CRAIN'S
PETROPHYSICAL
HANDBOOK
c. 1978 - 2008 E. R. (Ross) Crain, P.Eng.
Rocky Mountain House, Alberta Canada T4T 2A2
403-845-2527 email us
Please be fair to the author - pay your Shareware Fee HERE
and receive a copy of Crain’s Petrophysical Handbook on CD-ROM
at no extra cost.
Updated 7 July 2005
CHAPTER
THIRTY-ONE:
STRUCTURAL ANALYSIS
1 Table
of Contents Continue to Chapter Thirty-Two Publication History: Sections 31.01 to 31.05 are condensed from "Geology for Geophysical Engineers", a course prepared for Geophysical Services Inc by the author in 1967. This material and subsequent sections formed part of Chapter Six of Volume Two of The Log Analysis Handbook, part of a series of seminars offered by the author beginning in 1979. Updated 1985, 1993. Revised and re-organized for this electronic edition Oct 2002. CHAPTER
THIRTY-ONE:
STRUCTURAL
ANALYSIS
1 31.00
Introduction To This Chapter Additional non-traditional dipmeter analysis techniques used for structural analysis are covered in Chapter Thirty-Two. Stratigraphic traps, created by the juxtaposition of porous and nonporous rocks, are described in Chapter Thirty-Three. The key links in defining structural traps are geologic mapping from correlation of well logs, reflection seismic mapping, and a logging tool called the dipmeter, which provides data that can tell us about the dip angle and dip direction of the rock layers. These results in turn allow us to analyze rock structures as seen in one or more boreholes. Dipmeter tools and calculation methods are covered beginning in Chapter Twenty-Six, which is a prerequisite to this Chapter. If you plan to use existing dipmeter data for serious exploration, you must be aware of the differences and limitations of each tool and processing technique.
The planet earth has a molten core, covered by a relatively plastic mantle. The surface crust is quite rigid, but is broken into a number of plates which are free to move over the mantle. About 75% of the Earth's surface is covered by oceans, each of which is underlain by one or more plates. The continents are land masses, predominately above sea level, which are also comprised of one or more plates. The motions of the plates relative to each other is called plate tectonics. Plate tectonics and migration of continents is a central feature in the present theory of the earth's structure. The concept was first mentioned by Antonio Snyder-Pelligreni in 1858, who attributed it to the biblical flood. In 1912, Alfred Wegener, proposed a theory which accounted for the movement of the continents and the apparent wandering of the north and south poles. However, it was not until the mid-1960's, that Wegener's theory was widely accepted by the geological community, Tuzo Wilson of the University of Toronto being one of the significant contributors. The theory itself was originally described by the term continental drift, but it is evident that many other pieces of the surface move, and do not carry continents, so the term plate tectonics is preferred as it more correctly describes the real situation. Exploration of the ocean floor, undertaken in the 1960's by the Deep Sea Drilling Project, indicates a ridge system circling the globe roughly in the middle of each ocean. The rocks in these mid-oceanic ridges are very young compared to the rest of the sea floor. Magnetic polarity reversals are used to estimate the ages of the rocks. This material is constantly being created as the sea floors spread outward from their centers. Because of this, and the fact that the earth is not growing larger in circumference, Wegener's theory was extended to include the movement of sea floor rocks underneath the continents in a process called subduction. The
earth is divided into approximately eight large, rigid, but shifting,
major plates and many hundreds of minor plates. The major plates
support at least one massive continental plateau, often referred
to as a craton.
One edge of a plate is a subduction zone, usually marked by a trench where the plate dives deeply into the earth's mantle, underneath another plate. On the opposite side of the plate is a mid-ocean ridge or pull-apart zone. As the rift opens, the gap is quickly healed from below by the inflow of molten rock. The other two sides of the plates connecting the rifts to the trenches are shear zones, called transform faults. There
are thus three types of plate boundary, namely the divergent boundary
(the mid-ocean rift), the shear boundaries (where plates slip
past each other), and the convergent boundary (where two plates
collide, with one usually being subducted and consumed).
