Page 393 - Petroleum Geology

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In regressive sequences, primary migration is vertical to the nearest land,
then lateral towards the land of the time from which the regression comes.
In a general sense, the source rocks tend to be below the massive sands; but
here too the relationship is diachronous and the source rock is coeval with
reservoir rock. Fluctuations of environment lead to the alternating sequence.
Episodic transgressions and regressions within the dominant regression seem
to favour the mudstone overlying the sandstone as the petroleum source
rock because the more rapid accumulation of sediment favours the preserva-
tion of the organic matter. In detail, therefcre, there will be important down-
ward primary migration to the sandstone carrier beds, followed by lateral
secondary migration to traps that formed concurrently with the accumula-
tion of sediment.
The quality of the crude oil depends largely on the stratigraphy in the
sense that it depends on the facies of the source rock and the conditions to
which it was subjected during migration. The physical controls on migration
are also related to the stratigraphy, so that petroleum normally accumulates
in reservoirs that are stratigraphically close to their source rocks. Passage
through growth faults may move this petroleum stratigraphically downwards
as regards rock units (Fig. 16-7), but local migration may be in the reverse
direction towards a growth anticline, and so pass through a growth fault to
younger stratigraphic levels. The limits of this stratigraphic transfer are roughly
the throws of the growth faults between source and accumulation.
Stratigraphy is more important than time or temperature in determining
the quality of petroleum in a pool: there are too many examples of variations
that bear no relationship to thermal gradients, but could be explained by
facies changes. This is not to assert, of course, that temperature has no effect

Fig. 16-7. Migration through faults results in accumulation in reservoirs stratigraphically
removed from the source.