Page 372 - Petroleum Geology

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1958, both of which encountered abnormally high pore pressures in marine
mudstones at depths of about 1300 m (4300 ft), underlying the sands. The
ensuing drilling difficulties prevented their reaching their target, and no sig-
nificant accumulation of petroleum was found.
The extent of the abnormality is not clear because the data given by Visser
and Hermes (1962, fig. V-17, p. 230) cannot be taken at face value, andre-
quire careful analysis (after the correction of the pressure scale! An entertain-
ing hour can be spent on this diagram: it illustrates well the difficulty of draw-
ing valid conclusions from raw data). It seems quite certain that down to
the depths penetrated the fluid pressure never exceeded the overburden pres-
sure in spite of the mudweights used. It is inconceivable that the side-track
to Gesa 1 (Gesa 1A) penetrated rocks with significantly higher pore pressures
than those encountered in Gesa 1. So the troubles in Gesa 1A are to be attrib-
uted more to excessive mud weight and consequent loss of mud (as mention-
ed by Visser and Hermes on p. 228). The problems of Gesa 2, 500 m away,
are likely to be analogous, with spuriously high pore pressures indicated by
the mud weights. A value of X of about 0.75 (6 = 0.54) is indicated at adepth
of about 1800 m (5900 ft) in Gesa 1.
The regional extent of this abnormal pressure is indicated not only by the
mud volcanoes, but also by the seismic refraction survey, which detected a
low-speed refractor at about 1500 m.
The Gesa anticline has therefore strong indications of a diapiric origin. It
is clearly at a very early stage of development, and this may account for the
lack of significant petroleum accumulation. Gas was reported in the mudstones
only, so primary migration may be taken place.
Since Visser and Hermes mention that I suggested in 1954. that gravity
sliding was the cause of these structures (1962, p. 171), it is approprate to
record here that th.e data obtained from drilling suggest that this hypothesis
was wrong - at least for the Gesa anticline. This matter will be reconsidered
when the evidence of other areas has been introduced.

Seria field, Brunei

The Seria field on the coast of Brunei (Fig. 15-13) is an asymmetric anti-
cline of Tertiary sediments: it is about 20 km long and less than 5 km wide,
with the axis parallel and close to the present-day coastline. The stratigraphic
sequence is regressive, passing upwards from the neritic Setap Shale Forma-
tion (Oligo-Miocene), through the neritic mudstones and sandstones of the
Miri Formation and the neritic sandstones and mudstones of the Seria Forma-
tion, to the paralic Liang Formation (Schaub and Jackson, 1958; Liechti et
al., 1960). The regression is apparently still going on, towards the north or
north-west.
The structure has no surface expression. It was revealed by shallow core-
drilling under a flat, low-lying coastal swamp. The drilling of several hundred