5. The Fossil Fuels: Petroleum I - Finding Oil
5.1 Introduction Finding petroleum is a difficult and sometimes dangerous job. It involves a large number of individuals with a wide variety of skills. These include geologists, seismologists, various types of engineers and landmen. All of these individuals play vital but specific roles in the development and ultimate production of a play. This lab conveys just a portion of the complexities involved in this important search.
Figure 1: Drill pipe stacked on an offshore platform ready for drilling (photo by Erin Campbell- Stone).
This lab will investigate the: • different types of petroleum wells • difference between exploration and production • the costs of exploring and producing oil • the importance of understanding geology
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5.2 Petroleum Geology
5.2.1 Petroleum Traps
5.2.1.1 Intro Petroleum is less dense than the other fluids, mainly water, that it occurs with in the subsurface. Consequently, with time, it rises upward. If it does not encounter an impermeable layer, it will rise all the way to the Earth's surface where the lighter fractions will evaporate. This produces the oil seeps and tar pits that were important sources of early petroleum products. To prevent its loss and to form an oil field, petroleum must be trapped before it reaches the surface and allowed to accumulate. This combination of natural geologic conditions is a hydrocarbon trap. Thus, oil and natural gas companies spend considerable time, effort and money looking for the right combination of geologic conditions that in the past may have produced a hydrocarbon trap.
Figure 2: The Richat Structure in the Sahara desert of Mauritania is a prime example of the type of geologic structure petroleum exploration geologist search for in their quest to locate economic hydrocarbon traps. The structure, as known as the Eye of the Sahara, consists of sedimentary layers dipping outward at 10-20o. It is deeply eroded and approximately 40 km in diameter. Source: NASA Earth Observatory (http://earthobservatory.nasa.gov/IOTD/view.php?id=2561).
An accumulation of oil in a trap is a transient feature. Ultimately, the oil will seep through even "impermeable" units and reach the surface. The low permeability of a cap rock
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simply lengthens the time it takes for this to happen. If we are lucky, we find the oil still in the trap. Bacteria also devour oil. Thus, it is not uncommon to find a trap in which the oil is no longer present, but there is evidence that it may have existed in the trap in the past.
5.2.1.2 Components The formation of an oil trap requires a number of different components and their arrangement of these components in a specific vertical sequence in the proper geologic timing. The components of an oil trap include: • a source rock is a geologic unit that accumulated organic compounds that when exposed to increased temperature were converted to oil; • a reservoir rock is the unit that has high permeability and porosity which permits the petroleum to migrate into it and accumulate. This is the unit that an oil company would drill a well into; • a cap or seal rock is the impermeable unit that slows the rise of the petroleum thereby permitting trapping.
Figure 3: Left: Conventional hydrocarbon trap. Right: Stratigraphic sequence without a hydrocarbon trap.
To produce a trap, these units must be arranged with the cap rock at the shallowest depth and the reservoir rock just below it. They must also be arranged so that there is a restricted space in which the petroleum can accumulate. The source rock, i.e. the rock out of which the petroleum migrates, must be below both of these units.
5.2.1.3 Types The spatial arrangements of geologic units that produce traps are quite varied but fall into four broad categories. These are: • structural traps: traps created by tectonic stresses acting on the Earth's crust to produce folds and/or faults, they act after the reservoir beds have been deposited • stratigraphic traps: traps created by changes in lithology or rock type produced during deposition of the reservoir rocks or after deposition • combination: trap formed by a combination of tectonic processes and sedimentary processes. • salt dome traps: cylinder of salt that has risen upwards, penetrating, fracturing and bending pre-existing strata to form multiple traps
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• hydrodynamic: movement of formation water prevents upward migration of hydrocarbons. Each category of trap is the result of a different set of geologic conditions.
Figure 4: The three main classes of conventional hydrocarbon traps.
5.2.1.4 Structural
5.2.1.4.1 Intro Structural traps are the products of folding and faulting. They are produced where the crust has been subjected to stress. Either it has been squeezed (compression) or stretched (tension). If the stress is great enough, the crust breaks forming faults. At lower amounts of stress, it permanently deforms to form folds. Some of the types of structural traps include: anticlines; domes; and faults
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Figure 5: The principal structural hydrocarbon traps
5.2.1.4.2 Domes A dome is a roughly circular or elliptical fold in which strata dip outward and down from a central point with the oldest strata located in the structure’s core. It approximates an overturned bowl. On a geologic map, domes are recognized by concentric, circular and closed outcrop patterns. The strike and dip symbols are oriented with their dip tailings point radially outward from the center of the dome. On the map, the oldest rocks occur in the center of the dome and become younger toward the edge.
Figure 6: Cross-sections and geologic map of a domal hydrocarbon trap.
Domes are ideal hydrocarbon exploration targets because if they contain the proper sequence of rocks, they may form excellent oil and/or natural gas traps. To form a trap, a
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dome must contain a cap unit (impermeable), a reservoir rock and a source rock in this depth sequence. With the rocks in this sequence, the cap rock can capture any ascending hydrocarbons and let them accumulate.
5.2.1.4.3 Anticlines Intro Anticlines are folds with an arch-like structure in which limbs dip away from center of the fold and the oldest strata are found in the core of the structure. If the axis of the anticlines is horizontal, the map pattern produced consists of a series of parallel lithologic stripes. In this pattern, the oldest rocks are exposed along the axis of the fold and they become younger outwards. A key requirement for identifying an anticline is the age relations of the rocks. If the age relations cannot be established with certainty, the proper term to use of the structure is an antiform.
Figure 7: Geologic map (upper right) and cross-section (left and bottom) views of a symmetrical anticline.
Trap
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Anticlines form petroleum traps when there is a cap rock lying above a suitable reservoirs strata and both are above a source for petroleum.
Figure 8: A symmetrical anticline forming a petroleum trap. The axis of the anticline runs N-S on the map with the limbs dipping east and west.
Form The limbs of anticlines can dip in a wide variety of different ways. The orientation and nature of the anticline is important in determining where to drill on the anticline for hydrocarbons.
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Figure 9: Left: A symmetrical anticline. Right: An asymmetrical anticline
Symmetrical Symmetrical anticlines are symmetrical with respect to a vertical axial plane, i.e. the limbs of the anticline dip in opposite directions but at the same angle. As a result, any hydrocarbon fluids are evenly distributed along the center of the anticline. Thus, the prime place to drill on these types of structures that bear oil is along the crest of the anticline.
Figure 10: Cross-section through a symmetrical anticline showing that the optimum place to drill for hydrocarbons is along the crest of the structure.
Asymmetrical Asymmetrical anticlines have limbs that fold dip at the different angles in opposite directions. In addition a plane through the center of the anticline is tilted from the vertical. Because of the difference in dip of the two limbs, hydrocarbon fluids trapped in such structures are not likely to be concentrated along the center of the anticline. Rather, they will be displaced toward the shallower limb side of the anticline. In this case, drilling along the crest of the anticline may actually miss the hydrocarbon accumulation and at the minimum would be displaced to one edge of the reservoir. Thus, drainage would not be optimum and additional development wells might have to be drilled and completed later to effectively produce the reservoir.
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Figure 11: Cross-section through an asymmetrical anticline showing where to drill for hydrocarbons.
Plunging Anticline If the hinge line, i.e. the intersection of the axial plane and the Earth’s surface, is not horizontal, the anticline is plunging. Plunging anticlines can be identified by the V type lithologic patterns they produce on a map. The direction of the plunge of the anticline is indicated by an arrow pointing in the direction of the plunge. If known, the dip of the plunge is noted on the map next to the arrow.
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Figure 12: Illustration of the relation of the hinge line of an anticline and the Earth’s surface. Because the hinge line is not horizontal, this is a plunging anticline with the plunge directed toward the northeast.
Some anticlines have a hinge line that plunges in two directions. These are doubling plunging anticlines and they form closed lithologic patterns on geologic maps. Such structures can make ideal hydrocarbon traps because if a cap unit is present, the structure is closed in all directions to hydrocarbon migration. Thus, the structure can fill from the top downward as hydrocarbons migrate into the structure and are trapped below the cap rock.
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Figure 13: Map and cross-sections a doubling plunging anticline and the location of cap and reservoir rocks.
5.2.1.4.4 Faults Intro When crustal stresses exceed the strength, the rocks undergo brittle fracture, i.e. they break. A fault is a fracture (break) in rocks along which movement has occurred. Faults can be produced by both compressive, tensional (pulling), and shear (sliding) stresses. Because brittle behavior is more common at low temperatures and pressures, faulting is most common in the shallow part of the crust. Faults are important in energy and mineral exploration. In hydrocarbon production, they often mark potential traps for hydrocarbon accumulation. For mineral exploration, faults often are the site of ore veins. How faults are expressed on a structural contour map depends upon what type of fault it is, i.e. oblique or strike slip.
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Figure 14: A fault runs diagonally through the center of this photograph displacing the horizontal sedimentary unit. (Photo by J.E. McClurg).
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Figure 15: Fault trap.
Types Strike-slip faults are characterized by horizontal movement parallel the fault's strike (Figure 31). They also have fault planes that are oriented vertically. In this type of faulting, the blocks are simply moved laterally with respect to each. There is no vertical motion of the blocks. To visualize the type of movement, consider an imaginary plane running through the block of crust impacted by faulting. The fault breaks the plane into two parts that are shifted horizontally with respect to each other. However, the two sections of the plane remain at the same elevation or depth during faulting. Thus, if you were to walk along the plane and across the fault, you would no move up or down as you step from one plane to the other.
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Figure 16: Strike-slip faults experience only horizontal motion along a vertical fault plane.
Dip-slip faults are characterized by vertical motion parallel the dip of the fault plane. There are two classes of dip-slip faults depending upon how the blocks on either side of the fault have moved relative to each other (Figure 34).
Figure 17: Block diagram of a normal dip-slip fault.
Oblique-slip faults have both a vertical and horizontal component of motion along the fault (Figure 35). Thus, adjacent points on different sides of the fault have moved up or down and back or forward relative to each other. They are essentially a combination of strike-slip and dip-slip motion.
Figure 18: Oblique-slip faults have horizontal and vertical motion components.
Orientation attitude of strata compared to dip of fault Trap Closure
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Figure 19: Different types of fault hydrocarbon trap closures.
Roll-over
Figure 20: Cross-section of a roll-over hydrocarbon trap.
Secondary
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Figure 21: Secondary traps associated with faulting.
5.2.1.5 Stratigraphic
5.2.1.5.1 Intro Stratigraphic traps are produced by the processes of sedimentation not faulting or folding. A large number of traps can be produced when sedimentary processes deposit permeable and impermeable units adjacent to each other. These include: • unconformities: traps produced by breaks in the sequence of rocks deposited, break is called an unconformity • reefs; remains of carbonate reefs buried in impermeable surrounding sediments • facies changes (pinch-outs): a wedge of high porosity/permeability rock surrounded by impermeable rock
Figure 22: The principal stratigraphic hydrocarbon traps.
