Page 58 - Petroleum Geology

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the younger basins of South-East Asia, including Borneo and New Guinea;
and they occur in Australia. They have received little attention in the literature,
yet they are commonly figured. Experience suggests that many of the petro-
leum-bearing anticlines have a growth component, and that many were initi-
ated as growth anticlines.
As with growth faults, growth anticlines (and other growth structures) are
best understood in terms of differential subsidence relative to baselevel. The
whole area of the growth anticline was subsiding, but the area of the crest
was subsiding more slowly than the flank areas. Hence the flank areas had a
capacity to accumulate a greater thickness of sediment. Again, one must see
the sediment as being in transport across the area, not deposited from sus-
pension, because one cannot postulate such patterns of deposition from sus-
pension.
Growth anticlines range from those that show continuity of rock units
across it, with differences confined to thickness (and perhaps porosity and
permeability), through those that influenced sedimentation to such a degree
that different facies occurred in the crestal area from the flank areas, to those
in which rock units are discontinuous across the crestal area. Reservoir rocks
may therefore vary not only volumetrically, but also in reservoir properties.
As with growth faults, these are but variations on a theme of differential sub-
sidence.
It is important to bear in mind that the growth of an anticline only affects
those sediments that were accumulating during the growth period. This is
analogous to the intermittent movement of a growth fault. All sediments
that accumulated in the area prior to the beginning of growth of the anticline,
and all sediments that accumulated during periods of quiescence and after
growth had ceased, will show either no thickness variations, or variations
that are not related to the structure. This statement requires two qualifica-
tions: it applies to sediments above the agent or cause of growth; and the
processes of compaction tend to prolong the growth, as they do of growth
faults. One must therefore distinguish between the periods of growth on the
time scale of accumulated sediment, and the folding of sedimentary rocks
into an anticline subsequently to the accumulation of the sediment. The time
or times of deformation may be revealed by the younger sediments.
Growth anticlines may be faulted by growth faults. The very nature of
these requires, of course, that these are contemporaneous deformations. The
simultaneous deformation of a structure by folding and faulting requires
careful analysis. Qualitatively, the deformation of the curved fault plane of a
growth fault by contemporaneous growth of the anticline may be important
in that it sets up stresses that may be relieved by further faulting. Quantita-
tively, such deformation of the fault surface may be insignificant because it
will usually be far finer than the rather coarse control afforded by boreholes.
The deformation of a growth anticline by contemporaneous growth fault-
ing is, of course, significant in terms of the geometry of the anticline and its