Page 245 - Petroleum Geology

would originally consist of sands with kaolinitic-illitic mudstones on kaolini-
tirillitic mudstones on montmorillonitic mudstones - and a sandy trans-
gressive sequence, perhaps, the reverse. Because these are facies of mudstones
and petroleum source rocks are facies of mudstones, it is almost certain that
primary migration exposes petroleum to different clay minerals along its path
to the nearest sand or sandstone.

CRUDE OILS WITH HIGH WAX CONTENT

Petroleum waxes in crude oil are solid hydrocarbons, mainly normal alkanes
in the range Czz-C30. Their melting points are above 30-35"C, so they can
cause difficulties during production. They have an intrinsic value as a product
for the manufacture of candles, impregnated paper, polishes, and water-proof-
ing agents. Most crude oils contain some wax, but some contain up to 15%
wax by weight (rarely, more). Crude oils with more than 5% wax are regarded
as high-wax oils.
Hedberg (1968) suggested that the high wax content of some crude oils is
an original characteristic related to shale-sandstone stratigraphic sequences of
non-marine origin or origin in waters of less tha9 normal marine salinity in
continental, paralic, or near-shore marine environments, cominonly associated
with coals, of Devonian to Pliocene age. From these associations he inferred
that the wax content reflects a terrigenous organic contribution to the source
material. These associations have been widely accepted as causal, because
land plants synthesize waxes to C37 (see, for example, Tissot and Welte,
1978, pp. 394-396; Hunt, 1979, p. 91).
Reed (1969) tested Hedberg's hypothesis in the western offshore Niger
delta, assessing the environment of the sequences in which high-wax oil oc-
curred from the faunal content. He found that 34 out of the 41 samples stud-
ied supported the hypothesis. (Note that the hypothesis concerns the en-
vironment in which the sequence accumulated and the salinity of its waters,
not the salinity of present-day pore waters in the sequences.)
Because wax content is not always determined, reliance is also placed on
two properties of crude oil that are normally determined: pour-point, which
is the temperature at which the crude oil will just flow, is normally (but not
always) associated with high-wax crude oils; and cloud-point, which is the
temperature at which a cloud is formed in reduced pressure distillation frac-
tions by separation of wax on cooling.
The list of high-wax crudes that Hedberg assembled has a number of inter-
esting points. Apart from the associations already mentioned, it seems that
there is a positive association between measured wax content and API gravity
- the lighter the oil, the greater the measured wax content (but such associa-
tions must be taken with caution because analyses of the lighter, more valu-
able crudes may be more readily available than those of heavy crudes that