INTRODUCTION6 ORIGIN OF PETROLEUM7 Inorganic Theories7 Deep seated terrestrial hypothesis7 Extraterrestrial hypothesis. 7 Problems with inorganic hypotheses. 8 Generation of crude oil13 Generation of Natural Gas14 CHEMISTRY OF PETROLEUM15 Introduction:15 Hydrocarbons15 Paraffin Series16 Unsaturated Hydrocarbons18 Naphthene Hydrocarbons19 Aromatic Hydrocarbons19 Types of Crude Oils20 Paraffin-based Crude Oils20 Asphaltic Based Crude Oils20 Mixed Base Crude Oils21 Natural Gas21 PETROLEUM GEOLOGY23 The Rock Cycle23 The 3 basic types of rocks. 25 Igneous Rocks25 Texture26
Composition26 Sedimentary Rocks27 Clastic sedimentary rocks:28 Sandstone28 Conglomerate28 Shale29 Clays29 Bentonite30 Chemical sedimentary rocks:30 Organic sedimentary rocks30 Metamorphic Rocks31 The Geological Time Scale31 GEOLOGICAL FEATURES34 Reservoir Rock34 Traps34 Anticline Trap35 Fault trap36 Thrust Fault36 Salt Dome Trap38 Stratigraphic Trap38 PETROLEUM RESERVOIRS40 Reservoir Properties40 Permeability40 Darcy’s Equation for linear incompressible fluid flow41 Porosity and hydraulic conductivity43 Sorting and porosity43 Types of porosity43 Measuring Porosity43
Water Saturation44 Determining Fluids in Place45 PETROLEUM RESERVES DEFINITIONS46 Proved Reserves47 Unproved Reserves48 Probable Reserves48 Possible Reserves49 Reserve Status Categories49 Developed Reserves49 Producing Reserves50 Non-producing Reserves50 Undeveloped Reserves50 SURFACE EXPLORATION METHODS51 Field Reconnaissance51 Aerial surveys51 Surface Geochemical Analysis51 GEOPHYSICAL EXPLORATION52 Seismic Surveys53 Seismic Section53 Seismic data acquisition54 Seismic data processing55 Marine Seismic acquisition56 Seismic records and the synthetic seismogram57
Gravity Surveys62 Magnetic Surveys63 STRUCTURE CONTOUR MAPPING65 Rules for Construction66 Example67 Subsurface Exploration Methods69 Rock Cuttings69 Reservoir Fluid Samples69 Mud Logs69 Cores70 Well Logs71 The Spontaneous Potential (SP) log71 TheResistivity log77 The "Porosity" logs80 Drill Stem Testing86 Appraisal Wells86 Reservoir Development Plan87 Development Wells87 Producing Wells87 Injection Wells88 Reservoir Pressure Control88 Observation Wells89 The Drilling Process90 Rigging up91 Blowout prevention94 Drilling95 Well Completion96 Casing String and Design Factors96
Conductor Pipe98 The Surface String98 Intermediate String98 The Production String98 Production Choke99 Running the casing99 Primary Cementing100 Squeeze Cementing101 Well Completion102 Conventional Single Zone Completion102 Open Hole Completion102 Single Zone Cased Hole Completion102 Conventional Multiple Completion103 Tubingless Completion104 Tubing104 Packers104 Wellheads104 Casing Gun Perforating107 Through tubing perforating107 Tubing Conveyed Perforating108 Production EQUATIONS109 Productivity Index109 Inflow Performance Relationship110 Formation Damage and skin factor110
Flow Efficiency110 Darcy Equation for Radial Flow111 Artificial Lift113 Gas Lift113 Continuous Gas lift114 Intermittent Gas Lift114 Plunger Lift115 Advantages of plunger lift115 Beam Pumping116 Electric Submersible Pump117 Progressive Cavity Pump118 PCP System Applications119 Reservoir Development Practices120 Hydrocarbon Recovery Mechanisms121 Primary Recovery121 Dissolved Gas Drive121 Gas-Cap Drive122 Water Drive122 Secondary Recovery122 Water Flood123 Gas –Cap Injection124 Enhanced Recovery124 Thermal Processes124 Miscible Processes126 Chemical Processes127
Other EOR Processes128 Recovery Efficiencies129 REMEDIAL WELL WORK130 Gravel packing130 Acidising131 Acid Fracturing132 Hydraulic Fracturing132 Processing of Produced Fluids133 Oil Wells133 Oil Well Surface Processing System134 Gas Wells136 Gas Well Surface Processing System136 INTRODUCTION With the current oil prices in the $60US range, the cyclic interest in the petroleum industry has heightened once again. Just a couple years ago, some companies sold oil (heavy crude) at less than $10 US per barrel (bottled water may have fetched a higher price).
As a result some companies switched their focus to natural gas. Crude oil remains a commodity in demand, with alternative sources of energy still lagging way behind. Gasoline and fuel oil still remain prime fuels, resulting in high world demand for crude oil. Petroleum is a non-renewable commodity and the next generation may well experience shortages in supply, with increasing demand, resulting in ridiculously high prices. Through the process of generation, migration and trapping mechanisms, petroleum accumulates in the sub strata, waiting to be discovered by some innovative explorationist.
This “oil of rock”, as the name indicates, is found and produced from formations as shallow as a couple hundred feet to depths as deep at 3 miles beneath the earth’s surface. Technololgies employed range from simple to very complex. Problems experienced in “winning” the petroleum also lie in the same range. The challenge to companies is how to find and produce crude oil and natural gas, in the most cost effective way, in the timeliest fashion, capturing the markets at an opportune time when the prices are attractive. The general trend is to be reactionary to commodity prices.
When the price of oil is down, companies react and scale down their drilling and downsize their operations. When the price is up, they do the opposite. A company can reap the benefits of proper planning by drilling when the price of crude oil is low, and hence services such as rig rental are cheap, resulting in higher production rates when the price rebounds. This course seeks to trace the life petroleum from birth (generation) to the point of sales. Processes include generation, migration, accumulation, exploration, development and production phases.