5.2.1.5.2 Unconformities Traps produced by breaks in the sequence of rocks deposited, break is called an unconformity.
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Figure 23: Hydrocarbon trap formed by an unconfonformity.
5.2.1.5.3 Reefs Remains of carbonate reefs buried in impermeable surrounding sediments.
Figure 24: Stratigraphic trap formed by a reef.
5.2.1.5.4 Facies Changes
Figure 25: Stratigraphic trap formed by a facies cange.
5.2.1.6 Salt Domes Another very different but important type of oil trap is formed by salt domes. Rock salt is an evaporative rock produced when sea water evaporates and produces a brine. When the dissolved solids reach about 3 %, the brine begins to precipitate rock salt, which is less dense than most rocks and deforms plastically quite readily. Under the weight of overlying units, rock salt begins to flow and form diapirs (blobs) that rise toward the surface. This motion causes the overlying layers to be bent upward producing domes. It may also induce normal faulting above the rising diapir. Petroleum commonly accumulates along faults above the dome as well as immediately adjacent to the dome itself since rock salt is impermeable.
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Figure 26: The migration of salt forms domes that deform overlying rocks as they rise, thereby producing several potential trap sites.
Salt dome traps are the source of many of the oil fields along the Gulf Coast of Texas and Louisiana.
5.2.1.7 Combination
5.2.1.7.1 Introduction There are several types of traps produced by a combination of geologic processes. The most important of these include: • fold-fault traps; • roll-over anticlines; and • drape-compaction anticlines.
5.2.1.7.2 Fold-Fault Trap
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Figure 27: A fold-fault combination hydrocarbon trap
5.2.1.7.3 Roll-over Anticlines Intro not done Simple
Figure 28: Roll-over hydrocarbon trap.
Secondary Faulting
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Figure 29: Fault hydrocarbon trap with secondary faulting
Nigeria
Figure 30: Cross-section illustrating typical roll-over and secondary fault characteristic of oil fields in the Niger River Delta Region of Nigeria.
5.2.1.7.4 Drape-Compaction Anticlines Intro not done?
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Figure 31: Cross-section show faulted base with a horst. Subsequent sedimentary units have deep deposited over structure. Different compaction of overlying sediments forms a hydrocarbon trap.
Formation not done Ghawar Oil Field, Saudi Arabia
Figure 32: Cross-section through the supergiant Gahwar Oil field of Saudi Arabia. The oil-bearing sediments are draped over a basement horst (middle of section).
Summary not done yet
5.2.1.8 Formation: Hydrocarbon Traps – Terminology
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5.2.1.8.1 Introduction The oil industry uses a wide range of terminology to describe the practical aspects of a hydrocarbon trap. These terms are important because their dimensions and positions within a trap are useful in calculating the amounts of oil and gas in a trap.
Figure 33: The petroleum industry used many terms to describe the geometry and fluids of a trap.
5.2.1.8.2 Accumulation Initially, a trap starts out filled with only water. Slowly, secondary migration fills the trap with hydrocarbons from its highest point downward. As hydrocarbons accumulate in the trap, they displace the heavier water forcing it out of the trap.
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Figure 34: Empty trap
Figure 35: partially full trap
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Figure 36: more full trap
Figure 37: nearly full trap
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full trap
The highest point of the trap is the crest. The point where the oil can exit the trap is the spill point. The position of the spill point determines how much hydrocarbon fluids could be stored in the trap.
5.2.1.8.3 Closure The distance between the crest and the spill point represents the closure of the trap. It is important to note that closure need not be vertically oriented. It depends on the orientation of the trap itself.
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Figure 38: The closure is the distance between the crest and spill point of a trap.
5.2.1.8.4 Fluid Contacts Traps are filled with a variable combination of fluids. Because these fluids are immiscible, there will boundaries, i.e. contacts, between the different fluids. Because it is between two fluids, these contacts are mostly horizontal.
Figure 39: This trap has two contacts because it contains two hydrocarbon fluids.
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Traps containing both gas and oil have two fluid contacts: GOC (gas-oil contact) and OWC (oil-water contact). In these situations, the gas is termed associated gas, i.e. it is associated with oil. When there is only one hydrocarbon in the trap, there is a single contact. For gas-only reservoirs, the fluid contact is GWC (gas-water contact) and the gas is non-associated gas. If it cannot be economically moved to a market, it is also referred to as stranded gas. When no gas is present only oil, the single fluid contact in the reservoir is a OWC.
5.2.1.8.5 Summary The petroleum industry uses a wide variety of terms to describe the physical characteristics of hydrocarbon traps. Some of the most important include: • crest: highest point of the trap • spill point: points where oil/gas can exit the trap • closure: distance between the crest and spill point • fluid contacts o OWC: oil-water contact o GWC: gas-water contact o GOC: gas-oil contact • terms describing gas: o associated gas: gas co-existing with oil in a reservoir o non-associated gas: gas that does not occur with oil o stranded gas: non-associated gas that is spatially isolated from any potential market
Figure 40: The important terms used to describe a trap and the hydrocarbons it contains.
5.2.2 Formation Properties
5.2.2.1 Intro
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When they discuss petroleum traps, petroleum geologists often talk about formations. A formation is a mappable rock unit or layer that can be readily distinguished from other rock layers in the area. It is characterized by a sharp top and bottom and distinctive physical characteristics, e.g. color, mineral content, grain size, etc.
Figure 41: The different color regions on this geologic map represent different geologic formations.
5.2.2.2 Porosity Porosity is a measure of the void space in a rock and is calculated according to the following expression:
It determines the capacity of a rock to store fluid, e.g. petroleum, gas or water. The higher the porosity, the more voids in the rock and the greater the capacity to store fluid. Porosity can be measure using cuttings, cores or logs (neutron density, formation density or sonic- velocity).
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Figure 42: Thin section photomicrograph showing the voids, pores, between sedimentary grains. (Photo by J.E. McClurg)
The ranges for a typical oil reservoir rock are shown in the table below. Because it is compressible, a good natural gas reservoir is commonly characterized by lower porosities. Given their high pressures and the significant compressibility of gas, deep, reservoir rocks with very low porosity can store large, economic volumes of natural gas.
potential porosity (%) insignificant 0-5 poor 5-10 fair 10-15 good 15-20 excellent 20-25
Because it is easy to measure accurately, oil companies will often use a cut-off porosity to determine if a well should be completed or not. In sandstones, the cut-off porosity is typically 8-10 %. Although they are marked by lower porosities, limestones are often fractured making it possible to drain larger areas. Thus, the cut-off porosity for limestones is only 3-5 %.
5.2.2.3 Permeability Permeability is the ability of a rock to transmit a fluid, e.g. water, oil, gas, etc. It is related to the degree to which voids in a rock or soil are interconnected. Rocks with low permeability mean that it is difficult for fluid to flow through the rock thereby making fluid extraction difficult. Permeability is a function of: number of conduits; conduit size; and conduit straightness.
reservoir potential permeability (md) poor 1-20 good 10-100
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excellent 100-10,000
Permeability is measured in darcy (D) or milldarcies (md; 1 D = 1000 mD). A darcy is the permeability that produces a fluid flow (viscosity of 1 centipoise) of 1 cm3/s over a distance of 1 cm through an area of 1 cm2 and experiencing a differential pressure of 1 atmosphere.
5.2.2.4 Reservoirs A geologic formation or group of formations containing abundant quantities of petroleum or natural gas are termed reservoirs. Reservoirs are characterized by high porosity that allows them to store large volumes of fluid. The best reservoirs also have high permeability that permits easy extraction of the trapped hydrocarbons.
Figure 43: Reservoirs are geologic formation(s) that contain significant hydrocarbon fluids.
5.2.2.5 Lithology Only a few types of rocks (lithologies) commonly make good reservoirs. The two most important lithologies that commonly form reservoirs are the sedimentary rocks sandstone and limestone. Sandstones are clastic sedimentary rocks that form when solid sediment, i.e. clasts, is cemented together. Sandstones, because of their typically high porosity and permeability, make very good reservoir rocks. Tight sands refer to sandstone reservoirs in which there is low permeability. Thus, a lot of wells must be drilled into the reservoirs to recover the hydrocarbons.
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Figure 44: A typical sandstone showing its clastic grains.
Limestones are another common lithology that forms hydrocarbon reservoirs. A limestone is a chemical sedimentary rock formed when calcium carbonate, i.e. calcite (CaCO3), is precipitated from seawater. The high porosities needed to make limestones good reservoirs are often produced by fracturing of the rock after formation or by dissolution of some of the rock by circulating groundwater.
Figure 45: A fossiliferous limestone showing abundant fossils in a carbonate matrix.
5.2.3 Formation Fluids
5.2.3.1 Intro Petroleum traps can contain up to three different types of fluids. These are natural gas, petroleum (crude oil) and water. Not all three fluids are present in all traps. Because they are characterized by different density, the three fluids are typically arranged in a specific depth sequence. With increasing depth, the arrangement is gas, oil and water. These three phases are characterized by very different properties from one petroleum reservoir to another.
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5.2.3.2 Water The densest fluid in a reservoir is water. Thus, it occurs below any other fluids present. The water in a formation may be either freshwater or salt water. As the hydrocarbons are pumped from the reservoir, water rises thereby helping to maintain the reservoir pressure. This is crucial in determining how much hydrocarbon is ultimately produced from a well or oil field. If the well is pumped too fast, water may rise quickly in the vicinity of the well producing a cone and introducing water into the well. As a well matures, the percentage of water, the water cut, steadily increases. Ultimately, a point is reached where too much water is produced and too little hydrocarbons. At this point, the well becomes uneconomic and is shut-in and abandoned.
Figure 46: Because it is the densest fluid found in hydrocarbon traps, water always occurs at the deepest depths in a trap.
Regardless of the composition of the water in a reservoir, producers must be careful not to let it contaminate any shallow, freshwater aquifers. This requires casing the well promptly and handling produced water such that it is disposed of properly. This might involve re-injection into the reservoir or disposal away from the site.
5.2.3.3 Natural Gas
5.2.3.3.1 Intro Natural gas consists of hydrocarbons that exist as a gas or vapor at ordinary temperatures and pressures. Typically, these are hydrocarbon molecules with less than six carbon atoms. At reservoir pressures, some of these components may be present as liquid that vaporizes as they are brought to surface pressures.
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Figure 47: A natural gas processing plant in Wyoming (WSGS photo).
