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cuticle – кутикула

peat – торф

rank – категорія

lignite – буре вугілля

Volatile – леткий, непостійний

residue – осад

toluene – толуол

tar - смола

Petroleum is primarily a product of the diagenesis of fundamental organic compounds contained in organic matter that accumulated with fine-grained sediment in a low-energy environment deficient in oxygen. The diagenesis takes place during burial, under the influences of heat, time and pressure – probably in the presence of clay catalysts. Early diagenesis removes the more volatile and soluble components, leaving kerogen.

The portion that is insoluble in organic solvents is called kerogen, of which three main types have been recognized.

Type-I kerogen: derived from organic matter with lipids, dominantly marine, with H/C ratios greater than 1.5. It is composed dominantly of aliphatic chains, with some aromatic chains. It is considered to be a good source material for oil and gas.

Type-II kerogen: derived from marine organic matter mainly, with H/C ratios smaller than for Type I. It is composed of aromatic and naphthenic rings, and considered to be quite a good source material for oil and gas.

Type-III kerogen: derived from terrestrial higher plants mainly, with H/C ratios usually less than 1.0. Not a good source material, is may generate gas rather than oil.

Mudstones are important source rocks of petroleum, but not all mudstones are source rocks. Thicker mudstones are probably more important than thinner because greater volumes of water must be expelled from them eventually. Concentrations of residual organic matter may be less in thicker mudstones than in thinner mudstones, but the total quantity must be large for large volumes of petroleum to be generated.

Petroleum is probably generated as a separate phase in the source rock, and migrates as a separate phase in water to the accumulation. During migration, oil may be altered by removal of the more soluble components (water washing); and bacterial degradation may remove normal-alkanes in the range C₁₆ – C₂₅, then progressively those with higher carbon numbers.

High-wax crude oils seem to be genetically related to the environment in which the sediments accumulated. They are associated with sandstone/mudstone sequences, both transgressive and regressive, of Devonian age and younger. Not all petroleum provinces have waxy crude oils, and some important carbonate provinces appear to have no waxy crude oils.

It is natural that one should enquire whether the two great fuels, coal and petroleum, have any significant geological relationship; and this enquiry has been going on for more than a century. Coal results from the diagenesis of vegetable organic matter that accumulated in an environment largely devoid of sediment. Conditions on the actual surface of accumulation may have been reducing or oxidizing; but close below this surface, reducing conditions prevailed. Coal consists largely of carbonized plant tissues, wood and bark, with spores (particularly the more durable spore coatings), leaf cuticles, waxes and resins. Coals form a series, with peat at one end and graphite at the other, and they are ranked according to their degree of alteration from lignites to anthracites. The type of coal groups coals of similar composition. Cannel coal, for example, is a coal rich in volatiles that burns easily and commonly contains significant proportions of spore coatings. Heating of a “bituminous” coal results in distillation of a gas that consists largely of hydrogen and methane, with numerous other components in small proportions (some being hydrocarbons). The coal-tar residue contains hydrocarbon oils (benzene, toluene etc.) with other components.

Not surprisingly, early work on the association between coal and petroleum was carried out in Pennsylvania, U.S.A. – a coal mining area in which commercial oil production began in the United States. The early work culminated in White’s “Carbon Ratio Theory”, in which he related the occurrences of oil and gas to the percentage of “fixed carbon” in associated coals. He ranked petroleum from low-ranking heavy oils to high-ranking light oils, and noted that oil occurred where the fixed carbon is less than 65 per cent, mostly less than 60 per cent; no commercial accumulation occurred when the percentage of fixed carbon is greater than about 70 per cent; and gas occurred in the intervening zone. There was thus observed a ranking of oil with depth analogous to the ranking of coal with depth in many fields. This theory was considered to be widely applicable.