Page 29 - Petroleum Geology

10

the relative lowering of sea level results in a lowering of baseleuel while re-
gression of the second type results from a rising baseleuel - more sediment
being supplied than can be removed.
Regressions of the first type are important for the erosion and redistribu-
tion of sediment that had previously accumulated in the sedimentary basin.
This will accumulate elsewhere, and there will be a hiatus where the erosion
took place. Regressions of the second type are important for the accumula-
tion of major regressive sequences that persisted for much of the life of a
sedimentary basin.
Deltas are examples of the second type of regression (which we regard as
the normal type). They result from the volume of sediment supplied by the
river exceeding the volume that can be removed by the sea, with a conse-
quent shallowing of the sea and extension of the land area. Delta growth is
not uniform. There may be strong seasonal influences, with periods of high
river level and massive sediment transport. Distributary channels become
unstable as they build up their levees, and change their courses from time to
time, so changing the shape of the delta. The building of a delta is episodic,
but its very existence indicates that more sediment is supplied to it than can
be removed, and the net effect is regressive. The sequence that accumulates
as the delta progrades has paralic sediments overlying the deeper-water, pro-
delta sediments of finer grade.
“Transgression” has two aspects analogous to those of regressions. The
basic concept is that of a rising sea level relative to the land, with a conse-
quent migration of the strand line with its associated environments and
facies towards the land, and a reduction of land area. The derived concept is
this: when the energy of the environment is greater than that required to
remove the sediment supplied to the area, the surplus energy may erode and
redistribute sediment that accumulated previously (e.g. coastal erosion). This
also leads to the migration of environments and their facies landwards. Both
types of transgression may result in accumulation of sediment in a strati-
graphic sequence in which deeper-water facies overlie shallower-water facies
(or marine overlie terrestrial). Transgressions of the first type result in rising
baselevel and so a potential to accumulate sediment. Transgressions of the
second type result in a lowering of baselevel and the erosion and redistribu-
tion of sediment that will accumulate again elsewhere. (Note in passing that
a break in the stratigraphic sequence is not necessarily regressive.)
Petroleum geology is concerned with the accumulation of sediment as well
as the accumulation of petroleum. We are therefore concerned with transgres-
sions and regressions that tend to accumulate thick sedimentary sequences,
rather than those that tend to erode and redistribute sediment, in sedimentary
basins. Sedimentary basins typically begin with a transgressive sequence and
end with a regressive sequence. Transgressions tend to accumulate fine-
grained, porous but relatively impermeable rocks (potential petroleum source
rocks) on top of coarser-grained, porous and permeable rocks (potential