5.2.3.3.2 Hydrocarbon Gases Typical hydrocarbon molecules found in natural gas include: methane, ethane, propane, n- butane, isobutane. The abundances of these various molecules vary from one natural gas reservoir to another.
hydrocarbon formula percentage
methane CH4 70-90
ethane C2H6 1-10
propane C3H8 trace-5
butane C4H10 trace-2
pentane C5H12 trace-1
hexane C6H14 trace-0.5
5.2.3.3.3 Other Gases Other non-hydrocarbon gases may also be present in natural gas. These include nitrogen, carbon dioxide, hydrogen sulfide and helium among others. Typically, these cases must be removed before the gas is sold to customers because they lower the heating value of the gas or are dangerous. Some like carbon dioxide (used for enhanced oil recovery) and helium are valuable by themselves.
hydrocarbon formula percentage
nitrogen N2 trace-15
carbon dioxide COs trace-1
hydrogen sulfide H2S trace helium He trace-5
In some instances, the concentrations of these non-hydrocarbon gases can be very high. For example, natural gas produced from the Sleipner Platforms in the North Sea has up to 9 % carbon dioxide. In Wyoming, natural gas produced from the Moxa Arch in the southwest part of the state contains 66 % carbon dioxide, 21 % methane, 7 % nitrogen, 5 % hydrogen sulfide and 0.6 % helium.
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Figure 48: A natural gas processing plant in SW Wyoming.
5.2.3.3.4 Occurrence At reservoir pressures and temperature, natural gas will dissolve in petroleum. However if there is enough gas in the trap, it may saturate the crude oil, that is, the volume of oil in the trap will not be able to hold all the gas in solution. In this case, a free gas cap will sit on top of the oil because it is less dense than the oil.
Figure 49: Gas caps often occupy the shallowest parts of hydrocarbon traps.
5.2.3.3.5 Deposit Types Conventional natural gas reservoirs are classified into two types depending upon whether or not they occur with petroleum. The two types are:
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Figure 50: Cross-section illustrating two geologic settings of conventional natural gas accumulations. Source: EIA.
Because petroleum is much more valuable than natural gas, many non-associated gas deposits were not developed in the past. However increased global demand has lead to increasing development of non-associated gas fields around the world.
5.2.3.4 Oil
5.2.3.4.1 Intro Petroleum is comprised of hydrocarbons, molecules of hydrogen and carbon. There are many types of hydrocarbon molecules each with different amounts of hydrogen and carbon arranged in different structures. A single petroleum will have hundreds if not thousands of different hydrocarbon molecules. The nature of the hydrocarbons present in any given crude oil determines its physical and chemical characteristics.
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Figure 51: Being lighter than water, oil will always occur above formation water in a trap.
Because of the many different hydrocarbon molecules, petroleum varies greatly in physical characteristics. At one end of the petroleum spectrum are low density and viscosity oils that are straw yellow to greenish in color. At the other extreme are oils that are black or brown oils that are so viscous they are more wax-like that fluid. The combination of physical and chemical characteristic of a particular crude oil is what determines its economic value.
5.2.3.4.2 API gravity The density of a particular crude oil is described by its oAPI gravity. The scale uses the specific gravity of the oil measured at 60oF. Specific gravity is the ratio of the weight of a given volume of liquid to the weight of an equal volume of a standard liquid under the same conditions. oAPI gravity varies from 10, heavy oils, to 45, light oils. All other factors being equal, the lighter the oil, i.e. the higher the API number, the more valuable the oil.
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Figure 52: Crude oil APIs and their general characteristics.
5.2.3.4.3 Sulfur Content An important characteristic of any crude oil is its sulfur content. Crude oils are defined based on their sulfur content: • sweet crudes have less than 1 wt % sulfur • sour crudes have greater than 1 wt % sulfur Sulfur produces problems both during refining and combustion. In a refinery, sulfur can damage some of the equipment if it is not removed early during the process. During combustion, sulfur dioxide is released which will produce sulfuric acid when combined with moisture in the atmosphere. Because of these adverse effects, sulfur must be removed from crude oil during the refining process.
5.2.3.4.4 Pour Point Paraffins, long straight hydrocarbon chains, are components of all crude oils. Molecules with more than 18 carbons are waxes and have a significant influence on the physical characteristics of the crude oil. Waxes are liquid at the high temperatures found oil reservoirs. However, as the crude is lifted to the surface it cools and the waxes may solidify. Obviously, solidification of the waxes can have an adverse effect on crude oil handling. If waxes are a significant component of a crude oil, the oil can harden as it rises and clog production equipment thereby forcing a shutdown of the well for workover. In contrast, crude oils with low wax content are less likely to solidify as they are produced. The lowest temperature at which the oil will still pour is the pour point. The range of observed crude oil pour points is 125oF to -75oF (52oC to -60oC). The higher the pour point of an oil, the greater is its wax content. The color of the crude oil as varies with wax content. Low or no wax oils are black whereas very waxy crudes are greenish. A yellow color marks crudes with only small amounts of waxes.
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5.2.3.4.5 Chemistry Because petroleum is composed of hundreds, perhaps thousands of different hydrocarbon molecules, it does not boil at a single temperature. Rather it boils over a range of temperatures. Using this characteristic, crude oil can be fractionated or split into different fractions. This is the basis for the refining of petroleum. Since different crude oils have different combinations of hydrocarbons, they have different fractionation curves and when refined will produce different amounts of gas, gasoline, kerosene, jet fuel and asphalt.
Figure 53: As the different distillation curves for different crude oils clearly show petroleum is comprised of a large number of hydrocarbon molecules.
5.2.3.4.6 Crude Streams A crude stream is a characteristic crude oil that is produced by an oil-producing nation. It may be for export or internal consumption. Crude streams may represent petroleum from a single oil field or a blend produced by mixing one or more crude oils from different oil fields.
pour point crude stream country API S % (oF) Brass River Nigeria 43.0 0.08 -5 Bonny light Nigeria 37.6 0.13 36 Arabian light Saudi Arabia 34.4 1.80 -30 Iranian light Iran 33.5 1.40 -20 Dubai Dubai 32.5 1.68 -5 Kuwait Kuwait 31.2 2.50 0 Ekofisk Norway 35.8 0.18 15 North Sea Brent Great Britain 38 0.3 - North Slope USA 26.8 1.04 -5 Bachequero Venezuela 16.8 2.40 -10 after Hyne, 2001
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5.2.3.5 Fluid Contacts Because it contains multiple fluids, a hydrocarbon trap typically has a number of different types of fluid contacts. The number and types of contacts is determined by the types of fluids found in the trap. A trap with water-oil-gas will have two contacts. From top to bottom (see figure below), these contacts are: • GOC: gas-oil contact; and • OWC: oil-water contact. If a reservoir contains only gas and water, there is only one fluid contact, i.e. the GWC (gas-water contact).
Figure 54: Geologic map and cross-section illustrating the concept fluid contantcts.
Fluid contacts are often shown on cross-sections and maps (see above). They are useful for estimating the amounts of different hydrocarbons a reservoir contains as well as for planning for the production of a particular trap.
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5.3 Petroleum Exploration
5.3.1 Subsurface Maps
5.3.1.1 Intro When looking for hydrocarbons or mineral deposits, geologists often investigate subsurface geologic structures, i.e. structures not exposed at the Earth's surface. They must reconstruct or model these subsurface structures using data from well logs, drill cores, well cuttings and seismic sections. These data may be limited in number and commonly spread overly large geographic areas. To aid them in visualizing subsurface structures and plan exploration and extraction, geologists use three types of subsurface geologic maps: • structural maps • isopach maps • percentage maps
5.3.1.2 Structural
5.3.1.2.1 Intro A structural contour map or structural map is a map that shows the depth to the top of a specific subsurface rock unit. In essence, this type of depicts the geologic structure of the rocks at depth. Superficially, these structural contour maps look similar to topographic maps. That is, they have contour lines connecting points of equal elevation. However, these elevations are of an underground surface, not the Earth's surface. They are characterized by a contour interval and index contours. Just like topographic maps, structural contour maps also do not reveal anything about the geologic units below the mapped surface.
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Figure 55: This portion of a structural contour map shows the information typically conveyed on such maps including topography and geology as well as unit depth. The red lines on the map are the structural contours.
Superficially, these structural contour maps look similar to topographic maps. That is, they have contour lines connecting points of equal elevation. However, these elevations are of an underground surface, not the Earth's surface. They are characterized by a contour interval and index contours. Just like topographic maps, structural contour maps also do not reveal anything about the geologic units below the mapped surface.
5.3.1.2.2 Interpretation A good way of visualizing what a structural contour map is by thinking of a block diagram showing the units at depth (Figure 39a). If we were to draw contours of equal depth on the surface of the geologic unit of interest, we would have a construct similar to that of elevation on surface topography. By simply projecting these contours onto a flat surface, we would have constructed a structural contour map (Figure 39b).
(a) (b)
Figure 56: (a) Block diagram illustrating contouring depth of a geologic unit. (b) Schematic
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structural contour map produced for this unit.
A common geologic feature that is often structurally contoured and shown on geologic maps is a formation. A formation is a mappable, i.e. identifiable and recognizable, rock unit or package of such units. Thus, the unit has a thickness, but only the top surface of the formation is contoured. For oil and gas and groundwater studies, formations of interest are almost exclusively of sedimentary origin. Consequently, a large number of these are tabular in form with planar upper surfaces. Faults may also form planar structures that are depicted on structural contour maps. Structural contours are perhaps easiest to visualize for these types of dipping planar structures (Figure 18). In this case, the structural contours, if they could be drawn on the feature in the subsurface, would appear as a series of straight lines with trends parallel to the strike of the plane of the formation. On a structural contour map, the contour lines will be evenly spaced with a separation distance that is a function of the feature dip. The steeper the dip, the closer spaced the contour lines. As the dip angle decreases, the lines become more spaced out.
Figure 57: The structural contours of a planar structure, e.g. fault, formation, etc., are easier to visualize than for curving or complex surfaces. (Left) The contours on the surface of the structure or formation define a series of straight parallel lines. (Righ
Structural contour maps are interpreted just like topographic maps. The spacing of contours indicate the steepness of the dipping surface, i.e. close spacing contours indicate a steeply dipping surface, whereas widely spaced contours are indicative of gently dipping surfaces (Figure 54). Gradients can be calculated in the same manner as on topographic surfaces.
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Figure 58: Cross-section (top) and geologic map (bottom) illstratiing the impact of dip on structural contour spacing on a structural contour map.
5.3.1.2.3 Homocline Homoclines are simple tilted geologic structures that dip in uniformly in a single direction. On structural contour maps, they are identified by contours that steadily increase or decrease depending upon dip across the map.
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Figure 59: Photograph of a homocline in the middle far ground. Photo by J.E. McClurg.
