The source rock horizons of the Ionian Basin (NW Greece)

Abstract

The organic geochemical study of six geological sections and two wells in the Ionian Zone (NW Greece), allows us to distinguish five horizons of possible source rocks: The Vigla shales (Cenomanian–Turonian), the Upper Posidonia Beds (Callovian–Tithonian), the Lower Posidonia Beds (Toarcian–Aalenian), the marls at the base of the Ammonitico Rosso (Early Toarcian), and shale fragments incorporated within the Triassic breccias. These horizons have good hydrocarbon potential and an organic matter type I to II. In the deeper parts of the Botsara sub-basin, the oil window is located in the interval between 3700–5800 m depth. Consequently, the Triassic shales have already entered the gas window. The Lower and Upper Posidonia Beds, and the marls at the base of the Ammonitico Rosso are mature in terms of oil generation. The Vigla shales maturity corresponds to the early maturation stage. The timing of the principal oil-source horizons maturation is the Late Jurassic for the Triassic shale beds and the Serravalian for the Lower Posidonia Beds. The preservation of the organic matter in the Lower and Upper Posidonia Beds through Toarcian to Tithonian and in the marls at the base of the Ammonitico Rosso during Early Toarcian are directly related to the geometry of the syn-rift period of the Ionian Basin. The organic matter preservation in the Vigla shales is related to the sub-basins that were preserved by the continuation of halokinetic movements during the post-rift period. The geometry of the restricted sub-basins that were formed during the syn-rift and post-rift period of the Ionian Basin evolution favored water stagnation and the development of local euxinic conditions in the bottom waters; these conditions were accentuated during the anoxic events that are known to have affected the Tethys ocean during the Early Toarcian and Late Cenomanian. The organic rich shale fragments within the Triassic breccias were initially deposited as stratigraphic layers in sub-basins of the evaporitic basin. The establishment of evaporitic sedimentation in the entire basin favored the preservation of the organic matter. The processes that resulted in the formation of the evaporite dissolution collapse breccias caused also the fragmentation of the organic rich layers, which are present actually as organic rich shale fragments within the Triassic breccias.