Shallow Gas in the Southeastern Levant: the Variability of an Evolving Microbial Gas System

Abstract

A variety of seepage features were recently discovered in the southeastern Levant Basin. Here we provide a preliminary review of the scope, geological context and variability of gas seepage within the relatively small Israeli offshore. This review encompasses a set of works, and is based on geophysical analysis of 3D seismic data, coupled with ROV surveying of E/V Nautilus in 2010 and 2011, the EUROFLEETS2 2016 expedition SEMSEEPS onboard R/V Aegaeo and some of Leviathan Partnership extensive seafloor AUV and ROV surveying. The seepage features include pockmarks of various types and sizes; carbonate buildups, intense seafloor perturbations, chemosynthetic fauna and sporadic gas bubbles emission. The predominance of microbial methane within these seeps is preliminarily suggested by two short sediment cores, and low δC13 values of authigenic carbonates and sampled biota. The southeastern Levant seafloor is divided to two distinct depositional zones. The eastern deep-sea fan of the Nile, stretching across a major part of the basin, is riddled throughout with deep-sea channel and lobe systems, indicating direct transport of Nilotic sediments towards the Cypriot deep. In contrast, the southeastern (Israeli) margin is a sedimentary bypass of Nilotic sediments, carried northeastwards by currents and then transported down slope. Seafloor seepage found within the eastern Nile fan is associated with focused edifices that are primarily associated with seafloor ridges and channels, at water depths between 1100 to 1300m. These seepage edifice and associated phenomena consistently correlate with high amplitude sub-seafloor reflectivity, imaged on seismic data across a major part of the Levant and attributed to the accumulation and migration of gas throughout the Plio-Quaternary sedimentary section. The vertical distribution of this high amplitude reflectivity reveals two principal populations: (1) trending sub-parallel to the seafloor and attributed to shallow gas accumulations beneath the seafloor; and (2) deepening with water depth in correlation with our modeled base hydrates stability, and therefore is attributed to free gas accumulated beneath a scattered hydrates layer. This reflectivity and our modeling suggest the potential presence of natural methane hydrates at water depths >1240m across a substantial part of the basin. The spatial distribution of the first population reveals a multitude of extensive lobes, partly correlated with current seafloor channels, but buried tens of meters beneath them. Active seafloor seepage occur where these buried lobes are focused towards and truncated by the seabed. We therefore suggest the presence of a pervasive shallow sub-seafloor gas-bearing interval within the buried lobes of the deep sea fan of the Nile. This system may be fed, at least in part, by structurally controlled migration of gas from the deeper Plio-Pleistocene sedimentary levels. Seepage found along the base of the eastern margin of the Levant Basin appears to be characterized by distinctly different morphologies. Particularly, hundreds of clusters encompassing thousands of ~1 to ~40 m diameter, ~1 m deep, round seafloor pockmarks, were mapped by AUV surveying along the base of the continental slope offshore central Israel. These pockmarks are distributed over a 5 to 10 km wide and at least 25 km long belt between ~400 m to ~1150 m water depths. Commercial seismic reflection data image a sequence of high-amplitude reflections, underlying the seafloor pockmarks area ~200 m to ~400 m beneath the seafloor. This sequence is structurally modified by a north trending fault ramp syncline basin and rollover ramp structure, which is faulted by multiple, conjugate, extensional thin-skinned faults. The structure, sequence of high-amplitude reflections, and faults appear to correlate with the distribution of the seafloor pockmarks. We theorize that the high amplitude reflections represent a deformed and faulted multilevel gas-bearing interval, which is pervasively percolating to the surface. Future work aims to expand the mapping of gas seepage across the basin, and delineate the compositional context and evolution of these systems with relation to changing environmental conditions.

AAPG Datapages/Search and Discovery Article #90341 ©2019 AAPG Geoscience Technology Workshop, Exploration and Development of Siliciclastic and Carbonate Reservoirs in the Eastern Mediterranean, Tel Aviv, Israel, February 26-27, 2019