The role of clay mineral diagenesis in overpressure generation and compaction of siliciclastic mudstones

Abstract

Clay mineral diagenesis has a considerable effect on the physical properties of siliciclastic mudstones, with important implications for pore pressure prediction. The dominant clay mineral reaction, the conversion of smectite to illite, involves a series of dissolution and reprecipitation reactions which results in a significant change in the orientation of the clay mineral fabric. Unloading is a direct result of clay mineral diagenesis and concomitant fabric destabilisation, due to the local transfer of load from dissolving detrital clay grains to fluid. Pore pressure is then a function of the rate at which it is generated by clay mineral diagenesis (and other mechanisms such as disequilibrium compaction) and the rate at which it is dissipated by compaction and fluid flow.
Clear evidence has been found for chemical compaction (porosity loss/sediment volume reduction) associated with illitization of smectite in Miocene mudstones in the Central Malay Basin, in Cretaceous mudstones at Haltenbanken, offshore mid-Norway, in Cretaceous to Tertiary mudstones in the Sergipe-Alagoas Basin, offshore Brazil and in Triassic mudstones in the North Sea Central Graben from measured physical, textural, and mineralogical properties, and from log responses. In addition to this diagenetically mature, illitized mudstones continue to compact mechanically with increasing effective stress. The newly presented data have been interpreted to discriminate between two models for the chemical compaction of diagenetically altered mudstones proposed by previous researchers: (effective) stress-independent chemical compaction and chemically-enhanced mechanical compaction. Key evidence in favour of the chemically-enhanced mechanical compaction model comes from density logs of Cretaceous mudstones at the Halten Terrace, offshore mid-Norway in association with the pore pressure history inferred by previous pore pressure analysis. This model is also consistent with the petrographic evidence that clay-rich siliciclastic mudstones have a clay-supported matrix both before and after illitization. Established methods of pore pressure estimation do not correctly account for the mechanical and chemical contributions to mudstone compaction, except empirically or in favourable circumstances where use can be made of data from offset wells with similar lithology, burial history and temperature history.