Page 387 - Petroleum Geology

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ing sedimentary basins as a regressive sequence. The regressive sequence is
the stratigraphic expression in a sedimentary basin of an orogeny outside the
sedimentary basin. But just as the orogeny that created the Rocky Mountains
had no effect on the Devonian reef province of the Western Canada basin
(other than subsidence, perhaps), so the orogeny providing the sediment of a
regressive sequence was a vertical event because the stress field in the sedi-
mentary basin remained one with a component of horizontal tension.
In this respect, the island of Borneo is particularly interesting. The north-
west coast, with Sarawak, Brunei and Sabah, is a Tertiary sedimentary basin
that has had (and is having) a similar geological history to that in the east
coast of Kalimantan. Petroleum occurs in both basins in numerous structural
traps in the coastal region and offshore. A stratigraphic section down the
east coast is shown in Fig. 16-2. The onshore areas have been successfully ex-
plored, and the area west of the Mahakam delta (Fig. 16-3), with the Samar-
inda anticlinorium, formed an important part of Van Bemmelen’s (1949)
hypothesis of gravitational tectogenesis. Long, narrow anticlinal trends with
steep, sheared cores occur. These have the features of compressional struc-
tures, including thrust faults, and Van Bemmelen (1949, pp. 352 and 732) re-
garded these as the consequence of a tendency to slide into the “depressed

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Fig. 16-3. Sketch map of Mahakam delta, Kalimantan, Indonesia.