Microfacies and mineralogical analyses of the late cretaceous carbonate rocks from The Central Benue Trough, Nigeria

Abstract

The Cretaceous limestone facies from Yandev section in the central Benue Trough, Nigeria constitutes parts of the potential reservoirs  and sources rocks for petroleum prospects in the basin. The central Benue Trough is the least studied basin among its contemporaries  due to lack of subsurface data. Detailed mineralogical, facies analysis and diagenesis of these carbonate rocks in the basin have not  received significant study. The aim of this study was to employed multifaceted methodology such as sedimentological, facies analysis  petrographic and geochemical approaches which was lacking in previous research in the trough. The sedimentological analysis was done  to evaluate fabrics grain sizes, and lithology types. For petrographic analysis, thin sections were prepared and examined under a  petrographic microscope and categorized according to Dunham's classification. Geochemical analysis involved X-ray fluorescence (XRF),  X-ray diffraction (XRD), and scanning electron microscope - energy dispersive X-ray spectroscopy (SEM-EDX) techniques to unveil  mineralogical, oxide, and elemental compositions. The sedimentological results revealed light to dark grey medium to fine-grained limestone facies with intercalation of dark grey shales and siltstones. The XRD mineralogy data, revealed calcite (60.00 - 64.10%), quartz  (14.30 - 51.00%), albite (12.30 - 19.00%), and chlorite (6.30 - 8.10%). Microfacies results revealed distinct sandy bioclastic wackstone and  sparitic intraclastic packstone, as well as micritic intraclastic wackestone and sparitic bioclastic packstone. SEM images highlighted the  intricate composition, including elongated structures, whites and dark spots suggesting varying conditions of deposition. Elemental  analysis through EDX emphasized significant proportions of carbon, oxygen, sodium, potassium, calcium, silicon, aluminum, iron, and  magnesium. Based on sedimentological, biofacies, lithofacies and geochemical evidence we infer that the carbonate sediments were  mainly deposited in a shallow marine depositional environment (inner to outer neritic).