THE SOURCE-ROCK HORIZONS OF THE IONIAN BASIN (NW GREECE)

The organic geochemical study of six geological sections and two wells in the Ionian Zone (NW Greece), allows us to distinguish five horizon s of possible source rocks: The Vigla shales (Cenomanian-Turonian), th e Upper Posidonia Beds (Callovian-Tithonian), the Lower Posidonia Beds (Toarcian-Aalenian), the marls at the base of the Ammonitico Rosso (E arly Toarcian), and shale fragments incorporated within the Triassic b reccias. These horizons have good hydrocarbon potential and an organic matter type I to II. In the deeper parts of the Botsara sub-basin, th e oil window is located in the interval between 3700-5800 m depth. Con sequently, the Triassic shales have already entered the gas window. Th e Lower and Upper Posidonia Beds, and the marls at the base of the Amm onitico 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 Posidoni a Beds through Toarcian to Tithonian and in the marts at the base of t he Ammonitico Rosso during Early Toarcian are directly related to the geometry of the syn-rift period of the Ionian Basin. The organic matte r preservation in the Vigla shales is related to the sub-basins that w ere 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 ev olution favored water stagnation and the development of local euxinic conditions in the bottom waters; these conditions were accentuated dur ing 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 st ratigraphic layers in sub-basins of the evaporitic basin. The establis hment of evaporitic sedimentation in the entire basin favored the pres ervation of the organic matter. The processes that resulted in the for mation of the evaporite dissolution collapse breccias caused also the fragmentation of the organic rich layers, which are present actually a s organic rich shale fragments within the Triassic breccias. (C) 1998 Elsevier Science Ltd. All rights reserved.