Geophysical and geochemical evidence of large scale fluid flow within shallow sediments in the eastern Gulf of Mexico, offshore Louisiana

Abstract

We analyse the fluid flow regime within sediments on the Eastern levee of the modern Mississippi Canyon using 3D seismic data and downhole logging data acquired at Sites U1322 and U1324 during the 2005 Integrated Ocean Drilling Program (IODP) Expedition 308 in the Gulf of Mexico. Sulphate and methane concentrations in pore water show that sulphate-methane transition zone, at 74 and 94 m below seafloor, are amongst the deepest ever found in a sedimentary basin. This is in part due to a basinward fluid flow in a buried turbiditic channel (Blue Unit, 1000 mbsf), which separates sedimentary compartments located below and above this unit, preventing normal upward methane flux to the seafloor. Overpressure in the lower compartment leads to episodic and focused fluid migration through deep conduits that bypass the upper compartment, forming mud volcanoes at the seabed. This may also favour seawater circulation and we interpret the deep sulphate-methane transition zones as a result of high downward sulphate fluxes coming from seawater that are about 5-10 times above those measured in other basins. The results show that geochemical reactions within shallow sediments are dominated by seawater downwelling in the Mars-Ursa basin, compared to other basins in which the upward fluid flux is controlling methane-related reactions. This has implications for the occurrence of gas hydrates in the subsurface and is evidence of the active connection between buried sediments and the water column.

Deep-seated faults and hydrocarbon leakage in the Snøhvit Gas Field, Hammerfest Basin, Southwestern Barents Sea

“…Focused fluid-flow systems usually serve as conduits for fluids and sediment remobilization. They have been the interest of many studies and yet significant works are needed (Barnett et al, 1987;Cartwright et al, 2007;Dupré et al, 2010;Gay et al, 2011;Huuse et al, 2010).…”

“…Out of all these trigger mechanisms, the role of tectonism or faulting in hydrocarbon migration/leakage on continental margins is still poorly understood. Fluids can migrate laterally and vertically through fault zones, salt diapirs, erosional surfaces like active canyons and paloecanyons, polygonal faults or buried chimneys (Gay et al, 2011).…”

“…Fluid-flow or migration is associated with excess pore-fluid pressure which can be attributed to temporal and spatially varying processes, such as rapid sediment loading (Dugan and Flemings, 2000), uplift and erosion (Doré and Jensen, 1996), dissociation of gas hydrate (Mienert et al, 2005), polygonal faulting (Andresen and Huuse, 2011;Cartwright et al, 2007;Gay et al, 2011;Ostanin et al, 2012a), and leakage from deep and shallow source and reservoir rocks (Heggland, 1998;Hovland and Judd, 1988;Solheim and Elverhøi, 1985). Focused fluid-flow systems usually serve as conduits for fluids and sediment remobilization.…”

Natural Seepage Systems at Biloxi & Dauphin Domes & Mars Mud Volcano, North East Mississippi Canyon Protraction Area, Gulf of Mexico

“…Mud volcanoes are caused by the release of high pressure from shale horizons, and frequently act as vertical conduits for migrating oil and gas (Gay et al, 2011;Sawyer et al, 2007;Talukder, 2012) The Mars Mud Volcano we surveyed is the largest of 12 identified in the local area by Gay et al The feature is about 2km wide and rise 40 to 50 meters above the surrounding seafloor. Mud volcanoes are caused by the release of high pressure from shale horizons, and frequently act as vertical conduits for migrating oil and gas (Gay et al, 2011;Sawyer et al, 2007;Talukder, 2012) The Mars Mud Volcano we surveyed is the largest of 12 identified in the local area by Gay et al The feature is about 2km wide and rise 40 to 50 meters above the surrounding seafloor.…”

“…However, with the exception of the seafloor gas hydrate observatory in MC118 (Bowles et al, 2011;Ingram et al, 2010;Lutken et al, 2006;Sassen et al, 2006;Simonetti et al, 2011;Sleeper et al, 2006) and the Mars Mud Volcano (Gay et al, 2011) there was little detail available about the seeps in the deep water of North East Mississippi Canyon. In order to obtain information about the baseline of natural seepage in the vicinity of the Macondo well (drilled in block Mississippi Canyon 252 -MC252) BP Gulf Coast Restoration Organization (GCRO) operated a multi-vessel Natural Resource Damage Assessment (NRDA) Natural Seeps Survey during the late summer and fall of 2011.…”