AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Lithostratigraphic correlation between onshore and offshore units from the Algarve basin, Southern Portugal

Abstract

The Algarve basin is a Meso-cenozoic basin in Southern Portugal, broadly oriented E-W and related with the opening of the Atlantic and the closure of the Western Tethys. Hudrocarbon exploration has been focused in the offshore sectors of the basin, with seismic studies and five wells until now, whereas onshore areas only recently are being regarded as having some potential, with no seismic and no wells. Onshore geology has been studied for decades and formal lithostratigraphic units are well defined. However, the offshore wells show a different geological record, and correlation between both realities is still not well established. This work aimed to address this correlation and consisted mainly on a lithostratigraphic analysis of the five offshore exploration wells, linking them both with the onshore geology and with the offshore seismic lines. The onshore geological record and lithostratigraphy has been revisited and organized, as a base to look at the offshore record and try to correlate them, taking into account the important East-West lateral variations, as well as the expected proximal-distal variations. Well report analysis included lithological data, well logs, drilling issues, etc. Using the gathered data, synthetic age logs were made, subjected to corrections based on biostratigraphic data (nannofossils, palynostratigraphy and foraminifera record, vd. Roque, 2007), for a better age constrain. Combining ages, lithologies and GR+SP logs, new lithostratigraphic units were defined and correlated with the onshore units. From this correlation, lateral facies and thickness variations, both East-West and proximal-distal, have been identified and characterized. Nine regional seismic lines have been analyzed, 5 crossing the wells with a N-S orientation, 3 with a NW-SE orientation and 1 crossing all the other (to check interpretation accuracy). Based on what has been drilled in each well and connecting it to seismic horizons, the following units have been marked: Triassic (with some uncertainty), Jurassic (in some cases, separating L/M from Upper Jurassic), lower Cretaceous, upper Cretaceous, Paleocene, Eocene, Oligocene, Miocene, Pliocene and Quaternary. Regional structures and thickness variations have been identified and related with the onshore areas. This work is a contribution to a better understanding of the regional and paleogeographic evolution of this Basin and intended to help in hydrocarbon exploration on this promising frontier area.