Focused hydrocarbon‐migration in shallow sediments of a pockmark cluster in the Niger Delta (Off Nigeria)

Prunelé, Alexis De; Ruffine, Livio; Riboulot, Vincent; Peters, Carl A.; Croguennec, Claire; Guyader, Vivien; Pape, Thomas; Bollinger, Claire; Bayon, Germain; Caprais, Jean Claude; Germain, Yoan; Donval, Jean Pierre; Marsset, Tania; Bohrmann, Gerhard; Géli, Louis; Rabiu, Abdulkarim; Lescanne, Marc; Cauquil, E.; Sultan, Nabil

doi:10.1002/2016gc006554

Cited by 21 publications

(28 citation statements)

References 78 publications

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“…For core CL44, DIC concentrations and δ 13 C DIC values were determined using a continuous flow mass spectrometer (Thermo Fisher Delta-Plus). 0.5 ml porewater was treated with pure H 3 PO 4 in a glass vial at 25°C.…”

Section: Methodsmentioning

confidence: 99%

“…Enhancement in upward methane flux could also result in kink-type or concave-up porewater profiles [9][10][11][12]. Hence, these types of nonlinear porewater profiles can thus be used to estimate the timing of (sub)recent methane pulse and provide insights into the dynamics of methane seepage and the underlying gas hydrate reservoir [10,11,13,14].…”

Section: Introductionmentioning

confidence: 99%

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Methane Source and Turnover in the Shallow Sediments to the West of Haima Cold Seeps on the Northwestern Slope of the South China Sea

et al. 2019

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The Haima cold seeps are active cold seep areas that were recently discovered on the northwestern slope of the South China Sea (SCS). Three piston cores (CL30, CL44, and CL47) were collected within an area characterized by bottom simulating reflectors to the west of Haima cold seeps. Porewater profiles of the three cores exhibit typical kink-type feature, which is attributed to elevated methane flux (CL30) and bubble irrigation (CL44 and CL47). By simulating the porewater profiles of SO42-, CH4, PO43-, Ca2+, Mg2+, and dissolved inorganic carbon (DIC) in CL44 and CL47 using a steady-state reaction-transport model, we estimated that the dissolved SO42- was predominantly consumed by anaerobic oxidation of methane (AOM) at rates of 74.3 mmol m−2 yr−1 in CL44 and 85.0 mmol m−2 yr−1 in CL47. The relatively high AOM rates were sustained by free gas dissolution rather than local methanogenesis. Based on the diffusive Ba2+ fluxes and the excess barium contents in the sediments slightly above the current SMTZ, we estimated that methane fluxes at core CL44 and CL47 have persisted for ca. 3 kyr and 0.8-1.6 kyr, respectively. The non-steady-state modeling for CL30 predicted that a recent increase in upward dissolved methane flux was initiated ca. 85 yr ago. However, the required time for the formation of the barium front above the SMTZ at this core is much longer (ca. 2.2-4.2 kyr), which suggests that the depth of SMTZ possibly has fluctuated due to episodic changes in methane flux. Furthermore, using the model-derived fractions of different DIC sources and the δ13CDIC mass balance calculation, we estimated that the δ13C values of the external methane in cores CL30, CL44, and CL47 are -74.1‰, -75.4‰, and -66.7‰, respectively, indicating the microbial origin of methane. Our results suggest that methane seepage in the broader area surrounding the Haima cold seeps probably has persisted at least hundreds to thousands of years with changing methane fluxes.

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Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Methane Source and Turnover in the Shallow Sediments to the West of Haima Cold Seeps on the Northwestern Slope of the South China Sea

et al. 2019

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“…Relationship between the depth of the sulphate-methane transition zone (SMTZ) and the depth of occurrence of gas hydrate nodules in the study area. The plot was constructed using data for core N2-KS-44 (this study) and previously published data (Rongemaille, 2011;Ruffine et al, 2013;Sultan et al, 2014;de Prunelé et al, 2015). (Bayon et al, 2007), sediment accumulation rates, and methane-derived carbonate U-Th ages versus stratigraphic age of sediment core N2-KS-44.…”

