Diagenetic and oil migration history of the Kimmeridgian Ascla Formation, Maestrat Basin, Spain C. Rossi a, * , R.H. Goldstein b , R. Mar®l a , R. Salas c , M.I. Benito d , A. Permanyer c , J.A. de la Pen Äa a , M.A. Caja a a DepartamentodePetrologõ ÂayGeoquõ Âmica,FacultaddeCienciasGeolo Âgicas,UniversidadComplutense,28040Madrid,Spain b DepartmentofGeology,UniversityofKansas,Lawrence,KS66045-7613,USA c DepartamentodeGeoquõ Âmica,UniversidaddeBarcelona,08028Barcelona,Spain d Departamento de Estratigrafõ Âa,UniversidadComplutense,Madrid,Spain Received 17 November 2000; received in revised form 14 January 2001; accepted 19 January 2001 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 ¯uid-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 ®lls 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-®lling 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 1178C. Sequence 3 is dominated by fracture-®lling calcites with geochemistry and FIs indicating precipitation at low temperatures less than ,508C) 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. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Burial diagenesis; Fluid inclusions; Oil migration 1. Introduction The Kimmeridgian Ascla Formation was deposited in open marine environments in the Salzedella sub-basin, which was the main depocenter of the Maestrat Basin E Spain) Fig. 1). This formation contains intervals of lami- nated marly limestones with enough organic matter to be considered as potential oil source rocks Permanyer, Mar®l, de la Pen Äa, Dorronsoro, & Rossi, 1999; Salas, 1989). The source-rock potential of the Ascla was not recognized when the Maestrat basin and adjacent offshore areas were explored for hydrocarbons during the seventies. Onshore, exploration was unsuccessful. Offshore, in the Tarragona basin, several oil ®elds were discovered. Although most of the oils discovered in the Tarragona basin undoubtedly originated from Miocene source rocks Albaige Âs, Algaba, Clavell, & Grimalt, 1986), the oil from Amposta ®eld, which is located only 40 km to the west of La Salzedella, has unique geochemical characteristics and is not correlated with the other oils Albaige Âs et al., op. cit.). It has been postulated that the oil of the Amposta ®eld could have been generated from the Ascla Formation Seifert, Carlson, & Moldowan, 1983). Albaige Âs et al. 1986) discarded the Ascla as a potential source for the Amposta oil, arguing that the Ascla 1) does not have source-rock potential, and 2) it was overmature for oil generation at the time of trap formation the Miocene). Marine and Petroleum Geology 18 2001) 287±306 0264-8172/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0264-817201)00008-3 www.elsevier.com/locate/marpetgeo * Corresponding author. Fax: 134-91544-2535. E-mail address: crossi@eucmax.sim.ucm.es C. Rossi).