All of the above require experts who build careers in the various fields. These processes are costly and high risk, but the reward of success can be great, transforming companies, nations and individuals into multi-millionaires in a short space of time. The petroleum industry continues to attract individuals and companies who accept the challenge to take risk, hoping to reap the rewards. At the end of this course, non-technical participants will be able to understand and appreciate the various processes that are involved in the production of petroleum for sale to the customer. ORIGIN OF PETROLEUM
There are two basic schools of thought surrounding the formation of petroleum deep within the earth’s strata. There is the more widely accepted organic theory and the not so popular inorganic theory. Inorganic Theories Deep seated terrestrial hypothesis From as early as 1877, Dmitri Mendele'ev, a Russian who developed the periodic table, postulated an inorganic origin when it became apparent that there were widespread deposits of petroleum throughout the world. He reasoned that metallic carbides deep within Earth reacted with water at high temperatures to form acetylene (C2H2).
This acetylene condensed to form heavier hydrocarbons. This reaction can be easily performed under laboratory conditions. This theory was modified by Berthelot in 1860 and by Mendele'ev in 1902. Their theory was that the mantle of the earth contained iron carbide which would react with percolating water to form methane: FeC2 + 2H2O = CH4 + FeO2 The problem with this theory is the lack of evidence for the existence of iron carbide in the mantle. These theories are referred to as the deep-seated terrestrial hypothesis. Extraterrestrial hypothesis.
In 1890, Sokoloff proposed a cosmic origin for petroleum. His theory was that hydrocarbons precipitated as rain from original nebular matter from which the solar system was formed. The hydrocarbons were then ejected from earth's interior onto surface rocks. Interest in this inorganic theory heightened in the 20th Century as a result of two discoveries: The existence of carbonaceous chondrites (meteorites) and the discovery that atmospheres containing methane exists for some celestial bodies such as Saturn, Titan, Jupiter.
The only known source for methane would be through inorganic reactions. It has been postulated that the original atmosphere of earth contained methane, ammonia, hydrogen and water vapor which could result is the creation of an oily, waxy surface layer that may have been host to a variety of developing prebiotic compounds including the precursors of life as a result of photochemical reactions (due to UV radiation). The discovery (Mueller, 1963) of a type of meteorite called carbonaceous chondrites, also led to a renewed interest in an inorganic mechanism for creating organic compounds.
Chondritic meteorites contain greater than 6% organic matter (not graphite) and traces of various hydrocarbons including amino acids. The chief support of an inorganic origin is that the hydrocarbons methane, ethane, acetylene, and benzene have repeatedly been made from inorganic sources. For example, congealed magma has been found on the Kola Peninsula in Russia (Petersil'ye, 1962) containing gaseous and liquid hydrocarbons (90% methane, traces of ethane, propane, isobutane). Paraffinic hydrocarbons have also been found in other igneous rocks (Evans, Morton, and Cooper, 1964). Problems with inorganic hypotheses.
Firstly, there is no direct evidence that will show whether the source of the organic material in the chondritic meteorites is the result of a truly inorganic origin or was in an original parent material which was organically created. Similar reasoning applies to other celestial bodies. Secondly, there is no field evidence that inorganic processes have occurred in nature, yet there is mounting evidence for an organic origin. And thirdly, there should be large amounts of hydrocarbons emitted from volcanoes, congealed magma, and other igneous rocks if an inorganic origin is the primary methodology for the creation of hydrocarbons.
Gaseous hydrocarbons have been recorded (White and Waring, 1963) emanating from volcanoes, with methane (CH4) the most common. Volumes are generally less than 1%, but as high as 15% have been recorded. But the large pools are absent from igneous rocks. Where commercial accumulations do occur, they are in igneous rocks that have intruded into or are overlain by sedimentary materials; in other words, the hydrocarbons probably formed in the sedimentary sequence and migrated into the igneous material (more on this later when we discuss traps).
Conclusion: There are unquestioned instances of indigenous magmatic oil, but the occurrences are rare and the volumes of accumulated oil (pools) are low. Other problematic issues: Commercial accumulations are restricted to sedimentary basins, petroleum seeps and accumulations are absent from igneous and metamorphic rocks, and gas chromatography can fingerprint the organic matter in shales to that found in the adjacent pool. Thus current theory holds that most petroleum is formed by the thermal maturation of organic matter - An Organic Origin generated the vast reserves (pools) of oil and gas. Organic Theory:
There are a number of compelling reasons that support an organic development hypothesis. First and foremost, is the carbon-hydrogen-organic matter connection. Carbon and Hydrogen are the primary constituents of organic material, both plant and animal. Moreover, carbon, hydrogen, and hydrocarbons are continually produced by the life processes of plants and animals. A major breakthrough occurred when it was discovered that hydrocarbons and related compounds occur in many living organisms and are deposited in the sediments with little or no change. Second were observations dealing with the chemical characteristics of petroleum reservoirs.
Nitrogen and porphyrins (chlorophyll derivatives in plants, blood derivatives in animals) are found in all organic matter; they are also found in many petroleums. Presence of porphyrins also mean that anaerobic conditions must have developed early in the formation process because porphyrins are easily and rapidly oxidized and decompose under aerobic conditions. Additionally, low Oxygen content also implies a reducing environment. Thus there is a high probability that petroleum originates within an anaerobic and reducing environment. Third were observations dealing with the physical characteristics.
Nearly all petroleum occurs in sediments that are primarily of marine origin. Petroleum contained in non-marine sediments probably migrated into these areas from marine source materials located nearby. Furthermore, temperatures in the deeper petroleum reservoirs seldom exceed 300oF (141oC) . But temperatures never exceeded 392oF (200oC) where porphyrins are present because they are destroyed above this temperature. Therefore the origin of petroleum is most likely a low-temperature phenomenon. Finally, time requirements may be less than 1MM years; this is based on more recent oil discoveries in Pliocene sediments.
However, physical conditions on the Earth may have been different in the geologic past and therefore it may have taken considerably more time to develop liquid petroleum. [pic] Figure 1A Organic Hypothesis - Summary. The organic theory became the accepted theory about the turn of the century as the oil and gas industry began to fully develop and geologists were exploring for new deposits. Simply stated, the organic theory holds that the carbon and hydrogen necessary for the formation of oil and gas were derived from early marine life forms living on the Earth during the geologic past -- primarily marine plankton.
