Page 284 - Applied Petroleum Geomechanics

Abnormal pore pressure mechanisms 273

Haynesville and Bossier shale gas plays in Louisiana and Texas, the pore
pressure gradient is very high and ranges from 0.75 to 0.94 psi/ft
3
(1.73e2.17 g/cm ); the upside of the high pressure gradient has been
abnormally high well IPs (9.5 MMscf/d), almost five times those of the
benchmark Barnett (Sandrea and Sandrea, 2014). However, some shale gas
plays have normal pore pressures or even underpressures, such as the
Fayetteville, New Albany, and Antrim.

7.5.8.2 Bakken and Three Forks plays
The Bakken shale oil play is mainly located in the Williston Basin in North
Dakota and Montana of USA and Saskatchewan of Canada. The Missis-
sippian- and Devonian-aged Bakken formations consist of the Upper,
Middle, and Lower Bakken members. The Middle Bakken reservoir is a
calcitic and dolomitic siltstone lying between the Upper and Lower Bakken
shales, which are the source rocks for the play (Dohmen et al., 2013). The
Three Forks, beneath the Bakken formation, is also an oil-bearing forma-
tion. Pore pressures in the Bakken and Three Forks indicate an inverted
continuous system with pressure leaking off at top, apart from the Parshall
pressure cell (Fig. 7.29). The most likely regional normal pressure gradient is
0.47 psi/ft because of high salinity in the formations. The top of over-
pressure occurs at about 9000 ft; thereafter, the pore pressure is highly
overpressured in the Bakken and Three Forks formations, as shown in
Fig. 7.29. Comparisons indicate a higher overpressure in the Three Forks at
the same depth level than that in the Middle Bakken.
Meissner (1978a) pointed out that abnormal pressures associated with
mature Bakken source rocks are basically caused by:
1. the inhibited structural collapse of the rock framework as overburden-
supporting solid organic material (estimated to be at least 25 volume
percent of the rock) converted to nonoverburden-supporting hydrocar-
bon pore fluid (e.g., oil and/or gas); and
2. the increased volume occupied by metamorphosed organic residue plus
generated hydrocarbon fluids above those occupied by the unaltered
organic material.
Therefore, anomalous pressures in the Bakken formations are believed
to be maintained by the combination of large hydrocarbon volumes
generated at high rates and the relative isolation of the Bakken by extremely
tight rocks in the underlying Three Forks and overlying Lodgepole For-
mations (Meissner, 1978b). Theloy (2014) also found that the intense oil