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ABSTRACT: Geology and Hydrocarbon Reservoir Potential of the Pituil and Barreal Formations, Calingasta Valley, Western Argentina

J. S. Janks, O. R. Lopez-Gamundi, P. K. Siegele

The Calingasta basin is one of the norht-south-trending intermontane basins informally known as the Bolsones. The stratigraphy consists of

lower Paleozoic metamorphic basement overlain by sediments and volcanics of upper Paleozoic through Cenozoic age. Three distinct geological provinces are recognized within the Bolsones region: Sierras Pampeanas, Precordillera, and Cordillera Frontal.

Outcrop samples from the Permian Pituil and Triassic Barreal formations from the Tamberias region of the Sierras Pampeanas province were analyzed to determine the composition, porosity type, and diagenetic modification. The Pituil formation is a shallow marine sequence overlying Carboniferous glaciomarine sediments. They grade eastward into nonmarine lacustrine, deltaic, and fluvial sandstones. The rocks are fine- to medium-grained litharenites with porosities of 6-10%. Diagenetic modifications include quartz overgrowths, unstable grain dissolution, carbonate cements, pyrite, and kaolinite.

Triassic deposits occur on the western flank of the Precordillera, overlying a basement of volcanics and metasedimentary rocks. The Triassic sediments can be several hundreds of meters thick; deposition occurred in fluvial to lacustrine environments. These clastic sediments are considered to be northern extensions of the hydrocarbon-productive sediments in the Cuyo basin.

The Barreal formation ranges from clay-rich lithic wackes and shales to conglomeratic, volcaniclastic litharenites and sublitharenites. Framework grains consist of quartz, feldspars, rock fragments, and, rarely, glass shards. Cements include zeolites, carbonates, chalcedony, pyrite, and clays. Tuffs are found at certain intervals within the section; alteration to iron-rich smectite is common. Reservoir potential is highly variable. Porosities range from as low as 5% to greater than 25%.

The Pituil and Barreal formations offer significant potential for reservoir development within the Bolsones basins area of western Argentina. The quality of the reservoirs depends upon depositional environment, grain size and composition, and diagenetic modifications. These parameters will have to be delineated on a regional scale to fully predict reservoir potential.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990