Although plankton are microscopic, the ocean contains so many of them that over 95% of living matter in the ocean is plankton. The Sun's energy provides energy for all living things including plankton and other forms of marine life (Fig. 1 ; 1A). As these early life forms died, their remains were captured by the processes of erosion and sedimentation (Fig 2). Successive layers of organic-rich mud and silt covered preceding layers of organic rich sediments and over time created layers on the sea floor rich in the fossil remains of previous life (Fig. 3).
Thermal maturation processes (decay, heat, pressure) slowly converted the organic matter into oil and gas. Add additional geologic time (millions of years) and the organic rich sediments were converted into layers of rocks. Add more geologic time and the layers were deformed, buckled, broken, and uplifted; the liquid petroleum flowed upward through porous rock until it became trapped and could flow no further forming the oil and gas reservoirs that we explore for at present (Fig. 4). But the chemistry of the hydrocarbons found in the end product (oil, gas) differ somewhat from those we find in living things.
Thus changes, transformation, take place between the deposition of the organic remains and the creation of the end product. The basic formula for the creation of petroleum (oil, gas) is: Petroleum End Product = ([Raw Material + Accumulation + Transformation + Migration] + Geologic Time) Petroleum, according to the organic theory, is the product of altered organic material derived from the microscopic plant and animal life, which are carried in great volumes by streams and rivers to lakes or the sea, where they are deposited under deltaic, lacustrine and marine conditions with finely divided clastic sediments.
These environments produce their own microscopic plant and animal life, which are deposited with the organic materials introduced by the streams and rivers. As deposition of the organic material takes place in these environments, burial and protection by clay and silt accompany it. This prevents decomposition of the organic material and allows it to accumulate. Conversion of the organic material is called catagenesis. It is assisted by pressure caused by burial, temperature and thermal alteration and degradation. These factors result from depth, some bacterial action in a closed nonoxidising chemical system, radioactivity and catalysis.
Temperature, as thermogenic activity, appears to be the most important criterion, with assistance other factors as applicable. Accumulation of organic and clastic material on a sea or lake bottom is accompanied by bacterial action. If there is abundant oxygen, aerobic bacteria act upon the organic matter and destroy it. Plant and animal remains contain abundant carbon and hydrogen, which are fundamental elements in petroleum. Shale and some carbonates contain organic material that bears hydrocarbons of types similar to those in petroleum. These rocks are not reservoir rocks and could be considered ultimately to be source beds.
The hydrocarbons are of the same type as those found in living plants and animals and consist of asphalt, kerogen and liquid forms. The best source rocks are considered to be organically rich, black-coloured shales, deposited in a non-oxidising, quiet marine environment. Generation of crude oil Figure 5 – Organic composition in shales Organic material in shale averages approximately one (1) percent of the shale rock volume. Clay mineral constituents comprise the remaining 99 percent. Kerogen is an insoluble, high molecular weight, polymeric compound which comprises about 90 percent of the organic material in shale.
The remaining 10 percent comprises bitumens of varying composition, which, according to some researchers, is thermally altered kerogen. As alteration occurs, kerogen is developed by the increasing temperature in the closed system. Temperature increases with depth. Normal heat flow within the earth’s crust produces an average geothermal gradient of approximately 1. 5 oF for each 100 feet of depth. Maturation studies on various crude oil types indicate that temperatures required to produce oil occur between the depth of approximately 5,000 feet and 20,000 feet under average heat-flow conditions.
Pressure, like temperature, is a function of depth and increases 1 psi for each foot of depth. Pressure is caused by the weight of the sedimentary overburden. Bacterial action is important in the conversion of organic material to petroleum at shallow depths. It is involved in the process of breaking down the original material into hydrocarbon compounds, which eventually become biogenic gas. Kerogen is a primary factor in forming bitumens that increase and migrate to accumulate as crude oil. Thermal conversion of kerogen to bitumen is the important process of crude oil formation.
Thermal alteration increases the carbon content of the migratable hydrocarbons, which leaves the unmigratable kerogen components behind. Maturation of kerogen is a function of increased burial and temperature and is accompanied by chemical changes. As kerogen thermally matures and increases in carbon content, it changes from an immature light greenish-yellow color to an overmature black, which is representative of a higher coal rank. Generation of Natural Gas Natural gas comprises biogenic gas and thermogenic gas with differences contingent upon conditions of origin.
Biogenic gas forms at low temperatures at overburden depths of less than 3,000 feet under anaerobic or conditions associated with high rates of marine sediment accumulation. Oxygen in the sediments is consumed or eliminated early. And before reduction of sulfates in the system. Methane, the most common of natural gas constituents, forms after the sulfates are eliminated by hydrogen reduction of carbon dioxide. Anaerobic oxidation of carbon dioxide produces methane. Current estimates suggest that approximately 20 percent of the world’s known natural gas is biogenic.
Thermogenic gas forms at significantly higher temperatures and overburden pressures. It contains methane and significantly larger amounts of heavier hydrocarbons than biogenic gas. As time and temperature increase, progressively lighter hydrocarbons form as wet gas and condensate in the latter stages of thermogenesis. CHEMISTRY OF PETROLEUM Introduction: The smallest unit of a substance, which still retains the characteristics of that substance, is called a molecule. Molecules can only be divided into atoms - which are different elements.
For example, all molecules of water are identical and have the characteristics of water. Two atoms of hydrogen and an atom of oxygen (which made up the molecule) on their own have none of the characteristics of water. Crude oils are mixtures of many different substances, often difficult to separate, from which various petroleum products are derived, such as: gasoline, kerosene propane, fuel oil, lubricating oil, wax, and asphalt. These substances are mainly compounds of only two elements: carbon (C) and hydrogen (H). They are called, therefore: hydrocarbons.
Refining crude oil involves two kinds of processes to produce the products so essential to modern society. First, there are physical processes which simply refine the crude oil (without altering its molecular structure) into useful products such as lubricating oil or fuel oil. Second, there are chemical or other processes which alter the molecular structure and produce a wide range of products, some of them known by the general term petrochemicals. Hydrocarbons Hydrocarbons may be gaseous, liquid, or solid at normal temperature and pressure, depending on the number and arrangement of the carbon atoms in their molecules.
