Geochemistry of carbonate cements in Cretaceous sandstones, southeast Benue Trough, Nigeria: Implications for geochemical evolution of formation waters

Abstract

Carbonate cements including calcite, siderite, dolomite/ankerite were formed throughout the diagenetic history of the Asu River Group, Eze-Aku Group and Campano-Maastrichtain proto-Niger Delta sequences were analyzed for their major and trace element (Fe, Mn, Mg, Ca and Sr) and isotopic compositions. The earliest, ferroan carbonate cement has the following mean major and trace element concentration: Fe – 0.11 wt.%; Mn – 0.43 wt.%; Mg – 2.16 wt.%; Sr – 1050 ppm. The late ferroan has the following mean compositions: Fe – 0.48 wt.%; Mn – 0.22 wt.%; Mg – 0.25 wt.%; Sr – 1010 ppm. Earliest Fe-calcites, formed prior to significant compaction of the sediments, are relatively enriched in Mg (up to 4.12 mol.%), and have δ 18O values between −4.45% and −6.898% PDB close to the ideal original Cretaceous marine pore waters. Late calcites are relatively Fe-calcites (up to 4.2 mol.%). The earliest ferroan calcite occurs in both the Albian Asu River Group and Eze-Aku Group while late ferroan occurs in the three lithostratigraphic units studied. These geochemical variations appear to have resulted principally to reflect changes in pore water chemistry during diagenesis. The high value of Sr in cements is most likely due to interaction between pore waters and Sr-rich clay and possibly feldspar in the three lithostratigraphic units studied. Pore water Fe 2+ concentration was probably controlled by diagenetic alterations involving Fe-bearing minerals (e.g. pyrite precipitation). The low δ 18O value of some calcite cements (−11.62‰, −12.66‰, −14.31‰ PDB) suggests that an influx of meteoric water may have occurred in the Turonian–Coniacian, although the low value could also result from an abnormal geothermal gradient associated with tectonic activity in the trough. The elemental and isotopic composition of these cements varies as a function of the time of precipitation.