Relationship between diagenesis, porosity reduction, and oil emplacement in late Carboniferous sandstone reservoirs, Bothamsall Oilfield, E Midlands

In the Bothamsall Oilfield, sandstone reservoirs at the base of the Lower Coal Measures comprise distributary-channel and barrier-bar facies. Diagenetic modifications were achieved principally by chemical processes, with mixed-layer illite/montmorillonite clay, ferroandolomite and ferroancalcite widespread in the barrier-bar facies; quartz, ferroandolomite, and kaolinite important in the channel facies; and quartz, ferroandolomite and siderite prevalent in the overbank facies. Porosity retention is greatest in coarse-grained sandstone located in the axial region of channel facies, and least in fine-grained sandstone where quartz cementation predominates. Early oil emplacement in coarse-grained sandstone of the channel facies arrested quartz diagenesis and inhibited further diagenetic reactions, whereas in barren water-filled zones diagenesis proceeded unimpeded.