Page 254 - Applied Petroleum Geomechanics

Abnormal pore pressure mechanisms 247

maturity in the Bakken formation, North Dakota, suggest that extensive,
bedding-parallel microcracks exist in situ in most mature source rocks
(Vernik, 1994). Core samples of the Bossier and Haynesville shales show
that bedding-parallel fractures exist in most cores, and some cores are highly
fractured with poke-chipping fractures and other microfractures. This could
be caused by the fracturing from induced overpressures by hydrocarbon
generation.
Gas generation in low permeable rocks causes high overpressure in
many onshore gas plays in the United States. For example, in the Greater
Green River Basin in Wyoming, Colorado, and Utah, the overpressures are
mainly caused by gas generation and occurrence in low-permeability upper
Cretaceous and lower Tertiary rocks (Law, 1984; Law and Dickinson,
1985). Fig. 7.10 shows the pressure gradient in the Wagon Wheel well in
the Green River Basin, Wyoming. The pore pressures constructed by C.
W. Spencer (Law, 1984, 2002) were based on mud weight data from mud
logs, well logs, and drill stem tests (DST). The total gas-bearing interval has
been divided into three pressure-related intervals that are defined on the
basis of specific geologic conditions.
The uppermost interval, referred to as the inactive zone, extends from a
depth of 8035 ft to about 10,500 ft where there is an abrupt increase in the
pressure gradient. The inactive zone corresponds to an interval in which
significantly large volumes of gas have previously been thermally generated.

Figure 7.10 Pore pressure and temperature (left) and vitrinite reflectance (right) in the
Wagon Wheel well, the Green River Basin, Wyoming. (From Charpentier et al., 1987;
Law, 2002)