Potential Hydrocarbon Traps Within a Late Jurassic Strike-Slip Lacustrine Basin of North Spain

Victoriano Pujalte, Sergio Robles

Strike-slip basins in northern Spain were initiated during the Late Jurassic in association with crustal stretching that heralded the opening of the Bay of Biscay. One of these basins is the Aguilar basin, which has been studied using outcrop and published well data.

The basin subsided rapidly to accumulate up to 800 m of sediment. It extends at least 60 km along a major west-northwest-east-southeast-trending Hercynian basement fault and has a maximum width of 30 km. The internally drained closed basin was fringed to the north and south by alluvial systems which fed a shallow lake that repeatedly expanded and contracted in response to tectonic and climatic influences. This resulted in the deposition of marginal deposits of interfingering carbonates, shales, sandstones, and conglomerates that grade into a carbonate-dominated succession, in turn replaced in the basin center by carbonate-gypsum sequences.

Contemporary development of north-northeast-south-southwest tight anticlines and broad synclines influence sedimentation patterns; these are particularly prominent near the major boundary fault. Minor thrusts also occur. These structures influenced the later development of hydrocarbon traps. The Ayoluengo oil field is located over one of these structures, with the main reservoir unit occurring in Lower Cretaceous siliciclastics. Stratigraphic traps may occur in the synclines where lakeshore-related grainstones pinch out updip.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.