Page 252 - Applied Petroleum Geomechanics

Abnormal pore pressure mechanisms 245

temperature. Hydrocarbon generation is the creation of mobile fluids
(mainly oil and hydrocarbon gases) from an original solid immobile
kerogen, causing fluid volume or porosity increase if the fluids cannot be
expelled. The coincidence of overpressure and hydrocarbon generation was
given early prominence by the study of the Bakken shale in the Williston
Basin, Montana and North Dakota, USA (Meissner, 1978a,b). The
abnormal pressure was attributed by Meissner (1978a) to two processes: (1)
increased volume of hydrocarbons and residue relative to unaltered organic
material, and (2) inhibited structural collapse of the rock framework as
overburden-supporting solid organic matter was converted to hydrocarbon
pore fluid. He estimated the fluid volume increased at about 25%, with
even greater increases in volume when maturation proceeded from oil to
wet gas, and later to dry gas (Fig. 7.8). Spencer (1987) extended the link
between overpressure and volume increase during oil generation to most of
the deeper parts of the Rocky Mountain basins. Sweeney et al. (1995)
modeled volume increases from organic maturation during oil generation,
leading to 25% of the total overpressure found in the La Luna Formation
source rocks in the Maracaibo Basin, Venezuela. The distribution of
hydrocarbon generation with depth indicates that considerable hydrocar-
bon expulsion and migration occurred in the US Gulf Coast wells at below
10,000 ft, where vitrinite reflectance values ranged from Ro ¼ 0.65 to
0.9% through the hydrocarbon generation interval. Results in four wells
indicate a close relationship between the intervals of high overpressures
(pressure gradient ranging from 0.76 to 0.86 psi/ft) and the petroleum
generation and expulsion windows; however, the porosity versus depth plot
in each well does not show an obvious change (Hunt et al., 1998).
The conversion of kerogen to methane from source rocks is the most
aggressive overpressure mechanism. The rising temperature that accom-
panies increasing burial depth converts oil in a reservoir into thermal gas. A
consideration of hydrogen balance shows that approximately one volume of

Wet gas & Dry gas
condensate Volume
Oil increase

Figure 7.8 Estimation of volume change when Type II kerogen in the Bakken shale of
the Williston Basin matures to produce oil, then wet gas and condensate, and finally
dry gas. (Redrawn from Meissner, 1978b.)