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

The marine limestones of the Kimmeridgian Ascla Formation in the Maestrat B asin reached more than 3500 m in burial depth during the Cretaceous era. De spite containing organic-rich intervals, mature in parts of the basin, its potential as oil source-rock has been either overlooked or questioned. A pe trographic, geochemical and fluid-inclusion (FI) study of the cements of th e Ascla was performed in order to unravel its diagenetic and thermal evolut ion. We particularly sought evidence of oil migration and its timing. Three sequences of cement were distinguished. Sequence 1 fills the primary poros ity and began with Fe-poor calcites with geochemistry and ms consistent wit h precipitation from marine-derived waters during shallow burial. These cal cites were followed by burial cements, including ferroan calcite, dolomite, and minor celestite and barite, Sequence 2 consists of Mg-rich, fracture-f illing calcite cement,zones. The earlier ones are ferroan and contain prima ry aqueous and oil FIs with homogenization temperatures suggesting precipit ation at temperatures as high as 117 degreesC. Sequence 3 is dominated by f racture-filling calcites with geochemistry and FIs indicating precipitation at low temperatures (less than similar to 50 degreesC) from meteoric water s. 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 thr ough 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 generat ion; 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 b asin. Oil generation and migration occurred long before this event. Therefo re, it is probable that early traps were breached by the Alpine structures and that potential in this basin sector is low. (C) 2001 Elsevier Science L td. All rights reserved.