In sedimentary basins, compaction disequilibrium generates overpressure during rapid burial of fine-grained sediments in the mechanical compaction regime, at temperatures below ~70°C. Mudstones behave differently at greater depths in the chemical compaction regime, at temperatures above ~100°C, where evidence suggests that porosity reduction with increasing depth and temperature continues independently of effective stress up to high values of overpressure. We offer an explanation for this behaviour. The horizontal alignment of clay mineral grains is enhanced during clay diagenesis, creating sub-horizontal, flat pores. Because of their flexibility, the flat pores tend to close even under low values of normal effective stress acting across them. Thus, chemical compaction can proceed unless the net expulsion of pore water from the mudstones is inhibited sufficiently for the flat pores to be held open, which necessarily requires the pore pressure to approach the lithostatic stress.
In the Lower Kutai Basin, density log reversals are encountered in mudstones in the chemical compaction regime at depths of 3–4 km, where the pore pressure is close to the lithostatic stress. We attribute these reversals to the inhibition of dewatering during clay diagenesis at shallower depths, when the pore pressure was already close to lithostatic stress. Porosity was preserved by the very high pore pressure holding the flat pores open while the mudstone matrix was being cemented by the products of clay diagenesis. We coin the term ‘chemical undercompaction’ for this process.