02 Basic Background Geology

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MUSTAGH RESOURCES LTD. - NORM COOPERpage 2-1

Chapter2

Basic Background Geology

LIST OF FIGURES......................................................................................................................... 2-2

2. BASIC BACKGROUND GEOLOGY ................................................................................ 2-3

A. COMPOSITION OF THE EARTH AND CRUSTAL ROCK TYPES ............................................. 2-3

B. SEDIMENTARY BASINS THE LAYER CAKE MODEL ....................................................... 2-16C. SOME BASIC STRUCTURES IN A SEDIMENTARY BASIN .................................................... 2-18D. FLUID MIGRATION AND HYDROCARBON TRAPS ............................................................. 2-20E. TYPICAL HYDROCARBON TRAP TYPES ............................................................................ 2-26


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2. BASIC BACKGROUND GEOLOGY


Chapter 2

Basic Background Geology

List of Figures

Figure 2-1 Spreading centers and trenches...........................................................................................2-4Figure 2-2 Spreading centers and trenches- detail. ..............................................................................2-4Figure 2-3 A simplified cross section of the earth. ................................................................................2-5Figure 2-4 USGS cross section of the earth. .........................................................................................2-6Figure 2-5 A more detailed look at a spreading center and two continental margins. .........................2-7Figure 2-6 USGS continental margins. ................................................................................................. 2-8Figure 2-7 Geologic Time Scale. ...........................................................................................................2-9Figure 2-8 Our earth, 10 million years ago ........................................................................................ 2-11

Figure 2-9 Our earth, 100 million years ago. ..................................................................................... 2-11Figure 2-10 Our earth, 200 million years ago. ................................................................................... 2-12Figure 2-11 Our earth, 300 million years ago. ................................................................................... 2-12Figure 2-12 Our earth, 400 million years ago. ................................................................................... 2-13Figure 2-13 Our earth, 500 million years ago. ................................................................................... 2-13Figure 2-14 Our earth, 600 million years ago. ................................................................................... 2-14Figure 2-15 Our earth, 700 million years ago. ................................................................................... 2-14Figure 2-16 Spreading centers and trenches....................................................................................... 2-15Figure 2-17 Sea floor spreading..........................................................................................................2-15Figure 2-18 A simple schematic cross-section through Alberta.......................................................... 2-18Figure 2-19 A simplified depiction of several common types of faults. ............................................... 2-19Figure 2-20 Molecular structures........................................................................................................2-22Figure 2-21 Steam Assisted Gravity Drainage.................................................................................... 2-22

Figure 2-22 Porosity versus Permeablility..........................................................................................2-23Figure 2-23 Multi-phase fluid dynamics.............................................................................................. 2-25Figure 2-24 Some typical hydrocarbon traps. ..................................................................................... 2-26Figure 2-25 A model of a normal fault with a typical seismic expression........................................... 2-27Figure 2-26 Braided stream and deposition sequence. .......................................................................2-28Figure 2-27 Meandering stream and deposition sequence.................................................................. 2-28Figure 2-28 Basal cretaceous fluvial system. ...................................................................................... 2-29Figure 2-29 Flooding and regression of the Albian Sea. .................................................................... 2-29Figure 2-30 Model of a Lateral Sand Bar. .......................................................................................... 2-30Figure 2-31 Schematic cross-section of Little Bow Area.....................................................................2-30Figure 2-32 Cross-section through channel of interest and its seismic section. ................................. 2-31Figure 2-33 Shore margin deposits. .................................................................................................... 2-32Figure 2-34 3D data volume horizon slice. ......................................................................................... 2-32


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2. Basic Background Geology(or why Geologists have rocks in their heads)

This chapter provides a very brief description of some of thegeological background necessary to understand the purpose of exploration

geophysics as well as a few of the real-world problems we should anticipate.A simple overview of earth structure is included in order to put the field ofexploration geology and geophysics in proper perspective.

A. Composit ion of the Ear th and Crustal Rock Types

Figure 2-1 depicts some of the major elements of earthstructure which have, over long periods of time, influenced the arrangement ofpotential oil and gas bearing areas. The earth is a relatively young planet

(generally accepted as some 4,500,000,000 years old - four and a half billionyears in North America, four and a half milliard years in Europe). It wasoriginally formed from a condensing ball of vapors, gases and cosmic dust.Gravitational forces pulling on all this matter formed a hot ball of moltenmatter. As the resulting mass began to cool, thin crusts of solid rocks beganto form in patches on the surface. Kinetic forces and gravitational fieldsacting on the rotating mass along with patterns of heat flow resulted inconvection currents. These convection cells circulate the deeper, hotter fluidsto the surface and carry some of the cooled material back down into themolten depths to be re-melted. Eventually, plates of cooled rock prevailed onthe surface and are still moved about by the continuing flow of underlyingmolten rock in these convection cells. This process in known as platetectonics.

