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Abnormal pore pressure mechanisms 249

Figure 7.11 Pressureedepth plot and vitrinite reflectance data for wells H-9-B1, Handil
Field, and NLM-109X, Nilam Field (Ramdhan, 2010). The top of the overpressure occurs
at R o ¼ 0.6%.

7.2.3 Overpressures by uplift and unloading
Formation uplift and erosion cause overpressures, for example the Sabine
uplift in the Haynesville shale gas formation is one of reasons for over-
pressure generation (Zhang and Wieseneck, 2011). Evidence is also found
in other petroleum basins, for example, in the Cassia field, Trinidad
(Heppard et al., 2008), where the uplift made the top of the abnormal
pressure become shallower.
Assume that a formation is perfectly sealed at burial depth D 0 , retaining
a normal pressure of P f0 . If it is uplift to the depth D 1 , then the pore pressure
will keep the original value after uplift (the fluid expansion and other factors
are not considered here), i.e., P f 1 ¼ P f 0 (for a perfect seal), as shown in
Fig. 7.12. If the formation keeps the original pressure after uplift, it will
have a pressure elevation or overpressure compared to the current hydro-
static pore pressure at the new burial depth. The pore pressure elevation
due to uplift is

Dp f 1 ¼ r gD e (7.6)
f
where, D e is the height of the uplift or erosion.
Taking into account the factors of hydrocarbon compositions, thermal
history, initial pressure, and sealing conditions, Xia et al. (2013) analyzed
the fluid pressure variation of unconventional reservoirs during uplift. Their