AAPG Geoscience Technology Workshop

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Seafloor Seepage Features and Subsurface Expressions of Gas Accumulations in the SE of the Levant Basin, Offshore Israel


Recent seafloor exploration campaigns offshore Israel, namely E/V NAUTILUS 2010 and 2011 surveys and EUROFLEETS SEMSEEPS 2016 project, discovered several seafloor fluid seepage features (e.g. pockmarks, cold seeps communities and hydrocarbon-related microbial activities) at the base of the south-eastern continental margin of the Levant Basin and the eastern border of the Nile Deep Sea Fan. To-date many uncertainties exist as to the controls on the distribution of these features, the nature and sources of fluids feeding them and fluid migration pathways. Here, we employ detailed interpretation, attribute analyses and RGB blending techniques on commercial (12.5 m) resolution 3D seismic datasets to identify and characterize seafloor seepage edifice, and examine the controls on their distribution. The research is currently focused in water depths of 1000 to 1300 m around the compressional domain of the Palmahim Disturbance, a large-scale salt-controlled rotational slide in the southern continental margin of Israel. We identify possible fluid migration pathways within the Plio-Quaternary sedimentary stack, through which the seafloor seepages may be fed. Pockmarks are observed in the seismic data as: 1) ridge-crest centered seafloor depressions, underlain by high amplitude anomalies and sub-vertical acoustic masking effects; and 2) sub-circular seafloor depressions sitting atop a pervasive, multi-level and discontinuous system of sub-seafloor high amplitude reflectivity, to the west and south of the Disturbance. Maps created by RGB blending of multi-level seismic attribute extractions reveal that the pervasive high amplitude reflectivity represents a buried channel-levee system. The RGB maps also reveal a number of localized high amplitude responses on the seafloor, e.g. at areas where a seafloor channel incises the sub-seafloor high amplitude reflections. Our analyses reveal high amplitude anomalies, which are suggestive of gas accumulations around the base of fault-bounded Plio-Quaternary MTD blocks lying hundreds of meters below the ridges-confined seafloor pockmarks. Consequently, we hypothesize that the seafloor high amplitude responses represent seepage features and that channel incision of gas-rich paleo-depositional elements (channel-levee system) and breaching of sub-seafloor seals facilitate fluid escape along the walls of the channel. This hypothesis is supported by field observations of the 2016 SEEMSEEP cruise. We suggest that two closely-interacting fluid plumbing systems are active in the study area, within which fluids are conveyed via lateral-to-updip and/or sub-vertical routes: a) shallow gas bearing intervals, within sub-seafloor channel-levee complexes, from which fluids escape with a predominant paleo-depositional control; (b) deeper gas bearing intervals, within folded and faulted blocks, from which fluids escape with a predominant structural control. These results provide an example of the variability of gas accumulation and migration pathways during the formation stages of a microbial (biogenic) gas system