The cooling process can be likened to a pot of hot fudge. Hot,molten chocolate circulates vertically within the pot as hotter fudge rises tothe surface and cools. The currents can be witnessed when small bits of solidchocolate are dropped into the pot. As the fudge cools, plates of cooled,solid material float about the surface, occasionally being pulled under by thecurrents. As these bits are pulled under, they are re-melted and some heat isgiven up by the surrounding molten mass. Eventually, the circulating andcooling process causes the entire mass to gel and solidify as heat is given upto the surrounding atmosphere. Earths cooling process is extended by thefact that its atmosphere is contained by gravity and because it has its owninternal heat source (radiation). However, earth will eventually cool, itsplates will stop moving and this very dynamic planet will die.


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Figure 2-1

Spreading centers and trenches.

Reference: geoanalytic.com

Figure 2-2

Spreading centers and trenches- detail.

Reference: geoanalytic.com


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Figure 2-3

A simplified cross section of the earth.

The earth's crust is proportionately thinner than the shell on an egg. It is fractured into many "plates"

which float on convection currents which flow within the thick molten "Mantle". At spreading centers, the

plates are driven apart by an up-welling of molten rock. Where plates collide, one plate will be forcedback down into the mantle to be melted again. These areas are known as subduction zones.


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Figure 2-4

USGS cross section of the earth.

Reference: http://pubs.usgs.gov/publications/text/dynamic.html

Our earth is still in the early stages of this cooling process. Athin skin of overlapping plates of rock floats on circulating convection cells.Some heavy ores (believed to be rich in iron and nickel) have congealed into asolid core at the center of the earth. The outer skin is made up of continentalplates (relatively buoyant) and oceanic plates (relatively dense). This outerskin is thinner relative to the earth than the shell of an egg is relative to thewhole egg. This shell is called the crust (or lithosphere). The center of theearth is separated into an inner core (plasma or solid) and an outer core

(liquid). The remaining volume is molten rock and is known as the mantle(or mesosphere). The mantle behaves as a liquid or plastic and flows inconvection cells.


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Figure 2-5

A more detailed look at a spreading center and two continental margins.

Existing rocks are eroded by wind, rain and ground waters. Rivers and ocean currents carry the sediments

to the margins of the continental masses where they are deposited in deeper, calmer waters.

Some sedimentary rocks are formed by chemical or biological processes (evaporites, reefs, etc.)


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Figure 2-6

USGS continental margins.

Reference: http://pubs.usgs.gov/publications/text/dynamic.html

Plates are forced apart by the issuance of material from themantle to the ocean floors along weak zones in the crust known as spreadingcenters. Much volcanic activity occurs along such zones. The formation of

the island of Surtsey near Iceland in 1963 on the Mid-Atlantic Ridge is onerecent dramatic example of this activity. The Hawaiian Islands are an olderexample (although these islands are still active volcanoes today).

Regions where plates collide and override each other are calledsubduction zones and are characterized by frequent earthquakes. The entirePacific Rim is a series of subduction zones. The more buoyant of twocolliding plates will ride up over the more dense, forcing it back down into themantle to be re-melted. The physical stresses imposed by the collision ofcontinental sized masses of rock (twenty to sixty kilometers thick) result insevere deformation of the plates near the contact zone. Folding and faultingacting to relieve such stresses are the main mountain building mechanisms.


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The Rocky Mountains are a relatively recent example (about60 million years ago) of significant strain caused by such stresses. TheHimalayas are the highest range of mountains for a good reason. They wereformed by the collision of two continental plates - India and Asia. Since bothplates were relatively buoyant, the resulting mountains and plateaus werethrust up to very high elevations.

A word about the time scale of these events is warranted. Thecooling of the earth to this point has taken over four and a half billion years.The recently formed Rocky Mountains are some sixty million years old.(The Rockies have existed even longer than Gordie Howes career in theNHL.) The rate of relative motion between two plates at a subduction zone istypically measured in centimeters per year. (This is why one of the mostexciting things to do on a Saturday night in Vancouver is NOT to slip down tothe ocean and watch the subduction of the Pacific Plate ! ) Even very fastmovement is believed to be in the order of ten meters per year.

Figure 2-7

Geologic Time Scale.

