Page 63 - Introduction to Petroleum Engineering

CLASSIFICATION 47
There are many reasons for compositional variations such as those mentioned
earlier. First, the compositions of the fluids in the source rock depend on the organ-
isms that were present in the original sediment. Second, the processes of biological
and thermal maturation vary, depending on the type of organisms and temperature
history. Third, trapping processes vary, depending on geologic structures, properties
of those structures, and flux of water and other fluids through the structures. Given
all these variables, even oil samples taken from a single formation can vary with
depth and horizontal location. For example, exposure to an actively flowing aquifer
below an oil accumulation can produce compositional variations. A compositional
gradient, or variation of composition with spatial location, can also be induced by
thermal diffusion and gravity segregation.

3.2 CLASSIFICATION

Oils and gases can be classified in many ways. The most common of these are
summarized in this section.
Oils and gases are mixtures of predominantly hydrocarbon molecules with some
inorganic molecules, such as nitrogen, carbon dioxide, and hydrogen sulfide. Oils
and gases are called “sweet” if they contain only negligible amounts of sulfur com-
pounds such as hydrogen sulfide (H S) or mercaptans—organic molecules with a
2
sulfur–hydrogen functional group. If these fluids contain sulfur compounds such as
H S or a mercaptan, they are called “sour.”
2
Oils and gases are also classified by their specific gravities. For an oil, specific gravity
equals oil density divided by density of water at standard conditions. Heavy oils have
densities close to and even exceeding water density. Light oils are much less dense than
water. For gases, the reference density is that of air at standard conditions. Gases can be
rich or lean, depending on the amount of hydrocarbon molecules larger than methane.
Oil is also classified by the relative amounts of different kinds of hydrocarbon
molecules: paraffins, naphthenes, and aromatics. Paraffin molecules such as methane,
ethane, and propane have a single bond between carbon atoms and are considered
saturated hydrocarbons. Paraffins have the general chemical formula C H 2n+2 .
n
Naphthenes have the general chemical formula C H and are saturated hydrocarbons
n
2n
with a ringed structure, as in cyclopentane. The term “saturated” indicates that all
carbon–carbon bonds are single bonds. By contrast, unsaturated hydrocarbons have
at least one double or triple bond. Aromatics are unsaturated hydrocarbons with a
ringed structure. Aromatics are relatively stable and unreactive because of their
unique ring structure. Benzene is a well‐known example of an aromatic.
Petroleum can exist in a reservoir as a gas, liquid, or solid, depending on fluid
composition, temperature, and pressure. Natural gas is typically methane with lesser
amounts of heavier hydrocarbon molecules like ethane, propane, and butanes. Oil is
a liquid mixture of some natural gas dissolved in greater amounts of heavier hydro-
carbons with carbon numbers up to about 60. The carbon number designates the
number of carbon atoms in the molecule. Asphaltenes and waxes, the largest hydro-
carbon molecules in oil, can precipitate as solids under some conditions.