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Abstract: Neocomian Half Graben in the Western San Jorge Basin, Argentina: Petroleum Systems, Origin and Tectonic Inversion

Eduardo Figari, Maria Cid de la Paz, Guillermo Laffitte

During late Jurassic-early Cretaceous times, the extension related to the Gondwana break-up generated many isolated small half-graben basins, with a NW-SE structural trend, in the central part of Patagonia Terrane. The sedimentary record, which overlays an igneous metamorphic prejurassic basement, is made up by three megasequences that represent different stages in rifting evolution. The main source rock, composed of Neocomian lacustrine dark organic-rich shales, with type I kerogen, rests on a late Jurassic volcanic and sedimentary complex, and it is found within Megasequence I (early and late rift). This megasequence ends with fluvial deltaic and tidal sandstones as the oldest reservoir rocks. The main reservoirs, made up of upper Cretaceous braided and meander sandstones, are found within Megasequence II (early and late sag), which ends with thick pyroclastic overbank deposits as the regional seal. Megasequence III, composed of Tertiary marine and continental rocks, lacking source and reservo rs levels, solely acts as overburden rock.

After a W-E Tertiary compression, these basins were partially inverted, to different degrees, according to the orientation of pre-existing faults. Likewise, the thermal maturity history of the source rock, the migration pathways and the traps vary in each of these basins.

This study sets out to show how analogous basins with the same stratigraphic record became independent petroleum systems due to a different tectonic evolution.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela