Abstract: Controls of Hydrocarbon Distribution on the External Margin of a Back-arc Basin: Example in Neuquen Basin, Argentina

Veiga, Ricardo; Maria Ester Lara and Pedro Bruveris - YPF

The Neuquen basin, located in the west-central Argentina, produces the 44% and the 58% of Argentina?s oil and gas production respectively. Important hydrocarbon accumulations occur along the external margin (northeastern flank) where the cumulative production reaches 372 MM brls of oil and 150 BCF of gas.

Neuquen Basin is a back-arc ensialic basin originated in the Late Triassic as a result of an extensional collapse behind a stationary magmatic arc next to the actual Andean Cordillera. Its tectonic evolution reveals 3 steps: 1) tectonic subsidence controlled by faults (Late Triassic-Early Jurassic). 2) regional subsidence behind a subduction zone (Middle Jurassic - Paleocene) and 3) compressional deformation produced by Andina orogeny (Eocene-Pliocene).

The distribution of oil/gas fields, on the external margin, is driven by structural, stratigraphic and diagenetic controls. The main oil/gas fields display a NW - SE orientation as a result of half-grabens generated during Late Triassic. A NE - SW distensive field originated small-disconnected and big-interconnected half-grabens. The half-graben system Señal Cerro Bayo-Entre Lomas is the most important lineament. Several oil/gas fields are located over the footwalls and hanging walls of this system, which controlled the migration of hydrocarbons. On platform zone, the slightly reactivation of antitetic faults produced subtle structures and noses that affected the migration pathways developing commercial oil/gas fields.

The stratigraphic controls affect the distribution of source rocks, reservoirs and seals. The main source rock in Neuquen basin is the Vaca Muerta Formation which is related to an extensive drowning in the Early Tithonian. In spite of its widespread distribution, Vaca Muerta Formation presents poor geochemical properties on the external margin. Nevertheless the proliferation of oil/gas field in this sector indicates long migration pathways. The main reservoirs are Permian - Triassic weathered and fractured basement, Late Jurassic sandstones, Early Cretaceous sandstones and limestones and Hauterivian sandstones. The proliferation of reservoir allows the coexistence of different productive layers into an oil/gas field. The seals coincide with local sedimentary discontinuities and maximum flooding surfaces however as the several seals lose their conditions toward the basin margin due to lithological changes, the lack of efficient barriers that stops the vertical and lateral migration of hydrocarbons represents one of the most important problems in the occurrence of stratigraphic traps. The diagenetic mechanisms are responsible for the variation of petrophysical properties, mainly in carbonate reservoirs where the secondary porosity develops belts parallel to the basin margin.

Concluding, structural control is the predominant factor that regulates the main oil/gas field distribution due to the reactivation of previous faults. The proliferation of several reservoirs and the lack of good seals make it difficult to predict the existence of stratigraphic traps; however some oil/gas fields show stratigraphic closures. Finally, the volume of hydrocarbons generated allowed the partial filling of traps located far away of the generation zone.

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