How to evacuate 10 km3 of mud: saturate with gas and decrease the pressure!

Abstract

The crest of the Absheron anticline in the South Caspian Basin at a few hundred meters below the present seafloor shows a subcircular depression about 8 km in diameter and 200 m deep, bounded by steep edges dipping 15°to 45°into it. The depression and the surrounding series are respectively filled and overlain by a regional mass-transport deposit (MTD) 150 m thick outside the depression and 300 m thick inside, composed mostly of extensional blocks. Geometric and stratigraphic analyses indicate that 150 m of initially deposited sediment were removed from a closed area after burial. Seismic evidence of shallow gas accumulations below the crater-like feature suggests that gas likely played a significant role in its development. The model proposed for the emplacement of the crater is that the gas-bearing cover of a shallow gas reservoir underwent exsolution when its overburden thinned during an episode of extensional slope failure. This resulted in loss of resistance to shear and evacuation of the gas-bearing sediment, likely at the shearing base of the failed mass. This evacuation feature is considered an example where the presence of gas locally governs the morphology of an MTD. The interpreted process shows a positive feedback between slope failure and loss of strength at the base of the resulting MTD.

Evolution Model for the Absheron Mud Volcano: From In Situ Observations to Numerical Modeling

“…This can be due either to a quiescent phase of the MV activity when normal sedimentation was recorded or to very high sedimentation events, such as mass‐flows, that can drap MVs (Deville et al, ). The continuity of the seismic horizon separating the two mud masses is in favor of a normal sedimentation and does not seem related to any sediment instability present during the Post‐Absheron interval (Imbert et al, ). Normal faults flank the bowl‐shaped area from 1,500 to 2,000 m and show the motion toward the center of the bowl‐shaped geometry.…”

“…Methane was recently discovered in the Absheron field, and a giant active MV pierces the crest of the structure at the southwestern limit creating potential hazards for drilling operations (Contet & Unterseh, ). Previous studies on the Absheron anticline evidenced the presence of four mass transport deposits in the Post‐Absheron interval (Imbert et al, ). The most recent mass transport deposit covers the whole anticline with the exception of the MV location where it stumbles on the northern flank of the volcano.…”

Deep-seated hydrocarbons in the seep “Brecciated Limestones” of the Maiella area (Adriatic foreland basin): Evaporitic sealing and oil re-mobilization effects linked to the drawdown of the Messinian Salinity Crisis

“…The combination of pre-existing gas saturation and a rapid pressure change is believed to explain most of the mud volcanism and diapirism phenomena worldwide (Imbert et al, 2014). In the Maiella area, pressure release after the unloading following the MES provoked hydrofracturing in limestones mounds, venting in correspondence with discrete features , as well as evaporite disruption.…”

The spatial, temporal and volumetric analysis of a large mud volcano province within the Eastern Mediterranean

“…The presence of even modest amounts of methane in solution in the pore fluid could have had profound consequences and aid the process of dynamic liquefaction in the source units (c.f. Imbert et al (2014)). These essentially rapid overpressuring mechanisms may have had a role in taking the source region to the necessary state to achieve local liquefaction, but they would have been superimposed, in all likelihood, on a system that was already highly overpressured, possibly with a large, highly overpressured region developed within the entire presalt succession, such that only a minor trigger of a rapid event would be needed to exceed the necessary threshold.…”