Quantifying the role of hematite cement in controlling permeability in a deep tight gas reservoir from the North Sea

Recent work has shown the potential usefulness of magnetic susceptibility and hysteresis techniques in assessing the impact of fine grained hematite on permeability in red and white sandstone samples (Potter et al. 2009; Ali and Potter, 2011b). The present study demonstrates that hematite cementation is a major controlling factor on permeability in a deep tight gas reservoir in the North Sea. Magnetic susceptibility measurements undertaken on core plugs in this reservoir showed a strong correlation with probe permeability performed on the same plug samples. Moreover, samples with a higher content of hematite exhibited lower permeability values. Thin section analysis revealed the presence of a thin (approximately 10-15µm) rim of hematite cement surrounding quartz grains, which block pore connections and reduce permeability.

Magnetic hysteresis measurements on some representative samples indicated a very similar paramagnetic clay content in both the low and high permeability samples suggesting that the clay (mainly illite) is not the dominant controlling factor producing the variations in permeability that we observed. Since samples with higher hematite content exhibit lower permeability it appears that hematite is a major control on the permeability variations seen in this reservoir. Whilst the paramagnetic clays undoubtedly have an influence on the absolute permeability value, since increasing paramagnetic clay content has previously been shown to correlated with decreasing permeability (Potter, 2007), small amounts of hematite cement can significantly further reduce the permeability. Analysis of the magnetic hysteresis parameters on a Day plot indicated that the permeability was essentially independent of the hematite particle size for the fine particle sizes observed in this study.