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Petroleum and Gas Engineering Exploration-1 PGE 490
Dr. Aref Lashin
Petroleum and Gas Engineering Exploration-1
5. RESERVOIRS
A petroleum reservoir, or oil and gas reservoir, is a subsurface pool of hydrocarbons
contained in porous or fractured rock formations. The naturally occurring hydrocarbons,
such as crude oil or natural gas, are trapped by overlying rock formations with lower
permeability. Reservoirs are found using hydrocarbon exploration methods.
The petroleum reservoir is that portion of a container that contains the oil and gas found
underground.
Petroleum reservoir consists of two parts:
a. Reservoir rock
b. Reservoir pore spaces

Porosity consists of the tiny spaces in the rock that hold the oil or gas.

Permeability is a characteristic that allows the oil and gas to flow through the rock.
5.1 Classification of reservoirs
Nearly all the reservoirs are in the un-metamorphosed sedimentary rocks, and most of them
are in limestone and dolomites. However, any rock that contains connected pores may
become a reservoir.
A simple classification of the reservoir rocks, based mainly on the origin of the rock,
divides them into three categories:
5.1.1 Clastic reservoir rocks (fragmental reservoirs).
5.1.2 Calcareous reservoir rocks (Carbonate & Chemical reservoirs).
5.1.3 Miscellaneous reservoir rocks (Fractured metamorphic and basement rocks).
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Dr. Aref Lashin
5.1.1 Clastic reservoir rocks (fragmental reservoirs).
The clastic reservoirs are aggregates of particles, fragments of minerals, or fragments of
older rocks. They are also called clastic or detrital rocks because they consist of minerals and
rock particles washed from areas that have been eroded.
Most clastic sedimentary rocks consist of grains that are cemented together with minerals
like calcite or quartz. Friability is a measure of how well-cemented the grains of a clastic
rock are. Some clastic rocks are not very well cemented! You can rub them to pieces in your
hand! These rocks are a great find for the petroleum geologist, because the lack of cement
means there is more space to fill with oil or gas
The character of the clastic reservoirs varies with many factors such as;
 The nature of the eroded materials,
 The distance it is transported,
 The climate,
 The steepness of the gradients,
 The transporting agencies, etc.
The constituting particles of the clastic reservoirs vary in size from very fine particles up to
the very coarse grain and pebbles. The most important factor is the sorting of grains, i.e a
coarse-grain sandstone which is made
up mostly of the mineral quartz is
well-sorted. That means most of the
grains are about the same size in
relation to each other (Fig. 1).
The most important types of the clastic reservoirs are:

Sandstones

Conglomerates

Arkoses, gray wakes and siltstones
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Examples of Clastic reservoirs:

Ghawar Filed- Saudi Arabia (Fig. 2):
Deeper drilling in the Ghawar field has discovered vast amounts of natural gas in the
Paleozoic sediments. In 1994, Saudi Aramco began a vigorous plan for deep gas exploration
including prospects in Ghawar.
The main gas-bearing clastic reservoirs are Jauf and Unayzah sandstone (Fig. 3).

Burgan filed- Kuwait:
It produces oil from the Cretaceous sands that attains a thickness of 800 ft out of 1100 ft of
section.

Ramadan and October fields- Egypt:
They produce oil from the Nubia sandstone.
5.1.2 Calcareous reservoir rocks (Carbonate & Chemical reservoirs).
Chemical rocks are composed chiefly of chemical or biochemical precipitates. They consist
of mineral matter that was precipitated at the place where the rocks were formed and not
transported. The most important calcareous reservoirs rocks are the limestone and dolomite.
Examples of Carbonate reservoirs:

Ghawar Filed- Saudi Arabia (Fig. 2):
The Arab-D reservoir (carbonate rock) in Ghawar includes the lowermost zone of the Arab
formation and the uppermost part of the Jubaila formation (Fig. 4).

