Page 260 - Geology and Geochemistry of Oil and Gas

MODELS OF STATIC GEOLOGIC SYSTEMS 225
montmorillonite (40 to 100 wt%) and illite (0–35 wt%). The content of kaolinite,
chlorite, and mixed-layered minerals is approximately equal to 0 to 15–20 wt%. The
average content (in wt%) of clays are montmorillonite, 75%; illite, 12%; kaolinite,
5%; chlorite, 4%; and mixed-layered, 4%.
Difference in the content of montmorillonite and other clay minerals in these two
regions is quite low, except for clays present in the reservoir rocks. The montmo-
rillonite content in the clay cement of reservoir rocks of the Lower Kura Depression
is almost twice higher than in the reservoir rocks of the Baku Archipelago and South
Absheron Offshore Zone. This explains why the surface activity of reservoir rocks of
the Lower Kura Depression is higher than that in the offshore areas. The A da is
practically the same when the montmorillonite content in clays is the same. It is well
known that the surface activity of montmorillonite clay is higher in comparison with
other clay minerals.

11.2.2.1.2. Crude oil properties. Crude oil is a complex natural system consisting of
various components with a considerable predominance of different hydrocarbon
groups. Many different classifications of crude oils have been devised. A rational
basis of classification is found in some expression of the composition of oils. Com-
position of oil is determined by separating it into fractions according to the boiling
point (molecular weight), followed by establishing the types of hydrocarbons present
in each fraction.
To determine the fractional composition of oil, one can use a method of separating
it into fractions according to the boiling point (fractional distillation). Fractional
composition reflects relative content in percent by weight (wt%) of different oil
fractions boiling within definite boiling-point ranges. In Russia, the following main
fractions are distinguished: (1) ‘‘benzine’’ fraction with boiling points ranging from
401C to 2001C, (2) ‘‘ligroin’’ fraction with boiling points ranging from 2001C to
3501C, and (3) residual oil with boiling points ranging from 3501C to 5001C. 19
In order to evaluate the geochemical history, in addition to the fractional oil
composition, group-hydrocarbon composition of different fractions is also used, i.e.,
content of paraffinic, naphthenic, and aromatic groups of hydrocarbons. The group-
hydrocarbon composition of crude oil can be clearly presented on a mixture triangle.
Based on more than 100 samples of oils from the Absheron Archipelago fields of the
South Caspian Basin, the hydrocarbon-group composition of light fractions of crude
oil (gasoline and ligroin) was plotted on a triangular diagram in Fig. 11.9. Besides
hydrocarbon components, different non-hydrocarbon components are present in the
crude oil. Asphaltenes plus resins constitute the major portion of non-hydrocarbon
components.
Crude oil density and the contents of asphaltenes plus resins, gasoline, and ligroin
of the crude oil from Neft Dashlary oilfield in the Absheron Archipelago are pre-
sented as histograms in Fig. 11.10.

19
In USA, the ‘‘benzine’’ fraction as used in Russia, includes light naphtha, gasoline, and heavy naphtha;
‘‘ligroin’’ fraction includes kerosene, stove oil, and light gas oil; and residua includes heavy gas oil,
lubricating oil, and vacuum gas oil.