Convergent plate boundaries cause compression of the earth's crust, resulting in folding, overthrusting, trenching, or crustal thickening. Divergent boundaries, on the other hand, cause rifting, down dropping, or thinning. The collision of a sea floor plate with a continental plate generally produces mountains, such as the Rocky Mountains along the west coast of North America, the Andes along the west coast of South America, and the Appalachian Mountains along the east coast of North America. The collision of two continental plates also creates mountains, such as the Himalayas at the boundary of the Indian and Asian subcontinents. The driving force behind the motion of the plates appears to be convection currents in the molten core of the earth, which flows beneath the continental plates dragging them slowly in the same direction as the current. Hydrothermal processes have concentrated the majority of known metallic ore bodies along convergent plate boundaries, for example the gold fields of California and Alaska. These processes are occurring now at present continental margins and have occurred at ancient continental margins, some of which may be buried deeply under newer sediments. Hydrothermal processes are also active at divergent plate boundaries, such as the mid-Atlantic ridge and the Red Sea. An actual example of such an ore deposit can be found in Cypress where an ancient mid-oceanic ridge is now above sea level. In addition, convergent plate boundaries create conditions that permit accumulation of petroleum offshore or on land near shore. Because rocks are buckled and bent by plate movements, traps for hydrocarbons are formed. Moreover, the heat and pressure induced by subsiding plates helps to liberate petroleum from its source rocks, leaving it free to migrate into the traps. Since sea floors accumulate considerable biological material that is dragged under the continents, it is possible that hydrocarbon from biological sources is constantly being created. There are also plausible theories that predict the generation of hydrocarbons from both biological and non-biological sources below the crust. Divergent plate boundaries, on the other hand, create conditions that favour the development of oil and gas accumulations on the continental shelf and in the ancient deep sea basin under the continental rise, such as in the Gulf of Mexico.
Diastrophism is "the process by which the earth's crust is reshaped". The word is seldom heard today. More modern terms are "mountain building" and "tectonism". The word "orogeny" also means the process of mountain building, but is often used to mean a mountain building period of time in the earth's history. The cause of diastrophic movements is the stress created by the relative motion of the continental and sea floor plates. These are generally very slow processes so that extremely accurate observations would have to be made for us to see the results of such movements. For example, the Rocky Mountains are still rising at the rate of several inches per hundred years, due to the Pacific Plate sliding underneath the western edge of the North American Plate. Dramatic evidence exists to show the results of diastrophism. The fossil record shows that the rocks uplifted in the Rocky Mountains were once beneath a sea containing abundant marine life. Uplift and subsidence takes a long time, even by geological standards. The elevation of sea floors to form mountains and plateaus, and the subsidence of continental areas so that they are inundated by thousands of feet of ocean water and sediment, requires millions of years. More rapid displacements on smaller scales, such as those involved in earthquakes, also take place; but the magnitude of such movements can be measured in terms of a few feet or inches for any one disturbance. For example, the coastal region of California is moving northwest at the rate of about one half inch per year along the San Andreas fault. This fault is only one of many along the coast of California, which is a very complex series of tiny crustal plates jockeying their way northward. The
diastrophic processes of interest to petroleum geologists may
be classified as follows: These are described in more detail later in this Chapter.
The classic geosyncline is divided into two parts, namely a miogeocline, and a eugeocline which lies to the seaward side. The miogeocline is made up of sediments which form the continental shelf. The eugeocline consists of sediments on the continental rise in deeper water some distance offshore. If subsidence continues in spurts, more than one eugeocline can form, with the associated miogeocline lying on top of the previous eugeocline. A current example is the sedimentary section of the continental rise that lies seaward of the continental slope off the eastern United States. Landward
of the rise and capping the continental shelf is a wedge of sediments
that becomes progressively thicker as it extends towards the shelf
edge. This wedge is the miogeocline and is really a very young
geosyncline, before it is fully formed. The sediments are soft
and relatively un-compacted.
