In previous works, it has been suggested that dissolution of gas hydrate can be responsible for pockmark formation and evolution in deep water Nigeria. It was shown that those pockmarks which are at different stages of maturation are characterized by a common internal architecture associated to gas hydrate dynamics. New results obtained by drilling into gas hydrate-bearing sediments with the MeBo seafloor drill rig in concert with geotechnical in situ measurements and pore water analyses indicate that pockmark formation and evolution in the study area are mainly controlled by rapid hydrate growth opposed to slow hydrate dissolution. On one hand, positive temperature anomalies, free gas trapped in shallow microfractures near the seafloor and coexistence of free gas and gas hydrate indicate rapid hydrate growth. On the other hand, slow hydrate dissolution is evident by low methane concentrations and almost constant sulfate values 2 m above the Gas Hydrate Occurrence Zone. Study Area and Main ObjectiveThe investigated area is located in deep water of Nigeria. Bathymetry in the area ranges from 1100 to 1250 m ( Figure 1). This area was previously shown to host a field of (sub) circular pockmarks [Georges and Cauquil, 2007]. These range in shape from a slightly depressed, hummocky seafloor to a much more pronounced depression and each of them is several tens to a few hundreds of meters wide (Figure 1). The various morphologies of the pockmarks suggest either distinct modes of formation or different evolutionary stages [Sultan et al., 2010]. Most of the pockmarks are located in an area bounded by two NW-SE trending deeprooted normal faults, which delineate a graben linked to the axis of anticline in the subsurface. Several deep and shallow faults and three N-S trending buried channels were recognized with high-resolution 3-D seismic data (Figure 1). The buried channels, which are situated between 80 ms and 180 ms (two-way travel time, TWTT) below the seabed, may have the potential of accumulating amounts of free gas and play therefore an important role for the gas hydrate distributions.Based on geophysical and sedimentological data, and in situ piezocone measurements, Sultan et al. [2007] have shown that pockmark-associated gas hydrate accumulated within a few meters thick sediment layers at shallow depth. In addition, Sultan et al. [2010] proposed that the formation of a circular depression around the gas hydrate occurrence zone (GHOZ) is related to multiple steps in the pockmark evolution. The sequence is starting with hydrate formation induced by upward migration of fluids oversaturated in gas through fracture systems followed by decrease of fluid flow resulting in gas undersaturation, hydrate dissolution, generation of excess pore pressure, and by concurrent collapse of the gas hydrate-bearing sediment structures. Respective analyses were mainly based on subseabed approaches, using piston cores and in situ piezocone geotechnical measurements with a maximum penetration of 30 m below seafloor (mbsf). Howe...

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A joint research expedition between the French IFREMER and the German MARUM was conducted in 2011 using the R/V Pourquoi pas? to study gas hydrate distributions in a pockmark field (1141-1199 meters below sea surface) at the continental margin of Nigeria. The sea floor drill rig MeBo of MARUM was used to recover sediments as deep as 56.74 meters below seafloor. The presence of gas hydrates in specific core sections was deduced from temperature anomalies recorded during continuous records of infrared thermal scanning and anomalies in pore water chloride concentrations. In situ sediment temperature measurements showed elevated geothermal gradients of up to 258 °C/km in the center of the so-called pockmark A which is up to 4.6 times higher than that in the background sediment (72 °C/km). The gas hydrate distribution and thermal regime in the pockmark are largely controlled by the intensity, periodicity and direction of fluid flow. The joint interaction between fluid flow, gas hydrate formation and dissolution, and the thermal regime governs pockmark formation and evolution on the Nigerian continental margin.

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Twelve submarine mud volcanoes (MV) in the Kumano forearc basin within the Nankai Trough subduction zone were investigated for hydrocarbon origins and fluid dynamics. Gas hydrates diagnostic for methane concentrations exceeding solubilities were recovered from MVs 2, 4, 5, and 10. Molecular ratios (C 1 /C 2 < 250) and stable carbon isotopic compositions (d 13 C-CH 4 >240& V-PDB) indicate that hydratebound hydrocarbons (HCs) at MVs 2, 4, and 10 are derived from thermal cracking of organic matter. Considering thermal gradients at the nearby IODP Sites C0009 and C0002, the likely formation depth of such HCs ranges between 2300 and 4300 m below seafloor (mbsf). With respect to basin sediment thickness and the minimum distance to the top of the plate boundary thrust we propose that the majority of HCs fueling the MVs is derived from sediments of the Cretaceous to Tertiary Shimanto belt below Pliocene/Pleistocene to recent basin sediments. Considering their sizes and appearances hydrates are suggested to be relicts of higher MV activity in the past, although the sporadic presence of vesicomyid clams at MV 2 showed that fluid migration is sufficient to nourish chemosynthesis-based organisms in places. Distributions of dissolved methane at MVs 3, 4, 5, and 8 pointed at fluid supply through one or few MV conduits and effective methane oxidation in the immediate subsurface. The aged nature of the hydrates suggests that the major portion of methane immediately below the top of the methane-containing sediment interval is fueled by current hydrate dissolution rather than active migration from greater depth.

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