The Larsen Basin, on the northwest margin of the Weddell Sea, formed as a Mesozoic ensialic basin during Gondwana breakup. Deposition was either in half grabens on the extending Weddell Sea margin, or in a restricted back-arc basin. At the northern end of this basin 5–6 km of sedimentary rock crop out on James Ross Island, exposing elements of a large potential hydrocarbon system. Aeromagnetic and outcrop data suggest that the basin structure inferred from James Ross Island can be recognised at least as far south as 70°S.Upper Jurassic anoxic marine strata, deposited prior to the main phase of arc development, form a rich potential source (T.O.C. up to 3.5%) with both marine and terrestrial kerogens. Arc-derived volcaniclastic sediments of Barremian — Oligocene age form a regressive megasequence. Basal strata represent slope apron and rudaceous submarine fan deposits proximal to the margin; fan conglomerates form lenticular bodies hundreds of metres thick and tens of kilometres across, enveloped in slope-apron mudstones. Late Cretaceous fault reactivation and uplift led to dramatic shallowing of the basin, with deposition of shelf facies. Although there are many potentially attractive reservoir targets, there may be problems of pore occlusion due to the abundant labile volcanic grains. However, there is evidence of more quartzose sandstone towards the top of the section, and, inferentially, toward the basin centre.In the Larsen Basin, there is moderate potential for oil generated from Upper Jurassic source rocks and reservoired in Cretaceous and Tertiary sandstones and conglomerates, in large stratigraphic or structural traps caused by partial basin inversion during deposition.