Shallow gas and gas hydrate occurrences on the northwest Greenland shelf margin

David R Cox,Mads Huuse,Andrew M.W Newton,Arka D Sarkar,Paul C Knutz

doi:10.1016/j.margeo.2020.106382

Abstract

An extensive 3D seismic dataset was used to investigate the contemporary hydrocarbon distribution and historical fluid migration in Melville Bay offshore northwest Greenland, providing the first inventory of shallow gas and gas hydrate along this part of the Greenland margin. The shallow gas anomalies vary in seismic character and have been subdivided into four categories that represent (I) isolated shallow gas, (II) free gas trapped at the base of the gas hydrate stability zone (GHSZ), (III) gas charged glacial clinoforms and (IV) a giant mass transport deposit gas reservoir. Gas hydrate deposits have been identified across an area of 537 km2 via the identification of a discontinuous bottom simulating reflector (BSR) that marks the base of the GHSZ. The BSR has been used to estimate a geothermal gradient of 49 °C/km across the GHSZ and a heat flow of 70–90 mW/m2, providing the first publically available heat flow estimates offshore western Greenland. The contemporary hydrocarbon distribution and historical fluid migration is influenced by the underlying paleo-rift topography and multiple shelf edge glaciations since ~2.7 Ma. Continued uplift of the Melville Bay Ridge, as well as glacial-sediment redistribution and basinward margin tilting from isostatic compensation, have led to a concentration of gas within the Cenozoic stratigraphy above the ridge. Furthermore, repeated variations in subsurface conditions during glacial-interglacial cycles likely promoted fluid remigration, and possibly contributed to reservoir leakage and increased fluid migration through faults. The top of the gas hydrate occurrence at 650 m water depth is well below the hydrate-free gas phase boundary (~350 m) for the present bottom-water temperature of 1.5 °C, suggesting this hydrate province mainly adjusted to glacial-interglacial changes by expansion and dissociation at its base and is relatively inert to current levels of global warming. Glacial-related dissociation may have significantly contributed to the numerous free gas accumulations observed below the GHSZ at present day.

Highlights

Gas-rich sediments have been documented on most continental shelf margins worldwide (Fleischer et al, 2001), with these accumulations predominantly representing thermogenically or biogenically generated shallow gas or gas hydrate deposits (Floodgate and Judd, 1992; Kven volden, 1993; Minshull et al, 2020; Schoell, 1988; Stopler et al, 2014)
The shallow gas anomalies vary in seismic character and have been subdivided into four categories that represent (I) isolated shallow gas, (II) free gas trapped at the base of the gas hydrate stability zone (GHSZ), (III) gas charged glacial clinoforms and (IV) a giant mass transport deposit gas reservoir
Gas hydrate deposits have been identified across an area of 537 km2 via the identification of a discontinuous bottom simulating reflector (BSR) that marks the base of the GHSZ

Summary

Introduction

Gas-rich sediments have been documented on most continental shelf margins worldwide (Fleischer et al, 2001), with these accumulations predominantly representing thermogenically or biogenically generated shallow gas or gas hydrate deposits (Floodgate and Judd, 1992; Kven volden, 1993; Minshull et al, 2020; Schoell, 1988; Stopler et al, 2014) These deposits have attracted considerable interest over the last few decades as they: (1) represent potential drilling hazards (McConnell et al, 2012; Merey, 2016; Prince, 1990); (2) can impact the stability of seafloor sediments (Brown et al, 2006; Yang et al, 2018); (3) have been considered as a future lower-carbon energy source (Collett et al, 2009; Demirbas, 2010; McGlade and Ekins, 2015); (4) can be used to indirectly estimate shallow geothermal gradient and heat flow through identified bottom simulating reflectors (BSRs) (Dickens and Quinby-Hunt, 1994; Grevemeyer and Villinger, 2001); and (5), since methane is a powerful greenhouse gas, hydrate dissociation may pose a positive feedback mechanism for global climate warming, especially when found in rela tively shallow water depths (Karisiddaiah and Veerayya, 1994; Krey et al, 2009; Ruppel and Kessler, 2017; Zhao et al, 2017). Attempting to un derstand this cryosphere-methane interaction may reveal the sensitivity of gas hydrate deposits to environmental change; providing critical insight into how these deposits may respond to future oceanic warming (Biastoch et al, 2011; Krey et al, 2009; Ruppel and Kessler, 2017)

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