A petrographic study of the Rotliegendes Sandstone reservoir (Lower Permian) in the Rough Gas Field

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Abstract
The diagenetic history of the Rotliegendes Sandstone reservoir in the Rough Gas Field was studied using thin-sections, XRD analyses and SEM. The Rotliegendes comprises a sequence of fine-grained fluvial sheet-flood sandstones and coarse, gravelly, low-sinuosity channel sandstones, with thin aeolian interbeds, overlain by a sequence of aeolian dune and interdune sandstones. Early, environmentally-related diagnesis (eogenesis) shows a marked variability with sedimentary facies. Within aeolian sandstones, poikilotopic anhydrite and fine, rhombic dolomite are preserved. Fluvially-derived sandstones typically contain infiltrated detrital clays and early authigenic mixed-layer clays, together with coarse, framework-displacive dolomite. Feldspars show varying degrees of alteration within all facies. These eogenetic features reflect patterns of groundwater movement during the Rotliegendes and early Zechstein. Mineral dissolution and precipitation were controlled by the chemistry of the groundwaters. Burial diagenetic (mesogenetic) features are superimposed on eogenetic cements. Authigenic clays have been converted to illitic clays. In addition, mesogenetic chlorite has formed and quartz and strongly ferroan dolomite cements are recognized. These minerals may be related to clay diagenesis within the underlying Carboniferous Coal Measures. Early, framework-supporting anyhdrite, and both phases of dolomite, have been partially dissolved, creating secondary porosity. This is attributed to the action of acidic porewaters, generated by the maturation of organic material within the Carboniferous. Post-dissolution kaolinite, gypsum and minor pyrite infill secondary pores. Gas emplacement from the Late Cretaceous onwards effectively halted further diagenetic reactions.