Page 276 - Applied Petroleum Geomechanics

Abnormal pore pressure mechanisms 267

Figure 7.25 Cooper Basin pressureedepth trends with the Central Nappamerri Trough
deviating from the regional trend indicating overpressure (left) and schematic repre-
sentation of the potential thickness of gas saturated interval in Encounter-1 and
Holdfast-1 (right) (Trembath et al., 2012, with permission from the APPEA Journal).

Junggar basin are widely spread in the central and southern parts. Over-
pressures in the Tarim basin occur mainly in the Kuche depression (Hao
et al., 2007). Most shale gas and tight gas formations in the Sichuan basin are
overpressured, such as the Longmaxi shale gas formation in the Fushun-
Yongchuan block. Abnormally low pressures are widely spread in the
Ordos basin where basin-centered gas systems had been found.
Triassic-aged Xujiahe sandstones are tight gas formations located in the
western Sichuan basin, pore pressures in these tight sandstones and source
rocks are highly overpressured (Fig. 7.26). For example, pore pressure in gas
2
formation Xu 2 (T 3 X ) is 1.5e1.7 times greater than the hydrostatic
3 4
pressure; pore pressures in the Xu 3 (T 3 X ) and the Xu 4 (T 3 X ) have even
higher magnitudes, around 1.85e2.1 times greater than the hydrostatic
pressure. This region belongs to active tectonic stress regime because of the
strong compression in the Longmenshan fault belt, which might be one of
the reasons for abnormal pressure generation. In situ stresses measured from
borehole hydraulic fracturing tests (e.g., mini-frac) indicate that the
maximum horizontal stress is greater than the vertical stress
(s V ¼ 0.024e0.025Z). At the shallow depth, both horizontal stresses are