5.3.1.2.4 Basins/Domes Intro Domes and basins are folded geologic structures that are important in the production of both petroleum and natural gas and, less importantly, some metals. They are formed by compressive, i.e. squeezing, stresses acting on the Earth's crust. A dome consists of folded layers that are concave downward and dip radially outward from a central point (Figure 56). They approximate an overturned bowl. The oldest rocks are exposed in the center of the dome and become younger outward.
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Figure 60: NASA image of a dome in the Sahara.
Dome A dome consists of folded layers that are concave downward and dip radially outward from a central point. They approximate an overturned bowl. The oldest rocks are exposed in the center of the dome and become younger outward.
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Figure 61: Map and cross-sections illustrate age progression of geologic units in a dome.
When exposed at the surface, domes can be identified by roughly circular or elliptical outcrop patterns on geologic maps. These patterns are similar to those formed by basins. The key to distinguishing the two structures on geologic maps is to closely examine the strike and dip symbols of the units. For domes, the symbols all point outward and away from the center of the structure. Basin Folded layers that are concave upward and dip radially toward a central point form a basin (Figure 58). Basins mimic bowls that are right side up. The youngest rocks are exposed in the center of the basin and become older outward. On geologic maps, basins can be identified by roughly circular or elliptical patterns of geologic units. These patterns are similar to those formed by domes. The key to distinguishing the two structures on geologic maps is to closely examine the strike and dip symbols of the units. For basins, the symbols all point inward roughly toward the center of the structure (Figure 58).
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Figure 62: Map and cross-sections illustrating districtubion of geologic ages of formation comprising a basin.
On geologic maps, basins can be identified by roughly circular or elliptical patterns of geologic units. These patterns are similar to those formed by domes. The key to distinguishing the two structures on geologic maps is to closely examine the strike and dip symbols of the units. For basins, the symbols all point inward roughly toward the center of the structure. Structural Contour Expression Basins and domes have perhaps the simplest structural contour patterns to identify. Both have concentric, closed contours. For domes, the structural elevation of the contours increases toward the center of the pattern (they form upraised "hills"). In contrast, the structural contour elevation decreases inward for basins, i.e. they are "depressions".
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Figure 63: Structural contour map showing the contour patterns for a dome (upper right) and a basin (lower left-center). Although both form circular patterns the depth to the unit surface changes between the two.
5.3.1.2.5 Anticlines/Synclines Intro Two very important geologic structures are anticlines and synclines. Both are the result of folding of sequences (packages) of sedimentary rock. They are produced when compressive stresses shorten and thicken the crust. Because the rocks have deformed ductilely or plastically, they are formed at depth in the crust where temperature and pressure is high.
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Figure 64: Sheep Mountain anticline in Wyoming.
Anticline An anticline is a fold in layered strata in which the layers are convex upward and opposing sides dip away from the axis of the fold.
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Figure 65: Geologic map and cross-sections of a symmetrical anticline showing the age progression of the units.
If the axis of the anticlines is horizontal, the map pattern produced consists of a series of parallel stripes. In this pattern, the oldest rocks are exposed along the axis of the fold and they become younger outwards. Plunging Anticline A plunging anticline is an anticline in which the fold axis is not horizontal. Rather it dips at an angle to the horizontal.
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Figure 66: Geologic map and cross-sections of a doubly plunging anticline showing the age progression of units in the geologic structure
The map pattern produced by an isolated plunging anticline consists of an elongated ellipse. In this pattern, the oldest rocks are exposed along the axis of the fold and they become younger outwards. Syncline Synclines, morphologically the opposite of anticlines, consist of folded layers than are concave downward and dip inward toward an axis. Distinguishing the two types of geologic structures is important.
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Figure 67: Geologic map and cross-sections of a syncline showing the age relation of its geologic units.
If the fold axis of a syncline is horizontal, the map pattern produced consists of a series of alternating and repeating stripes parallel the fold axis. In this pattern, the youngest rocks are exposed along the axis of the fold and they become increasingly older as one moves out from the axis. Plunging Syncline A plunging syncline is a syncline in which the fold axis is not horizontal. Rather it dips at an angle to the horizontal.
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Figure 68: Geologic map and cross-sections of a doubly plunging syncline. These types of geologic structures commonly lack potential cap rocks.
The map pattern produced by an isolated plunging syncline consists of an elongated ellipse. In this pattern, the youngest rocks are exposed along the axis of the fold and they become older outwards. Structural Contour Expression Well-developed anticlines and synclines have structural contours that form closed loops like domes and basins. However, the contours are more elliptical in these structures. Anticlines have the shallowest contours in the center, whereas synclines are deepest in the center. Open ended anticlines and synclines form arches and troughs, respectively. Both structures may display crescent-shaped or straight contours depending upon their size relative to the map scale.
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Figure 69: A structural contour map showing an anticline (lower left) and a syncline (upper right) and the boreholes used to deduce the subsurface structure.
5.3.1.2.6 Faults Intro A fault is a fracture (break) in rocks along which movement has occurred. Faults are important in energy and mineral exploration. In hydrocarbon production, they often mark potential traps for hydrocarbon accumulation. For mineral exploration, faults often are the site of ore veins. The manner in which faults are expressed on a structural contour map depends upon what type of fault it is, i.e. oblique or strike slip. Strike-slip Faults Because movement is horizontal, the structural contours are simply displaced along the fault trace. Notice that in this type of faulting the structural contours run right up to the fault trace where they terminate. The behavior of the contours can be illustrated by a strike slip fault trending north-south (Figure 30). The geologic cross-section A-A’ runs across the fault. Because it is a strike-slip fault, all the movement is horizontal without vertical displacement. Thus in the cross-section, the right hand block has shifted into the page whereas the left side has moved out. Because of this motion, the top of the contoured strata (light purple strata) has not shifted vertically on either block. Thus, a contour on either side of the fault trace is shifted the same amount as the displacement on the fault and in the
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same sense as movement along the fault (Figure 30). In addition, the contours on both sides of the fault terminate at the fault trace. Sections B-B’ and C-C’ are oriented parallel the fault trace so do not cut the fault. Although the orientations of the two strata on opposite sides of the fault are the same, they are displaced in the same sense as the fault movement and to the same amount (Figure 30).
Figure 70: A structural contour map (center) of a vertical, strike-slip fault with three accompanying cross-sections, i.e. A-A’ (bottom), B-B’ (left), and C-C’ (right). The fault runs north-south on the right side of west side (left) of the map.
The geologic map of this feature with structural contours is shown in Figure 31, The parallel orientation of the structural contours is a clear indication that the feature contoured is a planar surface. The negative contours indicated the contoured strata lies below the datum plane, i.e. sea level. The increasing negative numbers to the north indicates the strata is dipping to the north.
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Figure 71: The geologic-structural contour maps for the area shown in Figure 30 as it would normally appear without the accompanying text explanations.
Dip-slip Faults Dip-slip faults are characterized by gaps in the contour lines. The shallower the dip of the fault, the wider the contour line gap. If the fault plane is vertical, the gap disappears and contours on either side of the fault terminate against the same line. Because there is no horizontal motion along the fault, there is no horizontal offset evident along a given contour line ().
missing figure
Oblique-slip Faults The fault motion of an oblique-slip fault produces a very distinctive pattern on a structural contour map (Figure 34). For both the hanging and footwall blocks the structural contours will terminate at the fault plane. The down dip motion of the fault serves to lower the surface being contoured on the hanging wall block. As with a purely dip-slip fault, this motion opens a gap between the contours on either side of the fault trace. Thus contours that line up across the fault are marked by a change in depth with the down dropped hanging wall contours having deeper values. The size of the gap between the two sets of contours is a function of the angle of the fault plane. Steeper fault plane angles produce wider separation gaps than shallower ones. Unlike a prue dip-slip fault, the countours are also shifted horizontally.
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Figure 72: A structural contour map (center) of a oblique-slip fault with three accompanying cross-sections, i.e. A-A’ (bottom), B-B’ (left), and C-C’ (right). The fault runs northwest southeast across the map.
On a structural contour map, oblique faults are recognized by offsets in contour lines and a gap in the contour lines (Figure 36).
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Figure 73: A structural contour map of an oblique-slip fault. The greater the vertical and horizontal displacements along the fault, the wider the gap on the structural contour map.
5.3.1.2.7 Construction Intro Structural contour maps are produced in much the same way that topographic maps or bathymetric charts are produced. In all three cases, spatially distributed data points with the depth to the unit of interest are plotted on a base map (either topographic or geologic). The resultant points are then contoured just like for elevation. Just like these other two types of maps, the reference or datum plane for the map is commonly taken as sea level.
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Figure 74: Simple structural contour map of a dome. The negative contour numbers indicate the contoured surface lies below the datum plane, i.e. sea level. The circles with dots identify the boreholes used to draw the map. Because the contour lines get deeper, i.e. more negative, outward from the central contour line, the structure is a dome.
Datum Just like for a surface contour map, the vertical position of a subsurface strata’s top can be determined only after a datum plane has been designated. Once a datum has been defined, the top of the unit contoured is measured relative to that plane. If the surface lies above the datum plane, the structural contours will be positive numbers. Negative structural contour numbers indicate the contoured surface lies below the datum plane. It is not uncommon to encounter structural contour maps in which a given geologic unit has both positive and negative contour values. This simply means the geologic surface crosses the datum plane with the regions with positive numbers lying above it and the regions with negative numbers position below it. One very common datum plane chosen for structural contour mapping is mean sea level. Such a choice commonly produces maps with positive contours in continental interiors and negative ones for crustal regions. Very thick, deep continental basins (20,000-30,000 feet thick), even in a continental will often yield negative contours for the deepest strata.
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Figure 75: Block diagram illustrating the procedure for constructing a structural contour map. In this example, the surface contoured lies below the datum plane, i.e. sea level, so the resultant map would have structural contours with negative numbers.
Boreholes The depth to a particular subsurface geologic unit can be determined from using both direct and indirect means. Indirect means typically involve measuring some geophysical property at the surface and constructing a theoretical model to describe the subsurface geology. One of the most common indirect methods used for determining unit depth is seismic surveying. Obviously, the quality of the subsurface model determines the accuracy of any resultant structural contour map. The direct method of depth determine is to drill boreholes or wells that penetrate the geologic surface or strata of interest. Once a well is drilled, there are several methods to determine the depths of contacts between different strata. One means is collect a core, i.e. solid cylinder of rock, that straddles the contact. Although perhaps the most accurate for position a contact’s subsurface position, coring is very expensive and only done in special cases. The more common method is to use well logs, i.e. records of the measurement of physical properties of the rocks penetrated by the well. The logs are used to mark changes in physical properties that indicate a change in rock type, i.e. lithology. The first step in map construction is to plot the spatial positions of the boreholes with data on a base map (Figure 83). Because wells are drilled primarily by private companies, the data from a particular record may not be in the public domain. A number of states require submitting of logs with the appropriate agencies regulating oil and gas activities in the states.
