Formation of Petroleum Reservoirs.
a. Sub-Surface Occurrences. In order for a petroleum reservoir to form, there must be three distinct sub-
surface occurrences. You must have:
(1) A source bed (the bed in which petroleum originally formed).
(2) A porous and permeable carrier and reservoir rock (rocks suitable for holding the fluid and
allowing the fluid to move, this will be discussed in more detail later in the ACCP).
(3) A trapping mechanism (a mechanism that "seals" the reservoir and prevents the fluid from
b. Source Bed (the origin of oil). In discussing the source bed (beds that are rich in organic material),
we must discuss the origin of oil because this is the phase in petroleum history in which the source bed is formed.
The origin of petroleum has provoked extensive argument throughout scientific and engineering circles. As yet, a
definite solution of the problem has not been found. The two most widely accepted theories by geologists and
petroleum engineers are the inorganic and the organic.
(1) The inorganic theory attempts to explain the formation of hydrocarbons (petroleum) by assuming
chemical reactions among water, carbon dioxide, and various inorganic substances (for example, carbides and
carbonates) in the earth.
(2) The organic theory, on the other hand, assumes that hydrocarbons evolved from decomposition of
vegetable and animal organisms that lived during previous geologic ages. In general, scientists have abandoned
the inorganic theory as untenable. Conversely, the organic theory is supported by much geologic evidence.
c. Porous and Permeable Media. The earth consists of a potentially liquid core, a vitreous shell, and a
crystalline crust. The earth's crust is composed of three main types of rocks classified according to their origin.
The three types of rocks are:
(1) Igneous. These are formed from cooled lava.
(2) Sedimentary. These rocks are formed by the action of wind, water, and ice; from the accumulated
deposition of the mineral remains of animals, from the deposits of evaporating land-locked seas, or from chemical
reactions. Sedimentary rocks are the most common type that form petroleum reservoirs. Sedimentary rocks can
be divided into two main classifications, clastics and carbonates.
(a) Clastic sediments are composed mainly of broken and worn particles of pre-existing minerals,
rocks, and/or shells. Due to weathering, erosion, and transport, these particles are eventually deposited with
organic debris in marine basins. This deposition is normally in successive layers. Clastic sedimentary reservoirs
are characteristically sands or fine-grained silica.
(b) Carbonate reservoirs are characteristically limestone and dolomite. The framework of these
firmations is chemically precipitated from seawater and/or removed from seawater by living organisms. Often
fossil remains of these organisms are found in the framework. These sediments undergo various chemical
processes, which alter and restructure the original sediment.