Deposition and alteration of carbonaceous series within a Neotethyan rift at the western boundary of the Arabian Plate: The Late Permian Um Irna Formation, NW Jordan, a petroleum system

Abstract

Abstract During the late Permian (Kungurian to Kazanian) a Neotethyan rift basin evolved at the western boundary of the Arabian Plate, in what is called today the Dead Sea Valley of western Jordan. The break-up of Pangaea was accompanied by low-sinuosity sandy braided- to meandering-fluvial drainage systems which were fed by the uplift of the Arabian Shield and by poorly aerated swamps and ponds that concentrated plant debris of the Cathaysian floral province in the Um Irna Formation. These proximal wet fan sediments are overlain by a dry fan characterized by extensive reddish floodplain deposits, anastomosing channel systems and paleosols. The wet fan is underlain by Cambrian sandstones. These units serve as the top and bottom seals of the OM-bearing system of the Um Irna Formation. The sedimentary rocks of the OM-bearing Um Irna Formation underwent supergene, diagenetic and epigenetic hydrothermal alteration under an elevated geothermal gradient. The temperature increased from the time of deposition of the wet to the time of deposition of the dry fan and caused remobilization of manganese already pre-concentrated in the Cambrian footwall rocks of the rift basin. The anomalous heat regime may be accounted for as a predecessor stage of the Dead Sea Rift which is still active today. Oil seeps are found along faults and fractures near this deep-seated lineamentary fault zone. The deposition and alteration of the organic matter in this late Permian rift are of great consequence for oil generation in the region. Organic petrographic investigations revealed that organic-rich terrestrial carbonaceous and coal rich sediments of mainly of type III kerogen are dominant in the Um Irna Formation. In addition, aquatic liptinite rich sedimentary input (fresh water lake and/or lacustrine swamp) of type I kerogen is also noted. Coal derived organic matter occurs in the form of coaly particles with ranks from subbituminous A to high volatile bituminous C. Higher plant-derived macerals as sporinite and cutinite with an oil generation potential typical of type II kerogen contribute to the source rock potential of the Um Irna Formation which is equivalent to the Khuff Formation in the oil provinces of Iraq, Kuwait and Saudi Arabia (thermal maturity of dispersed OM ranges between 0.48 and 0.55% R r giving rise to the mature stage of oil generation). Additionally, Botryococcus -type algae point towards the presence of type I kerogen. Moreover the presence of solid bitumen and oil droplets (petroleum fluid expulsions) in the sediments of the Um Irna Formation indicates formation of hydrocarbons. Organic chemistry not only supported the coal petrographic findings but also indicated inhospitable aquatic conditions near the P/Tr boundary evidenced by PAHs present in the sample and by lots of charcoal. The tropical areas were affected by vast wildfires raging in the forest and swamps during the late Permian. They contributed to a highly toxic atmosphere, a decrease of the oxygen contents speeded up by dead OM spilled into the water and creating anoxia approaching the P/Tr boundary. The elevated geothermal gradient sparking hydrothermal alteration in the OM-bearing Um Irna Formation during the Late Permian may be accounted for as related to a precursor stage of the Dead Sea Rift, which is still active today in terms of migration of hot brines and hydrocarbons.