Measurement of diagenetic compaction strain from quantitative analysis of fault plane dip

We developed a new technique for quantifying the amount of porosity loss during diagenetic transformation of opal-A to opal-CT. The technique is based on 3D seismic data from offshore Norway, where the widely developed biosiliceous sediments of the Brygge Formation are deformed by a polygonal fault system. Evidence is shown from two study areas that the dip of the polygonal fault planes reduces abruptly across the opal-A to opal-CT diagenetic boundary, suggesting that the fault planes were passively rotated into shallower dips due to diagenesis. The reduction of fault plane dip was used to quantify the vertical compaction strain due to diagenesis. Using an independent assessment of the magnitude of the porosity loss, we are able to validate the method based on fault plane dips, and also to evaluate whether the porosity loss results in an exclusively vertical strain. Additionally, the impact of silica diagenesis on the shear strength of the sediments and fault growth is investigated. We present evidence suggesting that deviations from classical Mohr–Coulomb behaviour may be expected during the combined processes of diagenesis and porosity collapse, and may indeed be sufficient to promote continued fault growth.