AAPG Geoscience Technology Workshop

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Internal Structure of the Triangular Horst-Like Jonah High in the Levant Basin, Eastern Mediterranean


A clear magnetic anomaly is evident across the central part of the Southern Levant Basin, coinciding with prominent structural features known as the Jonah High and Jonah Ridge. Jonah High is a triangular shaped, horst-like structure, bounded at each of its three rims by incoherent, steeply dipping seismic events and is capped in places by a high amplitude quazi-flat marker. Attempt to drill the structure by the Myra-1 exploration well in 2012 resulted in a loss of circulation at the level of the cap; the well was side-tracked to the east and penetrated a Lower Miocene and Upper Oligocene deep-water section including the Tamar Sands, the main reservoir found in all the other wells in the basin. Few models have been suggested since the 1980's to explain the combined magnetic and oddly-shaped structure; they include basement high, deep intrusive body and shallow volcanic edifice. All these remotely related models obtained a good fit between observed and calculated measurements of the magnetic data, an inherent problem in potential field methods. This study focuses on seismic imaging rather than using the magnetic data as a constraint. It takes advantage of a recently released 3D survey acquired over the Jonah High and few yet unpublished reports. The interior of the upper part of the high, from about 5 to 7 km, has been suggested by most previous works to be a carbonate buildup or a reef, yet contemporenous with the surrunding strata. These models are now challenged by the definition of the Oligo-Miocene section in the well to be of outer neritic to upper bathyal palaeoenvironment. Alternatively, this study suggests that this part of the structure is composed, as its surroundings, of an Oligocene and Lower Miocene deep water section and probably contains the Tamar Sands. It is further suggested that it has been intruded during the Lower to Middle Miocene by allochthonous bodies that formed its triangular shape. These are contemporaneous with the formation of all the gas-bearing structures in the basin. The intrusions formed local features, elevated by a few hundred meters, mainly along the eastern rim. The deep part of the high, below 7 km, may contain Mesozoic volcanic rocks that upwelled with the intrusive bodies and subsequently caused the magnetic anomaly to acquire the shape of the structure. The deep part is characterized by high amplitude events dipping away from its central axis but as yet these are not easily correlated to the surrounding strata. The Jonah High at all its levels requires further imaging efforts, even reshooting of a large aperture 3D survey, in order to solve this highly complex feature and maybe shed more light on the history of the Levant Basin as a whole.