Page 256 - Petroleum Geology

CHAPTER 11

ORIGIN, MIGRATION AND ACCUMULATION OF PETROLEUM:
DISCUSSION

SUMMARY

(1) High-wax crude oil that is found in sediments of inner neritic, paralic
or continental facies was probably generated in source rocks stratigraphically
close to the reservoir.
(2) Fields in which the composition of the crude oils and water varies
from reservoir to reservoir probably received their oil from source rocks strati-
graphically adjacent to the reservoir.
(3) Fields and provinces in which petroleum occurrences and petroleum
quality are associated with the sand/shale ratio probably derived their petro-
leum from source rocks interbedded with the reservoirs.
(4) Biodegradation of crude oil is evidence of early generation, dominantly
in rocks of continental and paralic environments, rather than marine. The as-
sociation of heavier crude oils with fresher formation waters suggests that
such crudes either have an inherently different composition or tend to be im-
mature.
(5) Density of crude oils is determined largely by facies of the source
rock, rather than by temperature and time.
(6) If crude oil generated early is heavier than the later, secondary migra-
tion directions may be dominated by water motion because the critical slope
may be 10 or 15 times the slope of the potentiometric surface.
(7) Fields with similar crude oils in reservoirs of different facies probably
had a source stratigraphically removed from the reservoirs.
(8) Faults generally act as barriers to lateral migration, but can leak if the
vertical dimension of the pooled crude oil or gas is sufficient to generate a
sufficiently large pressure differential across the fault to another permeable
rock unit.
(9) Fields with fault traps and petroleum in the same rock unit in both
blocks were either sourced from local source rocks or migration took place
during fault movement.
(10) Faults do not generally act as conduits for petroleum migration.

EVIDENCE OF STRATIGRAPHIC POSITION OF SOURCE FOR ACCUMULATIONS

It is a matter of considerable importance to petroleum geology and the in-