Aptian\u2013Albian shale oil unconventional system as registration of Cretaceous oceanic anoxic sub-events in the southern Tethys (Bir M\u2019Cherga basin, Tunisia)

Rachida Talbi,Raymond Levey,Rached Lakhdar,Reginal Spiller,Amor Smati

doi:10.1007/s13202-018-0577-6

Abstract

The Bir M’Cherga basin (North-east Tunisia), with about 600 km2 area, had recorded four Middle Cretaceous source rocks well stratigraphically correlated with the four known oceanic anoxic sub-events: OAE1a, OAE1b, OAE1c, and OAE1d. Variety of lithology, thickness and organic richness had characterized these source rocks. The sedimentary tectonic analysis, the petrology and geochemistry study established the petroleum system of these source rocks. Basin formation began early in the Barremian–Aptian interval by synsedimentary tectonics reactivating old basement faults. During the Aptian–Albian, the formed basin had a depocenter that recorded thicker black shales while its NW and SE edges remained raised under the Triassic halokinetic activities. The evolution of the sedimentary filling during this period generated two sedimentary cycles corresponding to two filling second-order fining and thickening upwards sequences. The black shales that constitute these source rocks are formed between subtidal and external platform environment and are interbedded with juxtaposed high organic rich layers and poor ones. The rich organic facies consists of dark shale that constitutes the source rock. The poor organic beds formed by light grey and nodular limestones constitute an intra host reservoir. Thereby, petroleum system consists in an “unconventional oil shale hybrid systems with a combination of juxtaposed organic-rich and organic-lean intervals associated to open fractures”. The kerogen is essentially amorphous, with marine planktonic origin and low ligneous organic matter contribution. This organic material of dark facies had been well preserved in an anoxic environment with little or no energy. Light grey limestones were of oxic-to-sub-oxic environment. The stage of the thermal evolution for these source rocks provided by IH/Tmax diagram is of the “oil window”. The average transformation ratio (TR) is estimated as 45% suggesting thus black shales are oil shale resources which still close about untransformed 55% of its hydrocarbon generating potential. The expulsion and release of oil into these source rocks are proven by the observation of hydrocarbons filling micro-cracks and by the variable values of the oil saturation index OSI ranging from 0 to 138%. The latter exceeds 100% near the paleo-high reliefs indicating two “oil crossover” areas attributed to the high degree of oil source rock saturation and accumulation of hydrocarbons considered ideal for hydraulic fracturing. This oil crossover is a consequence of secondary migration into black shale source rock, achieved by various faults created during the distensive phase that were reactivated again several times.

Highlights

Black shales are the consequence of oceanic anoxic events that had marked the geological history in the global ocean since the early Cretaceous and record the phases of Extended author information available on the last page of the article proliferation and diversification of marine biomass in sediments; which led to the genesis of good source rocks of oil and gas (Schlanger and Jenkyns 1976; Arthur et al 1985)
Most black shales were deposited during the Oceanic Anoxic Event OAE1 of the Middle Cretaceous and coincide with the unfolding of global phenomena related to the main phases of mid-ocean volcanism, generalizing transgressions and increasing CO2 levels in the atmosphere (Schlanger and Jenkyns 1976; Arthur et al 1985)
Journal of Petroleum Exploration and Production Technology (2019) 9:1007–1022. This Oceanic Anoxic Event occurred over a long period (113–109 Ma) with short-term paroxysms (≤ 1 Ma) that coincided with the accumulation and fossilization of significant amounts of organic matter in four black shale units during the four oceanic anoxic sub-events: OAE1a, OAE1b, OAE1c and OAE1d (Jenkyns et al 1990; Bralower et al 1994; Jarvis et al 2002; Leckie et al 2002; Soua 2016)

Summary

Introduction

Black shales are the consequence of oceanic anoxic events that had marked the geological history in the global ocean since the early Cretaceous and record the phases of Extended author information available on the last page of the article proliferation and diversification of marine biomass in sediments; which led to the genesis of good source rocks of oil and gas (Schlanger and Jenkyns 1976; Arthur et al 1985). Journal of Petroleum Exploration and Production Technology (2019) 9:1007–1022 This Oceanic Anoxic Event occurred over a long period (113–109 Ma) with short-term paroxysms (≤ 1 Ma) that coincided with the accumulation and fossilization of significant amounts of organic matter in four black shale units during the four oceanic anoxic sub-events: OAE1a, OAE1b, OAE1c and OAE1d (Jenkyns et al 1990; Bralower et al 1994; Jarvis et al 2002; Leckie et al 2002; Soua 2016). The second subevent (OAE1b) is of the Lower Albian with Ticinella bejaouaensis and Hedbergella planispira biozone, characterizes the western Tethyan (Bréhéret 1994; Bralower et al 1994) This interval is marked by a glaciation phase of the latest Aptian followed by an eustatic rise phase during the Lower Albian (Weissert et al 1998). The sub-event OAE1d is of latest Upper Albian with Rotalipora appenninica biozone and is widely represented across the Tethys Ocean but sporadically represented in the southern part of the Atlantic and Indian Ocean and the eastern part of the Pacific ocean (Bréhéret 1994; Erbacher et al 1996; Wilson and Norris 2001; Bornemann et al 2017)

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