Those with up to 4 carbon atoms are gaseous; those with 20 or more are solid; those in between are liquid. Crude oils are liquid but may contain gaseous or solid compounds (or both) in solution. The heavier a crude oil (i. e. the more carbon atoms its molecules contain) the closer it is to being a solid and this may be especially noticeable as its temperature cools. Light oils will remain liquid even at very low temperatures. Although hydrocarbons consist of two elements only (carbon and hydrogen), they exist in a wide variety of types and in large numbers. This arises from the ability of carbon atoms to form long chains.
The hydrocarbons may be classified according to their composition (type and number of atoms) and the structure (arrangements of atoms in space) of the molecule. Hydrocarbons are usually classified in the paraffin, unsaturated, naphtene and aromatic types. Paraffin Series This series, also known as alkane series, is characterized by the fact that the carbon atoms are arranged in open chains (not closed rings) and are joined by single bonds. The hydrocarbons of the paraffin type are thus saturated (single bonds only between carbon atoms) and have the general formula CnH2n+2.
The simplest hydrocarbon is methane, a gas consisting of one carbon atom and four hydrogen atoms: [pic] Figure 6 – Molecular structure of methane A carbon atom has four bonds that can unite with either one or more other carbon atoms (a property almost unique to carbon) or with atoms of other elements. A hydrogen atom has only one bond and can never unite with more than one other atom. The larger hydrocarbon molecules have two or more carbon atoms joined to one another as well as to hydrogen atoms. The carbon atoms may link together in a straight chain, a branched chain, or a ring.
The first three members of the paraffin series methane, propane and butane respectively have a single structural formula. Examples include: Propane (C3H8), a straight chain molecule, shown below. [pic] Figure 7 – Molecular structure of propane The remaining members may have two or more structural formulas for the same chemical formula. The phenomenon, known as isomerism, has a strong impact on the thermodynamic properties of the hydrocarbons. An example of a branched chain, Isobutane (C4H10), is shown below: [pic] Figure 7 – Molecular structure of Isobutane
Isobutene has a boiling point of 109 oF while normal butane boils at 31. 1 oF. The members of the paraffin series are very important constituents of crude oil. Some crude oils are largely composed of hydrocarbons of this series while others contain them to a lesser extent. Natural gas consists mainly of the more volatile members of the paraffin series containing from one to four carbon atoms per molecule. The paraffin series are characterized by their chemical inertness. They will not react with concentrated sulphuric or nitric acid at room temperature.
However, when ignited on the presence of air or oxygen, they give off large amounts of heat and under proper conditions, this combustion is explosive. The reaction with oxygen occurs only at elevated temperatures. The inertness of the paraffin hydrocarbons accounts for their presence in petroleum since their existence for geological periods of time would require a high degree of stability. Unsaturated Hydrocarbons The unsaturated hydrocarbons are characterized by the presence of double or triple bonds between the carbon atoms.
The multiple bonds allow the addition of hydrogen atoms, under appropriate conditions, which explains the name unsaturated. The olefin series of hydrocarbons is characterized by the presence of a double bond in the molecule and has the general formula CnH2n. The first three members (n=1…4) of this series, ethene, propene and butene are now commonly referred to using their traditional names ethylene, propylene and butylene. Isomerism occurs also with the olefins, not only due to the branching of the carbon chains, but also to the position of the double bond in the molecule.
Another series of unsaturated hydrocarbons is known as diolefins. They are characterized by the fact that there are two double bonds in the molecule. The general formula for the series is CnH2n-2. A third series of unsaturated hydrocarbons of considerable importance is the acetylene series. The compounds have a triple bond and general formula CnH2n -2 . Hence they are isomers with the diolefins. The first three members of this series (n=2…4) are ethine (commonly called acetylene), propine and butine. [pic] Figure 8 – Molecular structure of Ethine
The unsaturated hydrocarbons are very reactive, in contrast with the members of the paraffin series. They react rapidly with chlorine to form oily liquids ; hence the name olefins (oil forming). Under the proper conditions they react rapidly with hydrogen, which saturates the double bonds and forms the corresponding paraffin. Because of their high reactivity, these unsaturated hydrocarbons are not found in crude oil to any great extent. However, they are formed in large amounts in petroleum cracking processes and have considerable industrial importance. Naphthene Hydrocarbons
The naphthene hydrocarbons are also called cycloparaffins and, as ther name implies, they are saturated hydrocarbons in which the carbon chains form closed rings. The general formula for this series is CnH2n (n greater than 2) and consequently they are isometric with the olefins. They are named by placing the prefix cyclo before the names of the corresponding paraffin hydrocarbon. The first members of this series (n=3…6) are cyclopropane, cyclobutane and cyclohexane, and so on. These compounds, being saturated, are relatively stable and are important constituents of crude oil.
In general, the chemical properties of these hydrocarbons are very similar to those of the paraffins. Aromatic Hydrocarbons These hydrocarbons are also cyclic and may be considered to be derivatives of benzene and have general formula CnH2n-6 (n greater than 5). Benzene has the formula C6H6, and the structure consists of a six-fold ring, with alternate single and double bonds. This structure is so common in organic compounds that chemists use a hexagon with a circle in the middle as a special symbol to represent the benzene molecule.
Some of the simpler members of this series consist of benzene with one or more alkyl groups as side chains. An example, methylbenzene, also known as toluene, is of sufficient importance to warrant a common name. The fact that the benzene ring contains three double bonds suggests that the members of this series should be very reactive. However, this is not so and, although they are not as stable as the paraffins, they do not show the high reactivity that is so characteristic of the olefins. Compounds of this series do occur in crude oil.
Petroleum is one of the important sources of these important hydrocarbons. [pic] Figure 9 – Molecular structure of Aromatics The aromatic hydrocarbons are either liquids or solids under standard conditions of temperature and pressure. Benzene is a colorless liquid with as boiling point of 176oF. Many of the members of this series are characterized by fragrant odors; hencr the name aromatic given to this series. Types of Crude Oils Crude oils vary widely in appearance and viscosity from field to field. They range in colour, odour, and in the properties they contain.
