Mudstone compaction

China-clay rack is a kaolinized granite made up chiefly of quartz and kaolin, with sometimes tire presence of muscovite and tourmaline. The rock crumbles easily in the lingers. China stone is 11) a partially kaolinized granite, which contains quartz, kaolin, and sometimes mica and fluorite, is harder than china-clay rock and is used as a glaze in the production ol china or (2i a fine-grained, compact mudstone or limestone found in England and Wales... 

Bj0rlykke K. (1999) Principal aspects of compaction and fluid flow in mudstones. In Muds and Mudstone Physical and Fluid-flow Properties (eds. A. C. Aplin, A. J. Fleet, and J. S. Macquaker). Geological Society of London, vol. 158, pp. 73-78. 

The pressure data from the Smprbukk field area shows that the top of the overpressure occurs at different depths in the different wells. This suggests that the permeability distribution is mainly controlled by the stratigraphy and mineralogy, and not by the developments of depth controlled pressure seals. The Eocene and Oligocene mudstones have low permeabilities and are poorly compacted, probably due to high amounts of smectite. 

Compaction of mudstones is, as for sand, mostly a function of effective stress at low temperatures (<80°C) (Aplin et al. 1999, Bjprlykke 1997). Carbonate cementation sourced by biogenic carbonate may, however, occur at rather shallow depth so that carbonate concretions or layers can be rendered almost incompressible at shallow depth. The precipitation of quartz cement in mudstones follows much the same principles as in sandstones and is primarily controlled by the temperature. Dissolution of biogenic silica has a strong effect as a source of silica on quartz cementation at temperatures exceeding about 80 C. When modelling the compaction of mudstones the... 

Figure 6. Welt data from well 34/7-1 in the northern North Sea. Note from the log that the Tertiary sequence is generally muddy. The Pliocene and Pleistocene mud however compact much more readily than the underlying smectite-rich Oligocene and Eocene mudstones. This is clear from the velocity log which indicates nearly 3 km/s at the base of the Pliocene sequence but and less than 2 km/s in the underlying smectite rich mud. From Thyberg et al. (2000)...

The third essential requirement for a commercial accumulation of petroleum is a cap rock or seal. This is a sedimentary stratum that immediately overlies the reservoir and inhibits further upward movement. A cap rock need have only one property It must be impermeable. It can have porosity, and may indeed even contain petroleum, but it must not permit fluid to move through it. Theoretically atty impermeable rock may serve as a seal. In practice it is the shales and evaporites that provide most examples. Shales are probably the commonest, but evaporites are the more effective. We saw earlier how mud is compacted during burial into mudstone, or shale. These rocks are commonly porous, but because of the narrow diameter of the pore tluoats, they have negligible permeability. Thus shales generally make excellent seals to stop petroleum migration. When strata are folded or faulted, however, brittle shales may fracture. As described earlier, iiactures enhance permeability most dramatically. In such instances, petroleum may leak from an underlying reservoir and ultimately escape to the surface of the earth. 

Severe settlements may take place in low-grade compaction shales. Conversely, uplift frequently occurs in excavations in shales and is attributable to swelling and heave. Rebound on unloading of shales during excavation is attributed to heave due to the release of stored strain energy. The greatest amount of rebound occurs in heavily overconsolidated compaction shales. Sulphur compounds frequently are present in shales and mudstones. An expansion... 

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