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ment deficient in oxygen, and that this organic matter was derived from the
tissues of living plants and animals.
When living tissues die, they are either consumed by living organisms and
pass into the food chain, or they decompose with the aid of bacteria. If the
environment is sufficiently rich in oxygen, those tissues not consumed by
scavengers decompose to CO, and water (mainly): if the environment is defi-
cient in oxygen and free of scavengers, the products are more complicated
and varied, but are essentially compounds of carbon, hydrogen, oxygen, and
nitrogen.
We concluded in the general review that the lack of correspondence be-
tween the composition of crude oils and the nature of organic life on earth
when the crude oil was presumably formed, revealed by the fossils, indicated
a source for hydrocarbons in the fundamental biological molecules: proteins
(amino acids), lipids (fats, vegetable oils, waxes), carbohydrates, and, in the
higher plants, lignins. Some of these are present in all forms of life, and their
initial preservation, which is necessary for their subsequent alteration, depends
on their rapid transport to an environment free of scavengers and deficient
in oxygen, and their incorporation there into the accumulating sediment. This
anoxic environment is not necessarily at the sea water/sediment interface,
but it must exist at a very shallow depth of burial. What matters is the balance
between the amount of material to be oxidized and the available oxygen, be-
cause a large supply of organic material can create its own anoxic environ-
ment. These environments will be free from scavenging animals, but not free
from bacteria.
The physical environment most prone to develop anoxic chemical environ-
ments is one of little energy, that is, one in which only fine-grained sediment
is present. Thereafter, the preservation of organic matter in quantity depends
on its being buried as it is supplied, that is, the area must be one accumulating
fine-grained sediment in a sedimentary basin. Note that the accumulation of
fine-grained sediment is not in itself sufficient: there must also be a supply of
organic matter to an anoxic environment. So true source rock must be dis-
tinguished from source rock in a broad sense because not all of a mudstone
unit, for example, will necessarily be true petroleum source rock.
The requirement of large supplies of organic matter over considerable peri-
ods of time implies that we are concerned with sedimentary basins near con-
tinental margins mainly (but not exclusively) because these are areas of large
biomass where sediment is commonly accumulating into the stratigraphic
record. In such areas, great variations may exist between the relative supply
of terrestrial and marine organic matter, and these differ in their chemical
compositions. For large supplies of organic matter, organisms such as plank-
tonic algae in the marine environment and higher plants in the terrestrial are
more likely progenitors than vertebrates, for example. Marine planktonic
algae contain variable amounts of proteins, lipids and carbohydrates, whereas
higher land plants contain cellulose and lignin, with some lipids in spores,