JURASSIC PETROLEUM SYSTEMS IN THE LUSITANIAN BASIN, PORTUGAL: NEW INSIGHTS BASED ON OIL – SOURCE ROCK CORRELATIONS

New stable carbon isotope and biomarker data for oils and source rock extracts from the Lusitanian Basin, Portugal, were studied in order to investigate the petroleum systems which are present there. The new analytical data was combined with data presented in previous publications, and oil-oil and oil- source rock correlations were carried out. Three genetic groups of oils (Groups 1, 2 and 3), belonging to three different petroleum systems, were identified:

Group 1 oils occur in the northern sector of the Lusitanian Basin and were generated by the Coimbra Formation (Sinemurian) source rock. Reservoir rocks for oils in this group are the Coimbra, Água de Madeiros (upper Sinemurian – lower Pliensbachian), Boa Viagem (Kimmeridgian –Tithonian) and Figueira da Foz (upper Aptian – Cenomanian) Formations. Other potential source rocks in the northern sector of the basin, such as the Polvoeira Member of the Água de Madeiros Formation and the Marly Limestones with Organic Facies (MLOF) Member of the Vale das Fontes Formation (Pliensbachian), had biomarker characteristics which differed from those of the Group 1 oils and did not therefore generate them.

Group 2 oils occur in the central and southern sectors of the basin. The source rock is the Cabaços Formation (middle Oxfordian), and reservoir rocks are the Montejunto (middle-upper Oxfordian) and Abadia (Kimmeridgian) Formations.

Group 3 is represented by an oil sample from the central sector of the Lusitanian Basin. Both the source rock and the reservoir rock for the oil are the Montejunto Formation.

Geochemical data combined with the regional tectono-stratigraphic history suggest that the generation-migration-accumulation of most of the oil (critical moment) in the Coimbra – Coimbra - Água de Madeiros - Boa Viagem - Figueira da Foz (!) petroleum system occurred in the early Campanian. For the Cabaços – Montejunto - Abadia (!) and Montejunto – Montejunto (!) petroleum systems, the critical moment occurred in the late Cenomanian.