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Figure 76: Base structural contour map showing the spatial distribution of boreholes or wells that will be used to construct a structural contour map. Notice the positioning and density of the boreholes will determine how well the position of the subsurface unit will be determined.
Picking Tops There are number of factors that make determining the position of ‘tops’ in the subsurface less than straightforward. The first need is to establish what the datum will be for the map. Most common is to select sea level as the datum. With this datum, negative contours indicate the surface lies below sea level whereas positive number denotes a surface above sea level. Because of topographic effects, the starting point for measuring depth in a well varies from one well to another (Figure 84). Typically, depth in a borehole is measured from the kelly bushing, which coincides with the floor of the drilling platform. This position must be used to normalize all well logs to the same datum. Once the well logs are normalized to the same datum plane, depths (for below the datum) or elevations (above the datum plane) are measured to the top of the formation or surface in the well logs. Constructing a structural contour map for a large oil field or mineral deposit may entail picking tops from hundreds of boreholes and reading thousands of feet of well logs.
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Figure 77: Cross-section illustrating the concept of identifying tops. Before determining tops, the depths in the well record must be adjusted for topographic variations in where the depths are measured from. Drilling records will normally record the position of the Kelly bushing, the point from which well depths are measured. This data is then used to adjust depths in individual wells to the same datum plane.
Tops Once tops have been established for all the available wellbores, the depth/elevation to the geologic feature to be contoured are transfer to the base map next to the appropriate well location (Figure 85).
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Figure 78: Base map showing wells with subsurface information. The depths to the top of the formation to be structurally contoured are shown next to each well.
Contouring As with any spatial distributed data, the tops are now contoured by choosing an appropriate contour interval (Figure 86). As with surface topographic, there are number of different methods for contouring the data. Historically, data was contoured by hand, but it is often now done using sophisticated computer algorithms. Different programs will use different extrapolation methods for drawing contours. Regardless of the algorithm used, it is necessary to examine the resulting contour map to ensure it makes geologic sense and is consistent with other types of subsurface information.
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Figure 79: Completed structural contour map. The contour lies have been drawn using the same principles as for contour the Earth’s surface.
Finished Map The final structural contour map is typically overprinted on a surface geology map. To improve readability, it is common to remove the individual top positions thereby leaving on the smoothed structural contours (Figure 87).
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Figure 80: Completed structural contour map with top's depths removed for simplification.
5.3.1.3 Isopach Isopach maps, another type of subsurface contour map, are maps that record the thickness of geologic units, i.e. formations. On these types of maps, the contour lines, i.e. isopachs, represent positions of equal formation thickness with the thickness being measured perpendicular to the bedding. They are particularly useful in showing how a formations swells or shrinks with position and even pinches out. Geologic mapping, mining and underground mapping, drilling and seismic surveys all provide information about formation thickness. When plotted on a base map, these data are contoured like elevation and structural contours. The patterns of contours on isopach maps show thickness variation for a formation or series of formations.
Figure 81: Block diagram (left) illustrating the concept of a isopach map and the resultant map itself (left).
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Isopach maps are important for determining the volume of coal seams or potential hydrocarbon reservoirs. They are also important for planning how to produce particular oil and gas reservoirs. It is important to remember that an isopach map does not tell you how deep the formation might be. This information is found on the structural contour map for the reservoir unit.
Figure 82: Maps showing the thickness of thin sandstone units in the subsurface. These often represent good hydrocarbon reservoirs.
Figure 83: An isopach map showing the varying thicknesses of two subsurface geologic units.
Another type of thickness map is the isochore map, i.e. a map that connects points of equal vertical thickness in a geologic stratum. A constant true thickness unit that has been folded will produce an isopack map that is of constant thickness. However, an isochore map of the same unit will depict a variation in vertical thickness across the unit’s extent. Isopach and isochore maps will produce the same contour patterns only for horizontal geologic strata. To summarize, an isopach map depicts variations in formation thickness whereas an isochore map represents thickness trends.
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5.3.1.4 Percentage A percentage map is subsurface map that plots the amount of a certain type of rock in a formation. They are used extensively in petroleum exploration and production. A typical percentage map might show the amount of sandstone in the formation. Because sandstone makes a good petroleum reservoir rock knowing this type of information is important in estimating the amount of hydrocarbons a formation may contain as well as planning the drilling program to extract these fluids. Conversely, it might depict the amount of shale present in the formation. Shale, an impermeable sedimentary rock, reduces the potential amount of oil or gas and presents a barrier to pumping fluids.
Figure 84: A percentage map shows the amount of a specific rock type in a formation.
A formation is a mappable, i.e. identifiable and recognizable, rock unit or package of such units.
5.3.2 Logging
5.3.2.1 Intro Once an oil or gas well has been drilled and hydrocarbons found, a decision has to be made as to whether to complete the hole or to plug it and abandoned it. This decision revolves around the amount of hydrocarbons that can be extracted from the well. Because completing a well can cost far more than drilling it in the first place, sound information, e.g. porosity, permeability, lithology, etc., about the reservoir rocks that the well has penetrated must be known. The most common way of acquiring such information is by logging the well. Because of its importance, logging has become a major component of the petroleum industry. Companies devoted specifically to logging have developed over the last century. These companies employ sophisticated and complex instruments and hire highly trained specialists to interpret the data produced.
5.3.2.2 Logs In the petroleum industry, a well log is either a: • method of testing the characteristics of a bore hole; • the name of a particular test; or • the record made from a test. Logs, i.e. the records of tests, are used to determine if a well has the potential to produce commercial quantities of petroleum and/or natural gas. Logging provides the information to determine the formations penetrated by the borehole, their thickness and the depths to each. They also provide information on formation temperature, porosity, permeability and the presence/absence of hydrocarbons.
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5.3.2.3 Overview Historically, logging tools were contained in a cylindrical sonde, which was lowered into the borehole on a multi-conductor electric cable (the wireline). At the bottom of the hole, the logging tools are turned on and properties of penetrated rock recorded as the sonde is drawn up the hole to the surface. The sonde is extracted at different rates depending on whether the logging is being done for hydrocarbons or coal.
Figure 85: Schematic diagram illustrating the running of a wireline log.
Today, many logs are also run using miniature, computerized instruments inside the drill string. The instruments are positioned just above the drill bit and use telemetry to transmit data to the surface. These new logging instruments can withstand pressures of 20,000 pounds per square inch (psi) and temperatures up to 400oF.
5.3.2.4 Sonde
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The sonde is the package in which tools are lowered into the borehole to make the different log measurements. It is a cylinder ranging from 27 feet (8 m) to as much as 90 feet (27.5 m) in length depending upon how many tools are screwed together to build it. Tools vary in diameter from 3 inches (8 cm) to 4 inches (10 cm).
Figure 86: Contact (left) and centered (right) sonde positions.
Depending upon the types of logs being run, the sonde is either pressed against the borehole wall by a spring or arm or centered in the borehole by a series of arms or springs. The sonde is lowered and raised in the borehole using an armored steel cable (the wireline). In the center of the cable are the electrical wires that carry the various signals back to a recording truck on the surface. One trip up and down the hole is a run.
5.3.2.5 Record Historically, logs have been printed on paper about eight inches wide with one of two formats. In both types, well information is recorded in the header region of the log near the top. Down the middle of the log is a depth strip with depth (usually in feet) measured from below the kelly bushing (KB), rotary table (RT) or ground level (GL). To the left of the depth track is track 1. To the right of the depth track, there may be one, track 2 (Fig. 1), or two, tracks 2 and 3, tracks (Fig. 2). Each track records one, two or three measurements using lines of different weight or style and labeled. At the top of each track is one or more linear or logarithmic scales depending upon the information displayed in the track.
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Figure 87: (Left) A two-track log format. (Riight) A three-track log format..
Two different scales are used for the depth track: • correlation log: 1 in to 100 ft • detail log: 5 in to 100 ft
5.3.2.6 Hole Condition Some logs can be run only in certain types of holes. This condition leads to two classes of logs: • open-hole logs: run only in holes without casing and just bare rock - most common (Fig. 1) o compensated logs - special type of open-hole log in which the measurements are corrected for irregularities in borehole diameter and roughness • cased-hole logs: can be run in both open and cased holes, i.e. lined with steel pipe cemented to the rock formations (Fig. 2)
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Figure 88: (Left) An open hole typically varies in diameter depending upon the relative strengths of the formations penetrated. (Right) A cased hole is lined with steel pipe which is cemented to the formations intersected.
5.3.2.7 Log Types
5.3.2.7.1 Intro There are over 100 types of logs that can be run on a petroleum or gas well. Some of the properties recorded by logs include: formation resistivity, sonic velocity, density and radioactivity. Formation properties that can be determined from well logs include lithology and porosity. Also the types of fluid (gas, oil, water) that saturate a formation can be deduced. Logging a well is very expensive and can cost as much as a million dollars for a single log. Thus, logs are carefully chosen to collect specific information. Typically, a single log will not provide all the information necessary to bring a well on line. Thus, multiple logs are commonly run on a well. Crudely, logs can be divided into three main classes: • electric logs: logs that use instruments to measure the difference in electrical resistivity between oil, gas, and brine • radioactivity logs: measurement of natural radioactive signal of formations or signal from interaction of artificial radioactive signals and formation fluids or lithologies
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• miscellaneous logs: a host of other types of logs that measure a wide variety of rock, fluid, and borehole characteristics.
5.3.2.7.2 Electric Logs Electric logs are always run in uncased holes filled with water. Although they are easy to run, their interpretation is difficult. As it is drawn up, an electrode on the sonde emits an electric current that runs to a different electrode positioned at the Earth's surface. The current from the source in the sonde radiates outward as a sphere. A third electrode picks up the current after it has flowed through the formations along the borehole. Because the original current had a constant voltage, variations in it measure physical changes in the property of the formation it has passed through. The sensing electrodes can be positioned in different configurations thereby sampling different parts of the formations. This variation in sensor positioning produces four main types of electric logs: • spontaneous potential (or sp): • resistivity: o short normal: 16" between sensors o long normal: 64" between sensors • lateral: 18' 8" between sensors
5.3.2.7.3 Radioactive Logs Radioactive logs measure the interaction of the formations penetrated with radioactivity produced by a radioactive source located in the sonde. There are two main types of radioactive logs: • neutron: • density:
5.3.2.7.4 Miscellaneous Logs In addition to the electric and radioactive logs, there are number of other logs that provide different types of information about penetrated formations or the characteristics of the borehole itself. Four important ones are: • the sonic or acoustic log: • the dip meter log: • the caliper log: • the gamma and spectral gamma:
5.3.2.8 Electric Logs
5.3.2.8.1 Intro Because they would be shorted out by the steel casing, electric logs can only be run in open or uncased holes. The three types of electric logs are: • spontaneous potential (or sp): • resistivity: short normal, long normal & lateral: • conductivity: The sp log is recorded in track 1, resistivity logs (multiple or single) in track 2 and the conductivity log is shown in track 3.
