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Resumen:
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The diagenetic evolution of Permian (Autunian and Saxonian) and Triassic (Buntsandstein) sand-stones and mudrocks have been studied over 1000 m sequence from the Sig€uenza 44-3 drill core inthe Iberian Range, Spain. We compare and contrast the diagenetic processes in these differentlithologies and the timing of clay mineral formation. Moreover, we establish the relationshipbetween clay mineral diagenesis and reservoir potential. Both the Permian and Triassic successionsare characterised by conglomerates, sandstones and interbedded mudstones of fluvial origin thatchange upwards into distal deposits of a fluvio-deltaic system. The clay minerals are illite, illite-smectite mixed layers, kaolinite and dickite. The illite content in all sequences is not related todiminished feldspars; it is owing to the initial detrital mineralogical composition of the Autuniansandstones. The effect of feldspar alteration to kaolin minerals has a strong influence on the lost ofporosity-permeability in the Saxonian facies. In contrast, illite and mixed layers illite-smectite arethe main clay rims preserving porosity in the Buntsandstein sandstones. However, fibrous illite isthe dominant pore-filling in the Permian Autunian facies, closing porosity and permeability. Kaoli-nite and dickite show opposite trends: dickite increases yet kaolinite decreases from Triassic to Per-mian sandstones. Dickite replaced kaolinite during burial-thermal evolution of the succession. ThedD andd18O isotopic signatures from silt and clay fractions indicate a mixture of meteoric and mar-ine waters, and suggest a minimum temperature range between 60 and 150°C for diagenetic porefluids. The PermiandD values (?24&to?44&) are relatively similar to Buntsandstein values(?24&to?37&). However, the Permiand18O values (+7.6 and+15.3, average of+13.3&) aregenerally higher byca.6.2&compared to the Buntsandstein data (4.8–10.1&, average+7.1&).Such a variation is interpreted as the result of mesodiagenetic pore fluid changes. The extensivedickitisation of kaolinite is attributed to increased hydrogen ions resulting from maturation oforganic matter. The vitrinite reflectance of organic matter and the modelled thermal history suggesta maximum burial of 3400 m, accomplished 70 Ma ago. The Permo-Triassic reached the gas win-dow shortly before major uplift, at 65 Ma, when further maturation and hydrocarbon expulsion ceased
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