An insight Into the 3D Kinematics of a Thick Salt Sheet Using Fluids Escape Pipes as Natural Markers for Deformation

Abstract

Presented here through the interpretation of 2D and 3D seismic data is a suite of 5 linearly distributed trails of fluids escape pipes with pockmarks at their upper terminus. These pipe trails bypass the thick Messinian evaporites, in an area of frontier exploration offshore Lebanon. One of the pipe trails roots to the crest of a prominent pre-salt fold named the Oceanus Structure, while the other 4 pipe trails emanate from a different pre-salt fold named here the Saida-Tyr Structure. The pipes are interpreted to root in Early Miocene sandstone reservoirs analogous to the ‘Tamar’ sands, which are the main reservoir interval in the South Levant Basin following the discovery of >35 Tcf of biogenic methane. The pipe trails are oriented NW-SE and the pockmarks increase in age to the NW, with the first expulsion episode in each trail dated at 1.7 Ma (±0.3 Ma). Each expulsion episode has been systematically offset to the NW, away from the crestal foci of the fold by the flow of the salt, resulting in deformation of the fluids escape pipes in the salt. The outer most deformed pipe in each trail is imaged in 3D seismic data and extends obliquely through the salt at angles of 15°-16° from the pre-salt anticline to directly beneath the first pockmark, demonstrating that the top of the salt and the base of the Pliocene to Recent overburden are coupled. These deformed fluids escape pipes present a natural marker for the deformation through a salt sheet, revealing a Couette flow regime and flow velocities at the top of the salt sheet ranging from 2-4 mm/yr. The 4 pipe trails above the Saida-Tyr Structure presents the first direct calibration for the 3D kinematics of a salt sheet deforming under gravity. Flow orientation above the Saida-Tyr Structure remains consistently parallel over a distance of ~3.5 km, while flow velocities and maximum flow distance in each pipe trail varies by 1mm/yr and 2.5 km respectively. The onset of fluid expulsion from these pipe trails is synchronous with the onset of the most recent phase of extension at the salt basin margins. This means that the deformed fluid escape pipes record the cumulative strain of the salt sheet since the onset of the most recent post-Messinian phase of deformation of the salt in the Levant Basin, and implies a genetic link between the triggers for salt flow and fluid expulsion. Changes in flow direction and flow velocity in the salt can be used to highlight variability in the driving forces of differential loading and marginal uplift and titling around the basin margin, which has implications for migration, fluids expulsion and overpressure.