Page 392 - Petroleum Geology

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sedimentary basin in an area of low relief results in a carbonate-mudstone
transgressive sequence, favourable for the growth of coral reefs. As subsidence
in the basin and its complementary orogeny outside proceed, so the volume
of sediment generated increases. Transgression gives way to regression; and
as the regression develops, mechanical instability also develops in the regres-
sive sequence. This is shown diagrammatically in Fig. 16-6.
Petroleum source rocks are a facies of the sedimentary basin. So are the
carrier beds and reservoir rocks. Once petroleum is generated, it moves under
physical laws to positions of lower energy. These are largely determined by
the stratigraphy of the area: the superposition of permeable potential reser-
voirs and relatively impermeable source rocks and cap rocks normally restricts
primary migration to short vertical paths to the nearest permeable bed with
less energy in its pore fluids. From there it moves laterally until it is trapped
- or dissipates at the surface.
In transgressive sequences, primary migration is usually downwards to the
basal permeable unit, then lateral towards the land of the time, in the direc-
tion of the transgression. There is little structural relief on this surface, so
trapping depends on a stratigraphic cause - palaeogeomorphic traps and fossil
coral reefs being the most important. Secondary migration paths may be
quite long, but both primary and secondary migration are confined to a dia-
chronous sequence of lithologies, the source rock being coeval with the reser-
voir rock.
1. 2
-SEDIMENTARY BASIN OROGENY
TRANSGRESSION
>>>>>>>>>>>
u

‘Deformation
due to
instability
-u. in sequence
Fig. 16-6. Schematic diagrams showing development of sedimentary basin and nearby
orogeny.