Page 382 - Petroleum Geology

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DEFORMATION OF SEDIMENTARY BASINS

It is one of the remarkable features of petroleum geology that some large
petroleum provinces are virtually devoid of structure. Large areas of Silurian
and Devonian reefs in North America, some within sight of the Rocky Moun-
tains, have regional dips less than 5". A geological history lasting 300-400
m.y. has taken them from sea level to depths of three kilometres or so with
only the slightest deformation, and they were hardly affected by the orogeny
that built the Rocky Mountains. Moreover, the physiographic basin within
which the reefs grew remained favourable for them for 5-10 m.y. or 50 m.y.
if one takes the combined Silurian and Devonian reef provinces of northern
U.S.A. and western Canada. The geological picture is one of utmost tranquility.
Similarly, the Cretaceous reefs of northern Mexico, after 100 m.y. of geol-
ogical history, lie almost horizontally at shallow depths down to about two
kilometres. Were it not for the reefs that formed stratigraphic traps, these
would not have become petroleum provinces.
By contrast, there are large areas of the world in which rocks less than 25
m.y. old have suffered substantial deformation, with folds and faults. These
are areas that can be examined in outcrop and mapped. It is perhaps for this
reason that we were taught by the collective experience of surface geologists
that sedimentary basins were typically deformed by horizontally directed
tectonic forces of orogeny after the sedimentary history had come to a close,
or, alternatively, that these events terminated the sedimentary history of the
basin.
In those areas, petroleum occurs in structural traps, and their study has
revealed that much of this deformation took place during the sedimentary
history of the basin, and that one must distinguish between pre-orogenic and
orogenic deformation of such sedimentary basins. The offshore basins are no
exception to these rules: the petroleum is in structural traps in parts of the
basin that have not suffered orogeny, where deformation formed an integral
part of the sedimentary history of the basin.
Since about 1970, yet another type of sedimentary basin has been recog-
nized: the rift basin. It too has features that are shared by widely separated
provinces, so that the petroleum geology of the North Sea is very similar to
that of the north-west shelf of Australia, for example. The geological history
of these basins consists typically of three stages. First, the formation of one
or more graben (the bounding faults throwing perhaps 5-6 km eventually)
provides a basin in which sediment accumulated. These graben contain numer-
ous normal faults that moved while the sediment accumulated. A period of
non-accumulation followed, with erosion on the higher parts and the upthrow-
ing blocks of growth faults. During this period, the faults stopped moving,
and the disconformity/unconformity was completed by epeirogenic subsi-
dence, and the accumulation of fine-grained sediment followed. Faulting of
the younger sequence is confined to minor movement of some faults. Petro-