Reference: http://www.geo.ucalgary.ca/~macrae/timescale/timescale.html


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Figures 2-8 to 2-16 show the history, as we currentlyunderstand it, of the movement of the continents over the past 700 millionyears. These figures and the animation shown during the presentation wereobtained from the internet (University of California Berkeley Department ofIntegrated Biology and University of California Museum of Paleontology; W.Brian Simison, webmaster). This presentation was apparently constructed by

scanning images from a flip book authored by Christopher R. Scotese, withthe PALEOMAP Project at the University of Texas at Arlington.

Working from Figure 2-6 to 2-14, pick out the location of yourfavorite city (or province, or state, or oil/gas prospect) and try to follow itslocation in past times. Note that 300 to 500 million years ago, Alberta waslocated in tropical latitudes (near the equator). At this time, our Devonianreefs (Swan Hills, Leduc, Redwater, etc.) were being formed.


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Figure 2-8

Our earth, 10 million years agolooked pretty much the same as it does today.

Figure 2-9

Our earth, 100 million years ago.

The Atlantic ocean was much smaller. Note that the inland seaway was open through what is now Alberta.

India is not a part of Asia; in fact, it has just split off the southeast corner of Africa (along withMadagascar) and within a short 80 million years travels across the Indian Ocean to collide with Asia and

form the Himalayas.


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Figure 2-10

Our earth, 200 million years ago.There was no Atlantic Ocean. Alberta was dominated by the inland seaway and was located just north of

tropical latitudes.

Figure 2-11


All the worlds continents formed one large land mass known as Pangaea. Note Albertas inland sea is well

within tropical (reef building) environments.


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Figure 2-12

Our earth, 400 million years ago.North America is in the southern hemisphere, South America is upside down. The inland sea was still

open and reefs were growing in what is now northeast British Columbia.

Figure 2-13


Hudsons Bay was right on the equator sun tanning in Churchill anyone?


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Figure 2-14

Our earth, 600 million years ago.North America was almost upside down and near the south pole.

Figure 2-15


Even near the beginning of sedimentary history, North Americas inland seaway was open and Hudsons

bay was formed.


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Figure 2-16

Spreading centers and trenches..

Figure 2-17

Sea floor spreading.

Magnetometer up and bathymetry down.


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B. Sedimentary Basins the Layer Cake Model

Now that you are an expert on the history of the formation ofthe earth and its continents and oceans, we will deal with a topic which is

slightly more relevant to exploration for oil and gas. First we must appreciatethe distinction between the three major rock types which occur in the earthscrust.

There are three major geological rock types:

Igneous - formed from cooled magma (molten rock from the earth'smantle)

Sedimentary - formed from compaction of eroded sediments that havebeen carried and deposited by water or wind. Also chemical andbiological growths such as reefs.

Metamorphic - either of the previous rock types which have been

extensively altered by heat and pressure.

The crust accounts for only a small portion of the earthsvolume. Of this small portion, less than 5% consists of sedimentary basinswhere we find prospective oil and gas reservoirs.

Most oil and gas is found in basins where sedimentary rockshave accumulated. Particles of rocks eroded by wind and rain are transportedby rivers to depo-centers. The high energy water courses which transportthe sediment flow into lower energy basins (point bars, lakes, bays, andcontinental margins) where calmer waters allow the sediment load to settle tothe water bottom. Sedimentary rocks are formed when layers of accumulatedsediment are compressed by deep burial. Some of these layers may have beenformed at the floor of deep oceans. These types of sediments are calledclastics. Another type of sedimentary rock is chemical and biologicalaccumulation such as evaporite pans and organic reef structures (referred to ascarbonates).

Viewed on a large scale, sedimentary basins form with a layercake appearance. Each layer is formed by fairly uniform deposition of aparticular rock type. Periodically, the dominant rock type changes as sealevels change, plates move into different environments, or source materialschange. Furthermore, earth stresses act to tilt, fold and deform the layer cake.

Hydrocarbons are formed when the remains of biotic

organisms in these layers are subjected to heat and pressure of burial.Terrigenous hydrocarbons may also be formed from the remains of ancientmarshes and peat bogs.

Oil and gas formed from these "source" rocks is lighter thanthe brine fluids otherwise occupying pore spaces in the rocks. Therefore, thehydrocarbons try to move upward toward the surface. As the hydrocarbonsencounter more permeable layers, they will preferentially move along theselayers. This is called"migration".


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Often these migration paths are blocked by a stratigraphicchange or by some older geologic structure. The hydrocarbons will move intothese areas faster than they can move through relatively impermeableoverlying rocks (the seal). In this way, oil and gas accumulate in "traps". Theporosity of the rock and the volume of the trap determine the amount of oiland gas which can accumulate. The permeability of the rock determines how

effectively the reservoir can deliver its precious stores to a central gatheringarea (i.e. a well bore).