While all crude oils are essentially hydrocarbons, the differences in properties, especially the variations in molecular structure, mean that a crude is more or less easy to produce, pipeline, and refine. The variations may even influence its suitability for certain products and the quality of those products. Crudes are roughly classified into three groups, according to the nature of the hydrocarbons they contain. Paraffin-based Crude Oils These contain higher molecular weight paraffins which are solid at room temperature, but little or no asphaltic (bituminous) matter. They can produce high-grade lubricating oils.
Asphaltic Based Crude Oils Contain large proportions of asphaltic matter, and little or no paraffin. Some are predominantly naphthenes so yield a lubricating oil that is more sensitive to temperature changes than the paraffin-base crudes. Mixed Base Crude Oils The "gray area" between the two types above. Both paraffins and naphthenes are present, as well as aromatic hydrocarbons. Most crudes fit this category. Crude oils usually contain small amounts of combined oxygen, nitrogen and sulphur. Crude oils obtained from various localities have widely different characteristics indicating that the hydrocarbons have different properties.
Nearly all crude oils will give ultimate analyses within the limits shown below: |Element |Content | | |(% in weight) | | | | |Carbon |84 - 87 | |Hydrogen |11 - 14 | |Sulphur |0. 6 - 4. 0 | |Nitrogen |0. 1 - 2. 0 | |Oxygen |0. 1 - 2. 0 | | | | Table 1 – Composition of typical Crude Oil Classification of crude oils based on Gas Oil Ratio: Black Oil ; solution GOR, (Rs) less than 2,000 scf/bbl Volatile oil :solution GOR, (Rs) greater than 2,000 scf/bbl Natural Gas
Natural gas can occur by itself or in conjunction with liquid crude oils . It consists mainly of the more volatile members of the paraffin series containing from one to four carbon atoms per molecule. In addition, natural gases may contain varying amounts of carbon dioxide, nitrogen, hydrogen sulphide, helium and water vapour. Most natural gases consist predominantly of methane, the percentage of which may be as high as 98 percent. Natural gas can be classified as sweet and sour and as wet or dry.
A sour gas is one that contains appreciable amounts of hydrogen sulphide or carbon dioxide, and consequently can be quite corrosive. The designation wet gas has nothing to do with the presence of water vapour but signifies that the gas will yield appreciable quantities of liquid hydrocarbons with proper treatment. Water vapour is, however, often present in natural gas and sometimes causes stoppages in high pressure gas lines during cold weather. This is due to the fact that hydrocarbons form solid hydrates with water at high pressure and low temperature.
Typical Compositions of wet and dry natural gas: |Constituents |Content (% in volume) | | |Wet |Dry | | | | | |Hydrocarbons | | | |Methane |84. 6 |96 | |Ethane |6. 4 |2 | |Propane |5. 3 |0. 6 | |Isobutane |1. |0. 18 | |n-Butane |1. 4 |0. 12 | |Isopentane |0. 4 |0. 14 | |n-Pentane |0. 2 |0. 06 | |Hexanes |0. 4 |0. 01 | |Heptanes |0. 1 |0. 08 | |Non-hydrocarbons | | | |Carbon Dioxide |0. 5 | |Helium |0. 5 | |Hydrogen Sulphide |0. 5 | |Nitrogen |0. 1 | |Argon |0. 005 | |Radon, krypton, xenon |Trace | Table 2 – Composition of typical Natural Gas Classification of natural gas based on Condensate/Gas Ratio: Gas/condensate :gas/condensate ratio greater than 5 stb/million scf Dry gas:gas/condensate ratio less than 5 stb/million scf PETROLEUM GEOLOGY The Rock Cycle
There are four main layers that make up the earth: 1. Inner Core - A mass of iron with a temperature of about 7000 degrees F. Although such temperatures would normally melt iron, immense pressure on it keeps it in a solid form. The inner core is approximately 1,500 miles in diameter. 2. Outer Core - A mass of molten iron about 1,425 miles deep that surrounds the solid inner core. Electrical currents generated from this area produce the earth's magnetic field. 3. Mantle - A rock layer about 1,750 miles thick that reaches about half the distance to the center of the earth. arts of this layer become hot enough to liquify and become slow moving molten rock or magma. 4. Crust - A layer from 4-25 miles thick consisting of sand and rock. The core, mantle and crust of the earth can be envisioned as a giant rock recycling machine. However, the elements that make up rocks are never created or destroyed although they can be redistributed, transforming one rock type to another. The recycling machine works something like this. Liquid (molten) rock material solidifies either at or below the surface of the earth to form igneous rocks .
Uplifting occurs forming mountains made of rock. The exposure of rocks to weathering and erosion at the earth's surface breaks them down into smaller grains producing soil. The grains (soil) are transported by wind, water and gravity and eventually deposited as sediments. This process is referred to as erosion. The sediments are deposited in layers and become compacted and cemented (lithified) forming sedimentary rocks. Variation in temperature, pressure, and/or the chemistry of the rock can cause chemical and/or physical changes in igneous and sedimentary rocks to form metamorphic rocks.
When exposed to higher temperatures, metamorphic rocks (or any other rock type for that matter) may be partially melted resulting in the creation once again of igneous rocks starting the cycle all over again. [pic] Figure 10 – Rock Cycle As you might expect - since most of the earth's surface is covered by water - molten material from inside the earth often breaks through the floor of the ocean and flows from fissures where it is cooled by the water resulting in the formation of igneous rocks. Some low grade metamorphism often occurs during and after the formation of the rock due to the intrusion of the material by the magma.
As the molten material flows from the fissure, it begins forming ridges adjacent to it. If we examine the rock cycle in terms of plate tectonics, as depicted in figure 10 above, we see that igneous rocks form on the sea floor as spreading ridges. As the rocks cool, and more magma is introduced from below, the plate is forced away from the spreading ridge, and acquires a sediment cover. As shown in the figure, in this case, the oceanic plate eventually "dives" under the adjacent continental plate. As the oceanic plate travels deeper, high temperature conditions cause partial melting of the crustal slab.
