Page 73 - Petrophysics

ORIGIN OF PETROLEUM 47

TRANSFORMATION ORGANICS INTO KEROGEN
OF
Organic materials from dead plants and animals are either consumed
by living organisms or left to be decomposed by bacteria. If the orga-
nic material remains in an oxygen-rich, aerobic environment, aerobic
bacteria will decompose it to carbon dioxide and water. If the envi-
ronment is anaerobic, the products of decomposition will be essentially
compounds of carbon, hydrogen, and oxygen. The hydrocarbons of
crude oils can originate from the fundamental biological molecules:
proteins (amino acids), lipids (fats, waxes, and oils), carbohydrates
(sugars and starches), and lignins (polymeric hydrocarbons related to
cellulose) of plants. If these are preserved in a lowenergy environment
free of oxygen, they can be mixed with the clays and precipitates that
are forming the fine-grained sediments characteristic of the low-energy
transgressive phase of basin formation. Therefore, to be preserved, this
organic matter must be buried as it is supplied with fine-grained sedi-
ments. The source rocks of petroleum are, therefore, those rocks formed
from fine-grained sediments mixed with organic materials. Not all fine-
grained sediments are source rocks for petroleum, which implies that
a necessary criterion is the availability of abundant organic matter in an
area of fine-grained deposition. This implies a sedimentary basin along a
gentle continental slope and the presence of aquatic life (plankton, algae,
etc.), in addition to copious terrestrial plant life. Land vertebrates are not
a very likely source for organic matter in shallow marine sediments.
Higher-order land plants contain abundant quantities of cellulose and
lignin, yielding aromatic-type compounds with a low hydrogen-to-carbon
ratio (1 .O-1.5). Marine algae contain proteins, lipids and carbohydrates;
these are aliphatic in character with a high hydrogen-tocarbon ratio
of 1.7- 1.9. ("he hydrogen-to-carbon ratios of specific compounds are:
benzene-1 .O, cyclohexane-2.0, and n-pentane-2.4.)
The organic materials, fine-grained sediments, and bacteria that are
mixed together and deposited in the quiet, low-energy environments
are not in thermodynamic equilibrium. The system approaches thermo-
dynamic equilibrium during initial burial while it is undergoing diagenetic
transformations. Inasmuch as burial is shallow during this stage, the
temperature of the environment is low, and the sediment undergoes
diagenetic changes slowly under mild conditions. The first 10 feet or
so of sediment represents an interface where the biosphere passes into
the geosphere. The residence time in this shallow sediment, before
deeper burial, may range from 1,000 to 10,000 years. During this time,
the organic matter is subjected to both microbial and chemical action
that transforms it from the biopolymers (proteins, etc.) to more stable
polycondensed compounds that are the precursors of kerogen. In time