Unit-pockmarks and their potential significance for predicting fluid flow

Hovland, Martin; Heggland, Roar; Vries, Martine Helena de; Tjelta, Tor Inge

doi:10.1016/j.marpetgeo.2010.02.005

Cited by 123 publications

(117 citation statements)

References 34 publications

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“…As described by Berner (1980) Mapping unit pockmarks requires application of high-resolution bathymetry (at least 428 1m x 1m gridding) (Hovland et al, 2010). The resolution of the bathymetry equipment 429 which was used in this study is too low to locate unit pockmarks.…”

Section: Sulfate Reduction Rate (Srr) and Sulfate-profiles 371mentioning

confidence: 99%

Characterization of microbial activity in pockmark fields of the SW-Barents Sea

et al. 2012

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15 16Multibeam bathymetry revealed the occurrence of numerous craterlike depressions, 17 so-called pockmarks, on the sea floor of the Hammerfest Basin and the Loppa High, 18 south-western Barents Sea. To investigate whether these pockmarks are related to 19 ongoing gas seepage, microbial processes associated with methane metabolism 20 were analyzed using geochemical, biogeochemical and microbiological techniques. 21Gravity cores were collected along transects crossing individual pockmarks, allowing 22 a direct comparison between different locations inside (assumed activity center), on 23 the rim, and outside of a pockmark (reference sites). Concentrations of hydrocarbons 24 in the sediment, particularly methane, were measured as headspace (free) gas, and 25 in the occluded and adsorbed gas fraction. Down to a depth of 2.6 m below sea floor 26 2 (mbsf) sulfate reduction rates were quantified by radiotracer incubations. 27Concentrations of dissolved sulfate in the porewater were determined as well. Neither 28 the sulfate profiles nor the gas measurements show any evidence of microbial activity 29 or active fluid venting. Methane concentrations and sulfate reduction rates were 30 extremely low or even below the detection limit. The results show that the observed 31 sediment structures are most likely paleo-pockmarks, their formation probably 32 occurred during the last deglaciation. 33 34 Introduction 35Pockmarks are craterlike depressions in the seabed that form due to expulsion of 36

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Section: Sulfate Reduction Rate (Srr) and Sulfate-profiles 371mentioning

confidence: 99%

Characterization of microbial activity in pockmark fields of the SW-Barents Sea

et al. 2012

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“…Since initial pockmark formation through an undisturbed seabed is believed to have a violent, blow-out like characteristic, subsequent fluid migration is likely to utilize already established pathways including temporary reservoirs [Judd and Hovland, 2007]. However, more recent studies [e.g., Hovland et al, 2010] suggest that gentle venting events might occur periodically outside of main pockmark feature when the gas supply from the reservoir is low. The 'gas piston' will act as hydraulic pump causing pore water expulsion on the surface.…”

Section: Evidence Of Gas In the Sedimentmentioning

confidence: 99%

“…Alternatively, these spikes could coincide with shallow subsurface features of the pockmark, such as cracks or fractures, that could be utilized as migration pathway and accumulate pore fluids as described by Cathles et al [2010] and Judd and Hovland [2007]. Flow obstacles such as methane-derived authigenic carbonates (MDACs) can force alternative migration pathways and result in local pore fluid overpressure including pockmark walls in some scenarios [Hovland et al, 2010]. Low-permeability, fine-grained sediments can act as flow obstacles in a similar manner.…”

Section: Evidence Of Gas In the Sedimentmentioning

confidence: 99%

Geophysical and geochemical survey of a large marine pockmark on the Malin Shelf, Ireland

Szpak

Monteys

O’Reilly

et al. 2012

Geochem Geophys Geosyst

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[1] Marine pockmarks are a specific type of seabed geological setting resembling craters or pits and are considered seabed surface expressions of fluid flow in the subsurface. A large composite pockmark on the Malin Shelf, off the northern coast of Ireland was surveyed and ground truthed to assess its activity and investigate fluid related processes in the subsurface. Geophysical (including acoustic and electromagnetic) data confirmed the subsurface presence of signatures typical of fluids within the sediment. Shallow seismic profiling revealed a large shallow gas pocket and typical gas related indicators such as acoustic blanking and enhanced reflectors present underneath and around the large pockmark. Sulphate profiles indicate that gas from the shallow reservoir has been migrating upwards, at least recently. However, there are no chimney structures Copyright 2012 by the American Geophysical Union 1 of 18 observed in the sub-bottom data and the migration pathways are not apparent. Electromagnetic data show slightly elevated electrical conductivity on the edges of the pockmarks and a drop below regional levels within the confines of the pockmark, suggesting changes in physical properties of the sediment. Nuclear Magnetic Resonance (NMR) experiments were employed to characterize the organic component of sediments from selected depths. Very strong microbial signatures were evident in all NMR spectra but microbes outside the pockmark appear to be much more active than inside. These observations coincide with spikes in conductivity and the lateral gas bearing body suggesting that there is an increase in microbial activity and biomass when gas is present.

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“…One thing is certain, however, they have something to do with the hydraulics of the soft seafloor, which is both pressurized, and buoyed at the same time. One other important aspect is also that gases have something to do with their formation, as recently concluded by Cathles et al (2010) and Hovland et al (2010):…”

Section: What Are Pockmarks?mentioning

confidence: 92%

The Geomorphology and Nature of Seabed Seepage Processes

Hovland¹

2012

Bathymetry and Its Applications

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Unit-pockmarks and their potential significance for predicting fluid flow

Cited by 123 publications

References 34 publications

Characterization of microbial activity in pockmark fields of the SW-Barents Sea

Characterization of microbial activity in pockmark fields of the SW-Barents Sea

Geophysical and geochemical survey of a large marine pockmark on the Malin Shelf, Ireland

The Geomorphology and Nature of Seabed Seepage Processes

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