Abstract

Increasing domestic gas demand and declining hydrocarbon production from conventional reservoirs in the Magallanes foreland basin have recently prompted a redefinition of the regional exploration paradigm. As a result, commercial gas accumulations have been discovered in Paleogene unconventional reservoirs in the Arenal Block on Tierra del Fuego and the Dorado–Coirón Block on the mainland. These deposits were initially considered Eocene in age; however, new geochronologic and biostratigraphic data, integrated with well-log correlations and seismic interpretation, indicate a Paleocene age. This study is part of a broader project aimed at developing more predictive geological models to improve exploration strategies for identifying new hydrocarbon accumulations and managing existing discoveries.

The Paleocene succession of the basin unconformably overlies Cretaceous strata and is dominated by fine-grained marine siliciclastic deposits interrupted by episodic intervals of coarser-grained sedimentation, primarily sandstone. It is exposed along the Andean foothills, from southern Última Esperanza Province in the north to southern Tierra del Fuego Province in the south. In the subsurface, the Paleocene succession occurs at depths ranging from approximately 5,000 m in the western depocenter to approximately 2,300 m along the eastern foreland ramp. Its occurrence over the Springhill Platform remains uncertain. An unconformity representing a hiatus of approximately 1 m.y. separates the middle Danian–upper Selandian interval from the Thanetian. The Paleocene–Eocene transition is also marked by an unconformity.
From a petroleum perspective, the Paleocene succession is locally overpressured and contains reservoirs that have produced gas and condensate in the Arenal and Dorado–Coirón blocks. Total organic carbon content is generally low (<1%) but reaches as much as 2% in outcrop samples; values may be higher in the subsurface of the basin depocenter than currently available measurements indicate. Below approximately 2,000 m, vitrinite reflectance exceeds 0.6%, suggesting that the Paleocene succession has reached thermal maturity throughout the basin. Although the Magallanes Basin is considered mature to marginally prospective in terms of conventional exploration, it remains attractive for future exploration under the emerging unconventional-resource paradigm.