Palaeo‐climate controlled diagenesis of the Westphalian C & D fluvial sandstones in the Campine Basin (north‐east Belgium)

The Westphalian C and D fluvial sandstones in the Campine Basin (north‐east Belgium) are potential reservoirs for the sequestration of CO 2 and interesting analogues of the hydrocarbon reservoirs in the Southern North Sea. Although these sandstones were deposited in a relatively short period of time, their reservoir properties and mineralogical compositions are very different. A petrographic study complemented with stable isotope analyses, fluid inclusion microthermometry and X‐ray diffraction analyses of the clay fractions of the sandstones, which were sampled from deep boreholes (>1000 m) in the Campine Basin, revealed that these differences are related mainly to the climate at the time of deposition. The most important eogenetic processes affecting the Westphalian sandstones were the generation of a pseudomatrix by physical compaction of Al‐silicates and lithic fragments that were strongly altered by extensive meteoric leaching, kaolinitization of unstable silicates and precipitation of siderite. These processes had a detrimental influence on the reservoir properties of Westphalian C sandstones, but their impact on the Westphalian D sandstones was minimal. The difference is assumed to be related to the climate at the time of deposition, which changed from tropical humid in the Westphalian C to semi‐arid/arid during the Late Westphalian D. Both the Westphalian C and D sandstones were affected by similar mesogenetic processes. Mesogenetic quartz cementation resulted from chemical compaction and illitization of kaolinite, K‐feldspar and smectitic clays. Illitization of kaolinite was controlled by the available quantities of co‐existing kaolinite and K‐feldspar and mainly affected the Westphalian D sandstones. Illitization of K‐feldspar was controlled by the K‐feldspar content. It had a much larger impact on the reservoir properties of the Westphalian D as, in these sandstones, K‐feldspar was less affected by eogenetic alteration. The illitization of smectitic clays resulted in illite, quartz and ankerite cementation in both reservoirs. This process had a more important impact on the Westphalian C reservoir, since cementation here also resulted from smectite to illite conversion in the interbedded and underlying shales. The effect of mesogenetic alterations on the reservoir properties was much less drastic than the impact of eodiagenesis. Mesogenetic alterations do exert a significant control on the properties of the Westphalian D. The vast impact of eodiagenesis on the Westphalian C sandstones made them less susceptible to mesogenetic alteration. The effect of telogenetic processes on the porosity and permeability of the Westphalian sandstones was small and restricted to the top reservoir intervals that directly underlie the Cimmerian Unconformity. No significant telogenetic alterations related to the Variscan Unconformity were observed.