Preliminary Assessment of Hydrocarbon Potential of Larsen Basin, Antarctica

David I. M. MacDonald, P. F. Barker, S. W. Garrett, J. R. Ineson, R. R. F. Kinghorn, D. Pirrie, B. C. Storey, A. G. Whitham

The Larsen basin, on the northwest margin of the Weddell Sea, formed as a Mesozoic ensialic basin during Gondwana breakup. At the northern end of this basin, 5-6 km of sedimentary rock crop out on James Ross Island, exposing elements of a large hydrocarbon system. Aeromagnetic and outcrop data suggest that the basin history inferred from James Ross Island persists to 70°S. Deposition was in half-grabens on the extending Weddell Sea margin or in restricted back-arc basins.

Upper Jurassic anoxic marine strata, deposited prior to rifting, form a rich potential source (TOC up to 2.5%) 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 meters thick and tens of kilometers 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 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 tow rd the top of the section and toward the basin center.

In the northern Weddell 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.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.