67
Origin and Occurrence
In addition to maturation by thermal means, petroleum also undergoes nonthermal transformations after their accumulation in reservoirs. These changes are usually restricted to shallow oil
deposits located near surface outcrops or near faults, which provide access to surface waters containing dissolved oxygen.
3.3 OCCURRENCE
The reservoir rocks that yield crude oil range in age from Precambrian to recent geologic time but
rocks deposited during the Tertiary, Cretaceous, Permian, Pennsylvanian, Mississippian, Devonian,
and Ordovician periods are particularly productive. In contrast, rocks of Jurassic, Triassic, Silurian,
and Cambrian age are less productive and rocks of Precambrian age yield petroleum only under
exceptional circumstances.
Most of the crude oil currently recovered is produced from underground reservoirs. However,
surface seepage of crude oil and natural gas are common in many regions. In fact, it is the surface
seepage of oil that led to the first use of the high boiling material (bitumen) in the Fertile Crescent
(Chapter 1). It may also be stated that the presence of active seeps in an area is evidence that oil and
gas are still migrating.
The majority of crude oil reserves identified to date are located in a relatively small number of
very large fields, known as giants. In fact, approximately 300 of the largest oil fields contain almost
75% of the available crude oil. Although most of the world’s nations produce at least minor amounts
of oil, the primary concentrations are in Saudi Arabia, Russia, the United States (chiefly Texas,
California, Louisiana, Alaska, Oklahoma, and Kansas), Iran, China, Norway, Mexico, Venezuela,
Iraq, Great Britain, the United Arab Emirates, Nigeria, and Kuwait. The largest known reserves are
in the Middle East.
3.3.1 reserves
The definitions that are used to describe petroleum reserves are often misunderstood because they
are not adequately defined at the time of use. Therefore, as a means of alleviating this problem, it
is pertinent at this point to consider the definitions used to describe the amount of petroleum that
remains in subterranean reservoirs.
Petroleum is a resource; in particular, petroleum is a fossil fuel resource (Chapter 1). A resource
is the entire commodity that exists in the sediments and strata whereas the reserves represent
that fraction of a commodity that can be recovered economically. However, the use of the term
reserves as being descriptive of the resource is subject to much speculation. In fact, it is subject to
word variations! For example, reserves are classed as proved, unproved, probable, possible, and
undiscovered.
Proven reserves are those reserves of petroleum that are actually found by drilling operations
and are recoverable by means of current technology. They have a high degree of accuracy and are
frequently updated as the recovery operation proceeds. They may be updated by means of reservoir
characteristics, such as production data, pressure transient analysis, and reservoir modeling.
Probable reserves are those reserves of petroleum that are nearly certain but about which a
slight doubt exists. Possible reserves are those reserves of petroleum with an even greater degree of
uncertainty about recovery but about which there is some information. An additional term potential
reserves is also used on occasion; these reserves are based upon geological information about the
types of sediments where such resources are likely to occur and they are considered to represent
an educated guess. Then, there are the so-called undiscovered reserves, which are little more than
figments of the imagination! The terms undiscovered reserves or undiscovered resources should be
used with caution, especially when applied as a means of estimating reserves of petroleum reserves.
The data are very speculative and are regarded by many energy scientists as having little value other
than unbridled optimism.
Origin and Occurrence
In addition to maturation by thermal means, petroleum also undergoes nonthermal transformations after their accumulation in reservoirs. These changes are usually restricted to shallow oil
deposits located near surface outcrops or near faults, which provide access to surface waters containing dissolved oxygen.
3.3 OCCURRENCE
The reservoir rocks that yield crude oil range in age from Precambrian to recent geologic time but
rocks deposited during the Tertiary, Cretaceous, Permian, Pennsylvanian, Mississippian, Devonian,
and Ordovician periods are particularly productive. In contrast, rocks of Jurassic, Triassic, Silurian,
and Cambrian age are less productive and rocks of Precambrian age yield petroleum only under
exceptional circumstances.
Most of the crude oil currently recovered is produced from underground reservoirs. However,
surface seepage of crude oil and natural gas are common in many regions. In fact, it is the surface
seepage of oil that led to the first use of the high boiling material (bitumen) in the Fertile Crescent
(Chapter 1). It may also be stated that the presence of active seeps in an area is evidence that oil and
gas are still migrating.
The majority of crude oil reserves identified to date are located in a relatively small number of
very large fields, known as giants. In fact, approximately 300 of the largest oil fields contain almost
75% of the available crude oil. Although most of the world’s nations produce at least minor amounts
of oil, the primary concentrations are in Saudi Arabia, Russia, the United States (chiefly Texas,
California, Louisiana, Alaska, Oklahoma, and Kansas), Iran, China, Norway, Mexico, Venezuela,
Iraq, Great Britain, the United Arab Emirates, Nigeria, and Kuwait. The largest known reserves are
in the Middle East.
3.3.1 reserves
The definitions that are used to describe petroleum reserves are often misunderstood because they
are not adequately defined at the time of use. Therefore, as a means of alleviating this problem, it
is pertinent at this point to consider the definitions used to describe the amount of petroleum that
remains in subterranean reservoirs.
Petroleum is a resource; in particular, petroleum is a fossil fuel resource (Chapter 1). A resource
is the entire commodity that exists in the sediments and strata whereas the reserves represent
that fraction of a commodity that can be recovered economically. However, the use of the term
reserves as being descriptive of the resource is subject to much speculation. In fact, it is subject to
word variations! For example, reserves are classed as proved, unproved, probable, possible, and
undiscovered.
Proven reserves are those reserves of petroleum that are actually found by drilling operations
and are recoverable by means of current technology. They have a high degree of accuracy and are
frequently updated as the recovery operation proceeds. They may be updated by means of reservoir
characteristics, such as production data, pressure transient analysis, and reservoir modeling.
Probable reserves are those reserves of petroleum that are nearly certain but about which a
slight doubt exists. Possible reserves are those reserves of petroleum with an even greater degree of
uncertainty about recovery but about which there is some information. An additional term potential
reserves is also used on occasion; these reserves are based upon geological information about the
types of sediments where such resources are likely to occur and they are considered to represent
an educated guess. Then, there are the so-called undiscovered reserves, which are little more than
figments of the imagination! The terms undiscovered reserves or undiscovered resources should be
used with caution, especially when applied as a means of estimating reserves of petroleum reserves.
The data are very speculative and are regarded by many energy scientists as having little value other
than unbridled optimism.