Inversion structures and hydrocarbon occurrence in Argentina

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Abstract Positive inversion features are a common structural ingredient in broad regions of western and central Argentina. They show up in several geological provinces distributed from Salta at 24°S down to Tierra del Fuego below 54°S. Considerable variety in inversion style and intensity developed due to regional positioning within the Andean orogenic belt, near the mountain edge, or well within the South American foreland. Weakly inverted grabens, doubly plunging buckles and compressional anticlines may site as far as 600 km away from the present day oceanic trench and 300 km east of the Cordilleran front. Most of the studied examples are genetically linked to high-angle or listric normal fault families that developed across southern South America, induced by pervasive Triassic to Cretaceous extensional stress conditions. Old extensional structures involved in the inversion process include: segments of Triassic successor basins linked to extensional collapse of the Late Paleozoic orogenic tract, Jurassic faults belonging to a multicomponent rift system developed after incipient Basin-and-Range style deformation of Patagonia, segments of a network of linear Jurassic-Neocomian troughs and half-troughs related to pre-breakup fragmentation of the South American slab, and roughly linear Cretaceous-Palaeogene fault-bounded troughs of the arc-crest type induced after supracrustal extension driven by easterly subduction along the Pacific edge of the plate. As in other deformed areas around the world the inversion process did not advance as a wave from the orogenic front (‘bulldozer’ mode), but apparently occurred as a more or less synchronous reactivation of faults distributed across the inverted tract (‘accordion’ mode). In the Neuquén basin middle Jurassic to Cretaceous reversal from block subsidence to local uplift occurred along an intra-plate megashear aligned with the inboard projection of the South Atlantic oceanic transforms. Protracted strike-slip motion and related syndepositional block-inversion favoured pre-migration structuring and stratal anomalies prone to provide sedimentary and combination traps. Over most of the oil-producing Argentine basins inversion took place late, during the Cenozoic. Thus timing, with respect to the local maturation histories and onset of regional hydrocarbon charge, was a key control on field size and oil richness in each region. Late tectogenesis and late regional migration combined in the non-marine Cuyo basin to provide fields in the 250 MMBO range, out of inverted Triassic half-grabens. Less favourable convergence of inversion and migration is recorded in the western San Jorge basin, where regional contraction occurred 20–40 Ma after regional oil emplacement, and late inversion resulted in dispersive hydrocarbon reaccommodation. Finally, migration in the southern Magallanes basin happened at the time when burial beneath a thick foreland wedge had reduced reservoir properties and thermally degraded the source sections into the gas generation stage.