Late Cretaceous Oil Shale Deposits in the Levant: Stratigraphy, Geobiology, and Source Rock Potential

Abstract

The Levantine high productivity sequence is a product of an upwelling system that operated in the Late Cretaceous along the SE Tethyan margin. This system resulted in the deposition of carbonate, chert, porcellanite, phosphorite and organic-rich (oil shale) sediments in a series of basins located proximally and marginally to the upwelling center. In this presentation we summaries several years of research focused on these deposits, presenting a detailed chronostratigraphic framework, a geobiological investigation, and their source rock potential. A total of 23 datum levels were recognized using biostratigraphy, lithostratigraphy and well log markers. The varying lithostratigraphic units of the more proximal basins of southern Israel (Negev) were correlated to specific horizons within the monotonous organic-rich carbonates of the distal setting of the Shefela Basin in central Israel. The first occurrence of the Late Cretaceous organic-rich carbonates in Israel is documented in the Negev during the late Coniacian, within the upper Dicarinella concavata Zone. The regional unconformity around the Santonian/Campanian boundary was found to be less substantial at the distal localities in comparison to the proximal ones. The distinct appearance of the ‘Mishash Tongue’ chert in the Shefela was correlated to the massive Chert Member (Mishash Formation) in the Negev, and assigned to the middle Campanian. Deposition of the overlying phosphate series spans from the lower Contusotruncana plummerae to the base of the Pseudoguembelina palpebra zones (78.3-71.7 Ma) and co-occurs in both proximal and deeper distal areas. The top of the phosphatic unit is marked by a regional unconformity. The oil shale deposits in southern Israel coincide with the 100 m richest TOC interval (average 15.2% TOC) in the Shefela. The diminishing phase of organic-rich deposition occurred in a diachronous step-wise manner across Israel, from the top of the P. palpebra Zone to the upper part of the Abathomphalus mayaroensis Zone in southern and central Israel, respectively. The duration of the high productivity sequence in Israel spans approximately 19 myr. Our study has also revealed remarkable adaptations of species to extreme environmental conditions. It has generally been argued that the majority of fossil benthic foraminifera could not tolerate anoxia. Here we present evidence that in the Upper Cretaceous of the Levant, fossil foraminifera were able to successfully colonize anoxic–dysoxic bottom waters, by using adaptations similar to those found in living species. A shift from buliminid to diverse trochospiral dominated assemblages was recorded in an interval with a distinct anoxic geochemical signature coinciding with a regional change in lithology. This change was triggered by an alteration in the type of primary producers from diatoms to calcareous nannoplankton, possibly causing modifications in benthic foraminiferal morphological and physiological adaptations to life in the absence of oxygen. Finally, we address the complexity of sedimentary organic-matter formation, its source to burial, its influence on bottom-water communities, and the effect of these criteria on the source rock potential of the section. The siliceous and phosphatic-bearing sediments of the Campanian gave way to Maastrichtian uniform and regionally distributed organic-rich carbonates, accompanied by an increase in sedimentation rates from ∼1 to>6 cm/kyr. This has facilitated both the increase in organic-matter production and its excellent preservation. We propose that shifts in the type of primary producers promote a change in the amount and type of preserved organic-matter, in bottom water communities, on the lithological composition of the rock-mass, and ultimately on the hydrocarbon generation potential of the studied source rock. Oil shales from Jordan and Israel stand out as preferable source rocks with higher generative potential than similar deposits to the north (Lebanon) or to the more proximal southern locations (Egypt, Eastern and Western Desert).

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