--> Eastern Algerian offshore: Tectono-stratigraphy and Related Petroleum Systems

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Eastern Algerian offshore: Tectono-stratigraphy and Related Petroleum Systems


The Eastern Algerian offshore basin is located to the south-east of the Balearic Islands and north of the African continental margin, extending from Bejaia to Annaba localities. As the rest of the western Mediterranean domain, it is considered as a back arc basin initiated during the Late Oligocene to Aquitanian due to the southeastward migration of the AlKaPeCa blocks. The lack of high resolution seismic profiles and deep wells did not allow a direct identification of the pre-salt sedimentary series.

In such frontier domain, seismic stratigraphy interpretation ant outcrop analogs are the only tools that can be used in order to precise the stratigraphy and tectonic agenda of the basin. Three main sedimentary packages were defined as (1) pre-Messinian, (2) Messinian and (3) Plio-Quaternary on long listening MCS profiles (new data). Their tentative ages have been proposed by extrapolation from outcrops and using analogy to what is known in the western Mediterranean. The basin initiated during Late Oligocene extension, whereas its compressional/transpressional reactivation began during Late Tortonian.

The 3D facies modeling undertaken with Dionisos shows an occurrence of coarse siliciclastic facies that can constitute reservoirs and fine-grained sediments which may contain organic-rich layers. The Burdigalian and Messinian series were already recognized as good source rocks in the westernmost Algerian offshore and southern Sicily, whereas nearby Langhian outcrops in the Kabylides show organic-rich layers, as documented by our new geochemical analyses. The 3D basin modeling of the petroleum systems was realized considering various scenarii for the basal heat flow conditions. Minimum and maximum values were successively tested for the rifting phase, with mean values at about 110-120 mw, the heat flow subsequently decreasing down to 80 to 90 mw/m2 at present day. The overall crustal/lithospheric architecture of the section was constrained by wide angle seismic. These deep data and published parameters such as beta factor were ultimately used to derive a tentative present day heat flow at the base of sediments.

Results of the 3D model indicate that oil and gas expulsion occurred in the deep troughs at the toe of the margin since Late Miocene. Burdigalian and Langhian source rocks might have charged the Lower Langhian, Late Tortonian and Lower Messinian reservoirs. At places where it has been sufficiently buried, the Messinian lower unit may also constitute a local source to charge Messinian sandstones (channels) and Lower Pliocene reservoirs (deep sea fans). In addition to stratigraphic traps such as the Messinian channels, Pliocene fans and reefs (observed in seismic), structural traps such as ramp anticlines and inverted normal faults, involving infra-salt series, occur in the upper margin and deep basin, respectively.