Transformation of Organic Material Into Petroleum Under Geological Conditions: "the Geological Fence"

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The recognition of source beds of petroleum is essential to the evaluation and successful prospecting of many great areas throughout the world where some of the conditions common to oil-yielding areas are known to be present but where thus far little or no oil has been discovered. This includes areas where there are important oil fields in restricted parts of a great sedimentary basin but where prospects for petroleum in much larger parts of this same basin are undetermined. Laboratory studies might be directed toward explaining whether there is a fundamental reason for this connected with the original nature of the organic material, the environment of deposition, and the history of accumulation or alteration which took place during and after burial. If no reason can be found, then the industry must continue to explore these apparently unpromising areas. If criteria for the recognition of source beds are to be established, then a coordinated attack within the limits of geologic history must be made by all the fields of science involved—chemistry, physics, biology, and geology.

A few observations bearing on the problem are briefly noted. Although some of these observations may be proved irrelevant, five because of multiple occurrence are considered irrefutable and are dubbed "posts" in the "geological fence." It may be desirable to do some browsing in the range outside the fence as the grass may look greener there, but safety lies within the "corral." The herd of facts and data must be brought within the corral. Additional observations and others which might be added are considered as "stakes" in the fence. They may be broken and discarded and new stakes supplied, provided they fit conditions of observed geologic history.

The "posts" are: organic origin, marine environment, temperature, pressure, time.

ORGANIC ORIGIN

Most geologists believe that petroleum is of organic origin because it is closely associated with sediments containing relatively high content of organic matter. Nitrogenous compounds, optical activity, and chlorophyll derivatives have also been advanced as evidence of organic origin but these conceivably could have been picked up by petroleum migrating through rocks containing such substances. Petroleum-like products can be formed in the laboratory from inorganic substances, but the natural occurrence and distribution of such substances do not fit geological history. Products so formed are inert optically, and do not contain chlorophyll derivatives or nitrogenous compounds.

Equally important is the absence of oil fields where all essentials, except source, are present. The requirements for oil fields are: (a) source, (b) porous reservoir, and (c) trap that will stop upward movement of oil. The second and third requirements can be found in association with rocks of all types—granites, lavas, sediments that accumulated under desert conditions, on the flood plains of rivers and in the sea. No oil pools have been found in such traps and reservoirs in sections devoid of or lean in organic matter, and this has not been due to lack of search.

MARINE ENVIRONMENT

Ninety-nine per cent of the world’s oil fields are associated with marine sediments. Known producing formations are limited to certain associations with certain types of marine sedimentary rocks. Rocks deposited in fresh water, non-marine basins, not different in lithologic character from marine rocks which yield petroleum, contain no petroleum. Multiple occurrence of reservoirs and their associated fine-grained marine sediments containing organic matter appear to be significant. In about one per cent of known petroleum occurrence non-marine sections yield petroleum but the sediments were deposited in brackish water. No crude oil has been found in a sedimentary basin containing only fresh water, terrestrial sediments. Many geologists believe that sediments deposited in enclosed, or partly enclosed, shallow, marine basins yield more petroleum than sediments deposited in more open parts of epi-continental seas.

Trask compared the organic content of many recent marine sediments with that of ancient marine sediments, the latter likewise based on thousands of samples. The average organic content of recent marine sediments near shore is approximately 2.5 per cent, of ancient marine sediments 1.5 per cent. Ninety per cent of the samples analyzed had less than 4 per cent. These averages, however, are greatly exceeded in local geographic areas and in limited parts of the stratigraphic section. Organic content of more than 8 to 10 per cent has been reported from ancient rocks in some cases. Many geologists believe that the high organic content in marine rocks may be important in oil origin, but Trask did not find any statistical relationship between quantity of organic matter and the occurrence of petroleum. There is ample residual solid organic material, if it could be converted into petroleum, to account for far more petroleum than is known in the world.

Analyses of solvent extracts of four recent marine sediments were made by Trask and his associates. These studies indicate that liquid petroleum probably does not occur in sediments at the time of their deposition although small quantities of solid paraffins are found. Trask concluded that if any liquid petroleum is present in recent marine sediments, it is less than one per cent of the organic content. ZoBell reports finding 10 to 20 milligrams of liquid hydrocarbons in 100-gram samples of fresh recent marine sediments; if we assume an average organic content in these sediments of 2.5 per cent, the liquid hydrocarbons detected would amount to only 0.4 to 0.8 per cent of the organic content of the sediment. Much of the oil detected in ZoBell’s studies disappeared after a few days’ storage at room temperature, ostensibly due to bacterial activity.

Fatty and oily substances constitute less than one per cent of the organic content of the four organic-rich marine sediments which Trask studied. He considered that this was not enough to account for all of the known petroleum. Fatty acids ranging from cerotic

to melissic constitute 0.002 to 0.006 per cent of the sediment. Organic sulphur compounds form about 0.03 per cent of the deposits. Free sulphur is a common minor constituent of all sediments and ranges from 0.02 to 0.1 per cent. Trask’s studies did not completely exhaust the possibilities of water-soluble organic matter.

