Effect of diagenesis on pore pressures in fine-grained rocks in the Egersund Basin, Central North Sea
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Original versionNorsk Geologisk Tidsskrift 2015, 95(2):171-189 10.17850/njg95-2-03
Pore pressure in fine-grained rocks is important with respect to drilling problems such as kicks, blowouts, borehole instability, stuck pipe and lostcirculation. In this study, a succession of overpressured, fine-grained, sedimentary rocks located in the Egersund Basin, Central North Sea, wasanalysed with respect to mineralogical composition, source-rock maturation and log-derived petrophysical properties to highlight the effect ofdiagenetic processes on the pore pressure. Petrographic and geochemical analyses showed that the overpressure in the study area is largely linked todisequilibrium compaction, illitisation and source-rock maturation shown by log-derived physical properties. Pore-pressure prediction based on thedifference of log-derived sonic transit time compared to the normal compaction trend (NCT) of fine-grained rocks can be used to infer the generaltrends of pore-pressure changes. However, during such pore-pressure prediction (e.g., using Eaton’s approach), one should note that with regard tosonic response of the above-mentioned processes, the sonic log-derived, predicted pore pressure in the chemically compacted intervals and organicrichthermally mature successions may show either underestimations or overestimations, respectively.