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The sandstones of the Middle Jurassic Brent Group are major reservoir rocks in the North Sea. The overall reservoir quality of the sandstones is generally good, but porosity and permeability vary greatly within the reservoirs. Mica, particularly biotite, has been the most reactive mineral component during diagenesis of these sandstones. The primary distribution of mica and its degree of alteration to illite and kaolinite control, to a large extent, the porosity and especially the permeability. Detailed mineralogical analyses of cores from the Statfjord field show that most diagenetic kaolinite formed from alteration of mica; feldspar was less important as a precursor. The alteration of muscovite to kaolinite is accompanied by an increase in volume that significantly reduces permeability. Abundant siderite has been precipitated between sheets of biotite. Microprobe analyses of the iron carbonate show that it has a distinctive composition which is influenced by the geochemical microenvironment within the biotite. Carbonate-cemented layers or nodular lenses may be important as permeability barriers. There is some evidence supporting leaching of carbonate cement but none suggesting that large parts of the sandstones of the Brent Group were ever carbonate-cemented and subsequently leached. Secondary porosity, formed by leaching of feldspar, is commonly observed but does not contribute much to the overall porosity. Plagioclase and albite lamellae in perthites are selectively dissolved, whereas potassium feldspars have secondary overgrowths. The main leaching phase is interpreted to have been relatively early and probably caused by flushing of meteoric water. Later (post-mid-Cretaceous) diagenesis was essentially isochemical, but some leaching may have occurred during deep burial. Alteration of unstable mica buffered pore water with respect to potassium, which protected the K-feldspar in relation to Na-feldspar. Micas also have an important effect on compaction and seems to enhance pressure dissolution against quartz and feldspar.

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