Porosity-depth trends of chalk. Calculating effective burial by using Biot's coefficient

Water zone chalk from the over-pressured northern saltdome province of Danish central North Sea was sampled, and porosity, stable isotopes and acoustic properties were compared to those of samples from the hydrocarbon bearing South Arne chalk field. Data from the western Pacific chalk of the Ontong Java Plateau were used as reference. Porosity-decline as a function of depth was presented in two alternative ways taking pore pressure into account. Either by Terzaghi's equation, where pore-pressure is 100% effective in supporting the mineral grains, or by taking Biot's coefficient into account, where pore-pressure in cemented material is less than 100% efficient in supporting the mineral frame. Our data suggest that cement is supplied by pressure solution at stylolites according to Terzaghi's equation, because stylolites are mechanically similar to fractures. The cementation takes place at different temperature at different localities as indicated by δ ¹⁸O data. The chalk between the stylolites is cemented and has porosity on a smooth effective depth-trend, when effective depth is calculated by taking Biot's factor into account.