Page 255 - Applied Petroleum Geomechanics

248 Applied Petroleum Geomechanics

However, because a cooling event occurred 2e4 million years ago, the
thermal generation of gas has either ceased or diminished to low rates such
that it is lost from the reservoirs faster than it accumulates (Charpentier
et al., 1987).
Below the inactive zone is the active zone that is further subdivided into
two intervals. Within the active zone thermogenic gas is thought to be
actively generated. The upper part of the active zone extends from a depth
of about 10,500 ft to about 14,700 ft where another abrupt increase in the
pressure gradient occurs. The pressure gradient in the upper part of the
active zone appears to be due to a thermal effect elaborated on by the lower
part of the active zone that extends from a depth of about 14,700 ft to
17,700 ft where the base of the Rock Springs Formation occurs. The
pressure gradient in the lower part of the active zone is coincident with the
top of a coal-bearing zone. The elevated pressure gradient through this
interval has been interpreted to the increased gas generation of the coal
zone (Charpentier et al., 1987).
With subsequent burial and exposure to higher temperatures, the
accumulated oil undergoes thermal cracking to gas, accompanied by a
significant increase of fluid volume and pressures (Barker, 1990). The level
of thermal maturity at which oil is transformed to gas is commonly thought
to be about 1.35% vitrinite reflectance (R o ) for a liquid-prone (Type I/II
organic matter) source rock (Tissot and Welte, 1984; Hunt, 1996). For a
gas-prone source rock (Type III organic matter), when R o > 0.6%, gas
generation and overpressuring start, such as in the Greater Green River
Basin (Law, 2002), as shown in Fig. 7.10, where the increase of R o is closely
related to the overpressures.
Oudin and Picard (1982) found that there is a good correlation between
the top of hard overpressure and the increase in vitrinite reflectance in the
Handil Field in Indonesia while Bates (1996) obtained a similar correlation
in the Nilam Field. According to Lambert et al. (2003), gas generation starts
at a vitrinite reflectance of 0.6%. In the fields of the Sisi-Nubi, Tunu,
Peciko, Handil, and Nilam in the Lower Kutai Basin, Indonesia, the top of
the transition zone from normal pressure into hard overpressure coincides
with the vitrinite reflectance threshold value of 0.6% for gas generation
(Ramdhan, 2010). Vitrinite reflectance data and measured pore pressures
show coincidence in gas generation and overpressures, as shown in
Fig. 7.11.