Page 391 - Petroleum Geology

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to the reservoirs (see p. 243). These four fields appear to be a perfect natural
laboratory for the study of all the problems discussed in Chapter 11 in an
attempt at reconciling the geological, geochemical and hydraulic aspects of
generation, migration and entrapment of oil and gas in these fields.
Returning to the regional geology of the island of Borneo, we see that
structures of both Kalimantan in the east and Sabah, Brunei and Sarawak in
the north-west, result from orogeny in the interior of Borneo, and the progress
of this orogeny is recorded in the sedimentary basins around the island.
That Borneo is not a special case is seen in south Sumatra. The South
Sumatra basin consists of an Eocene-Miocene-Pliocene sedimentary cycle
(Fig. 16-4), its history beginning with a transgression across a pre-existing
topography on pre-Tertiary metamorphic and igneous rocks (Wennekers,
1958; Adiwidjaja and De Coster, 1974; De Coster, 1975). The transgression
led to palaeogeomorphic traps on the old topography: the regression led to
anticlinal traps. Figure 16-5 shows how these two classes of accumulation are
separated in space, suggesting that the folding of the regressive sequence was
largely independent of basement relief.
The petroleum geology of basins with a simple sedimentary cycle, begin-
ning with transgression and ending with regression, is dominated by the strati-
graphy, which acquires its dominant characteristics from events outside the
sedimentary basin. The sedimentary basin is the complement of orogeny,
and its stratigraphy records the development of both. The inception of a

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0 300 km
0 200 miles
Fig. 16-5. Distribution of petroleum occurrences in South Sumatra basin. Triangles, in
transgressive sequence; dots, in regressive sequence. (After Koesoemadinata, 1969, p. 2374,
fig. 4.)