HYDROCARBONS HABITAT IN A TRIASSIC‐TO‐CRETACEOUS SUB‐ANDEAN SETTING: NEUQUÉN BASIN, ARGENTINA

Abstract

The Triassic‐Cretaceous Neuquén Basin of west‐central Argentina covers an area of nearly 200,000 sq. km, encompassing the edge of the Andean Cordillera and the adjacent South American foreland. A seven‐km thick sedimentary succession has been developed after three main geotectonic stages: (1) Basin onset and fault‐induced subsidence after Triassic—Early Jurassic extensional collapse of a Late Paleozoic marginal orogenic belt; (2) Middle Jurassic—Paleogene regional subsidence behind a retreating subduction system; (3) Neogene contractional deformation and local load subsidence forced by advancing subduction conditions.Throughout the Jurassic and Cretaceous periods, depositional patterns were related to waxing and waning of a narrow seaway that was connected to the Pacific realm through gaps in the magmatic arc. Widespread anoxia and Type II organic matter preservation occurred in the context of peak transgressions during the Toarcian, Oxfordian, Tithonian, late Valanginian and Hauterivian. A protracted history of regular subsidence and moderate heat flow at a back‐arc position, together with the presence of widespread evaporite seals, has resulted in regional zonation of the hydrocarbon occurrences. The inverted Cordilleran province became largely overmature; an intermediate gas‐condensate to light‐oil belt developed after local drainage from mature source rocks; and a basin fringe belt, that includes mixed and partially‐degraded oil, was charged after lateral migration 20–50 km updip from effective source units.Reservoirs are provided by Jurassic‐Cretaceous marine‐to‐deltaic and fluvial‐to‐aeolian deposits, and ooidal or skeletal limestones and dolostones. A diverse trapping style includes drapes over basement blocks, variably inverted half‐graben features, detached and wrench‐related anticlines, and closures linked to the intrusion of Tertiary magmatic rocks. Perhaps as much as 50% of the discovered reserves lie in fields with a significant‐to‐dominant stratigraphic component. Exploration activity since the beginning of the century established the presence of recoverable reserves of about 2 B brls of oil and 15 T cu. ft of gas.