Discovery of an extraordinary Carboniferous cutinite-rich coal seam from Wadi Abu Thora, southern Sinai, Egypt: Organic petrographical and geochemical characterization

Abstract

An extraordinary cutinite-rich coal seam from the Abu Thora Formation in the Wadi Abu Thora area, west central Sinai, Egypt, was studied in detail. A sample taken from the seam consisted almost of pure cutinite with minor sporinite (megaspores) and secretinite, along with solid bitumen veins and exsudatinite. The cutinite-producing cuticles appear to have single plant origin as indicated by their morphological features and geochemistry. The cuticles display yellow fluorescence under under blue light illumination and were arranged in bands that present the ability to trace a single cuticle for the full length of the sample. The detrital component present between the cuticle bands was minimal. Organic geochemical analysis by Rock-Eval pyrolysis revealed a S2 of 214.2 mg/g rock and a hydrogen index of 477 mg HC/g TOC, which point to liptinite-rich kerogen Type II. Pyrolysis gas chromatography showed a well-defined chromatogram with abundant compounds in the range between C11 and C21. Organic matter in the coal seam is in the early mature stage based on cutinite fluorescence color, the Tmax of 437 °C, and the solid bitumen reflectance (0.30%), which equates to % vitrinite Ro of 0.60. Inorganic geochemical analysis revealed a low concentration of trace elements and deposition in swamp conditions without any influence by marine waters. Transmission FTIR analysis revealed spectra typical of cutinite, marked by a strong peak at 1710–1700 cm−1 attributed to the fatty acids CO group. The cutinite was produced by xerophytic pteridosperm plants as indicated by the chemotaxonomic correlation based on FTIR analysis. These plants produced larger quantities of cutinite relative to other maceral components, such as vitrinite or sporinite because of their relatively high frond biomass. Pinnule-bearing fronds that fell off the plants and accumulated in the flooded swamp were removed quickly from acrotelm to catotelm by rapid subsidence, where they were eventually preserved intact and with little alteration by microorganisms.