Page 278 - Applied Petroleum Geomechanics

Abnormal pore pressure mechanisms 269

formations, particularly in the center of the basin, the pore pressure and
temperature are markedly high, and these pose a direct threat to safe dril-
ling. It was reported that in the past 80% of the exploration and appraisal
wells in the Malay Basin were terminated because of overpressure (Shariff
and Leslie, 1995).
Singh and Ford (1982) suggested that the abnormal pressure in the
Malay Basin is a complex origin attributed to a combination of rapid burial
of certain stratigraphic units, uplift of initially normally pressured strata,
faulting, and hydrocarbon column effects. Shariff and Leslie (1995) found
that the margins of the Malay Basin are normally pressured, and the central
portion of the Malay Basin is overpressured. Madon (2004, 2007) consid-
ered that compaction disequilibrium was the primary causal mechanism for
overpressures in the basin center, when the sedimentation burial rates were
very high (>1000 m/m.y.). Hoesni (2004) proposed that the overpressure
in the Malay Basin was mainly caused by compaction disequilibrium with
potential contribution from clay diagenesis or chemical compaction pro-
cesses. He also pointed out that the occurrence of chemical compaction was
evident from a distinctive profile (rapidly increasing density with almost
constant velocity) observed on the velocityedensity cross-plots, and the
onset of chemical compaction was observed between 105 and 120 C.

Duffy et al. (2011) found that chemical compaction existed in the South
Malay Basin, and their XRD analysis on mudstone cuttings showed evi-
dence of different mineralogy with depth, with reduction of expandable
clays occurring below 2200 m. Tingay et al. (2013) concluded that
overpressure was generated by kerogen to gas maturation in the northern
Malay Basin. Satti et al. (2015) considered that the overpressure in the
southwestern Malay Basin was mainly caused by unloading. Reilly and
Zhang (2015) pointed out that pore pressure generations in the northern
Malay Basin were related to multiple factors, and the clay diagenesis could
be one of the causes of overpressures.
Measured pore pressure data in dozens of wells in the northern Malay
Basin show that wells drilled in different areas in the basin have very
different pore pressure regimes (Fig. 7.27). Wells drilled in the basin flank
have normal pressures or slight overpressures even in very deep formations
(>10,000 ft). However, wells drilled in the basin center are highly over-
pressured, probably due to more shaly and much thicker shales for building
more pressures. Many overpressured wells also have a pressure regression
following a significant pressure increase. That is, the normal pressure and
hard overpressure coexist in deep formations. In this case, the normally