origin and occurrence

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Origin and Occurrence

INTRODUCTION

Petroleum (also called crude oil) is a mixture of gaseous, liquid, and solid hydrocarboncompounds. Petroleum occurs in sedimentary rock deposits throughout the world and also contains

small quantities of nitrogen-, oxygen-, and sulfur-containing compounds as well as trace amounts

of metallic constituents (Bestougeff, 1967; Colombo, 1967; Hobson and Pohl, 1973; Thornton,

1977; Considine and Considine, 1984). Thus, the classification of petroleum as a hydrocarbonmixture should follow from this definition. But some clarification is required.

Petroleum is by far the most commonly used source of energy, especially as the source of liquid

fuel, and its use is projected to continue at least at the current levels for at least two decades.

However, there is petroleum and there is petroleum and all are not equal. In recent years, theaverage quality of crude oil has worsened. This is reflected in a progressive decrease in API

gravity (i.e., increase in density) and a rise in sulfur content (Swain, 1991, 1993, 1997, 2000).However, it is now believed that there has been a recent tendency for the quality of crude oilfeedstock to stabilize. Be that as it may, the nature of crude oil refining has changed considerably.

ORIGINThere are two theories on the origin of carbon fuels: the abiogenic theory and the biogenic theory.

ABIOGENIC ORIGIN

There have been several attempts at formulating theories that describe the detail of the origin ofpetroleum, of which the early postulates started with inorganic substances as source material. For

example, in 1866, Berthelot considered acetylene the basic material and crude oil constituents wereproduced from the acetylene. Initially, inorganic carbides were formed by the action of alkalimetals on carbonates after which acetylene was produced by the reaction of the carbides with

water.

Mendelejeff, who proposed that the action of dilute acids or hot water on mixed iron and

manganese carbides produces a mixture of hydrocarbons from which petroleum evolved, described

another theory in which acetylene is considered to be the basic material:

From the chemical point of view the inorganic theories are interesting because of their historical

importance, but these theories have not received much attention. Geological and chemical methodshave demonstrated the optical activity of petroleum constituents, the presence of thermally labile

organic compounds, and the almost exclusive occurrence of oil in sedimentary rocks. Other

theories have attempted to correlate the occurrence of coal strata and oil in the earths crust withcoal being the precursor to crude oil or both fossil fuels are formed from the same precursors at the


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same time but are the result of divergent paths. Chemical and geological investigations do not

support this concept and the idea is considered obsolete.

BIOGENIC ORIGIN

Petroleum is a naturally occurring hydrocarbon mixture but hydrocarbons that are synthesized byliving organisms usually account for less than 20% of the petroleum (Hunt, 1996). The remainder

of the hydrocarbons in petroleum is produced by a variety of processes that convert other organic

material to hydrocarbons as part of the maturation processes generally referred to as diagenesis,

catagenesis, and metagenesis. These three processes are a combination of bacteriological actionand low-temperature reactions that convert the source material into petroleum. During these

processes, migration of the liquid products from the source sediment to the reservoir rock may also

occur.

Most geologists view crude oil and natural gas as the products of compression and heating of

ancient vegetation over geological time scales. According to this theory, it is formed from the

decayed remains of prehistoric marine animals and terrestrial plants. Over many centuries thisorganic matter, mixed with mud, is buried under thick sedimentary layers of material. The resultinghigh levels of heat and pressure cause the remains to metamorphose, first into a waxy material

known as kerogen, and then into liquid and gaseous hydrocarbons in a process known as

catagenesis. These then migrate through adjacent rock layers until they become trappedunderground in porous rocks called reservoirs, forming an oil field, from which the liquid can be

extracted by drilling and pumping.

Chemically, it is generally proposed that petroleum is formed through the progressive chemical

change of materials provided by microscopic aquatic organisms that were incorporated over eonsin marine or near-marine sedimentary rocks. In fact, the details of petroleum genesis (diagenesis,

catagenesis, and metagenesis) have long been a topic of interest. However, the details of thistransformation and the mechanism by which petroleum is expelled from the source sediment andaccumulates in the reservoir rock are still uncertain.

Thus, it is generally believed that the generation of petroleum is associated with the deposition of

organic detritus. Detritus deposition occurs during the development of, fine grained sedimentary

rocks that occur in marine, near-marine, or even nonmarine environments (Vassoyevitch et al.,1969; Hood et al., 1975; Tissot and Welte, 1978; Brooks and Welte, 1984; Bjoroy et al., 1987).

Petroleum is believed to be the product arising from the decay of plant and animal debris that was

incorporated into sediments at the time of deposition. However, the details of this transformation

and the mechanism by which petroleum is expelled from the source sediment and accumulates inthe reservoir rock are still uncertain but progress has been made (Nagy and Colombo, 1967;

Hobson and Tiratsoo, 1975; Hunt, 1996).

OCCURRENCE

The reservoir rocks that yield crude oil range in age from pre-Cambrian 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 pre-Cambrian age yieldpetroleum only under exceptional circumstances. Most of the crude oil currently recovered is


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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. It may also be stated that the presence of activeseeps 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 three hundred of the largest oil fields containalmost 75% of the available crude oil. Although most of the worlds nations produce at least minoramounts 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 knownreserves are in the Middle East.