When that occurs, the surrounding "country rock" (existing adjacent rock) is metamorphosed at high temperature conditions by the contact. The molten material is either driven to the surface as volcanic eruptions, or crystallizes to form plutonic igneous rocks. The 3 basic types of rocks. Just as any person can be put into one of two main categories of human being, all rocks can be put into one of three fundamentally different types of rocks. They are igneous, sedimentary and metamorphic rocks: [pic] Figure 11 – Types of rocks Igneous Rocks
Igneous rocks are crystalline solids, which form directly from the cooling of magma. This is an exothermic process (it loses heat) and involves a phase change from the liquid to the solid state. Each mineral forms a characteristic type of crystal. For example, the well known igneous rock, Granite, is composed of three main minerals, Quartz, Mica and Feldspar, all of which look different and can be clearly seen in a sample. [pic] Figure 12 - The three main minerals in granite Black=Mica, White=Feldspar, Grey =Quartz The size of the crystals is usually determined by the speed at which the molten rock material cools.
Quick cooling produces small crystals, slow cooling produces larger crystals. The earth is made of igneous rock - at least at the surface where our planet is exposed to the coldness of space. Igneous rocks are given names based upon two things: composition (what they are made of) and texture (how big the crystals are). Magmas occur at depth in the crust, and are said to exist in "magma chambers," a rather loose term indicating an area where the temperature is great enough to melt the rock, and the pressure is low enough to allow the material to expand and exist in the liquid state.
Many different types of igneous rocks can be produced. The key factors to use in determining which rock you have are the rock's texture and composition. Texture Texture relates to how large the individual mineral grains are in the final, solid rock. In most cases, the resulting grain size depends on how quickly the magma cooled. In general, the slower the cooling, the larger the crystals in the final rock. Because of this, we assume that coarse grained igneous rocks are "intrusive," in that they cooled at depth in the crust where they were insulated by layers of rock and sediment.
Fine grained rocks are called "extrusive" and are generally produced through volcanic eruptions. Grain size can vary greatly, from extremely coarse grained rocks with crystals the size of your fist, down to glassy material which cooled so quickly that there are no mineral grains at all. Coarse grain varieties (with mineral grains large enough to see without a magnifying glass) are called phaneritic. Granite and gabbro are examples of phaneritic igneous rocks. Fine grained rocks, where the individual grains are too small to see, are called aphanitic. Basalt is an example.
The most common glassy rock is obsidian. Obviously, there are innumerable intermediate stages to confuse the issue. Composition The other factor is composition: the elements in the magma directly affect which minerals are formed when the magma cools. Again, we will describe the extremes, but there are countless intermediate compositions. The composition of igneous magmas is directly related to where the magma is formed. Magmas associated with crustal spreading are generally mafic, and produce basalt if the magma erupts at the surface, or gabbro if the magma never makes it out f the magma chamber. It is important to remember that basalt and gabbro are two different rocks based purely on textural differences - they are compositionally the same. Intermediate and felsic magmas are associated with crustal compression and subduction. In these areas, rock and sediment from the surface is subducted back into the crust, where it re-melts. This allows the differentiation process to continue, and the resulting magma is enriched in the lighter elements. Intermediate magmas produce diorite (intrusive) and andesite (extrusive).
Felsic magmas, the final purified result of the differentiation process, lead to the formation of granite (intrusive) or rhyolite (extrusive). Sedimentary Rocks [pic] Figure 13 – Sedimentary Rock Sedimentary rocks are formed at the surface of the Earth, either in water or on land. They are layered accumulations of sediments-fragments of rocks, minerals, or animal or plant material. Temperatures and pressures are low at the Earth's surface, and sedimentary rocks show this fact by their appearance and the minerals they contain.
Most sedimentary rocks become cemented together by minerals and chemicals or are held together by electrical attraction; some, however, remain loose and unconsolidated. The layers are normally parallel or nearly parallel to the Earth's surface; if they are at high angles to the surface or are twisted or broken, some kind of Earth movement has occurred since the rock was formed. Sedimentary rocks are forming around us all the time. Sand and gravel on beaches or in river bars, look like the sandstone and conglomerate they will become. Compacted and dried mud flats harden into shale.
Scuba divers who have seen mud and shells settling on the floors of lagoons find it easy to understand how sedimentary rocks form. Sedimentary rocks are called secondary, because they are often the result of the accumulation of small pieces broken off of pre-existing rocks. There are three main types of sedimentary rocks: Clastic sedimentary rocks: Clastic sedimentary rocks are accumulations of clasts: little pieces of broken up rock which have piled up and been "lithified" by compaction and cementation. Sandstone [pic] Figure 14 – Sandstone rocks
Sandstone is composed of mineral grains (commonly quartz) cemented together by silica, iron oxide, or calcium carbonate. Sandstones are typically white, gray, brown, or red. The red and brown sandstone is colored by iron oxide impurities. Most sandstones feel gritty, and some are easily crushed (friable) and break up to form sand. Sandstones have pore spaces between each grain of sand; this property, called porosity, makes them good reservoirs for oil and natural gas. Sandstones are very resistant to erosion and form bluffs, cliffs, ridges, rapids, arches, and waterfalls. Conglomerate
Conglomerate is a sedimentary rock usually composed of rounded quartz pebbles, cobbles, and boulders surrounded by a matrix of sand and finer material, and cemented with silica, iron oxide, or calcium carbonate. The rock fragments are rounded from being rolled along a stream bed or a beach during transportation. If the fragments embedded in the matrix are angular instead of rounded, the rock is called a breccia (pronounced BRECH-i-a). [pic] Figure 15 -Conglomerate Rock Shale |[pic] |[pic] |
Figure 16 - Shales Shale is the most abundant of all sedimentary rocks. It is composed primarily of soft clay minerals, but may include variable amounts of organic matter, calcareous material, and quartz grains. Shale may be any color, but is generally greenish gray to grayish black. It is relatively soft and has a smooth, greasy feel when freshly exposed, but is hard and brittle when dry. Most shales split into thin plates or sheets and are termed fissile, but others are massive (nonfissile) and break into irregular blocks. Shales weather very easily to form mud and clay.
