Significance to hydrocarbon exploration of terrestrial organic matter introduced into deep marine systems: Insights from the Lower Cretaceous in the Levant Basin

Abstract

Oil shows from wells in the Levant Basin and Egypt are suggestive of potential economic discoveries within unpenetrated Mesozoic reservoirs in the Eastern Mediterranean. Analysis of some of these oils indicate a non-marine source of mixed deltaic-terrestrial or lacustrine origin for the organic matter. Data from Barremian-Aptian sediments from the MNUPH-1 core in northern Israel opens a window on potential importance of terrestrial-sourced kerogen deposited in source rocks in a marine depositional environment.Core analyses of the Barremian Nabi Sa'id Fm. demonstrate the occurrence of a gas-prone immature source rock of >77 m gross thickness averaging 4.6% TOC, peaking at 37% TOC. Rock-Eval pyrolysis, biomarker n-alkane, sterane, and tricyclic terpane indices, atomic C/N ratios, and δ13Corg ranges indicate mixed terrestrial-marine organic source rocks for the Barremian, while higher up, the Aptian Hidra Fm. contains source rocks with organic matter derived dominantly from carbohydrate-rich land plants. The interval at MNUPH-1 includes abundant evidence for off-shelf mass transport of terrestrial and proximal marine sediments into the deep marine basin. These features are used to explain the anomaly of a terrestrial source for organic matter in this setting. Mass transport and consequent mixing of distal marine, proximal marine and land-derived sediment is, in fact, the norm at the foot of continental slopes globally.The dominantly type III source rocks described here would be unlikely to expel oil but might produce thermogenic gas. Thermogenic gas identified in similar marine settings might wrongly be interpreted as having been generated from marine organic matter, misleadingly indicative of a prospective oil source. When prospecting for deep-rooted hydrocarbon accumulations in offshore marine settings, an alternative system dominated by basinward sediment transport of gas-prone ‘terrestrial’ source rocks into strategically-positioned toe-of-slope marine settings, is supported here, and could well be producing gas in the Levant Basin.