Gas-bearing clastic mounds left by salt dissolution

Abstract

Tall mounds buried beneath a Pleistocene/Pliocene cover in the ancient El Arish and Afiq Canyons on the Levant margin of the eastern Mediterranean are interpreted as inversion structures consisting of sands that had once filled solution pits in Messinian-age halite. The mounds serve as reservoirs of natural gas. The sands belong to the Yafo Sand Member (YSM) of the Yafo Fm. and were previously attributed to the early Pliocene. However, the presence of ostracods and benthic foraminifera diagnostic of the Lago Mare stage of the Messinian Salinity Crisis assign the YSM to a late Miocene age when Mediterranean had transformed from a brine-filled sea to brackish lakes. The YSM and its equivalent Afiq Member of the Mavqiim Formation are present in cuttings from numerous exploration wells at elevations on the margin from 2500 to 500 m below today's sea level, whereas the preceding halite has mostly disappeared and is only preserved on the margin at a few locations above 1700 m. The YSM, Afiq Mbr. and a nearly 2-km-thick halite layer is observed on the basin floor in a network of reflection profiles. Profiles show that beyond the canyon mouths where the YSM overlies halite, the YSM fills circular cavities in the halite that resemble dolines, indicating that dissolution of salt had commenced earlier. The halite buries deep-sea channels emanating from the slope canyons and reaching far into the distal Levant basin where exceptionally large fields of gas have been discovered. This study suggests that the gas in the mounds might be linked to the same source as in the offshore deep-water fields and was delivered up dip to the canyon mounds via channelized sands underneath a halite seal that broke beneath the canyon floors during subsequent dissolution.