Origin And Occurrence 6w1c6t

Origin and Occurrence INTRODUCTION Petroleum (also called crude oil) is a mixture of gaseous, liquid, and solid hydrocarbon compounds. 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 hydrocarbon mixture 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, the average 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 oil feedstock to stabilize. Be that as it may, the nature of crude oil refining has changed considerably.

ORIGIN There 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 of petroleum, 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 were produced from the acetylene. Initially, inorganic carbides were formed by the action of alkali metals 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 methods have 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 earth’s crust with coal being the precursor to crude oil or both fossil fuels are formed from the same precursors at the

same time but are the result of divergent paths. Chemical and geological investigations do not this concept and the idea is considered obsolete.

BIOGENIC ORIGIN Petroleum is a naturally occurring hydrocarbon mixture but hydrocarbons that are synthesized by living organisms usually 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 action and 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 this organic matter, mixed with mud, is buried under thick sedimentary layers of material. The resulting high 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 trapped underground 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 eons in 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 this transformation and the mechanism by which petroleum is expelled from the source sediment and accumulates 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 in the 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 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. 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 three hundred 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.

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 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 on 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 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. 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 reservoir frameworks. 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 available reserves. 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

approximately 1 trillion be 2.6%.

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

In addition, the crude oils available today to the refinery are quite different in composition and properties 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 would have been classified as heavy feedstock, bearing in mind that they may not approach the definitions 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 economical form 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 earth’s 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 the impervious 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 been applied 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 is natural 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 pumping operations 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, and primary 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 an API 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 require thermal 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.

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 only available with some difficulty and optional refinery scenarios will be necessary for conversion of these 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 immobile under reservoir conditions.