Page 117 - Geology and Geochemistry of Oil and Gas

CLASSIFICATION OF CRUDE OILS 89
technically complex and had not been commonly used, although scientific exper-
imentation is in progress.
On the basis of the extensive knowledge of composition and structure for all
classes of hydrocarbons, the presence of biomarkers directly related to the parent
biomass had been established. Genetic relationship of crude oils and parent organic
matter, genetic uniformity (or nonuniformity) of oils in different stratigraphic se-
quences have been established. Transformations of the specific hydrocarbons within
the catagenetic, weathering, etc. zones had been elucidated. This information is
important in the petroleum exploration as it enables to (1) forecast the type and
composition of hydrocarbon fluids, (2) identify the potential cross-flow zones, and
(3) determine the paths of lateral and vertical migration.

5.2. CLASSIFICATION OF CRUDE OILS

Many methods of classification of crude oils have been devised. Systems based on
a superficial inspection involving some physical property, such as specific gravity, are
easily applied and specific gravity is actually used to a large extent in expressing the
quality of crude oils.
Specific weight (or specific gravity) of liquids are compared to that of fresh water
(Table 5.1). For example, if a brine is 1.04 times heavier than fresh water, its specific
gravity is 1.04 and the pressure gradient is equal to 0.45 psi/ft ( ¼ 1.04 0.434). The
relationship between the pressure gradient and salt content in water is presented in
Fig. 5.1. If the API gravity of the oil is given, its specific gravity (SG) at 601F can be
determined as follows:
o
SG ¼ 141:5=ð131:5 þ APIÞ
For example, the specific gravity of oil at 371API is equal to
SG ¼ 141:5=ð131:5 þ 37Þ ¼ 0:84.
A more rational basis of classification is found in some expression of the com-
position of the oils. In American practice, crude oils long have been roughly clas-
sified as (a) paraffin base, (b) naphthenic or asphaltic base, and (c) mixed base if they
contain both paraffin and asphalt. (Fig. 5.2). This system was derived on the basis of
differences in the nature of the lubricating oil portion of the crude after a non-
destructive distillation. In other words, the crude oil is carefully distilled and the
portion boiling in the lubricating oil range is examined. If this portion is waxy and
has the physical properties of paraffins, the crude oil is termed paraffin base. On the
other hand, if the lubricating oil portion contains little or no wax and contains
asphaltic material, the crude oil is termed asphaltic. Crude oils, lubricating oil frac-
tion of which contains both paraffins and asphalts, are termed mixed base.
Using this basis for the classification of petroleum, it has been found that crude oil
occurring in various sections of the United States can also be classified. The Penn-
sylvania type of crude oil is paraffinic. This type of crude oil is found in the eastern
states of USA: Pennsylvania, West Virginia, New York, Michigan, and Ohio.