Ras Fanar Field – Gulf of Suez-Egypt
Ras Fanar field is one among the largest oil-bearing carbonate reservoirs in the Gulf of Suez.
The field produces from the Middle Miocene Nullipore carbonate reservoir, which
consists mainly of algal rich dolomite and dolomitic limestone rocks and range in thickness
between 400 ft and 980 ft.
5.1.3 Miscellaneous reservoir rocks (Fractured metamorphic and basement rocks).
Miscellaneous reservoirs include the igneous and metamorphic rocks, mixtures of both
frequently forming the basement complexes. The reservoir space is represented mainly by
fracture system in the brittle basement rocks.
Examples of Miscellaneous reservoirs:
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
Dr. Aref Lashin
Zeit Bay Field – Gulf of Suez-Egypt
Zeit Bay field is located at the southern part of the Gulf of Suez. It produces oil mainly from
the highly fractured basement rocks.
Fig. 2 Ghawar oil field, Saudi Arabia.
Petroleum and Gas Engineering Exploration-1 PGE 490
Dr. Aref Lashin
Fig. 3 Sandstone reservoirs, Ghawar oil field- Saudi Arabia.
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Dr. Aref Lashin
Fig. 4 Carbonate reservoirs, Ghawar oil field- Saudi Arabia.
Petroleum and Gas Engineering Exploration-1 PGE 490
Dr. Aref Lashin
5.2 Reservoir Pore Spaces
The first essential element of a petroleum reservoir is the reservoir rock and the essential
feature of a reservoir rock is the porosity.
Porosity and permeability are related properties of any rock or loose sediment. Most oil and
gas has been produced from sandstones. These rocks usually have high porosity, and are
usually “high perm.” Porosity and permeability are absolutely necessary to make a
productive oil or gas well. The petroleum geologist must stay focused on the porosity and
permeability of the prospective reservoir.
5.2.1 Porosity
Porosity of a rock is a measure of its ability to hold a fluid. Mathematically, porosity is the
open space in a rock divided by the total rock volume (solid + space or holes). Porosity is
normally expressed as a pecentage of the total rock which is taken up by pore space. For
example, a sandstone may have 8% porosity. This means 92 percent is solid rock and 8
percent is open space containing oil, gas, or water. Eight percent is about the minimum
porosity that is required to make a decent oil well, though many poorer (and often noneconomic) wells are completed with less porosity.
Porosity is defined as the ratio of pore space to total volume of reservoir rock and is
commonly expressed as a percentage ( Fig. 5).
Fig. 5. Shows a sandstone
thin section. The grain size
and the pore spaces are
clear.
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There are two types of porosity:

Total porosity:
It is ratio of total volume of pore spaces to total volume of the rock is called absolute
of total porosity.
 Total

= (Total volume of pore spaces / Bulk volume of rock) x 100
Effective porosity:
Is the ratio of interconnected pore spaces to the bulk volume of the rock.
 Effective
= (Connected pore spaces / Bulk volume of rock) x 100
5.2.2 Permeability
Permeability is the property of rocks that is an indication of the ability for gases or fluids to
flow through rocks. High permeability will allow fluids and gases to move rapidly through
rocks Permeability is affected by the pressure in a rock. The unit of measure is called the
darcy, named after Henry Darcy (1856).
Sandstones may vary in permeability from less than one to over 50,000 millidarcies (md).
Permeabilities are more commonly in the range of tens to hundreds of millidarcies. A rock
with 25% porosity and a permeability of 1 md will not yield a significant flow of fluids or
gases. Such “tight” rocks are usually artificially stimulated (fractured or acidized) to create
permeability and yield a flow (Fig. 6).
Fig. 6. Permeability
measures how easily fluid
passes through a rock.
Petroleum and Gas Engineering Exploration-1 PGE 490
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The permeability of a rock is a measure of the resistance to the flow of a fluid through a
rock. If it takes a lot of pressure to squeeze fluid through a rock, that rock has “low
permeability” or “low perm.” If fluid passes through the rock easily, it has “high
permeability,” or “high perm.” The permeability of a rock is a measure of the resistance to
the flow of a fluid through a rock. If it takes a lot of pressure to squeeze fluid through a rock,
that rock has “low permeability” or “low perm.” If fluid passes through the rock easily, it has
“high permeability,” or “high perm.”
A rough field appraisal of reservoir permeability’s is:
Poor
0.1 - 1.0 mD
Fair
1.0 – 10 mD
Good
10 – 100 mD
In the last 10 years, an increasing amount of US gas production is coming from shale gas
wells. Shale has a lot of porosity (much more than sandstone), but extremely low
permeability. That means shale has historically been a poor producer of hydrocarbons.
While gas has been produced from shales for over a hundred years, quantities were small.
Two things have changed the situation, allowing for increased shale gas development. These
concepts have allowed petroleum companies to artificially induce more permeability into
shale gas rocks:

Horizontal Drilling – The widespread adoptance of horizontal drilling technology,
in which the drill bit is made to turn from the vertical to the horizontal (a 90-degree
turn), where it can continue to drill horizontally through the formation. The
horizontal track can be as much as a mile.

Advances In Hydraulic Fracturing (fracking) – Fracking is not a new technology;
it has been around well over 50 years (despite what you might hear on the news).
However, advances in fracking techniques in horizontally-drilled holes, particularly
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Dr. Aref Lashin
in shale formations, has led to a tremendous increase in shale gas production. These
new techniques allow the oil and gas operator to render low-permeability shale
reservoirs more permeable, by artificially introducing small fractures into the
formation.
The permeability chart for typical sediments is shown below.