Marine geological surveys in Asköfjärden in the southern Stockholm Archipelago hasrevealed step like features in the sediments on the Baltic sea floor. The aim of this project is toanalyse the steps and possible formation processes from the survey data. The data used aretaken from the acoustic instruments multibeam echosunder and Chirp sonar sediment profiler.The multibeam reveal the seafloor topography and can detect water column features. TheChirp sonar produce sub-bottom profiles showing the sediments down to some tens of metersunderneath the sea floor. The multibeam data show multiple crescent shaped steps as well aselliptically shaped “pockmarks”. Water column data show what could be interpreted as seepsfrom the sea floor under these features. The sub bottom profile show deformation and fluidsignatures under the steps. Gas signatures and what could be other fluids are present. Theinterpretation concludes that the formation could be from either gas or groundwater seeps. Nodefinitive conclusions can be made from the data available for the present study, andapetrophysical or geochemical study of the study area cold provide further understanding ofthe formation of the steps.

La marge algérienne est un domaine complexe, bordé au sud par les zones internes des Maghrébides (AlKaPeCa) qui chevauchent les zones externes telliennes vers le sud. La partie offshore de ce système constitue un bassin d‘arrière arc où s‘est déposée une série sédimentaire mio-pliocène reposant sur un substratum de nature et d‘origine variables spatialement. L'objectif de ce travail a consisté à analyser et modéliser le fonctionnement des systèmes pétroliers et à quantifier les volumes d‘hydrocarbures accumulés. Avant de procéder aux différentes modélisations numériques, stratigraphiques et thermiques, un modèle géologique conceptuel a été élaboré sur la base des données de géologie de terrain et de sismiques. La continuité terre- mer du socle kabyle a permis une extrapolation stratigraphique entre des formations oligo-miocènes en affleurement et les unités acoustiques définies dans le bassin offshore. L‘analyse tectonique dans les deux domaines a abouti à une évolution en trois phases : (1) syn-rift où le bassin a été ouvert en transtension à partir de l‘Oligocène supérieur-Aquitanien, (2) phase post-rift durant laquelle il y a eu le développement des bassins intra-arcs, la collision AlKaPeCa- Afrique et le magmatisme, (3) phase d‘inversion où la marge devient active essentiellement à partir du Quaternaire. En définissant l‘ensemble des éléments des systèmes pétroliers avec des incertitudes variables, la simulation du fonctionnement de ces derniers par modélisation de bassin 2D/3D a montré des possibilités d‘accumulation d‘huile et de gaz majoritairement près de la marge, entre 20 et 65 km de la côte avec un maximum de portée de 70 km dans le golfe de Bejaia. / The Algerian margin is a complex domain, limited to the south by the Maghrebian internal zones (AlKaPeCa) that overthrust the external Tellian zones southward. The offshore part of this system constitutes a back-arc basin, where a mio-pliocene sedimentary series were deposited over a substratum of laterally variable origin and nature. The goal of the present work consisted in analyzing and modelling the petroleum system at work and calculating hydrocarbon volumes. Before proceeding to different numerical basin, stratigraphic and thermal modelling, a conceptual geological model is required and was performed based on field geological studies and stratigraphic and structural interpretations of the seismic profiles. The extension of the continental crust beyond the foot of the margin, allows determining a chronostratigraphic model by extrapolation of the outcroping oligo-miocene formations onshore to the acoustic pre-messinian units defined in the offshore basin. Besides, a tectonic analysis in both onshore and offshore domains gave rise to three main steps of evolution: (1) syn-rift phase where the basin was opened by transtension since Late Oligocene- Aquitanian, (2) post-rift phase in which intra-arc basins were developed, AlKaPeCa and Africa docked and magmatism activity took place, (3) inversion phase where the margin became active mainly since Quaternary times. After defining the different petroleum system elements taking into account variable uncertainties, the 2D/3D petroleum system model depicts possibilities of oil and gas accumulations mainly close to the margin, between 20 to 65 km from the coastline to the north with a maximum range of 70 km in the Bejaia Gulf.