Clays The term "clay" is applied to various earthy materials composed dominantly of hydrous aluminum magnesium silicate minerals. The most familiar characteristic of clay is plasticity or the ability of moist clay to be fashioned into a desired shape. The physical properties of a clay are plasticity, strength, and refractoriness. Plasticity enables the clay to be molded; strength permits it to be handled during the forming, drying, and burning processes; and refractoriness permits it to be burned into a hard body of permanent form
Bentonite Bentonite is a soft, low-specific-gravity, expandable clay. It is altered volcanic ash and is found in central Kentucky in beds up to 3 feet thick near the top of the Tyrone Limestone. Drillers have labeled these bentonite beds the Mud Cave and Pencil Cave. Because of its peculiar property of expanding when wet, bentonite is effective as a water sealer, especially to prevent pond leakage, and is also used in rotary drilling muds to prevent contaminating formations with drilling fluid. Chemical sedimentary rocks:
Mny of these form when standing water evaporates, leaving dissolved minerals behind. These are very common in arid lands, where seasonal "playa lakes" occur in closed depressions. Thick deposits of salt and gypsum can form due to repeated flooding and evaporation over long periods of time. Other chemical sedimentary rocks include sedimentary iron ores, evaporites such as rock salt (Halite), and to some extent flint, limestone and chert. Organic sedimentary rocks Any accumulation of sedimentary debris caused by organic processes.
Many animals use calcium for shells, bones, and teeth. These bits of calcium can pile up on the seafloor and accumulate into a thick enough layer to form an "organic" sedimentary rock. These include Limestone, Chalk and Coal. Clues that may help you recognize a sedimentary rock are... • It looks like bits of other rocks stuck together. • It has a gritty feel and bits can be rubbed off it. • It contains fossils, bits of shell or pebbles. • There are no, or very few crystals in it. • All the grains look rounded and worn. Metamorphic Rocks
The metamorphics get their name from "meta" (change) and "morph" (form). Any rock can become a metamorphic rock. All that is required is for the rock to be moved into an environment in which the minerals which make up the rock become unstable and out of equilibrium with the new environmental conditions. The process of metamorphism does not melt the rocks, but instead transforms them into denser, more compact rocks. New minerals are created either by rearrangement of mineral components or by reactions with fluids that enter the rocks.
Some kinds of metamorphic rocks--granite gneiss and biotite schist are two examples--are strongly banded or foliated. (Foliated means the parallel arrangement of certain mineral grains that gives the rock a striped appearance. ) Pressure or temperature can even change previously metamorphosed rocks into new types. In most cases, this involves burial which leads to a rise in temperature and pressure. The metamorphic changes in the minerals always move in a direction designed to restore equilibrium. Common metamorphic rocks include slate, schist, gneiss, and marble. The Geological Time Scale
A sequence of divisions of geological time comprising in order from oldest to youngest: Precambrian, Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Tertiary and Quaternary. Each of the geological periods is characterised by groups, or suites, of fossils. The picture below shows a typical fossil embedded in a rock. [pic] Figure 17 – Fossil embedded in a rock The geological periods are grouped into three major divisions of Phanerozoic time. The block of "ancient life" is dated from some 540 million years before present (the Cambrian) to about 245 million years before present (the Permian).
Fossils such as trilobites, graptolites, early fish and ancestral plants belong to this "Era", known as the Paleozoic. The Paleozoic Era is replaced by the time of "middle life" (the Mesozoic Era), charcterised by dinosaurs and marine organisms such as the great marine reptiles and the ammonites. The Mesozoic Era commenced with the Triassic Period (starting about 245 million years ago) and concluded with the Cretaceous Period (66. 4 million years ago). The last block of geological time is the Cenozoic Era with two geological periods, the Tertiary and the Quaternary.
This era is characterised by widespead evolution of the mammals, and concludes with the appearance of modern Homo sapiens (our own species), in late Quaternary time. We are living in the Quaternary Period. |EON |ERA |PERIOD | |P |Cenozoic Era |Quaternary Period | |h |"Age of Mammals" |"The Age of Man" | |a | |1. mya to today | |n |65 mya through today | | |e | | | |r | | | |o | | | |z | | | |o | | | |i | | | |c | | | | | | | |E | | | |o | | | |n | | | | | | | |"Visible Life" | | | | | | | |Organisms with skeletons or hard shells. | | | | | | | |540 mya through today | | | | | |Tertiary Period |Neogene | | | |65 to 1. 8 mya |24-1. mya | | | | |Paleogene | | | | |65-24 mya | | |Mesozoic Era |Cretaceous Period | | | |146 to 65 mya | | |"Age of Reptiles" | | | | | | | |248 to 65 mya | | | | |Jurassic Period | | | |208 to 146 mya | | | |Triassic Period | | | |248 to 208 mya | | |Paleozoic Era |Permian Period | | |540 to 248 mya |"Age of Amphibians" | | | |280 to 248 mya | | | |Carboniferous |Pennsylvanian Period | | | |360 to 280 mya |325 to 280 mya | | | | |Mississippian Period | | | | |360 to 325 mya | | | |Devonian Period,"The Age of
Fishes" | | | |408 to 360 mya | | | |Silurian Period, 438 to 408 mya | | | |Ordovician Period, 505 to 438 mya | | | |Cambrian Period, 540 to 500 mya | |Proterozoic Eon |- |Vendian Period, 600 to 540 mya | |2. billion years ago to | | | |540 mya | | | | | |- | |Archeozoic Eon |- |- | |(Archean) | | | |3. 9 to 2. 5 billion years ago | | | |Hadean Eon |- |- | |4. 6 to 3. 9 billion years ago | | | GEOLOGICAL FEATURES There are three geological features that need to be present before oil may be present underground: source rock, reservoir rock, and geological traps.
The source rock is where the oil was formed (if you accept the organic theory), but because it is relatively non-porous, it cannot hold oil in appreciable amounts. Instead, the oil migrates to more porous rock like sandstone or limestone. These are examples of reservoir rock. It is possible for the oil to move through the reservoir rock all the way to the surface of the earth. However, this rarely happens because its progress is blocked by some impermeable rock barrier. This causes the oil to accumulate to form a reservoir. The barrier and the resulting reservoir form what is known as a trap. [pic] Figure 18 – Typical Traps Reservoir Rock The oil that migrates through the reservoir rock is not pure oil.