The source of sediments for the geosynclinal prism is from the continental craton. In the North American example, the majority of sediments from the continent are eventually dumped into the Atlantic Ocean and the Gulf of Mexico. Geosynclinal prisms are deposited along the trailing edge of a plate. If the continental plate changes its relative direction of motion, and the trailing edge becomes a leading edge, the geosyncline is compressed and folded. This has happened in eastern North America and caused the folding of the Appalachian Mountains. Sedimentation to form a geosyncline and subsequent folding constitutes a basic geologic cycle which evolves over several hundred million years, and may be repeated several times. Currently, the opening of the North Atlantic is progressing at the rate of 3 cm per year. The great sedimentary basin stretching from the Canadian Arctic through western Canada and the western United States to the Gulf of Mexico is another example of a geosyncline. The continental edge (miogeocline) was to the east and the seaward edge (eugeocline) was to the west. This geosyncline is being uplifted and folded by the pressure created by the collision of the North American plate with the Pacific Ocean plate.
1. dome mountains result from the upward bending of rocks into the form of a dome. The Black Hills of South Dakota are a typical example of a naturally dissected dome mountain. 2. fold mountains consist of a series of long, more or less parallel wrinkles, or folds. The mountains of West Virginia and central Pennsylvania are good examples of fold mountains in the mature stage of erosion. This type of lateral compressional folding, whether it occurs in mountain building or on a much more localized scale in the subsurface, creates structures which may contain hydrocarbons. These are anticlines, synclines, and monoclines. 3. block mountains are formed by faulting and uplifting of angular blocks of rock. Some of the blocks are pushed and tilted at various angles into block mountains. The depressed blocks between the uplifted ones are block basins. 1. Folds Anticlines are the raised peaks of folds (the tops of the hills) and may be evident at the surface, although deeply buried anticlines may be covered by flat lying sediments. Synclines are the valleys between folds. A monocline is a long, relatively flat, sloping surface usually with a more or less flat plain at the bottom end and a flat plateau at the top end. Formation dip is the angle relative to horizontal at which the top surface of a formation rests. The dip direction, or dip azimuth, is the direction relative to north at which the formation dips downward. Strike is the direction of a line along the surface at right angles to the dip direction.
A
fold is made of: 2.
the dip of both flanks with regard to the axial plane; 3.
the direction of the angle of folding: 4.
the angle of folding: A dome is a fold where the beds dip in all directions away from the point of folding. The axial plane may be a warped surface. 2. Faults The
same type of forces that act on a major scale to build mountains
act in a smaller localized way to create faults. The gravity,
or normal, fault is caused by tensional forces. Removal of compression
allows one block of the earth to move downward with respect to
another.
When compressional forces are exerted, a reverse or thrust fault results. In a thrust fault, pressure forces cause the hanging wall (the rock above the fault plane) to move upward and over the top of the foot wall (the rock below the fault plane). Reverse faults are similar except that there is no motion of the upper block along the top of the lower block. Movement of either wall in a direction parallel with the fault plane is called a rift or tear or shear fault. In some instances a long and narrow block drops down between two or more faults, and is called a graben. Blocks that remain raised between two faults are called horsts. The faults bounding both grabens and horsts are gravity (or normal) faults. A
fault is a fracture with relative movement of the two blocks of
rock on either side of the fracture. It is made of: A
fault can be described: 2.
according to the relative movement of both blocks with respect
to the dip of the fault plane 3.
according to the magnitude of dip of the fault plane 4.
according to the time of occurrence with respect to deposition Other
descriptive terms used are: 2. throw: the vertical component of separation; i.e., separation measured in a vertical plane normal to the fault plane. The term 45-foot fault means that 45 feet of vertical section are missing at the well bore (if a normal fault) or 45 feet of section is repeated (if a reverse fault). 3. slip: the movement along the fault, being the distance between two formerly adjacent points. Slip can be in any direction within the fault plane; same as separation if there is no horizontal component of movement. 4. extent: a fault is limited in horizontal and vertical extent by sudden disappearance against another fault, or against an unconformity, or by gradual decrease of throw along the fault plane. In rotational faults, throw decreases in the direction of the plane of rotation. 5. correlation gaps or repeats: when a well crosses a normal fault, correlation of the well logs will show a missing section. When a well crosses a reverse fault, the same formation is crossed twice, and appears as a repeat section on the log. A reversed repeat can occur when drilling through a recumbent fold. 6.
drag and rollover: are fold-like deformations of the bedding planes
in the vicinity of a fault and associated with the fault. Such
folding is called drag when it is caused by friction of the two
blocks against each other along the fault. The result is:
In certain regions, normal faulting occurs with reverse drag, or rollover in the downthrown block. In this configuration, the bedding planes dip into the fault plane instead of away from it. Rollover is frequently associated with growth faults; i.e., faulting contemporaneous with deposition, with greater formation thickness on the downthrown side of the fault.
