Shallow Gas Hydrate Accumulations at a Nigerian Deepwater Pockmark—Quantities and Dynamics

• Published in: Journal of geophysical research. Solid earth 2020-09, Vol.125 (9), p.n/a
• Main Authors: Pape, Thomas, Ruffine, Livio, Hong, Wei‐Li, Sultan, Nabil, Riboulot, Vincent, Peters, Carl A., Kölling, Martin, Zabel, Matthias, Garziglia, Sébastien, Bohrmann, Gerhard
• Format: Article
• Language: English
• Subjects: Ascent; Biodegradation; Core analysis; Core sampling; Cores; Coring; Deep sea; Deep water; Drilling; Gas formation; gas hydrate; Gas hydrates; Gas pressure; Geofag: 450; Geologic depressions; Geophysics; Geosciences: 450; Hydrates; Hydrocarbons; Isotope composition; Matematikk og Naturvitenskap: 400; Mathematics and natural science: 400; MeBo; Methane; Microorganisms; Ocean floor; Offshore; Offshore drilling rigs; Petroleum; Petroleum hydrocarbons; pockmark; Pore water; pore water modeling; pressure coring; Sediment; Sediments; Stability; Sulfates; VDP
• ISSN: 2169-9313; 2169-9356; 2169-9356
• DOI: 10.1029/2019JB018283

The evolution of submarine pockmarks is often related to the ascent of fluid from the subsurface. For pockmarks located within the gas hydrate stability zone, methane oversaturation can result in the formation of gas hydrates in the sediment. An ~600 m‐wide sea floor depression in deep waters offshore Nigeria, Pockmark A, was investigated for distributions and quantities of shallow gas hydrates, origins of hydrocarbons, and time elapsed since the last major fluid ascent event. For the first time, pressure coring of shallow sediments and drilling of more than 50 m‐long cores with the sea floor drill rig MARUM‐MeBo70 were conducted in this pockmark. Unusually, high hydrate saturations of up to 51% of pore volume in the uppermost 2.5 m of sediment in the pockmark center substantiate that deepwater pockmarks are a relevant methane reservoir. Molecular and stable C and H isotopic compositions suggest that thermogenic hydrocarbons and secondary microbial methane resulting from petroleum biodegradation are injected into shallower sediments and mixed with primary microbial hydrocarbons. Two independent pore water chloride and sulfate modeling approaches suggest that a major methane migration event occurred during the past one to three centuries. A rough sea floor topography within the pockmark most likely results from combined sediment removal through ascending gas bubbles, hydrate clogging and deflection of migration pathways, gas pressure build‐up, and hydrate sea floor detachment. This study shows for the first time the chronological interrelationship between gas migration events, hydrate formation, and sea floor shaping in a deep sea pockmark. Key Points Shallow pressure coring and MeBo drilling in a deepwater pockmark off Nigeria revealed gas hydrate saturations of up to 51% of pore space Shallow hydrates are fueled by thermogenic hydrocarbons mixed with microbial methane from petroleum degradation and from carbonate reduction Modeling of pore water chloride and sulfate independently suggests that last major methane injection event occurred during the past three centuries