Page 63 - Geology and Geochemistry of Oil and Gas

40 TEMPERATURE AND PRESSURE IN THE SUBSURFACE

The effective stress (p e ) determines the degree of compaction. Of importance here
is the escape of water occupying the pore space (mainly upward). This process is
determined by the permeability that changes with degree of lithification. Naturally,
during burial different rocks compact differently. Fig. 3.2 shows compaction for
different rock types. Sometimes, argillaceous rocks are more compactable than
sandstones (also see Rieke and Chilingarian, 1974; Chilingarian and Wolf, 1975,
1976). Shale compaction is significantly affected by mineralogy. Fig. 3.3 shows
changes occurring in montmorillonite, illite, and kaolinite clays upon compaction.
Numerous experiments established that the most rapid water loss (hence
compaction) occurs during the initial burial of sediments to a depth of 25–30 m.
During this stage, the sediments lose 50–60% of their originally contained water.
Consequently, the compaction rate significantly slows down. Most of the sediment
compaction occurs at a depth of 600–800 m. After that, the process becomes almost
imperceptible. Moreover, reservoir pressure increases due to the decrease in
permeability and consequent decrease in the rate of water escape. Thus, under-
compacted rocks form in some areas. Their major characteristic is the anomalously
high pressure. The change in the clay’s water content causes modifications in the clay
texture. Mutual orientation of mineral microblocks and microaggregates changes.
Also, the deflocculated clays are capable of plugging pores, pore throats, and canals,
thereby significantly decreasing the reservoir–rock permeability.
Carbonates in the process of compaction also lose water, and rapidly change from
the domain of plastic deformations into the domain of disruptive deformations. This
results in microfracturing, and the rock that was a seal may become a reservoir. The
carbonates with argillaceous and organic matter components usually have laminated

Fig. 3.2. Compaction of clays and sandstones (after Allen and Allen, 1990) 1 – Oklahoma (clay);
2 – Venezuela (clay); 3 – North Sea (clay); 4 – North Sea (sandstone); 5 – South Louisiana (sandstone).