Rocks which form a trap do not always contain hydrocarbons, but all hydrocarbons are contained in some kind of trap. Otherwise they would flow to the surface of the earth and be dissipated into the atmosphere or the oceans. Some oil and gas does reach the surface, and are called seeps. Most early oil discoveries were located because of a seep at the surface. The trapping mechanism is one of the most important aspects of petroleum geology. For example, if porous rock has been folded upward into a dome or anticline, oil and gas may collect at the top of such a structure. It may be kept from escaping by an overlying nonporous layer of rock. Traps can also be formed by faults, which allow a porous layer to be thrust against a nonporous layer, thereby sealing in the hydrocarbon. Examples are shown in Figure 31.07.
FIGURE 31.07: Structural traps Traps filled with hydrocarbons can be rearranged structurally by diastrophic forces, thus un-trapping the hydrocarbons. The presence of oil staining or fluorescence on rock samples from depleted reservoirs is relatively common, indicating that trapped oil has been released by geologic forces on numerous occasions. Stratigraphic traps are created by the nature of the depositional sequence itself and do not depend on deformation of the rocks. For example, a sandstone that may have once been an old beach usually tapers off to a wedge ending between two layers of rock that are not porous. If one edge of the wedge is high relative to the other, oil and gas could collect in the updip edge and could go no further. Ancient river valleys, sand bars, beaches, and delta fronts can be sealed by impervious rock above and below so as to create traps of many varied shapes and sizes. Some traps have both a structural and a stratigraphic component, such as reef and salt dome traps. The fluids trapped in a reservoir are segregated by gravity. Since gas is lighter than oil, it will rise to the top of the trap. Some reservoirs do not contain sufficient gas to create a separate gas cap. Since gas can be dissolved in oil, most oil wells produce some dissolved gas. Water is heavier than conventional light oil, so the free water in the reservoir will be forced to the bottom of the trap. In heavy oil wells, it is possible to have water on top of the oil column, or even interbedded within the oil, depending on the oil gravity and the timing of the oil degradation. The vertical relief of anticlines varies, ranging from a few feet to several thousand feet. Some of the largest oil fields of the world are anticlinal traps. The Abqaiq pool in Saudi Arabia is a large anticline, 25 miles long and 6 miles wide, This structure produced about one billion barrels of oil in 8 years. The Abqaiq anticline contrasts sharply with the small Wilcox structures of central Oklahoma that cover only 100 acres. Asymmetrical anticlines are also formed by lateral compression. Typically these structures are elongate and often occur in trends that parallel the major uplifts of the geologic province. Many of the surface anticlines in the Rocky Mountains fall in this category. Frequently the center of the surface anticline forms a topographic low where erosion has cut through otherwise resistant beds, which have been fractured by the folding process. The resistant beds form prominent scarps or rim rock around the basin that are easily recognized. The axis of the asymmetrical anticline may shift with depth. As a consequence, the deep seated fold may not lie directly under the surface anticline. Many of the larger anticlines of the world are faulted. The fault patterns range from simple to exceedingly complex, such as those found in the Rocky Mountains and California. Production is often confined to the upthrown block, but this does not entirely eliminate the downthrown side of the fault as a potential trap. In this type of structure, the role of faulting appears to be of minor importance in controlling accumulation on the crest of the fold. Faulted anticlines occur in nearly all geologic provinces and can usually be mapped by geophysical surveys. The Gulf of Suez fields in Egypt is a classic faulted anticline structures. Overthrust anticlines are formed by compression and occur along the edges of major uplifts as narrow folded belts. The geology in these thrust belts is very complex, but some structures are prolific and may develop into major fields. Turner Valley in the Alberta foothills of Canada is a classic example of an overthrust fold and has produced 100 million barrels since its discovery in 1915. Normal or gravity faults control production in many oil fields in most of the geologic provinces of the world. This type of trap occurs wherever the region has been folded or uplifted. Often the trap is formed by a combination of both folding and faulting. Fields can occur in narrow belts along a fault system for hundreds of miles. Good examples of gravity faults occur in the Niger Delta. Fault
planes formed by gravity or tension usually dip at 45-60 degrees.
