Hydrocarbon Group Composition and Main Properties of Crude

Composition of crude oil is determined by dividing it into fractions according to the molecular weight, followed by estimation of hydrocarbon-group composition of each fraction. Division of crude oil into fractions can be made according to their boiling point ranges. Fractional composition shows relative contents in percent by weight (wt %) of different oil fractions boiling within definite boiling-point ranges. The following main fractions are distinguished in Russia and other CIS countries: "benzine" with boiling point range of 40 to 200°C, "ligroin" with boiling point range of 200 to 350°C, and "residual oil" with boiling point range of 350 to 500°C. "Benzine" fractions of the Azerbaijan oils constitute 40% of distillate at 100°C. (See p. 177.)

Entropy, as a measure of heterogeneity of crude oil composition, differs in fractions having different boiling-point ranges (Buryakovsky, 1968). If relative entropy of oil as a heterogeneous system of light-boiling fractions (from 65 to 150°C) is 0.6-0.7, it increases to 0.8 in fractions with boiling-point range of 150 to 225°C. For high-boiling fractions (from 225 to 350°C), the relative entropy reaches a maximum value of 1.

Entropy evaluation, as a measure of complexity of an oil composition, has a certain advantage over the other classifications of oils, because it allows one to assign a numerical value to the oil heterogeneity. The values range from 0 to 1, with 0 characterizing vertexes of the mixture triangle, and 1 characterizing the center of the triangle (see Figure 10-9).

In order to evaluate the geochemical history, in addition to the fractional oil composition, hydrocarbon-group composition of different fractions is also used (content of paraffinic, naphthenic, and aromatic groups of hydrocarbons). The hydrocarbon-group composition of crude oil can be clearly presented on a mixture triangle (Gibbs' triangle). Based on more than 100 samples of oils from the Apsheron Archipelago fields, hydrocarbon-group composition of light fractions of crude oil (gasoline and ligroin) is plotted on a triangular diagram in Figure 10-19.

100 o

100 o

Figure 10-19. Hydrocarbon-group composition of light fractions of oils from Neft Dashlary Field (Modified after Buryakovsky and Dzhevanshir, 1992). • = "benzine" (gasoline); x = "ligroin."

Naphthenic

Figure 10-19. Hydrocarbon-group composition of light fractions of oils from Neft Dashlary Field (Modified after Buryakovsky and Dzhevanshir, 1992). • = "benzine" (gasoline); x = "ligroin."

According to the experimental data, Figure 10-20 shows the dependence of content of various hydrocarbon groups on boiling point for crude oils of the Apsheron Archipelago. The results obtained by Dobryanskiy (1948) and Kartsev (1950) for "world" oils (weighted average data for many oil fields) are given for a comparison. With increasing boiling point, the aromatic hydrocarbons content increases, whereas the content of paraffinic hydrocarbons decreases.

Besides hydrocarbon components, different non-hydrocarbon components are present in the crude oil. Asphaltenes and resins constitute the major portion of non-hydrocarbon components.

Crude oil density and contents of asphaltenes and resins, gasoline, and ligroin of the crude oil from Neft Dashlary oilfield in the Apsheron

Figure 10-20. Hydrocarbon-group composition of crude oils. 1—Average "world" crude oil according to A. A. Kartsev; 2—average "world" crude oil according to A. F. Dobryanskiy; 3—oil from Neft Dashlary Field; 4—oil from Palchygh Pilpilasi Field (Modified after Buryakovsky and Dzhevanshir, 1992).

Figure 10-20. Hydrocarbon-group composition of crude oils. 1—Average "world" crude oil according to A. A. Kartsev; 2—average "world" crude oil according to A. F. Dobryanskiy; 3—oil from Neft Dashlary Field; 4—oil from Palchygh Pilpilasi Field (Modified after Buryakovsky and Dzhevanshir, 1992).

Archipelago (Buryakovsky, 1974a) are presented as histograms in Figure 10-21.

In the northwestern part of the Apsheron Archipelago (Darvin Bank, Pirallaghi Adasi, and Gyurgyany Deniz fields), average crude oil parameters [with evaluation of their variation within two-sigma limits (95% of confidence)], based on 1,642 analyses, can be presented as follows:

Y = 0.9137 ± 0.0240 g/cm3 Yave = 0.9137 ± 0.0006 g/cm3

where Y is density, R is the content of asphaltenes and resins, B is the "benzine" (gasoline) content, and L is the content of ligroin.

Figure 10-21. Histograms of crude oil density (a), content of resins (b), and "benzine" (gasoline) content (c) in the crude oils of Neft Dashlary Field (Modified after Buryakovsky and Dzhevanshir, 1992). 1—Balakhany Suite, 2— "Pereryv" Suite, 3—NKP, 4—KS, 5—PK, 6—KaS.

Based on 820 analyses, in the southeastern part of the Apsheron Archipelago (Chalov Adasi, Palchygh Pilpilasi, and Neft Dashlary oilfields) these crude oil parameters are equal to:

Y = 0.8800 ± 0.0380 g/cm3 yave = 0.8800 ± 0.0013 g/cm3

The crude oil of the northwestern part of the Apsheron Archipelago contains more asphaltenes and resins and less low-boiling fractions; hence, its density is higher than that of the oil from the southeastern part of archipelago.

Using both numerical characteristics and histograms or as frequency distributions (relatived frequencies), one can solve different geological and geochemical problems. For example, Figure 10-21 shows the distribution of oil densities, content of asphaltenes and resins, and gasoline content in different suites of the Neft Dashlary Field. With increase in burial depth, density of crude oil and content of asphaltenes and resins increase, whereas the gasoline content decreases. These trends, however, are not present in the Upper Productive Series. The increase in density of oil in the Upper Productive Series is related to the oxidation of oil by near-surface agents, which increases the content of asphaltenes and resins and decreases the gasoline content.

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