Organic material present in recent marine sediments also includes carbon-hydrogen-oxygen compounds such as cellulose, as well as haemoglobin and chlorophyll and their derivatives. Lignin occurs but geologists do not believe that oil was derived from lignin for the same reason they do not believe it was derived from torbanite and the like. Most simple carbohydrates and proteins are readily decomposed by bacteria soon after reaching the sea bottom.

Geologists do not believe that non-marine, pyrobituminous substances such as oil shale, tasmanite, torbanite, and boghead coal are a source of crude petroleum. Many sedimentary basins which yield petroleum are devoid of such rocks. Oil fields have not been found in non-marine basins where high concentrations of such pyrobituminous substances occur.

Torbanitic deposits have a matrix of partly decayed plant debris, probably ulmic or humic in nature, with variable amounts of unaltered algal cups, cuticles, cuticular secretions, resins, and oily spores, and pollen exines.

The oil fractions which can be retorted from such rocks are derived chiefly from oily or resinous materials which were formed during the life cycle of the plant. Lesser amounts of hydrocarbon products result from the pyrolysis of the matrix. Pond algae for instance, which convert starch to terpenes in their life processes, are preserved in torbanites unaltered throughout geologic time under overburden pressures as great as those which have prevailed in petroliferous areas. These algae and spores are exceedingly resistant to bacterial alteration. Laboratory pilotplant attempts to liberate the oil from torbanitic deposits by pressures far greater than pressures commonly occurring in petroliferous basins have failed.

A single living alga before cell division and before the oil formed is absorbed by its cup will release a small drop under moderate pressure.

Most oil-field reservoir rocks are hydrophilic. The interstitial water which coats the pore walls commonly has high chloride content which in some cases is much higher than m sea water. This is also true of the bottom and edge waters of most oil fields. Occurrences of relatively fresh interstitial and edge water are believed by the geologists to be exceptions caused by either: migration of meteoric water into the reservoir at the outcrop, resulting in dilution as in the case of the Kern River oil field, California; or migration of oil into sands already saturated with meteoric water.

TEMPERATURE

In rare cases, temperatures slightly higher than the boiling point of water occur in petroliferous basins. Temperature of at least 113°C. (235°F.) has been recorded. This is a salt-dome temperature where the salt mass may be acting as a conduit for higher, deeper-seated temperatures. Geologic history, however, indicates that most oil-basin temperatures have been low throughout geologic time. The maximum temperatures have probably been approximately 100°C. for most oil basins. The presence of chlorophyll derivatives in petroleum indicates that a maximum temperature has been less than 200°C. Geologic history does not permit the temperatures required under hypotheses of Engler and his followers.

Geologic observation suggests that the critical original thickness of sediment in which petroleum has been formed is a vertical section of about 5,000 feet. If the average temperature gradient was 1°C. per 100 feet and the average surface temperature 15°C., the minimum temperature under which petroleum could have been formed would have been 65°C. (149°F.). This could have been increased by heat due to exothermic chemical or biochemical reaction or possibly to shear pressures generating heat in fine-grained sediments. Shear-pressure experiments of Hawley and Rand did not raise temperatures. Their samples, however, were air-dried so that they did not completely simulate geologic history.

Temperatures in oil fields are commonly but not always higher than temperatures in adjacent synclinal areas after correction for depth, possibly suggesting some type of exothermic reaction.

PRESSURE

Since some petroleum occurs in basins which have had a stratigraphic thickness of about 5,000 feet but not much more than 5,000 feet, minimum pressures are limited. The hydrostatic head in such a thickness of rock at the deepest part of the basin would not greatly exceed 2,000 pounds per square inch but this pressure probably was exceeded in fine-grained or colloidal sediments at point contacts by the weight of overburden sediment or by shear pressures. Overburden pressure of 5,000 feet would be approximately 5,000 p.s.i. However, shear pressure would exceed this by an unknown amount.

TIME

Petroleum occurs in rocks of all ages from the Cambrian to the Pliocene inclusive, but no evidence has been found to prove that any petroleum has been formed since the Pliocene, although sedimentation patterns and thicknesses in Pleistocene and Recent sediments are similar to those in the Pliocene where petroleum has formed. Residual carbon and gas which may have been derived from petroleum occur in metasedimentary pre-Cambrian rocks. The scale factor for time since the Pliocene cannot be reckoned accurately in calendar years but may be taken for scale purposes as about a million years for the formation of the youngest known petroleum in geologic history. Time since the Cambrian can be taken as one billion years. The apparent absence of formation of petroleum subsequent to the Pliocene must be explained in any study of the transformation of organic material into petroleum.

^* From American Association of Petroleum Geologists Bulletin 30 (1946), 645–659.