Assessment of β the compression coefficient of mudstones and its relationship with detailed lithology

Abstract

The mechanical compaction of soils and fine-grained sediments is commonly described in soil mechanics by a relationship between void ratio and effective stress. The relationship is defined by two parameters: β, derived from the slope of the line relating void ratio and log effective stress, and e ∗ , the void ratio at a chosen effective stress (e.g. 100 kPa). β and e ∗ are strongly correlated and are related to the Atterburg liquid limit, which is itself related, in a general way, to the grain size and mineralogy of the sediment. In general, finer grained sediments have higher initial void ratios and compact faster than coarser sediments. Porosity-depth trends for mudstones in sedimentary basins are therefore strongly influenced by lithology; steps or changes in the slope of porosity-depth trends can be caused by lithological change rather than changes in the pressure regime. A method is presented to evaluate β for mechanically compacted mudstones, using porosities determined from the density log (FDC) and based on the relationship between effective stress and void ratio developed in soil mechanics. The method allows one to obtain, rapidly, approximate values for materials for which it is essentially impossible to obtain compaction parameters experimentally. Calculated β values for mudstones in wells 8/3-2 and 30/2-1 on the Norwegian Margin vary between 0.2 and 0.65. The β values correlate positively with the clay particle content of the mudstones, which we have determined for 50 mudstone samples from the same wells. The strong link between mudstone lithology and β suggests that a downhole ‘β log’ may be a useful way to visualize downhole variations in mudstone lithology. The β log also shows how lithology affects the rate of porosity loss with depth or effective stress. For example, porosity in mudstones within both wells 8/3-2 and 30/2-1 is essentially constant over 700 m sections of normally pressured mudstones. The β log and grain size data indicate that this reflects changes in the lithological character (and β) of the muds and not, as commonly inferred, a change in the pressure regime.