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Sedimentary Sequences and Petroleum Systems in the Austral Foreland Basin

Carlos M. Urien1 and Craig F. Schiefelbein2
1 Urien & Associates, Buenos Aires, Argentina
2 Geochemical Solutions International, The Woodlands, TX

The Austral-Malvinas Basin is one of the few basins on the Argentine continental shelf that contains proven petroleum systems. It evolved through three main tectonic phases: rift, sag and foredeep. Sedimentary fill is closely related to the tectonic history of the basin associated with the breakup of Gondwana. The Austral-Malvinas Basin initially formed as an extensional trough during Late Triassic-Jurassic continental rifting of Patagonia. Following breakup, the basin evolved into a tectonically quiet Cretaceous sag basin. During the Tertiary, the southern part of the basin was affected by intense tectonism that resulted in the formation of the Malvinas foredeep.

The Late Jurassic Tobifera Formation overlying basement consists of volcanic and volcanoclastic rocks interbedded with nonmarine sediments. Unconformably overlying the Tobifera are basal fluvial and shallow marine sandstones of the Spring Hill Formation or lateral equivalent shales and mudstones of the Estratos con Favrella Formation. These are succeeded by organic-rich dark shales of the Lower Inoceramus Formation previously considered as the main source for oils in Spring Hill reservoirs.

Recent evidence supported by 2.5D modeling indicates that although organic rich sequences are frequent in both the Mesozoic and Tertiary sequences, the proven and probable petroleum systems are associated with Late Jurassic and Lower Neocomian sequences. At least two different oil families have been identified: continental and marine. Continental oils originate from Lower Jurassic lacustrine shales as well as from fluvial shales of the Springhill. Marine oils are derived from lagoonal Springhill coastal facies and/or Estratos con Favrellas marine shales.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005