Page 74 - Origin and Prediction of Abnormal Formation Pressures

56 G.V. CHILINGAR, J.O. ROBERTSON JR. AND H.H. RIEKE III

interlayered water. According to Harkins and Baugher (1969), in order for abnormal
pressures to develop, the shales must be over 200 ft in thickness.
Continued sedimentary deposition can result in a shear zone developed by overload-
ing the undercompacted shale. Expulsion of water from these sediments is accompanied
by the subsidence of blocks of sediments. Many contemporaneous faults found in the
Gulf Coast Basin (USA) exhibit the following cycles: (1) deposition, (2) expulsion of
water, (3) subsidence of blocks of sediments, and (4) temperature increase. (Also see
Chapter 1.)

Tectonics

Tectonic activity may be the cause of AHFPs, including local and regional faulting,
folding, lateral sliding and slipping, squeezing resulting from down-dropping of fault
blocks, diapiric salt/shale movements, and earthquakes.
In their classical book, Poston and Berg (1997) stated that some of the earliest
recorded AHFPs were reported from areas where recent tectonic activity caused the
principal normal stress to be horizontal. Transfer of tectonic stress to the fluids can result
in overpressures, as exemplified by the Ventura oilfield of California (USA). Fig. 2-21
illustrates the effect of tectonic activity on oilfield pressures. Other pertinent references
appear in Chapter 8.

%
%
",, Khaur
o
o
o
\ o~k, Chia-Surkh
Ventura \ 'O~,
Q. "-'~\ . ,,
s , \ e -,
~ ~ - Qum

10 \
G~ "G,

12 \
0 2 4 6 8 10 12
Pressure, 1,000 psig

Fig. 2-21. Overpressures recorded in wells drilled in or near active tectonic belts of compressional loading
and faulting. (Modified after Hubbert and Rubey, 1959, p. 115.)