Page 374 - Petroleum Geology

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drome: regressive stratigraphic sequence, abnormal pressures in the thick
mudstone formation, growth structures and petroleum trapped in them, and
lighter oil in deeper reservoirs. The evidence is strong, if not unambiguous,
that deformation began early and continued through Miocene and Pliocene
times at least. The fact that the structure was discovered by shallow core-
drilling indicates that deformation has been continuing to the very recent
past, if not the present. The evidence of growth faults is unambiguous that
the deformation took place - is taking place - in a stress field in which the
least compressive stress is horizontal, and it took place while the sequence in
the area was subsiding. The younger growth faults (Fig. 15-13) lie north of
the older, on their downthrown sides, in the direction of regression, affecting
progressively younger sediments.
Schaub and Jackson (1958, p. 1335) noted the structural style of “wide,
gentle synclines separated by relatively narrow and structurally complicated
anticlinal zones” on one of which Seria is a culmination. They also noted
that, in view of widespread evidence of diapirism, the Seria structure could
“be due to, or have been modified by, a diapiric core of relatively plastic
Setap shale”.
We would go further than this: the internal evidence of stratigraphic se-
quence and stress field during contemporaneous deformation during subsi-
dence indicates mechanical instability in the sequence, and the structural style
should be regarded as the expression of the dominant wavelength of diapirism.
A density inversion is unlikely now, but it seems an inevitable consequence
of severe abnormal pressures that a density inversion did exist when the Setap
shale was less deeply buried.

Miri field, Sarawak

Down the north coast of Borneo to the south-west from Seria field lies the
Miri field. This field was discovered in 1910, and most of the development
was done before modern logging techniques had been invented. According to
Schaub and Jackson (1958), Miri lies on the same anticlinal trend as Seria,
and contains a similar regressive sequence (Fig. 15-14). The cross-section
shows that it is a steeper structure than Seria, and the shale/mudstone core
is prominent in a fault block bounded on one side by a reverse fault. This
reverse fault seems clearly caused by the shale core because the net throw
across the block is small.
Miri differs from Seria in some respects. The oil is confined to the north
flank of Miri, and there are two types of crude in the many reservoirs. The
shallow crude is non-waxy 26”API, while the deeper crude is waxy 30”API.
The normal faults are growth faults that dip more steeply than usual on
account of the dip of the beds they cut. The reverse fault is not to be inter-
preted as evidence of horizontal “compression” because its dip is consistent
with a stress field with the greatest principal compressive stress vertical, not