Abstract

The General Levalle basin forms a long, narrow, and deep Early Cretaceous intracratonic rift in southern Córdoba province, Argentina. It trends approximately north-south for more than 150 km (93 mi), ranges from 5 to 50 km (3 to 31 mi) wide, and is more than 6500 m (21,325 ft) deep. Below a prominent middle Cretaceous unconformity, steeply dipping normal faults bound tilted graben and half-graben fault blocks. The lower rift-fill section, the General Levalle Formation (new formation name), is a Valanginian–Hauterivian siliciclastic and evaporite package more than 3200 m (10,500 ft) thick. It was deposited in an arid, restricted, rift basin that included a hydrologically closed saline lake. Nine lithology-based members represent one continuous cycle of deposition, with a lower coarse clastic sequence gradually fining upward into an evaporite member and then coarsening upward again to an upper sandstone. The uppermost rift-fill sequence, the Guardia Vieja Basalt (new formation name), is a series of Aptian basalt flows and sills more than 800 m (2625 ft) thick, with some thin red-bed intervals. Unstructured Upper Cretaceous to Pleistocene strata overlie the buried rift basin.

Following an extensive exploration campaign, in 1995–1996 the first exploratory well in the basin tested a deep-seated anticline to 5179 m (16,991 ft), but encountered just one minor show. Reservoir-quality sandstone occurred only in the upper rift sandstone member, but this lacked adequate seals. Deeper sandstone beds were tightly cemented, and basin-center dark shale below the evaporite member was thin, surprisingly low in total organic carbon, and overmature for oil. Although additional geological, geophysical, and geochemical work could have improved predrill understanding and risk evaluation, in the end, only drilling the wildcat determined the actual subsurface situation. It is now evident that given the narrow, deep depocenter, unfavorable reservoir-seal relationships, and a paucity of source facies, an effective petroleum system probably never existed in the basin.

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