So the key elements of a productive oil or gas field are:

Source rocks - somewhere in the basin, we must have the necessary rocktypes and thermal / pressure history to generate hydrocarbons

Migration paths - we must have permeable layers to conduct the movementof hydrocarbons throughout the basin

Reservoir - rocks of sufficient porosity and permeability to hold and deliverstored hydrocarbons must be available

Trap - some structural or stratigraphic barrier to the migration ofhydrocarbons must exist in the immediate vicinity of the reservoir

Seal - The overlying rocks must be sufficiently impermeable as to contain theaccumulation of hydrocarbons.


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C. Some Basic Structures in a Sedimentary Basin

Figure 2-18

A simple schematic cross-section through Alberta

representing some typical hydrocarbon traps in the Western Canada Basin.

Can you find: a roll-over on a thrust fault, a normal fault, a reef bank, a pinnacle reef, the Calgary Tower,

a Granite Wash play, a Cretaceous channel, Mt. Rundle, an erosional truncation, a shoaling environment, a

facies change?

Figure 2-15 shows a diagrammatic cross section through theWestern Canada Sedimentary Basin. This section could run from near Banffin the southwest (left side) to Lake Athabasca in the northeast (right side).although the earths crust is 20 - 40 kilometers thick over most of this area,only the upper 1 - 4 kilometers is sedimentary basin (the underlying crust iscrystalline rock - igneous and metamorphic).


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Figure 2-19

A simplified depiction of several common types of faults.

a) Normal Fault occurs under extensional stress

b) Reverse Fault occurs under compressional stress

c) Thrust Fault results from basinal compression with movement over long distancesd) Geologists Fault results from drilling without using seismic


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D. Fluid Migration and HydroCarbon Traps

We often refer to oil and gas products collectively ashydrocarbons. These are combinations of hydrogen and carbon atoms inmolecules that have attractive properties as fuels and lubricants and are a keycomponent of the petro-chemical industry.

Stable molecular structures are formed when a carbon atom isclosely surrounded by (bonded to) four other atoms. Each hydrogen atom in amolecule likes to be bonded to one other atom. On the following page aresome examples of basic hydrocarbon molecular structures. The simpleststructures are known as the "light ends" of hydrocarbons. Mixtures of thesemolecules form natural gas. The medium molecules form condensates andfuels. The heavy ends form lubricants. Very heavy oil is often formed bybiodegradation of the light ends, leaving behind only the very heavy ends.

These products form tars and asphalts.

Oil and gas is lighter than water (H2O) and will rise upwardswhen introduced into a water-saturated environment. All of the subsurfacesedimentary basins are saturated with fluid (mostly briny water) below thelevel of the local water table.

Since oil and gas are formed from simple combinations ofcarbon and hydrogen, and because hydrogen is abundantly available in ourwater and atmosphere, the necessary component in the formation ofhydrocarbons is carbon. There is a popular belief that oil comes from burieddinosaurs and swamps. The decaying animals and woody materials providethe necessary carbon. In fact, most of the world's oil and gas is not

terregenous (formed from land based materials). Most oil we produce wasformed from dead and decaying marine biota forming layers at the bottom ofthe oceans and large waterways.

Hydrocarbon elements must be "cooked" to form oil and gas.This requires burying the source rock layers at great depth and subjectingthem to high temperatures and pressures. This happens in the naturalevolution of most sedimentary basins.


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HH C H Met hane CH4

H

H H

H C C H Et hane C2H6H H

H H HH C C C H Pr opane C3H8

H H H

H H H HH C C C C H But ane C4H10

H H H H

H H H H HH C C C C C H Pent ane C5H12

H H H H H

H H H H H HH C C C C C C H Hexane C6H14

H H H H H H

H H H H H H H

H C C C C C C C H Hept ane C7H16H H H H H H H

H H H H H H H HH C C C C C C C C H Oct ane C8H18

H H H H H H H H

H H H H H H H HH C C C C ~ C C C C H CNH2N+2

H H H H H H H H


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Figure 2-20

Molecular structures

Photo taken from Oil Sands Discovery Center, Ft. McMurray.

Figure 2-21

Steam Assisted Gravity Drainage

Photo taken from Oil Sands Discovery Center, Ft. McMurray.


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A sedimentary rock not a mass of solid material. Whenviewed through a magnifying glass or under a microscope, we can see it ismade up of many small grains cemented together to some degree. Thegeometry of these grains determines the reservoir qualities of the rock. The

spaces between the grains are called pore spaces while the extent to whichthese pore spaces are connected determines the permeability.