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Figure 89: Typical configuration for the electric logs.
5.3.2.8.2 SP Intro The spontaneous potential (sp) log measures the electrical potential between an electrode fixed on the surface and one in the sonde, which is at depth in the well. The sp log, which is the oldest log, can be used only in uncased (open) boreholes filled with a conductive drilling mud. The response of the sp log indicates the difference in salinity between the drilling mud and the fluid in the formation. During the test, charged ions (Na+ or Cl-) flow from concentrated to dilute solutions. Sonde sonde description Record The sp log is related to formation porosity and is plotted to the left of the depth track with values increasing to the right (see diagram to right). sp values are small, measured in millivolts (0-100 mV) and plotted increasing to the right. A low baseline on the right of the plot indicates low porosity formations and is referred to as the shale baseline. Deflections to the left indicate high porosity units such as limestone or sandstone. An important point to remember about the sp log is that it does not reveal any information about what type of fluid might be in a reservoir formation only whether or not the reservoir has the capacity to store large or small volumes of fluid.
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Figure 90: The sp log is displayed to the left of the well lithology column and with low values (shale baseline) on left and high values on right.
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Interpretation
Figure 91: The sp log is displayed to the left of the well lithology column and with low values (shale baseline) on left and high values on right.
5.3.2.8.3 Resistivity Intro The resistivity log measures the resistance of the formation and its fluids to an electric current. An electrical current passes between two electrodes on the sonde (Fig. 1) and into the adjacent formation and its fluid. By varying the distance between the sonde electrodes (short and long normal), the resistance at different distances from the borehole is measured. Rocks themselves typically have high resistivity. In addition, rocks that are porous and saturated with freshwater, oil and/or gas also have high resistivity and plot on the left of the log. In contrast, shale with salty water or brine has lower resistivity (plotting to the left of the log). Unfortunately, resistivity cannot distinguish between oil and gas formation fluids.
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Figure 92: Electrode configuration in the sonde for making a resistivity log.
Sonde sonde description
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Figure 93: Electrode configuration in the sonde for making a resistivity log.
Long Normal The long normal resistivity tool has the sonde electrodes separated by 64 inches (162.5 cm) apart. The greater separation sends the current that much further into the formation and beyond the invaded zone. Thus, the true resistivity of the formation and its natural fluids are measured.
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Figure 94: Electrode configuration in the sonde for making a resistivity log.
Short Normal Short normal resistivity log have the electrodes in the sonde positioned only 16 inches (40.5 cm) apart. This produces a current that penetrates the formation only a short distance. In this distance often means, the short normal log is measuring the resistance in the invaded zone of the formation. This is the region around the borehole that has been infiltrated by the drilling fluid or the region of the well with drilling mud caked onto the walls. Depending upon the permeability and porosity of the rock, this zone may be 0-100 inches (0-254 cm) thick. Obviously, the resistivity measured may not reflect the primary formation characteristics.
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Figure 95: Electrode configuration in the sonde for making a resistivity log.
Record The resistivity logs are shown to the right of the depth track and increase to the right. They may be both plotted in track 2 or the short normal resistivity is in track 2 and the long normal in track 3.
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Figure 96: (Left) In this log, short and long normal resistivity are both plotted in track 2 to the right of the depth track. (Right) In a three-track log, short normal is plotted in track 2 and long normal in track 3.
Interpretation
Figure 97: In a three-track log, short normal is plotted in track 2 and long normal in track 3
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5.3.2.8.4 Conductivity not done yet
5.3.2.9 Radioactive Logs
5.3.2.9.1 Intro Radioactive logs measure either a formation's natural radioactivity or the radioactivity bounced back by the formation from a radioactive source in the sonde. They can be run in either cased or open holes. The two main radioactivity logs are: • neutron porosity: • density: • neutron-gamma: induced gamma radioactivity resulting from bombardment of rocks by fast neutrons from sonde. High-porosity rocks produced low signals because of capture of neutrons, especially by chlorine • neutron: neutrons or gamma rays produced by neutron bombardment of rocks along borehole, indicates presence of fluid, but cannot distinguish oil and water. With gamma-ray log differentiates porous from non-porous formations. • neutron-neutron: measures neutrons in specific energy ranges, neutrons arriving at detector have been scattered and slow as move through rock, hydrogen nuclei are most efficient at this,
5.3.2.9.2 Neutron Porosity Intro Whereas the gamma-ray and natural gamma-ray logs measure the natural radioactivity along the borehole, the neutron log uses a radioactive source in the sonde to induce the emission of gamma radiation from the formation. The neutrons when they strike hydrogen excite it and cause it to emit gamma-rays. Since hydrogen is abundant in formation fluids, e.g. water, gas, oil, high neutron log values indicate formations with high porosity. Formations with low readings lack porosity. Sonde Commonly, an americium-beryllium source is used to bombard the formations with neutrons. Because neutrons and gamma-rays readily penetrate steel, the neutron log can be run in cased holes.
The radioactive source in the neutron porosity tool bombards formations with neutrons which excite hydrogen causing it to emit gamma radiation. Record The neutron log is displayed to the right of the lithology log with values increasing to the left.
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Figure 98: The neutron log is displayed in track 2 with high values on right and low values on left.
Interpretation
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Figure 99: Interpretation of a neutron porosity log.
5.3.2.9.3 Density Intro The density or formation density log measures the variation in a formation's specific density, which is function of porosity. It emits gamma radiation from the sonde that penetrates the formation (Fig. 1). Some of the radiation is reflected back and is measured by the tool. The amount of radiation returned is related to the electron density in the formation. In turn, the electron density is a function of the bulk density of the formation. Because it both emits and records gamma radiation, this log is sometimes called the gamma-gamma tool. Density logs can only be run in open holes because rock contact with the sonde is necessary. The number of gamma rays bounced back is a function of formation density. Sonde Only gamma rays that entered the formation and were bounced back reach the detector.
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Figure 100: The density log provides a measure of the bulk density of formations.
Record From the density, the porosity of the formations can be calculated. Both tracks are plotted in track 2 to the right of the depth track.
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Figure 101: The density log is displayed in track 2 with high values on left and low values on the right.
Interpretation not done yet
5.3.2.9.4 Gas Effect Most logs do not distinguish between the type of hydrocarbon fluid occupying the pores of a reservoir. To separate gas from oil, the density log and neutron logs must be run simultaneously and the two logs plotted in the same track. The gas effect appears on the two logs when the density log kicks to the left and the neutron log to the right.
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Figure 102: The gas effect allows the differentiation of oil and gas fluids.
5.3.2.10 Miscellaneous Logs
5.3.2.10.1 Intro A variety of other logs provide a range of information about the well itself in addition to the formations it penetrates. Four important miscellaneous logs are: • sonic or acoustic log: • dip meter log: • caliper log: • gamma log: • spectral gamma log:
5.3.2.10.2 Sonic Intro A sonic log (SL) or acoustic velocity log (AVL) measures the sound velocity through individual rock layers to calculate their porosity. Sound velocity is a function of the rock and fluid materials through which it passes (Table 1). Sound velocity is less through a fluid than solid rock. The more porous the rock, the more fluid or gas it can contain. Rocks of high porosity and fluid content will, therefore, have slower sonic velocities.
velocity velocity Δt material (ft/s) (m/s) (Δs/ft) shale 7,000-17,000 2,134-5,182 144-59 sandstone 11,500-16,000 3,505-4,877 87-62 limestone 13,000-18,500 3,962-5,639 77-54 dolomite 15,000-20,000 4,475-6,096 67-50
natural gas 1,500 456 667 water 5,000 1,524 200
Characteristic sound velocities through typical lithologic units and fluids.
Sonic logs are run in open holes filled with fluid. Sonde
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The sonic log sonde has a sound transmitter at the top of the tool and two sound receivers below it. The transmitter sends a sound pulse into the surrounding rock formations. After passing through the rock and any contained fluid the pulse is picked up by the two receivers. The lag time is a measure of the lithology and fluid content of the formations traversed.
Figure 103: Sonde for measuring sound velocities.
Record The sonic log is displayed in either track 2 or 3. It plots the interval transit time in milliseconds per foot. High values are plotted on the left and low ones on the right.
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Figure 104: The sonic log is typically recorded in track 2.
Interpretation Sound travels faster through solids that liquids or gases. With increased porosity, a greater percentage of the rock volume is occupied by fluids so the average sound velocity is less and the interval transit time greater. Thus, formations that plot to the left of the track have higher porosities and larger interval transit times. In contrast, formations with little porosity and hence fluids will have small interval transit times and plot to the right in the track. When the rock composition is known, porosity can be calculated from the sonic log.
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Figure 105: Sonic logs are useful for determining formation porosity and hence identifying potential reservoirs.
Example The sonic log shown to the right has been used to construct a potential lithologic column for the formations penetrated by the borehole. When the sonic log kicks to the left, formation porosity is high. In contrast, low porosity formations are suggested by portions of the log that plot on the right hand side of the log. Notice that the sonic log tells us nothing about the lithology of the formation and its relative porosity.
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Figure 106: Example of the interpretation of a sonic log. In this section, formations 2, 4 and 6 would be potential reservoirs.
5.3.2.10.3 Dipmeter Intro The dipmeter or dip log determines the dip (direction and angle) of geologic formations intercepted relative to the direction of the borehole. A gyroscope is used to determine the orientation of the borehole. Dipmeters are very useful for determining geologic structure in exploratory wells. Dipmeters are run in uncased holes.
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Figure 107: Cross-section illustrating dipping strata intersected by borehole.
Sonde It consists of four arms that remain in contact with the sides of the well. On each arm are two electrodes or electrode pads that measure the resistivity of the rocks they touch. If the formations are horizontal, the resistivity curves from each arm will plot on top of each. However, when the units are dipping, the resistivity curves for the four arms are displaced from each other because the arms contact the same layer at different depths. From these curves, the dips of formations can be calculated.
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Figure 108: Schematic cross-section of dipmeter sonde.
Record Dipmeters logs are represented in one of two ways. A tadpole plot consists of a small circle with a short tail (a). The vertical position of the circle indicates the depth and its horizontal position the dip (horizontal dip is plotted on the left). The small line indicates the direction of the dip, with straight up representing north. stick plot uses sticks (lines) to show dip (b). Where the stick crosses a vertical line in the center of the plot marks the depth and the stick's angle is the dip angle. Unlike tadpole plots, stick plot logs do not indicate the dip direction of the strata logged.
