Page 41 - Petroleum Geology

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In plan, sedimentary basins may be circular, oval or almost rectangular
(asymmetrical basins usually have one fairly straight side), and they vary in
size from a few tens of kilometres to several hundreds in horizontal dimen-
sions. Geosynclines may be regarded as particular forms of sedimentary
basin of which the length is considerably greater than the width, and in
which a great thickness of characteristic sediments and associations of. sedi-
ments, and volcanic rocks, accumulated (Kay, 1951; Kundig, 1959).
Physiographic basins may be open or closed, silled or unsilled, continen-
tal or marine, or a combination of these. The Gulf of Mexico, the northern
part of the Bay of Bengal, and the South China Sea, are examples of open
basins. The Black Sea, the Baltic and the Mediterranean are examples of
closed basins. The area around Lake Eyre in South Australia is an example
of a closed continental basin. Lake Eyre is 12 m below sea level, and is evi-
dently subsiding, so it could become a marine basin in time. The environ-
ments of physiographic basins are very variable according to the climate (or
range of climates) and the general geography and geology of the area. Of
particular significance in petroleum geology is the alternation of environ-
ments in the course of time between euxinic, or areas of anaerobic, reducing
conditions, during the accumulation of fine-grained sediment, and those en-
vironments that led to the accumulation of coarser-grained porous and per-
meable sediments.
The discrete concepts of physiographic and sedimentary basins merge in
this respect. Reducing conditions, which are essential for the preservation of
organic matter, occur in the aqueous environment near the water/sediment
interface, or within the sediment. Hence sediments may either accumulate in
a reducing environment of the physiographic basin or be buried to a reducing
environment by rapid subsidence.

Generalizing, sedimentary basins usually begin with a dominantly trans-
gressive phase during which the volume created by subsidence exceeds the
volume of sediment supplied, and end with a dominantly regressive phase
during which the volume of sediment supplied exceeds the volume created
by subsidence. Some sedimentary basins record a single, simple dominant
cycle of sediment accumulation (Fig. 1-3). Others record a transgressive se-
quence only, and yet others record a long and complicated history with
several cycles, doubtless with eustatic changes imposed on the record. The
stratigraphic record is nearly everywhere incomplete due to diastems and dis-
conformities, but it records the changes of the physiographic environments
on the sedimentary basin with time. An initial carbonate transgression in-
dicates that the physiographic basin had no significant terrestrial topographic
relief, whereas regressions, invariably arenaceous, indicate the presence of
mountains from which the sediment was derived. Sedimentary basins are
commonly intimately associated with orogeny during their development,
particularly the later stages of their development.