Figure 2-22

Porosity versus Permeablility

Fluid accumulation in porous, permeable rocks. Porosity is the volume of a rock capable of holding fluids;

permeability is the ability of fluids to move between the grains and through the rock.

Top Left: well-sorted, well rounded grains (point bar) porosity about 25%, good permeability.

Bottom Left: poorly-sorted, well rounded grains (levee splay) low porosity, fair permeabilityTop Right: well-sorted, angular grains (aolean - sand dune) good porosity, fair permeability (may be

anisotropic)

Bottom Right: poorly-sorted, angular grains (abandoned channel, shales) poor porosity, poor permeability

(may be anisotropic)


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The porosity of a rock is the percentage of the total volume(matrix grains and pore space together) which is available to contain fluids(gas, oil or water). If all the grains in a rock are well sorted (the same size andshape) then the porosity is fairly great. If they are poorly sorted, the smallgrains tend to fill the pore space between larger grains and the result is a low

porosity rock. Some depositional environments (such as point bars alongrivers) provide excellent natural sorting of grain sizes.

The permeability of a rock is a measure of the ability of fluidsto move through the rock. Smaller grained rocks will have lower permeabilitythan larger grained rocks because the pore throats (restricted areas betweenpore spaces) will be smaller. Secondary mineralization (known as diageneticalteration) can also plug up the pore throats and reduce permeability.

Some rocks (like shales) are made up of flat grains. Thepermeability along the grains will be good while the permeability across thegrains will be poor. This difference in properties depending on direction ofmeasurement is known as anisotropy. An isotropic material will exhibit

similar permeability in all directions while shale is an example of ananisotropic material.

If a rocks grain type and geometry is similar over a large scale,it is said to be homogeneous. If it is made of a variety of grain types (aconglomerate for example) it is inhomogeneous. Note that a uniform shalecan be homogeneous but anisotropic.

Once we have located a good reservoir rock near a migrationpath which connects it to a good source rock, and the reservoir is capped andtrapped, we have a potential oil or gas field. If we drill a well into thereservoir, the trapped oil / gas will flow into the well bore where we cancollect it and pump it to storage and transportation facilities at the surface.

As long as nothing happens to damage the reservoir near thewell bore, oil / gas will continue to flow into the well as we produce it.Eventually, as we deplete the hydrocarbon reserve of the formation, it will bereplaced by surrounding water. Figure 2-18 shows that the water will adheremore closely to the rock grains than the oil or gas. As the proportion of waterin a reservoir increases the thickness of this water jacket around each grainwill also increase. When the water chokes off the pore throats, the oil / gaswill no longer flow through the rock. Only the water will flow whilesubstantial amounts of oil / gas remain bound in the remaining pore spaces.The amount of oil we can produce from a reservoir as a percentage of theoriginal volume of oil contained in the reservoir is called the recovery factor.

It is determined by grain size and geometry, degree of cementation, proximityto oil / gas - water contact, and production rates.


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Figure 2-23Multi-phase fluid dynamics.

Four grains of sandstone are depicted surrounded by a thin layer of water (top left) and a thicker layer of

water (bottom right). The rest of the pore space is occupied by oil (black). The water adheres to the grains

more closely than oil because it is more wettable.

Note that the pore throats in the upper picture are open for the passage of oil. As oil is produced from a

reservoir, it is replaced by water and the water jacket thickens. Eventually, the water pinches off the pore

throats so that the remaining oil cannot be produced (bound oil). At this point, the reservoir is said to have

watered out.

Enhanced recovery techniques use methods to break down the water jacket and free the bound oil.


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E. Typical HydroCarbon Trap Types

Oil and gas can become trapped in a variety of geological

settings. Some traps are formed by structural deformation of the layers ofsedimentary rocks (faulting and folding). Others result from changes in thethickness or type of material in a particular layer and are known asstratigraphic traps.

Figure 2-24

Some typical hydrocarbon traps.


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Figure 2-25

A model of a normal fault with a typical seismic expression.

The small building on the right represents the Calgary Tower, a local landmark over 600 feet high (200 m).


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Figure 2-26

Braided stream and deposition sequence.

Figure 2-27

Meandering stream and deposition sequence.


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Figure 2-28

Basal cretaceous fluvial system.

Figure 2-29

Flooding and regression of the Albian Sea.


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Figure 2-30

Model of a Lateral Sand Bar.

Figure 2-31

Schematic cross-section of Little Bow Area.


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Figure 2-32

Cross-section through channel of interest and its seismic section.

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02 Basic Background Geology