Page 275 - Applied Petroleum Geomechanics

266 Applied Petroleum Geomechanics

the range 38e52 MPa, which might be caused by thermal cracking from oil
to gas.
Gaarensstroom et al. (1993) studied the generation of overpressure and
sealing mechanisms in the Central Graben of the United Kingdom and
proposed that overpressure was initiated by rapid sedimentation and burial
of low permeability claystones. This early pressure generation was then
followed by an increase in the magnitude of the overpressure due to
hydrocarbon generation. Finally, as the pore pressure continued to increase,
the fracture gradient was exceeded with subsequent loss of fluid and
pressure.

7.5.5 Abnormal pressures in the Cooper Basin, Australia
The Nappamerri Trough contains a thick Permian section of sandstones,
coals, siltstones, and shales, and extensive drilling has proved up many
commercial gas fields. The Roseneath Shale, Epsilon Formation, and
Murteree Shale (REM) were initially the main focus of shale gas assessment
in the Nappamerri Trough of the Cooper Basin in Australia. The thick
shale units of the Roseneath and Murteree are considered to be regional
seals. The Nappamerri Trough contains a thick Permian section of sand-
stones, coals, siltstones, and shales deposited in a cold climate fluviolacus-
trine setting (Trembath et al., 2012).
The prospective REM section has a high vitrinite reflectance of 2%e4%
with the level of thermal maturity depending on location within the
Trough. With the high level of maturity, the Permian sequence is expected
to be within the dry gas window. DSTs and mud weights in offset wells
indicate that the Epsilon and Patchawarra Formations are highly over-
pressured and that the overpressure is confined to the Nappamerri Trough.
The regional pressure gradient is 0.43 psi/ft (hydrostatic pressure gradient),
while the pressure gradient in the Nappamerri Trough, based on DST data
over the Epsilon and Patchawarra Formations, is about 0.72 psi/ft
(Fig. 7.25). The top of overpressure starts at 2800 m and the maximum
overpressure compared to the hydrostatic pressure can reach 3000 psi. This
overpressure might be caused by gas generation.

7.5.6 Abnormal pressures in China
Abnormal pore pressures exist in many geologic basins in China. For
example, overpressures have developed over large parts of the Yinggehai,
Qiongdongnan, Sichuan, and Bohai Bay basins. Overpressures in the