Abstract: The Source Rocks of the Mesozoic Petroleum Systems of Argentina: A Comparative Overview on their Geochemistry, Paleoenvironments and Hydrocarbon Generation Patterns

Villar, H. J. - Cirgeo; G. A. Laffitte - YPF; L. Legarreta - P. A. San Jorge

The petroleum source rocks of five Argentine Basins (Figures 1 and 2), involving Mesozoic hydrocarbon systems, are comparatively described on the basis of their geochemical and stratigraphic characteristics.

The Cuyo Basin contains Triassic freshwater lacustrine shales bearing unstructured, lipid-rich (Type I) kerogen, which originated early-mature high-wax oils. A relatively reduced hydrocarbon generation took place late in the history of the basin, after an important Tertiary overburden, around 10 m.y. ago. In the Neuquén Basin, the three main source rocks, Early-Middle Jurassic Molles, Late Jurassic Vaca Muerta and Early Cretaceous Agrio, were deposited in rather restricted marine settings, locally affected by strongly anoxic conditions. Organic-rich and oil-prone lacustrine shales accumulated in areally restricted E. Triassic-E. Jurassic halfgrabens. The Los Molies terrestrially influenced shales (Type II to II-III kerogen) mostly generated light hydrocarbons, whereas the algae-rich Vaca Muerta and Agrio marly shales (Type II kerogen) sourced paraffinic-to-naphthenic oils. The Vaca Muerta shale is the most prolific hydrocarbon source rock and has also generated significant volumes of gas/condensate in thermally post-mature areas, particularly to the West of the basin. Within the Austral Basin, dark shales accumulated throughout the Early Cretaceous in suboxic marine settings. Its poorly preserved amorphous, mostly algal, kerogen generated light-oil and gas. The kitchens are located considerably apart from the hydrocarbon accumulations, in the eastern areas of the basin, thus implying long distance lateral migrations in the order of 50-200 km. In the San Jorge Basin, Early Cretaceous shales were deposited in anoxic freshwater-to-brackish lakes, fringed by occasional shallow carbonate shorelines. The phytoplankton-rich kerogen of the D-129 source rock mostly generated paraffinic oils that show at present variable extents of biodegradation, water-washing and a generalized pattern of multiple reservoir charges and hydrocarbon alteration. A minor oil source is known as ? Neocomiano?, in the NW of the basin. The Northwest Late Cretaceous Basin consisted of an extended lac-mer where deposition was associated with widespread shallow water algal-mats (stromatolites) and affected by anoxic conditions during short-lived flooding events. The limited volume of mature source rock, represented by thin packages of shales containing mixed planktonic and terrestrial contributions, resulted in non-prolific yields of dominantly paraffinic oils. Gas and condensate occurrences are restricted to localized deep burial of the source rock.

A peculiar stratigraphic framework and tectonic evolution explain the development and preservation of hydrocarbons in San Jorge and Neuquén basins with a total EUR of 1,050 MMm³ oil and 570 MMMm³ gas.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil