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Diagenetic Evolution and Porosity Destruction of Turbiditic Hybrid Arenites and Siliciclastic Sandstones of Foreland Basins: Evidence from the Eocene Hecho Group, Pyrenees, Spain

¹ Department of Earth Sciences, Uppsala University, Villavägen 16, SE 752 36 Uppsala, Sweden; present address: AGR Petroleum, Karenslyst allé 4, P.O. Box 444, NO-0278 Oslo, Norway
² Departamento de Petrología y Geoquímica, Facultad de Geología, Universidad Complutense de Madrid, Avenida Jose Antonio Novais, s/n., 28040-Madrid, Spain
³ Departamento de Petrología y Geoquímica, Facultad de Geología, Universidad Complutense de Madrid, Avenida Jose Antonio Novais, s/n., 28040-Madrid, Spain
⁴ Department of Earth Sciences, Uppsala University, Villavägen 16, SE 752 36 Uppsala, Sweden; Department of Petroleum Geosciences, The Petroleum Institute, P.O. Box 2533, Abu Dhabi, United Arab Emirates
⁵ Departament de Geologia, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
⁶ Centre SPIN, Ecole Nationale Supérieure des Mines de Saint Etienne, and UMR CNRS 524, 158 cours Fauriel, 42023 Saint Etienne, France
⁷ ESCET, Universidad Rey Juan Carlos, Edificio Departamental I, 28933 Madrid, Spain
⁸ Department of Petroleum Geosciences, University Brunei Darussalam, Tungku Link, Gadong BE 1410, Brunei Darussalam
⁹ Department of Geosciences, University of Oslo, P. O. Box 1047, Blindern, Oslo

This study aims to unravel the impact of diagenetic alterations on porosity loss of foreland-basin turbiditic hybrid arenites and associated siliciclastic sandstones of the Eocene Hecho Group (south-central Pyrenees, Spain). In this succession, hybrid arenites and calclithites are extensively cemented by mesogenetic calcite cement (¹⁸O_VPDB = –10.0 to –5.8; T_h, mode = 80° C; salinity mode = 18.8 wt% eq. NaCl), Fe-dolomite (¹⁸O_VPDB = –8.5 to –6.3) and trace amounts of siderite. The extent of carbonate cementation is interpreted to be related to the amounts of extrabasinal and intrabasinal carbonate grains, which provided nuclei and sources for the precipitation and growth of carbonate cements. Other diagenetic alterations, such as pyrite and albitization, had no impact on reservoir quality. Scarce early diagenetic cements, coupled with abundant ductile carbonate and siliciclastic framework grains, have led to rapid porosity loss owing to compaction. Conversely, abundant quartz in the sandstones prevented rapid loss of porosity by mechanical compaction. Reservoir quality was affected by mesogenetic cementation by quartz overgrowths, calcite and dolomite intergranular pressure dissolution of quartz grains, and formation of fracture-filling calcite cement (¹⁸O_V-PDB values from –10.4 to –7.8; T_h temperatures of 150° C), which are attributed to deep circulation of hot meteoric waters during extensional stages of tectonism.

The results of this study illustrate that diagenetic evolution pathways of the arenites and sandstones are closely linked to the variation in detrital composition, particularly the proportion and types of extrabasinal noncarbonates, extrabasinal carbonates, and intrabasinal carbonate grains. These insights suggest that marine turbiditic hybrid arenites and calclithites of foreland basins are subjected to more rapid and extensive porosity loss owing to compaction and cementation than associated siliciclastic sandstones. Degradation of reservoir quality makes these hybrid arenites, calclithites, and sandstones suitable as tight gas reservoirs, but only if fracture porosity and permeability develop during tectonic deformation.