Page 370 - Petroleum Geology

340

association between mudstone diapirism and petroleum accumulations is in-
ferred to be close and, from a practical point of view, causal. It is therefore
of major importance in the geology of petroleum.
In its geological context, mechanical instability can develop whenever a
sequence of greater density and equivalent viscosity accumulates on a sequence
of smaller density and equivalent viscosity. There are examples of unstable
sequences of carbonates on mudstones (see Chapman, 1981, pp. 162-168),
but the dominant association is with regressive sequences.

GROWTH STRUCTURES AND INCIPIENT DIAPIRISM

Just as the development of a sedimentary basin is reflected in its strati-
graphy, so are the details reflected in the local detailed stratigraphy (as Trus-
heim and Sannemann demonstrated for northern Germany, discussed above).
Of particular importance are the variations of thicknesses of rock units due
to contemporaneous developments of traps.
Petroleum occurs in dominantly regressive sequences in many parts of the
world, and some of them are major producing areas: for example, California
(Santa Barbara Channel region), the United States’ Gulf Coast, Trinidad, the
Niger delta, and many areas of Southeast Asia. They have several features in
common: (a) growth structures; (b) petroleum accumulations almost exclu-
sively in structural traps; (c) underlying mudstones with abnormal pressures;
(d) multiple sandstone reservoirs, with a tendency for the crude oils to be
lighter in deeper reservoirs; and (e) Tertiary age.
Examples are taken from South-east Asia because they can be set in a se-
quence that illustrates the development of such traps and accumulations.
In Irian Jaya (W. New Guinea) in the early 1950s, field geologists used to
say that the geology of the Tertiary basins was the geology of anticlines with-
out synclines. The pattern of folding is not a wave-like sequence of anticlines
and synclines, but rather of narrow, steep anticlines separating broad gentle
synclines. This structural style is common in South-East Asia in onshore areas
(with hints that it is less developed offshore). The anticlinal trends are com-
monly asymmetrical in section and sinuous in plan. They are not strictly
parallel, but are generally parallel to the depositional strike. This structural
style has been found in Irian Jaya (Visser and Hermes, 1962, pp. 140,165-
170, 227) and Papua New Guinea (Tallis, 1975, p. 59), and in Brunei, Sabah
and Sarawak (Schaub and Jackson, 1958; Liechti et al., 1960, pp. 270-283).
It also occurs onshore in Kalimantan, where it formed an important part of
Van Bemmelen’s hypothesis of gruuitutionul tectogenesis (Van Bemmelen,
1949, e.g., p. 732). The steepness of the anticlines varies from gentle domes
and elongated structures with dips to 30” in offshore and coastal areas, with
some very steep anticlines with narrow, deformed cores, further inland. The