly CH 4 and small amounts of N 2 and CO 2. • CH 4 being generated by the reduction of CO 2. • 2D-Raman reveals spatial variability in guests' distribution and hydrate composition. • Raman imaging analysis shows a spatial variability of the cage occupancy. [ABSTRACT FROM AUTHOR]

A range of plumbing systems has been identified in deep-water Nigeria by interpreting multi-scale seismic data sets (reprocessed 3D exploration seismic reflection data and 2D very high-resolution near-bottom seismic reflection data) as well as sedimentological and geochemical data obtained from a long sediment core recovered by the seafloor drill rig MARUM-MeBo70. The plumbing systems are characterized by diverse fault networks and lie above tectonic features including thrust/fold structures and strike-slip faults linked to gravity-driven deformation caused by underlying over-pressured shales. The plumbing systems are associated with fluid seeping structures at the seafloor such as pockmarks, mud volcanoes and seafloor zonations colonized by living benthic macrofauna typical of active fluid flow. The comparison of seismic stratigraphic sequences with climate and environmental proxies determined for sediments including lithology, element ratios such as Ca/Fe and Zr/Rb, sedimentation rates, and planktonic foraminifera δ18O records shows a control of short-term (0.1–0.4 Myr) and long-term (around 1 Myr) sedimentary cycles. During short-term fluctuations, the recurrence of fluid seeping structures is controlled by the lithology (coarse-grained versus fine-grained sediments) linked to glacial-interglacial fluctuations and a monsoon regime. Conditions favorable for fluid flow are related to the deposition of coarse-grained layers, glacial periods, sea-level lowstands, and low sedimentation rates. We hypothesize that cyclic gravity-driven sediment deformations controlled by glacial-interglacial and long-term variations in accommodation and sedimentation, have led to the cyclic evolution of the plumbing systems and associated seeping structures since the Plio-Pleistocene. • Diverse faults networks are associated with varied fluid flow structures. • The recurrence of fluid flow structures is coeval to glacial periods. • Gravity-driven deformations possibly control fluid flow during the Pleistocene. [ABSTRACT FROM AUTHOR]