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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 74 (1990)

Issue: 6. (June)

First Page: 879

Last Page: 920

Title: Evolution of the San Jorge Basin, Argentina (1)

Author(s): M. G. FITZGERALD (2), R. M. MITCHUM, JR. (3), M. A. ULIANA (4), and K. T. BIDDLE (5)

Abstract:

The San Jorge basin, although small, is the most important hydrocarbon-producing basin in Argentina. Remaining untested potential is high because of the presence of good source rock, favorable structural complexity, and multiple reservoirs. Reservoir quality is commonly low because of the highly tuffaceous sandstones.

The sedimentary fill of the basin is closely related to its tectonic history. Northwest-southeast-trending grabens formed and filled during a Triassic and Early Jurassic early rift phase, climaxing with a pervasive Middle Jurassic volcanic episode; continued growth and filling of the basin occurred during a Late Jurassic-earliest Cretaceous late rift phase and Cretaceous early and late sag phases. Late Cretaceous-early Tertiary extension set up many of the present-day structural traps along normal faults. Middle Tertiary Andean compression produced the narrow, north-south San Bernardo structural belt, which exhibits reversed movement on older, normal, graben-bounding faults and on local, low-angle thrust faults. Marked early to middle Tertiary erosion produced a significant unconformi y within Cretaceous beds around basin margins.

Origin of Upper Jurassic and lowermost Cretaceous sedimentary fill is primarily lacustrine or fluvial in origin. Lacustrine, organic-rich black shales are fringed by oolitic and other limestones and fluvial-deltaic sandstones derived mostly from the north. A significant southern source of sand existed during the Valanginian. Interbedded marine shales occur mostly to the west toward a presumed marine seaway connection to the northern Magallanes basin. Middle to Upper Cretaceous sedimentary rocks, sourced mostly from the north, are mainly fluvial sandstone-shale successions with some minor lacustrine influence. Reservoir-quality glauconitic sands were deposited during a Late Cretaceous-early Tertiary marine incursion from the Atlantic. After development of a lower Tertiary regional unco formity, relatively undisturbed younger Tertiary sediments completed the basin fill.

Lower Cretaceous lacustrine shales of the D-129 Formation are the best source rocks yet identified. However, most hydrocarbon production occurs above the source in middle and Upper Cretaceous sandstones along the basin flanks. The most obvious hydrocarbon emplacement mechanism is vertical migration along reactivated graben faults.

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