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Figure 109: Two formats used for dip logs. Left: A tadpole version of a dip log. Right: A Stick plot.
Interpretation dip meter
5.3.2.10.4 Caliper Intro The diameter of the borehole is a function of the drill bit size, strength of the formations penetrated and the thickness of the filter cake. Soft rocks, e.g. shales, coal, break off and sluff into the well making it larger than the drill bit (Fig. 1). If fresh-water drilling mud is used, salt layers will dissolve thereby increasing the borehole diameter. In contrast stronger rocks, e.g. limestones, dolomites, well-cemented sandstones, produced holes very near the diameter of the drill bit. The true diameter of the borehole is measured with the caliper log. The size of the hole is important for calculating the amount of cement necessary to case a hole or plug it to abandon it. It is also used to calibrate logs that vary with well width, i.e. compensated logs.
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Figure 110: Cross-section showing variation in borehole diameter
Sonde The caliper tool has four expandable arms that expand when the tool reaches the bottom of the hole. This positions the tool in the center of the borehole. As the tool is raised, the arms flex in and out in response to changes in the hole diameter. These changes cause changes in the resistance to a current thereby generating a record of the hole diameter.
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Figure 111: Schematic cross-section of caliper tool.
Record The caliper log is normally plotted in track 1. It can be represented in one of two ways. The profile log shows profiles for both sides of the well. Thus, it portrays an accurate picture of the actual shape of the well. The hole diameter format simply shows how the diameter of the hole changes with position. Diameter is plotting increasing to the right with the scale normally going from 0 to 36 inches.
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Figure 112: Profile (left) and hole diameter (right) formats of the caliper log.
Interpretation The caliper log serves two main functions: • to calculate the volume of the hole for casing and plugging. It tells how much concrete will be needed for these tasks. • to use with compensated logs. These are logs in which the signal is a function of the hole size. The results from the caliper log allow corrections to the raw signals to account for variations in hole diameter.
5.3.2.10.5 Gamma Intro A gamma-ray log or natural gamma-ray log uses a scintillation counter in the sonde to measure gamma radiation produced by the natural decay of radioactively unstable elements in a rock formation. Naturally-occurring radioactive elements include potassium, uranium and thorium. • Potassium (K) is high in clay minerals which, in turn, are an important constituent of shales (a likely cap rock). • Uranium (U) and thorium (Th) are high in organic matter so oil source rocks have high gamma-ray logs. Unfortunately, the gamma-ray log cannot distinguish between the different types of radioactive elements. Sonde
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The gamma-ray log measures the gamma-ray radiation produced by penetrated strata. These are produced by the natural radioactive decay of potassium (K), uranium (U) and thorium (Th) in the minerals of the formations. Gamma-ray logs can be run in cased wells since the radiation easily passes through the steel of the casing. The gamma-ray reading is affected by the diameter of the borehole so it is commonly run simultaneously with a caliper log which measures borehole diameter. The readings from the caliper log are used to correct the gamma-ray log.
Figure 113: Schematic cross-section of gamma-ray sonde.
Record The gamma-ray log is displayed to the left of the well lithology column (if included) and the depth track. High values (lots of radioactive elements in the formation) are plotted on the right whereas low values (limited radioactivity) are shown on the left.
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Figure 114: Gamma ray log. Interpretation The primary purpose of the gamma-ray log is to identify lithology and the shale content of potential hydrocarbon reservoirs. A signal on the right of the track indicates high concentrations of minerals with radioactive elements. These are typically clays which occur in the important sedimentary rock type shales. Shales typically form excellent cap rocks. In contrast, a signal plotting on the left of the track indicates low radioactive element contents. Typically, more sand or carbonate. This are likely to be sandstones or limestones, both of which are potential hydrocarbon reservoirs. Also note the further to the left the more sand and the further to the right the more clay in the penetrated formations.
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Figure 115: The gamma-ray log.
Example The diagram on the right shows a typical gamma ray log (highly idealized) and the resultant lithologic column interpreted from it. The top of the log shows a region plotting on the right of the log so it has a high gamma signal indicating lots of clay, which is suggestive of a shale formation. There are two similar regions on the log suggesting the presence of two more shale units. Two sections of the log have low signals, i.e. limited clays. These are likely to be sandstones or limestones. Using just the gamma log, it is impossible to distinguish between the two. Other logs would have to be consulted to determine the specific lithologies of these units. Regardless, they are likely to be reservoir units. Again however, we do not know if they contain hydrocarbon fluids or not. Combining the gamma log with others should reveal if hydrocarbons are present. At the base of the log is a unit with high sand and clay content. It is likely a sandstone or limestone with high clay content.
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Figure 116: The gamma-ray log (left) with interpreted lithologic column (right).
5.3.2.10.6 Spectral Gamma The gamma-ray log cannot distinguish which of the radioactive decay series produced the radiation measured. Since the energy of the gamma-ray radiation differs with the parent element, e.g. K, U or Th, determining the specific energy of the gamma-ray radiation can identify the parent element. The spectral gamma-ray log uses a spectrometer to measure the gamma-ray energy spectrum thereby pinpointing the radioactive parent element. Its log is similar to the natural gamma-ray log except it has three lines for each element, but the different units respond in the same way, i.e. high for shale and low for other lithologies.
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Figure 117: Spectral gamma log.
5.3.2.11 Using Logs
5.3.2.11.1 Intro No single log provides all the information necessary to locate and evaluate potential hydrocarbon reservoirs along a borehole. Rather each provides a different piece of information. Some tell about formation characteristics whereas others are useful for detecting and identifying formation fluids. The most successful logging programs rely on combining different logs and using their individual results to paint a composite picture of the subsurface formations and their hydrocarbon potential.
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Figure 118: Schematic representation of a suite of well logs and the information that can be gleaned from each.
5.3.2.11.2 Example 1 In Figure 103, a suite of four logs are used to interpret the geology and fluid distribution along the borehole. The two logs on the right reveal the rock types encountered whereas the fluid distribution can be discerned from the right two logs. The low readings in the middle of the gamma ray (GR) log on the left below indicate the presence of a reservoir(s) unit. The slight dip in the readings suggests, in fact, there are two reservoir units (depicted in the lithologic column on the right). The spontaneous potential (sp) log has a step in the reservoir interval suggesting two different lithologies for the two reservoirs.
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Figure 119: A demonstration of how to interpret logs for determining lithology (i.e. rock type) and fluid distribution.
Turning to the logs on the right, the location of the hydrocarbons in the well can be assessed. The two plateaus in the resistivity (R) log indicate the presence (from top to bottom) of hydrocarbons (gas and/or oil) and water. The divergence in the two logs (dotted vs solid lines) on the porosity log indicates the presence of a gas cap in the hydrocarbon column. This distribution of fluids is also shown on to the right of the lithologic column.
5.3.2.11.3 Example 2 Another example of log interpretation is presented in Figure 88. The gamma ray (GR) log below indicates a single sandstone reservoir near the top of the section (shown on the lithologic column). The single low in spontaneous potential log confirms a single reservoir lithology.
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Figure 120: Second example of how logs can be interpreted.
On the right, the two peaks on the resistivity (R) log suggest this reservoir contains a hydrocarbon fluid column underlain by water (the lower plateau). The absence of a gas effect on the porosity log indicates that gas is not present in the reservoir.
5.3.3 Types of Petroleum Wells
5.3.3.1 Intro Petroleum and gas wells are drilled for a wide variety of reasons. They can be divided into two general classes: • exploration wells: oil wells drilled to find hydrocarbon strata • production wells: oil wells drilled to extract effectively hyrdocarbons from known reservoirs
5.3.3.2 Exploration Wells Exploration wells are drilled to find and evaluate oil deposits. Often they are drilled in regions where the subsurface geology is known only from surface surveys. Consequently, drillers have little knowledge of the conditions they will encounter as they drill the well. For this reason, exploration wells often require bigger rigs and more safety equipment than production wells to allow for a greater margin of safety. Exploration wells include: • wildcat well: wells drilled to find new oil or gas reserves in unproven area far from any existing producing well o rank wildcat well: drilled 3 km (2 mi) from any known production o dry well (or dry hole): wildcat or exploratory well that encounters no significant oil or gas o discovery well: successful wildcat or exploratory well that finds oil or gas in a previously unknown reservoir
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• exploratory well: similar to wildcat well but not as far from existing reserves, less risk • step-out or delineation well: drilled next to a proven well but in an unproven area, also used to determine oil or gas bearing formation limits
5.3.3.3 Production Wells Production wells are drilled to develop an oil field. Consequently, they are drilled in regions of known geology. Because previous wells drilled in the area have provided a large amount of geologic information about the area. The wells can, therefore, be drilled with smaller rigs, which makes them less costly. The types of production wells include: • development well: drilled in proven area • infill well: drilled between established producing wells to reduce spacing between wells and to increase production rate A stripper well is any producing well in the late stages of oil production where the well's output is less than 10 b/d. Production from stripper wells is taxed at a different rate than from other production wells. In the United States, several hundred thousand stripper wells produce a million barrels of oil a day.
5.3.3.4 Well Symbols The location of oil and gas wells are commonly shown on maps. To differentiate between the different types of wells, a system of symbols have been developed. The common well symbols for different types of oil and gas wells are shown below.
Figure 121: Map symbols to indicate different types of oil and gas wells.
5.3.3.5 Base Map A base map shows the locations and types of all wells drilled in an area. Well locations may be plotted on a topographic map or a geologic map.
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Figure 122: Base map showing the types of oil and gas wells drilled on a doubly plunging anticline.
Locating a wellsite and placing it on a base map is spotting a well. In addition to well locations, a base map may show seismic lines or other data.
5.4 Saudi Arabia: Geology and Geography
5.4.1 Physical Geography
5.4.1.1 Intro Occupying four-fifths of the Arabian Peninsula, the Kingdom of Saudi Arabia is the largest country in the Middle East, with the Red Sea and the Gulf of Aqaba to the west and the Persian Gulf to the east. Saudi Arabia contains the world's largest continuous sand desert, the Rub Al-Khali, or Empty Quarter. Its oil region lies primarily in the eastern province along the Persian Gulf.
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Figure 123: Saudi Arabia is the largest country in the Middle East.
5.4.1.2 Geography Neighboring countries are Jordan, Iraq, Kuwait, Qatar, the United Arab Emirates, the Sultanate of Oman, Yemen, and Bahrain, connected to the Saudi mainland by a causeway.
