Page 99 - Petroleum Geology

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but there are difficulties with this concept. The compaction of sediments,
as we saw in Chapter 3, results in important redistribution of the original
interstitial water. Changes in salinity may be related to this redistribution.
There are therefore not only the well-known difficulties of reconciling the
composition of sea water with the compositions of the surface waters dis-
charging into the sea, but also those of reconciling the compositions of for-
mation waters with those of sea water and fresh surface water. Reservoir
engineers call the water associated with petroleum in reservoirs “connate”,
and the composition of this water may differ from that of the water below
the petroleum in the same rock unit.
The salinity of formation water is also of interest to geologists because it
affects its density, and so also the pressure exerted by a column of water.
Water also exists in sedimentary rocks as part of the molecular structure
of some minerals - such as gypsum, CaSO, - 2H20, and smectite, A14Si8 Oz0
(OH), - nH,O. Under certain conditions of temperature and pressure, this
water can be released to the free interstitial water. The role of molecular or
lattice water is not yet fully understood, but it may have significance in both
petroleum geology and structural geology through the influence its release
could have on interstitial fluid pressures.
Generation of petroleum

The occurrences of petroleum in the world strongly suggest that petroleum
rarely, if ever, originates in the reservoir in which it is found, but rather in
other rocks, known as source rocks, from which the petroleum migrates to
the trap.
The study of petroleum generation is hampered by the fact that we can-
not scale the possible processes in the laboratory with confidence, so all
hypotheses are based on interpretations of geochemical observations. It is
also hampered by the fact that we do not know with confidence what the
source rocks are for most known petroleum accumulations. If source rocks
cannot be identified with certainty, they cannot be studied with understand-
ing.
There is general agreement that the main source of petroleum is organic
matter buried with a fine-grained sediment, usually a mudstone; and that
diagenesis of this organic matter leads to a “proto-petroleum” which, be-
fore or during migration, becomes modified by the physical and chemical
environment - particularly by increasing temperature during burial - until
it sooner or later becomes the petroleum we find in the accumulation.
There is general agreement that the conditions at the depositional surface
are critical for the preservation of organic matter, that aerated, oxidizing
conditions are inimicable to the preservation of organic matter for subse-
quent alteration to petroleum. This must not be interpreted too qualitatively,
because the amount of organic matter supplied to an environment is an im-