Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000geoji.143..608w&link_type=abstract
Geophysical Journal International, Volume 143, Issue 3, pp. 608-620.
Astronomy and Astrophysics
Astronomy
1
Cementation, Overpressure, Permeability, Porosity
Scientific paper
Overpressure build-up is studied when the main cause for porosity reduction is cementation of the pore space sourced locally. The average porosity reduction for siliclastic sediments is modelled with nth-order kinetics. It is shown that the overpressure in one layer at a constant depth will decrease exponentially with time in the case of first-order kinetics for the porosity reduction. The overpressure is studied in one layer subjected to cementation during constant burial along a thermal gradient. A small overpressure build-up is shown above the window for cementation, with a steep rise in overpressure in the upper part of the window. The overpressure build-up is then seen to decrease rapidly towards the end of the window for cementation. Overpressure build-up is also studied when cementation is the main cause for porosity reduction in the entire column of sediments during deposition and burial. The overpressure regime characterized by gravity numbers larger than one is studied. This regime corresponds to low or moderate overpressures in the case of mechanical compaction. Cementation is shown to imply a steep pressure build-up in the window of cementation, which will easily exceed the lithostatic pressure. The porosity loss due to cementation is seen to have a strong impact on the permeability, which leads to the formation of a pressure seal. Although most of the potential for fluid expulsion is exhausted below the seal, because most of the porosity is cemented up, the permeability of the seal is sufficiently low for hydrofracturing to take place. This scenario is consistent with overpressure observations in many wells.
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