Section: Figurementioning

confidence: 99%

U-Th isotope constraints on gas hydrate and pockmark dynamics at the Niger delta margin

et al. 2015

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The application of uranium-thorium dating methods to authigenic carbonates provides unique constraints on the temporal evolution of methane seeps at ocean margins. In this study, we report U-Th isotope measurements for carbonate breccias collected from within a hydrate-bearing pockmark located at the Niger Delta margin. These concretions were extracted from a carbonate-rich layer in the upper two meters of a sediment core (N2-KS-44; ~ 1200 m water depth), well above the present-day sulphatemethane transition zone (about 3 m depth) and the presence of gas hydrates in the sediment. The stratigraphy of core N2-KS-44 was established by tuning its downcore Al/Ti profile to a well-dated nearby reference core, and carbonate 230Th/U ages were calculated using isochron methods.Our results indicate that a major event of aragonite precipitation occurred between about 13 and 2.5 ka at the studied location. Comparison of sediment accumulation rates at both core N2-KS-44 and the nearby reference site suggests that the initial stage of carbonate precipitation, between 13 and 10 ka, was associated with sediment winnowing, probably related to intense fluid seepage. In contrast, our data indicate that sedimentation rates rapidly increased within the pockmark after 7 ka. In agreement with the presence of carbonate breccias exhibiting U-Th ages older than their corresponding stratigraphic age, this observation would suggest that sediment reworking took place after that time, possibly caused by erosion of the surrounding sediment within the pockmark. We hypothesize that the period of carbonate formation between 13 and 2.5 ka was related to an upward migration of gas-hydrate reservoirs to the near seafloor environment. After this pulse of enhanced fluid flow, the diminution of methane fluxes at the base of the local gas-hydrate occurrence zone would have led to hydrate dissolution in sub-surface sediments and pockmark formation, thereby explaining the progressive increase in sedimentation rates, the absence of recent carbonate concretions and the deepening of the sulphate-methane transition zone at site N2-KS-44 inferred from pore water data. Overall, these results provide further constraints about the relationship between gas hydrate dynamics and the evolution of pockmarks at ocean margins through time. Highlights► Breccias from hydrate-bearing sediments can be successfully dated with U-series. ► Seep carbonates precipitated between 13.0 and 2.5 kyr at the studied pockmark. ► Evidence for hydrate dissolution and SMTZ fluctuations during the Holocene. ► Co-evolution of gas hydrate and pockmark dynamics through time.

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“…Gas migration mechanisms in soft sediments control gas release events (Dupre et al, 2015;Scandella et al, 2017), determine the formation of craters and pockmarks (Andreassen et al, 2017;de Prunele et al, 2017;Riboulot et al, 2016), and constrain the potential gas recovery strategies from methane hydrate accumulations in shallow marine sediments (Fauria & Rempel, 2011;Jang, 2014;Moridis et al, 2011). Gas migration pathways are also relevant to other degassing processes such as subsurface CO 2 leakage (Bang et al, 2013;Cevatoglu et al, 2015;Lewicki et al, 2007) and gas-driven volcanic eruptions (Suckale et al, 2016).…”

Section: Introductionmentioning

confidence: 99%

Grain‐Displacive Gas Migration in Fine‐Grained Sediments

Sun

Santamarina

2019

JGR Solid Earth

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Gas migration mechanisms control the release of gas from seafloor sediments. We study underlying phenomena using transparent sediments subjected to controlled effective stress; this experimental approach allows high‐resolution real‐time monitoring of gas migration through cohesionless granular materials under 3‐D boundary conditions. Observed migration patterns depend on the effective stress at the time of injection and the stress history. Gas migration transitions from pore invasive to grain displacive when the capillary pressure for air entry ΔPAE is greater than the effective stress σ′. This study focuses on grain‐displacive gas migration. The morphology of grain‐displacive gas bodies changes with depth as the sediment stiffness G increases and the effect of surface tension γ vanishes: spheroidal gas bubbles form in the near‐surface, faceted cavities further down, and eventually open‐mode fractures develop at depth. The gas injection pressure is proportional to the effective stress in grain‐displacive migration. Preloading and overconsolidation cause the rotation of principal stresses and gas‐driven openings align with the new minimum principal stress direction. Cyclic loading promotes the upward migration of gas‐filled openings, and there is mechanical memory of previous gas pathways in sediments.

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Focused hydrocarbon‐migration in shallow sediments of a pockmark cluster in the Niger Delta (Off Nigeria)

Cited by 21 publications

References 78 publications

Methane Source and Turnover in the Shallow Sediments to the West of Haima Cold Seeps on the Northwestern Slope of the South China Sea

Methane Source and Turnover in the Shallow Sediments to the West of Haima Cold Seeps on the Northwestern Slope of the South China Sea

U-Th isotope constraints on gas hydrate and pockmark dynamics at the Niger delta margin

Grain‐Displacive Gas Migration in Fine‐Grained Sediments

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