Rather it is a mixture of oil, water, and natural gas. When the reservoir forms, the three components will separate, with the gas at the top, the oil in the middle, and the water at the bottom. Depending on the pressure in the reservoir, the gas may stay in solution. If the gas does form a separate layer at the top, it is referred to as the gas cap. It is important to note that the oil/water/gas mixture does not form a large pool of liquid as some people often envision; it is actually dispersed throughout the reservoir rock. Traps There are two basic kinds of traps: structural and stratigraphic. Structural traps are the result of deformations of the rock layer.
Examples of structural traps are anticlines and fault traps. The fault trap is associated with the shifting of fault layers along a fault line, something that we are familiar with as the cause of earthquakes if the shifting motion is strong enough. Stratigraphic traps form when reservoir rock is cut off by a horizontal layer of impermeable rock. The figure above shows oil pooling in the two different types of structural traps. The dome-like structure on the right is an anticline, while the structure on the left is a trap formed along a fault. There are three basic forms of a structural trap in petroleum geology: • Anticline trap • Fault Trap • Salt Dome Trap
The common link between these three is simple: some part of the earth has moved in the past, creating an impedence to oil flow. Anticline Trap An anticline is an example of rocks which were previously flat, but have been bent into an arch. Oil that finds its way into a reservoir rock that has been bent into an arch will flow to the crest of the arch, and get stuck (provided, of course, that there is an impermeable trap rock above the arch to seal the oil in place). [pic] Figure 19 A cross section of the Earth showing typical Anticline Traps. Reseroir rock that isn't completely filled with oil also contains large amounts of salt water. Figure 20 – Outcrop Anticline Fault trap
Fault traps are formed by movement of rock along a fault line. In some cases, the reservoir rock has moved opposite a layer of impermeable rock. The impermeable rock thus prevents the oil from escaping. In other cases, the fault itself can be a very effective trap. Clays within the fault zone are smeared as the layers of rock slip past one another. This is known as fault gouge. [pic] Figure 21 A cross section of rock showing a fault trap - in this case, an example of gouge. This is because the reservoir rock on both sides of the fault would be connected, if not for the fault seperating the two. In this example, it is the fault itself that is trapping the oil. Thrust Fault
Thrust faulting occurs when one section of the Earth is pushed up and over another section, and they most often occur in areas where two continental plates are running into one another. However, the photos below show sediments that were deposited by glaciers only 10,000 years ago, and these sediments were then run over by a glacial readvance. When the glacier moved back over the sediments, faulting occured. The faults below can be clearly seen. [pic] Figure 22 – Outcrop Thrust Faults Below you can see the faults and rock horizons drawn in If the conditions were right, oil might become trapped in this rock. [pic] Figure 23 – Interpretation of Figure 22
Also drawn in is the possibility of oil being trapped by the shale above it, as well as by the fault and the shale to the left of it. Of course, this outcrop is only a couple of meters wide, there really is no oil here, and the layers that we've assigned to the rock are mostly imaginary in this case. But the point is, this is exactly how many structural traps are set up below the Earth's surface. Salt Dome Trap Salt is a peculiar substance. If you put enough heat and pressure on it, the salt will slowly flow, much like a glacier that slowly but continually moves downhill. Unlike glaciers, salt which is buried kilometers below the surface of the Earth can move upward until it breaks through to the Earth's surface, where it is then dissolved by ground- and rain-water.
To get all the way to the Earth's surface, salt has to push aside and break through many layers of rock in its path. This is what ultimately will create the oil trap. [pic] Figure 24 Here we see salt that has moved up through the Earth, punching through and bending rock along the way. Oil can come to rest right up against the salt, which makes salt an effective trap. However, many times, the salt chemically changes the rocks next to it in such a way that oil will no longer seep into them. In a sense, it destroys the porosity of a reservoir rock. Stratigraphic Trap A stratigraphic trap accumulates oil due to changes of rock character rather than faulting or folding of the rock.
The term "stratigraphy" basically means "the study of the rocks and their variations". One thing stratigraphy has shown us is that many layers of rock change, sometimes over short distances, even within the same rock layer. As an example, it is possible that a layer of rock which is a sandstone at one location is a siltstone or a shale at another location. In between, the rock grades between the two rock types. From the section on reservoir rocks, we learned that sandstones make good reservoirs because of the many pore spaces contained within. On the other hand, shales, made up of clay particles, do not make good reservoirs, because they do not contain large pore spaces.
Therefore, if oil migrates into a sandstone, it will flow along this rock layer until it hits the low-porosity shale, thus forming a stratigraphic trap. Figure 25- An example of a stratigraphic trap [pic] The above series of diagrams is an attempt to illustrate a type of stratigraphic trap. In the diagram at the upper left, we see a river that is meandering. As it does so, it deposits sand along its bank. Further away from the river is the floodplain, where broad layers of mud are deposited during a flood. Though they seem fairly constant, rivers actually change course frequently, eventually moving to new locations. Sometimes these new locations are miles away from their former path. In the diagram at the upper right, we show what happens when a river changes its course.
The sand bars that were deposited earlier are now covered by the mud of the new floodplain. These lenses of sand, when looked at from the side many years later (the bottom diagram), become cut off from each other, and are surrounded by the mud of the river's floodplain - which will eventually turn to shale. This makes for a perfect stratigraphic trap. PETROLEUM RESERVOIRS The term reservoir implies storage. Reservoir rock, therefore, is that rock in which the hydrocarbon can be stored and from which it can be produced. The fluids of the subsurface migrate according to density with the dominant fluids in hydrocarbon regions being hydrocarbon gas, hydrocarbon liquids and salt water.
Since the hydrocarbons are the less dense of these fluids, they will tend to migrate upward, displacing the heavier salt water down elevation. Hydrocarbons may be forced from their source rock during lithification, and migrate into the reservoir rock in which they are stored. The fluids present will separate according to density as migration occurs. Reservoir Properties The key properties for describing a petroleum reservoir are porosity, pore saturation, and permeability. Definitions of these terms are as follows. Porosity refers to the capacity of the reservoir to hold fluids. It is basically