51
Origin and Occurrence
and Welte, 1978; Brooks and Welte, 1984; Bjorøy et al., 1987). Petroleum is believed to be the product
arising from the decay of plant and animal debris that was incorporated into sediments at the time
of deposition. However, the details of this transformation and the mechanism by which petroleum is
expelled from the source sediment and accumulates in the reservoir rock are still uncertain but progress has been made (Nagy and Colombo, 1967; Hobson and Tiratsoo, 1975; Hunt, 1996).
Nevertheless, the composition of petroleum is greatly influenced not only by the nature of the
precursors that eventually form petroleum but also by the relative amounts of these precursors (that
are dependent upon the local flora and fauna) that occur in the source material. Hence, it is not surprising that petroleum composition can vary with the location and age of the field in addition to any
variations that occur with the depth of the individual well. Two adjacent wells are more than likely
to produce petroleum with very different characteristics.
3.2.2.1 Deposition of Organic Matter
Only a minute fraction of the carbon cycling through the biosphere becomes incorporated into sediments and of this only a small part is preserved to become fossil fuels. The actual mechanism by
which this is achieved in aqueous environments involves photosynthesis by (1) phytoplankton, in
cooler waters, and (2) phytobacteria, in warmer waters. The photosynthetic process is ultimately
responsible for producing nearly all the organic matter in the oceans (Sorokin, 1971).
Photosynthetic activity is controlled by light, temperature, and the availability of nutrients. Light
does not seem to be the major controlling factor, except in deep water, polar regions, or turbid
coastal regions. The availability of carbon dioxide, which is essential for photosynthesis, is not
limiting in the eutrophic zone, the top 200 m, where productivity is highest.
In fact, nutrient availability (often nitrogen as ammonium, NH 4
+
, or as nitrate, NO 3 ) in marine
environments or as phosphate (PO 3
2− ) in lacustrine environments seems to be the most common
limit on photosynthetic productivity. Productivity in lakes follows the same general pattern as in
oceans, but deposition in the confines of a lake is much more subject to local climatic variations and
periodic changes in input (e.g., volcanic ash) than is marine deposition.
3.2.2.2 Establishment of Source Beds
Flowing water can erode particles from rocks and transport them to sites where the current is less
strong. The water also contains suspended organic matter, which will settle with the minerals and
hence serve as the oil-generating substance. There are arguments in favor of marine plankton as the
prominent source material, but local conditions may favor the accumulation of other organisms, such
as marine algae, the remains of larger marine animals, or even material from terrestrial sources.
Both the nature and the quantity of the organic and inorganic settling matter may vary. As a consequence, it must be expected that the characteristics of the crude oils found in different reservoirs
will vary. The nature of crude oil is related not only to the nature of the source material but also to
the relative amounts of the different constituents in the source material. In addition, the temperature, pressure, and any other physical conditions prevailing in the area will also affect the character
of the petroleum.
For example, high-wax crude oils are commonly associated with source material containing high
proportions of lipids of terrestrial higher plants and of microbial organisms. On the other hand,
high-sulfur crude oils are frequently related to carbonate-type source rocks. In addition, alteration
of the petroleum in the reservoir may be equally responsible for variations in composition. In short,
crude oil alteration will very likely make the original character of the oil difficult to assess, thereby
also masking the true nature of the precursors.
The debris that settles at the bottom of the sea is attacked by bottom-dwelling (benthic) organisms that feed both on the sediment and on any water-soluble material. The benthic bacteria are
largely responsible for the decomposition of organic compounds and the synthesis of new organic
compounds through enzymatic transformations. The remaining material is partly transformed by
bacteria and buried under the steadily increasing cover of sediments.
Origin and Occurrence
and Welte, 1978; Brooks and Welte, 1984; Bjorøy et al., 1987). Petroleum is believed to be the product
arising from the decay of plant and animal debris that was incorporated into sediments at the time
of deposition. However, the details of this transformation and the mechanism by which petroleum is
expelled from the source sediment and accumulates in the reservoir rock are still uncertain but progress has been made (Nagy and Colombo, 1967; Hobson and Tiratsoo, 1975; Hunt, 1996).
Nevertheless, the composition of petroleum is greatly influenced not only by the nature of the
precursors that eventually form petroleum but also by the relative amounts of these precursors (that
are dependent upon the local flora and fauna) that occur in the source material. Hence, it is not surprising that petroleum composition can vary with the location and age of the field in addition to any
variations that occur with the depth of the individual well. Two adjacent wells are more than likely
to produce petroleum with very different characteristics.
3.2.2.1 Deposition of Organic Matter
Only a minute fraction of the carbon cycling through the biosphere becomes incorporated into sediments and of this only a small part is preserved to become fossil fuels. The actual mechanism by
which this is achieved in aqueous environments involves photosynthesis by (1) phytoplankton, in
cooler waters, and (2) phytobacteria, in warmer waters. The photosynthetic process is ultimately
responsible for producing nearly all the organic matter in the oceans (Sorokin, 1971).
Photosynthetic activity is controlled by light, temperature, and the availability of nutrients. Light
does not seem to be the major controlling factor, except in deep water, polar regions, or turbid
coastal regions. The availability of carbon dioxide, which is essential for photosynthesis, is not
limiting in the eutrophic zone, the top 200 m, where productivity is highest.
In fact, nutrient availability (often nitrogen as ammonium, NH 4
+
, or as nitrate, NO 3 ) in marine
environments or as phosphate (PO 3
2− ) in lacustrine environments seems to be the most common
limit on photosynthetic productivity. Productivity in lakes follows the same general pattern as in
oceans, but deposition in the confines of a lake is much more subject to local climatic variations and
periodic changes in input (e.g., volcanic ash) than is marine deposition.
3.2.2.2 Establishment of Source Beds
Flowing water can erode particles from rocks and transport them to sites where the current is less
strong. The water also contains suspended organic matter, which will settle with the minerals and
hence serve as the oil-generating substance. There are arguments in favor of marine plankton as the
prominent source material, but local conditions may favor the accumulation of other organisms, such
as marine algae, the remains of larger marine animals, or even material from terrestrial sources.
Both the nature and the quantity of the organic and inorganic settling matter may vary. As a consequence, it must be expected that the characteristics of the crude oils found in different reservoirs
will vary. The nature of crude oil is related not only to the nature of the source material but also to
the relative amounts of the different constituents in the source material. In addition, the temperature, pressure, and any other physical conditions prevailing in the area will also affect the character
of the petroleum.
For example, high-wax crude oils are commonly associated with source material containing high
proportions of lipids of terrestrial higher plants and of microbial organisms. On the other hand,
high-sulfur crude oils are frequently related to carbonate-type source rocks. In addition, alteration
of the petroleum in the reservoir may be equally responsible for variations in composition. In short,
crude oil alteration will very likely make the original character of the oil difficult to assess, thereby
also masking the true nature of the precursors.
The debris that settles at the bottom of the sea is attacked by bottom-dwelling (benthic) organisms that feed both on the sediment and on any water-soluble material. The benthic bacteria are
largely responsible for the decomposition of organic compounds and the synthesis of new organic
compounds through enzymatic transformations. The remaining material is partly transformed by
bacteria and buried under the steadily increasing cover of sediments.