Diagenesis, carbonate cementation and resevoir quality evolution of Eocene deep-water marine turbidite sandstones of the Hecho Group, South-Central Pyrenees M. A. Caja1, R. Marfil2, D. Garcia3, H. Mansurbeg4 and E. Remacha5 1 Departament de Geoquímica, Petrología i Prospecció Geológica, Universität de Barcelona, Martí i Franqués s/n, 08028 Barcelona. miguelangel.caja@ub.edu 2 Departamento de Petrología y Geoquímica, Facultad C.C. Geológicas, Universidad Complutense de Madrid, avda. José Antonio Nováis s/n, 28040 Madrid, Spain, marfil@geo.ucm.es 3 Centre SPIN - departement GENERIC Ecole Nationale Supérieure des Mines de St. Etienne, 158 Cours Fauriel, 42023 Saint-Etienne, France. garcia@emse.fr 4 Department of Earth Sciences, Uppsala Universitet, Villavägen, 16, SE-752 36, Uppsala, Suecia, howri.mansurbeg@geo.uu.se 5 Departament de Geología, Universität Autónoma de Barcelona, 08193 Barcelona, eduard.remacha@uab.es ABSTRACT Sandstone turbidites from the Hecho Group in the South-Central Pyrenees are considered excep - tional examples for reservoir modelling and outcrop analogous studies. The Hecho Group is divid- ed into four major tectosedimentary units (TSU-2 to TSU-5) and the sandstone composition varies from quartzarenites to arkoses (TSU-2), lithoarenites to hybrid arenites (TSU-3 and TSU-4), and hybrid arenites (rich in carbonate bioclasts; TSU-5). In TSU-2, the lowermost and most deformed unit, calcite cement precipitation was related to tectonic deformation. In the other turbidite systems (TSU-3, 4 and 5) eodiagenesis is evidenced by precipitation of dolomite cement and pyrite, which are locally abundant in all sandstones. Overall, compaction was more important than cementation in destroying porosity. However, the precipitation of dolomite overgrowth and intragranular meso- genetic ferroan calcite occluded nearly completely the remaining porosity and halted further com- paction. Dissolution of calcite and dolomite cements has resulted in creation of minor amounts of secondary porosity. Key words: carbonate cementation, compaction, sandstones, turbidite systems, Hecho Group South-Central Pyrenees. INTRODUCTION The deep-water turbiditic sandstones are increasingly becoming major hydrocarbon targets for the oil companies (Pettingill, 2000). Reservoir modelling is a powerful method in deciphering reservoir compartmentalization and good outcrop analogous models are crucial for better understand- ing complicated subsurface geometries. Diagenetic alter- ations are of key importance in understanding the dynamics of sedimentary basins and their reservoir quality evolution. For better elucidation of porosity and permeability evolution within hydrocarbon reservoir, diagenetic pathways must be integrated into conventional sedimentological models. Tur- bidite sandstones of the Hecho Group, South-Central Pyre - nees (Fig. 1) is a potential analogue for the study of other deep-sea reservoirs in similar basinal settings in which reser - voir quality assessment is biased by great deal of uncertain- ties. The carbonate rich sandstones with abundant extrabasi- nal limestones and dolostones occur within the orogenic settings, as the survival of chemically unstable carbonate grains depend on rapid erosion, transport and burial. Although these hybrid arenites are common in deep-sea environments, their diagenetic alteration is poorly explored in the literature (Spadafora et ai, 1998). In this paper, main diagenetic alterations (carbonate cementation and com- paction) of the Eocene (lower Ilerdian to the upper Lutetian) turbiditic sandstones, which are responsible in destroying reservoir quality will be quantified and discussed. Diagenet- ic processes and fluid evolution will be discussed within the geological context of the Pyrenees Basin. SAMPLING AND ANALYTICAL METHODS The turbidite systems from the Hecho Group were stud- ied and sampled in several locations: TSU-2 turbidite system (early Eocene) was sampled in the Torla, Gerbe, Castilgaleu and Figols sections. TSU-3, early Eocene in age, comprised the Arro channels, the upper Broto channel-lobe transition and the Broto-Fanlo lobes. TSU-4, early-middle Eocene, is made up of the Banaston 1 to 4, all of them corresponding to Geo-Temas 9, 2006