Stable isotope and geochemical evidence of formation pore fluid evolution during diagenesis of Tertiary sandstones and mudrocks of the Niger Delta

Abstract

Petrological data provide evidence that framboidal pyrite, Fe-carbonates and kaolinite are the major diagenetic minerals developed during burial diagenesis in the Tertiary Niger Delta sandstones and associated mudrocks. The pyrite sulphur, carbonate carbon and oxygen and kaolinite oxygen and hydrogen isotope compositions have been determined. These data (pyrite, δ 34 S = −24.8 to 21.0‰ ; “siderite”, δ 13 C(PDB) = −14.7 to +5.0‰ , δ 18 O(PDB) = −19.1 to −0.6‰ ; Fe-calcite, δ 13 C(PDB) = +17.5 to 17.9‰, δ 18 O(PDB) = −8.3 to −8.0‰ ; kaolinite, δ 18 O(SMOW) = +14.7 to 17.5‰ , δD (SMOW) = −86 to −43‰) have been used to interpret the isotopic compositions of the precipitating pore fluids and/or the temperatures of mineral formation. The interpretation of these results indicate that in the deltaic depositional setting the syndepositional pore waters had a significant but variable marine influence that favoured the early formation of pyrite. Subsequently the subsurface influence of meteoric waters, showing varying degrees of modification involving organic and/or water-rock reactions, played an increasingly significant role in the development of later diagenetic cements in the sediments when abundant authigenic carbonates and kaolinites were formed.