GASTIME

Gas seeps and submarine slides in the East Mediterranean: toward comprehensive geohazard prevention (GASTIME)

Massive hydrocarbon reservoirs have been discovered in the until recently under-explored Levantine basin. Wells and boreholes show high quality gas trapped under Messinian salt units in lower Miocene sandstones at water depths over 1500 m. The estimated amount of gas in place for these fields is world class with 25 trillion cubic feet, thus bringing the eastern Mediterranean into the limelight. During the past few years increasing numbers of offshore constructions including undersea pipelines and oil rigs have been placed in relation to the development of these hydrocarbon reservoirs. The presence of fluid flow signatures piercing these deep Messinian salt layers and reaching shallower depths requires the implementation of a comprehensive geological study of the Levant basin based on the interpretation of 2D and 3D surveys, coupled with information retrieved from boreholes and cores. This exercise serves as a cornerstone for understanding the mechanisms controlling fluid flow in hydrocarbon prone sedimentary basins. The main objectives of GASTIME were to study the shaping mechanisms behind gas seepage and their relation to slope stability. Fluid escape features and mass transport deposits are common in hydrocarbon-prone basins and thus, studying the relationship between these features is crucial for hazard assessment and risk mitigation for safe hydrocarbon recovery.

Seismic profiles showing fractures emanating from the Messinian Unconformity towards the seafloor. A major pockmark area is located above the faults, probably pointing towards a direct connection allowing fluid seepage. Drawing courtesy of Emmanuel Eruteya.

The GASTIME project aimed to study the shaping mechanisms behind gas seepage in the deep Levant basin. For this purpose, the study was concentrated in seismic interpretation of 2D and 3D datasets and amalgamation of the data with logs and core samples from boreholes in the Levant Basin. The results show evidence for fluid flow and associated features both in the deep Levant basin and the adjacent Israeli continental shelf. These features are mainly identified as pipe structures in the post-evaporitic overburden. In the deep Levant basin, the pipes are spatially contextualized into western and eastern provinces according to differences in their morphology. While all piping structures are rooted in the Messinian evaporite substratum (MES), their ending time greatly differs. The western and eastern group pipes indicate episodes of fluid flow activity during the mid-Pliocene and late Pliocene, respectively. The fluid expulsion phenomena documented in this study has wider implications for the hydrodynamics of many basins where thick evaporite layers are widely developed. Moreover, these pipe structures may pose a potential risk to deep-water exploration and development.

As for the shallower Israeli margins, the analysis of high-resolution 3D seismic datasets reveal >140 discrete mounded structures situated on the outer continental shelf off-central Israel. These structures are conical to semi-rounded with asymmetric flanks and domed or ridge-like crest. They are spatially distributed along an elongated confined zone embedded within the Late Pleistocene erosional surface and overlying a structural high of the MES. Several pipe-like wipeout zones and bright spots coupled by phase reversals are identified below these mounded features indicating their possible connectivity with deep-rooted fluids. The study carried out through GASTIME suggests three potential sources for the possible fluids: 1) a shallow Quaternary source, 2) a deep source related to the Early Pliocene and 3) a deep source emanating from the Mesozoic source in the underlined Cenozoic Syrian Arc fold. Normal faults crosscutting the Pliocene-Pleistocene succession may facilitate cross-stratal flow of fluids from the deeper overburden strata to the shallower units. The main mechanism proposed for the formation of these mounded structures is in relation to biogenic cementation of carbonate following methane seepage during early stages of Holocene sea level rise or in conjunction to the latest Pleistocene sea level drop, in a similar manner as was documented for other shallow continental shelves worldwide.

Moreover, as a result of the high resolution investigation of 3D geophysical datasets, mass transport deposit units (MTD) were mapped and interpreted on the Levant slopes. Previously unrecognized structures were identified and include novel palm-like erosional morphologies, blocky facies, arcuate facies, mounded facies, syn-depositional thrust systems and a channelized geometry. These configurations indicate different transport distance, mechanisms and kinematic for each MTD probably suggesting different triggers. Moreover, the study shows that the MTD plays an important role in the interaction with the ascending fluids from deep hydrocarbon sources as the MTD emplacement constrains and limits their pathways. The results of this study shed new light into the nature and formation of the MTDs offshore Israel and it serves as an analogue to understand the configurations of these structures in similar basins with well-developed evaporite substratum units.

The above-mentioned results from the GASTIME project will provide comprehensive and improved insights into the geological mechanisms shaping fluid flow systems in the Levant basin as well as in its adjacent continental shelf. Furthermore, understanding the behavior of shallow fluid flow in the continental shelf is crucial for risk mitigation in view of their possible release into sea water and destabilization of the sedimentary overburden.

The project was funded by the Marie Curie FP7 Actions of the European Research Council.

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