RESERVES

Petroleum is a resource; in particular, petroleum is a fossil fuel resource. 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 beingdescriptive 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 arefrequently 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 ofuncertainty about recovery but about which there is some information. An additional term potential

reserves is also used on occasion; these reserves are based on geological information about thetypes of sediments where such resources are likely to occur and they are considered to represent aneducated 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 littlevalue other than unbridled optimism.

The term inferred reserves is also commonly used in addition to, or in place of, potential reserves.

Inferred reserves are regarded as of a higher degree of accuracy than potential reserves, and the

term is applied to those reserves that are estimated using an improved understanding of reservoirframeworks. The term also usually includes those reserves that can be recovered by further

development of recovery technologies.

CONVENTIONAL PETROLEUM

At the present time, several countries are recognized as producers of petroleum and have availablereserves. For example, on a worldwide basis the produced conventional crude oil is estimated to be

approximately 784 billion bbl with approximately 836 billion bbl remaining as

reserves. It is also estimated that there are 180 billion bbl which remain to be discovered with


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approximately 1 trillion bbl yet-to-be-produced. The annual depletion rate is estimated to

be 2.6%.

In addition, the crude oils available today to the refinery are quite different in composition andproperties to those available some 50 years ago (Swain, 1991, 1993, 1997, 2000). The current

crude oils are somewhat heavier, as they have higher proportions of nonvolatile (asphaltic)

constituents. In fact, by the standards of yesteryear, many of the crude oils currently in use wouldhave been classified as heavy feedstock, bearing in mind that they may not approach thedefinitions used today for heavy crude oil. Changes in feedstock character, such as this tendency to

heavier materials, require adjustments to refinery operations to handle these heavier crude oils to

reduce the amount of coke formed during processing and to balance the overall product slate.

NATURAL GAS

Natural gas is the gaseous mixture associated with petroleum reservoirs and is predominantly

methane, but does contain other combustible hydrocarbon compounds as well as nonhydrocarbon

compounds (Speight, 1993). In fact, associated natural gas is believed to be the most economicalform of ethane (Farry, 1998). Natural gas has no distinct odor and the main use is for fuel, but it

can also be used to make chemicals and liquefied petroleum gas (LPG). The gas occurs in the

porous rock of the earths crust either alone or with accumulations of petroleum. In the latter case,

the gas forms the gas cap, which is the mass of gas trapped between the liquid petroleum and theimpervious cap rock of the petroleum reservoir. When the pressure in the reservoir is sufficiently

high, the natural gas may be dissolved in the petroleum and is released upon penetration of the

reservoir as a result of drilling operations. There are several general definitions that have beenapplied to natural gas. Thus, lean gas is gas in which methane is the major constituent. Wet gas

contains considerable amounts of the higher molecular weight hydrocarbons. Sour gas contains

hydrogen sulfide, whereas sweet gas contains very little, if any, hydrogen sulfide. Residue gas isnatural gas from which the higher molecular weight hydrocarbons have been extracted and casing

head gas is derived from petroleum but is separated at the separation facility at the wellhead.

HEAVY OIL

When petroleum occurs in a reservoir that allows the crude material to be recovered by pumpingoperations as a free-flowing dark- to light-colored liquid, it is often referred to as conventional

petroleum. Heavy oils are the other types of petroleum that are different from conventional

petroleum insofar as they are much more difficult to recover from the subsurface reservoir. These

materials have a much higher viscosity (and lower API gravity) than conventional petroleum, andprimary recovery of these petroleum types usually requires thermal stimulation of the reservoir.

The definition of heavy oil is usually based on the API gravity or viscosity, and the definition is

quite arbitrary although there have been attempts to rationalize the definition based upon viscosity,

API gravity, and density. Thus, the generic term heavy oil is often applied to petroleum that has anAPI gravity of less than 208 and usually, but not always, sulfur content higher than 2% by weight.

Further, in contrast to conventional crude oils, heavy oils are darker in color and may even be

black. The term heavy oil has also been arbitrarily used to describe both the heavy oils that requirethermal stimulation of recovery from the reservoir and the bitumen in bituminous sand (tar sand)

formations from which the heavy bituminous material is recovered by a mining operation.


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BITUMEN (EXTRA HEAVY OIL)

In addition to conventional petroleum and heavy crude oil, there remains an even more viscous

material that offers some relief to the potential shortfalls in supply; such material is the bitumen(extra heavy oil) found in tar sand (oil sand) deposits. It is therefore worth noting here the

occurrence and potential supply of these materials. However, many of these reserves are onlyavailable with some difficulty and optional refinery scenarios will be necessary for conversion ofthese materials to liquid products, because of the substantial differences in character between

conventional petroleum and tar sand bitumen. Tar sands, also variously called oil sands or

bituminous sands, are loose-to-consolidated sandstone or a porous carbonate rock, impregnated

with bitumen, a high boiling asphaltic material with an extremely high viscosity that is immobileunder reservoir conditions.

origin and occurrence

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