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Figure 124: -
5.4.1.3 Physiography Almost all of Saudi Arabia consists of semi-desert and desert, with oases spotted throughout, rendering over half of the total surface area uninhabitable due to the vast desert. The country's terrain does contain slight variation, but on the whole it is an unbroken expanse of salt flats, gravel plains, and sand dunes with few lakes or permanent streams. The Empty Quarter (in Arabic, Rub al Khali), the largest continuous sand desert in the world, occupies the south of the country. It is linked to another large sandy desert, the Nafud, located in the country's north. The south-west contains a mountainous region with a few peaks rising to over 9,000 feet.
5.4.1.4 Climate Saudi Arabia has a typical desert climate of blistering hot days with cool nights as the heat captured in the sand during the day escapes quickly in the absence of sunlight. It is one of the driest countries in the world with a brief rainy season in the winter months, though the difference in rainfall is slight. All over the country there is very little rain and, for the most part, the climate in the entire country is consistently hot and dry. Given its proximity to the equator and the slight monthly difference in rainfall, the country does not have noticeable seasons.
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Figure 125: Saudi Arabia is one of the driest countries in the world with an average annual rainfall varies between 3 and 11 inches.
Most of the western parts of Saudi Arabia is plateau; the east is lowland, with very hot climates. The southwestern mountains are the area of the greenest and freshest climate in the entire country, but even this is negligible. Average rainfall is 3 inches per year throughout most of the country and 11 inches per year in the wettest region of Asir. Summers can be extremely hot with temperatures rising to 130ºF in some areas while winters have a daytime average temperature of 85ºF.
5.4.1.5 Vegetation Vegetation in the sandy desert is scant though there can be considerable life in the desert, especially after winter rains. Plants such as desert chamomile, scarlet pimpernel, heliotrope and wild iris are common, and the vegetation consists mostly of xerophytic herbs and shrubs. Animal life includes ibex, wildcats, baboons, wolves, and hyenas in the highlands as well as small animals such as lizards, porcupines, hedgehogs and rabbits in the desert. Small birds are found in the oases.
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Figure 126: Vegetation in Saudi Arabia must be adapted to very dry conditions.
5.4.2 Human Geography
5.4.2.1 Intro The Kingdom of Saudi Arabia was founded in its modern form in 1932 by King Ibn Saud (1882–1953). A descendant of Wahhabi leaders, he seized Riyadh in 1901 and set himself up as leader of the Arab nationalist movement. By 1906 he had established Wahhabi dominance in Nejd and conquered Hejaz in 1924–1925, thereby two of the largest regions on the Arabian Peninsula. The Hejaz and Nejd regions were merged to form the kingdom of Saudi Arabia in 1932 under an absolute monarchy. King Saud, a follower of Wahabism (a fundamentalist version of Islam) established an Islamic legal system, Sharia, as the main form of justice throughout his kingdom, Saudi Arabia uses the Quran, the teachings of Mohammad and including significant portions of the Old and New Testaments of the Bible, as its constitution. The system of enforced public and private behavior has changed little since the time King Saud ruled Saudi Arabia.
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Figure 127: -
5.4.2.2 Regions There are thirteen administrative regions in the country. Each is governed by a prince or member of the royal family who is appointed directly by the king.
Figure 128: Saudi Arabian provinces vary greatly in geographic extent.
5.4.2.3 Cities
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Riyadh is the capital city and also has the largest population, 4,700,000 people. Jeddah is the country's main sea port on the Red Sea and has the second highest population at 3,600,000. The two holiest cities of Islam, Mecca (with the tomb of Mohammad) and Medina (the birthplace of Mohammad) are also located in Saudi Arabia, making the kingdom the destination of millions of pilgrims each year. The main sea port on the Persian Gulf is Dammam with a population of 1,300,000.
Figure 129: Saudi Arabian cities.
5.4.2.4 Population In 2008, Saudi Arabia's population was estimated at just over 28 million people (28,161,417). The country's population is relatively young with a median age of only 21.5 years. Life expectancy is 76 years and the country has a growth rate of slightly under 2 % (1.945 %).
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Figure 130: Saudi Arabia's age-sex pyramids: in millions.
Figure 131: Saudi Arabia's age-sex pyramids: by percent.
Although Saudi Arabia's population is small when compared to some of the world's other nations, it has grown considerably in recent years. In 1995, its population was only 19 million. The country's demographic distribution means that a large number of young people will be entering the work force annually. Already unemployment levels in the country are
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high (estimated at 25-30 %). With expansion and diversification of the economic, the lack of jobs may present increasing social and political pressure.
5.4.2.5 Population Density Most of the population was nomadic or semi-nomadic until the 1960s when urbanization began to occur on a large scale. Due to the rapid economic and urban growth following the 1960s, more than 95 percent of the population now is settled. This has had significant consequences for infant and adult mortality, contributing to the rapid population growth of the country. Some cities and oases have population densities of more than 2,600 people per square mile while vast stretches of the sandy desert are uninhabited, making the average population density of 17.69 people per square mile a bit misleading.
Figure 132: The population density in Saudi Arabia varies greatly from that of modern large cities to empty desert spaces.
5.4.3 Cultural Geography
5.4.3.1 Intro The culture of Saudi Arabia has a long, deep-rooted tradition. Ethnic homogeneity and partial isolation in an extreme environment contributed to the formation of the culture and the absence of foreign cultures (leading to acculturation) for millennia. The culture has experienced, however, considerable change in the twentieth century. Limited though inevitable exposure to the West, the religious dominance of Wahhabism, the abundance of expatriate (especially oil) workers, and rapid urbanization have altered the Saudi Arabian cultural landscape.
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Figure 133: -
5.4.3.2 Ethnicity Most Saudis are ethnically Arab, descended from the indigenous tribes who still maintain their tribal affiliation today. There is a small percentage of Suadis who are of mixed ethnic origin, descended from Turks, Iranians, Indonesians, Indians, Africans, and others, most of whom immigrated as pilgrims and reside in the Hijaz region along the Red Sea coast. Saudi Arabia's 2005 population was estimated to be about 27 million, including 5.6 million resident foreigners. Of the foreigners, many are Arabs from nearby countries who are employed in the kingdom. A large number of Asian expatriates, mostly from India, Pakistan, Bangladesh, Indonesia, and the Philippines, are also employed in the kingdom, often in jobs Saudis prefer not to do. There are less than 100,000 Westerners living in the country. The economy is very dependent on foreign labor, though systematic efforts are beginning to be made to make it less dependent. Foreign residents are not able to establish citizenship and their civil rights are limited.
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Figure 134: A foreign worker in Saudi Arabia.
5.4.3.3 Society Saudi Arabian society is characterized by male dominance and religious fundamentalism. The particular form of fundamentalism prevalent in this society, Wahhabism, holds women in a subordinate position to men. They are not allowed to leave the house without a male escort, preferably a close family member, and must wear unrevealing clothing from head to toe whenever they are outside of the home. Unlike most countries where Muslims live, women must be veiled in public. Because of the strict rules governing women's behavior, e. g women cannot drive, they do not make up a large percentage of the workforce. Families and lineage are extremely important in Saudi Arabia. Many households are made up of three generations, with brothers and their families often living in the same domicile with their parents. Sharing and mutual responsibility among family members is paramount, despite the forces of globalization that have caused Saudi men (and some women) to study and work in other parts of the world. There are strict restrictions on the media and the importation of Western entertainment, literature and art, adding to the insularity and traditionalism of Saudi society.
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Figure 135: -
5.4.3.4 Religion The majority of the Saudi population is Muslim, mostly consisting of followers of Wahhabism. Wahhabism does not constitute a separate branch of Islam but is instead an interpretation of Sunni Islam which takes a very fundamentalist view of the Quran and the Sunnas (teachings) of Mohammad, as interpreted by Muhammad Ibn Abdul Wahhab (1703- 1792), a writer and scholar whose influence owes much to the early support of King Saud. The majority of the population adheres to this theology, and the political elites in the monarchy are staunch followers.
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Figure 136: -
The Shia population of the country is estimated at around 15 percent, primarily in the eastern provinces and larger cities where there are large Iranian immigrant populations. Some members of the immigrant population follow other religions, though they are not allowed to practice in the kingdom. The government requires that all citizens of Saudi Arabia be Muslims.
5.4.3.5 Language The main language spoken in Saudi Arabia is Arabic which found its roots on the Arabian Peninsula thousands of years ago. Arabic is a Semitic language spoken and understood in various forms by millions of people throughout the Middle East and beyond. It is used in two forms, both the written and the spoken, which differ greatly from one another. Classical Arabic, the language of the Quran, is used only in writing and rarely in speech. A standardized modern Arabic is used for newspapers, television and conversation, with local variations.
5.4.3.6 Arts The Arabian peninsula has an oral tradition of storytelling and poetry that goes back to pre- Islamic times. Language is greatly respected and the poetic tradition has been cited as one of the factors that helped the spread of Islam, inasmuch as the Quran is written as poetry. Poetry and storytelling are popular folk traditions that continue today. Hand-lettered Qurans, replete with complex geometric and floral designs, are one of the main forms of artisanship in the country. The beauty of mosques also makes apparent the artistic skills valued by the Saudis, with colorful tile, elegant metal work, and a very sophisticated sense of composition, proportion and space. The Arabian Peninsula has an oral tradition of storytelling and poetry that goes back to pre-Islamic times. Language is greatly respected and the poetic tradition has been cited as
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one of the factors that helped the spread of Islam, inasmuch as the Quran is written as poetry. Poetry and storytelling are popular folk traditions that continue today. Hand-lettered Qurans, replete with complex geometric and floral designs, are one of the main forms of artisanship in the country. The beauty of mosques also makes apparent the artistic skills valued by the Saudis, with colorful tile, elegant metal work, and a very sophisticated sense of composition, proportion and space.
Figure 137: -
5.4.3.7 Law Saudi Arabian law is based on the Sharia system of justice. For Sharia law, the Quran and the Sunna of Mohammed are the main statutory materials which are interpreted by judges, and all courts are governed by religious canon (unlike many Islamic countries and Israel where religious law governs only family matters and relations between the men and women). Capital and corporal punishment are used to punish many crimes. Amputation, for example, may be the penalty for robbery; the punishment for adultery can be death by stoning.
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Figure 138: -
Many human rights organizations condemn the Saudi Kingdom for its use of what they see as severe punishments while the Saudi government rejects these complaints as the attempted imposition of Western culture on longstanding Middle Eastern forms of justice. Legal statutes do not apply equally to men and women, nor are they the same for Saudi citizens and foreigners/expatriates.
5.4.4 General Geology
5.4.4.1 Intro not done
5.4.4.2 Plate Boundaries not done
5.4.4.3 Regional Geology
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5.4.4.4 Geologic Hazards
5.4.4.4.1 Intro not done
5.4.4.4.2 Volcanism not done
5.4.4.4.3 Earthquakes not done
5.4.4.4.4 Tsunamis not done
5.4.4.4.5 Landslides not done
5.4.4.4.6 Flooding not done
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