Diagenetic and oil migration history of the Kimmeridgian Ascla Formation, Maestrat Basin, Spain

Abstract

The marine limestones of the Kimmeridgian Ascla Formation in the Maestrat Basin reached more than 3500 m in burial depth during the Cretaceous era. Despite containing organic-rich intervals, mature in parts of the basin, its potential as oil source-rock has been either overlooked or questioned. A petrographic, geochemical and fluid-inclusion (FI) study of the cements of the Ascla was performed in order to unravel its diagenetic and thermal evolution. We particularly sought evidence of oil migration and its timing. Three sequences of cement were distinguished. Sequence 1 fills the primary porosity and began with Fe-poor calcites with geochemistry and FIs consistent with precipitation from marine-derived waters during shallow burial. These calcites were followed by burial cements, including ferroan calcite, dolomite, and minor celestite and barite. Sequence 2 consists of Mg-rich, fracture-filling calcite cement zones. The earlier ones are ferroan and contain primary aqueous and oil FIs with homogenization temperatures suggesting precipitation at temperatures as high as 117°C. Sequence 3 is dominated by fracture-filling calcites with geochemistry and FIs indicating precipitation at low temperatures (less than ∼50°C) from meteoric waters. Cross-cutting relationships with compressional microstructures indicate that Sequence 3 formed after the Eocene–Oligocene tectonic inversion of the basin. Oil FIs in Sequence 2 provide evidence that light oils migrated through the Ascla Formation via fractures and microfractures. These oils were likely generated in the organic-rich marls of the basal part of the Ascla. The paragenetic sequence and burial history are consistent with oil generation when the Ascla was at or close to maximum burial depth, but before the Eocene Alpine tectonism, which likely formed the structural traps in the basin. Oil generation and migration occurred long before this event. Therefore, it is probable that early traps were breached by the Alpine structures and that potential in this basin sector is low.