Invariably, wells that pass through a normal fault drill an abnormally
short section of sediments. The thickness of the missing section
is equal to the throw of the fault. This is a good criterion for
locating the fault if there is enough well control. In the absence
of sufficient data, it is easy to mistake the missing section
caused by faulting for an erosional surface, which also creates
missing section, as shown in Figure 31.08.
Reverse faulting occurs in areas that have been subjected to compression. Wells that pass through these fault planes will normally repeat section, going from older beds above the fault into younger beds below the fault. Reverse or thrust faulting occurs around the flanks of mountain uplifts where horizontal compression plays a dominant role in the forming of local and regional structures. Faulting often breaks an oil field into several separate pools or fault blocks. The faults may parallel each other, or they may intersect to form several traps. The fault patterns can be related to the regional and local geology and drilling along these trends is usually more successful than random drilling. An example is the Austin Chalk play in the southern USA, where production is greatly enhanced by fractures associated with minor faulting. An example of a field with many semi-radial faults is Hibernia, offshore east coast Canada.
FIGURE 31.09: Salt dome traps The caprock is a reservoir in certain Gulf Coast fields. Both gravity and seismic surveys are extremely useful methods of searching for salt dome features. Geologists suspect that many of the structural features in east Texas and north Louisiana are also related to deep seated salt movement. Salt solution plays an important role in other areas of the world, forming traps across the tops or flanks of un-dissolved salt. A striking example involves a map of the oilfields of Saskatchewan - the vast majority form a semicircle around the edge of the major salt solution basin in the Devonian, nearly 400 miles in diameter. Impact craters, shown in Figure 31.10, also form structural traps. They are usually small and difficult to find, but several are known in Saskatchewan, North Dakota, and other areas of the USA, in formations of various ages.
FIGURE
31.10: Impact crater traps Impact crater traps have hundreds of small faults and complex structure. Regional dip superimposed on such a trap makes the accumulation of hydrocarbons exceedingly complicated.
An
overview of the thought processes were described by E. L. Bigelow
in "Making More Intelligent Use of Dipmeter Information",
published in The Log Analyst in five parts beginning January 1985.
Figure 6.30, taken from that paper, demonstrates the complexity
and interrelated nature of the data acquisition. processing, and
analysis problem. A review of Mr. Bigelow's paper will provide
many insights not covered in this Chapter, due to lack of space
Structural analysis begins with a review of the arrow plot. Dips fit one of five general patterns, each defined by the color of the pencil used to mark them: GREEN PATTERNS: nearly constant dip and direction, representing regional dip, sometimes called structural dip. RED PATTERNS: increasing dip with depth, representing drape, down dip thickening, differential compaction, drag on faults, or folding. BLUE PATTERNS: decreasing dip with depth, representing drag on faults or folding. BLACK PATTERNS: abrupt change in dip and/or direction, representing unconformities, fault planes, or erosional boundaries between stratigraphic units. CIRCLED PATTERNS or YELLOW PATTERNS: random dip angles or directions indicates bad hole conditions, contorted bedding, fractures, slumping, or brecchia, sometimes associated with fault planes.
FIGURE 31.12: Colour patterns for dipmeter interpretation The
color assignments, namely green, red, blue, black, and yellow,
are purely arbitrary but have become an industry standard by common
usage. Appropriately colored pencils or ink markers are used to
join dip arrows to emphasize the patterns. The five patterns are
illustrated schematically in Figure 31.13. The features associated
with each pattern are listed on the illustrations.
All but black patterns should have roughly constant dip direction, or else they are not real patterns. An exception is a pattern that passes through zero or ninety degrees, where dip direction will reverse, as shown on the bottom of Figure 31.14 (right). To begin analysis, start at the top of the log (or somewhere above the zone of interest) and draw in the green, red, blue, black. and yellow patterns, in the order listed. Be careful not to cross a major change in dip direction with one of these patterns, unless it is an exception as described above. Remember that such a change in direction is normally a black pattern. Join arrows which are fairly close in depth. The end of a blue pattern can be the beginning of a red pattern and vice versa. Not all the results need to be included in every pattern. You may decide some are due to noise, rough hole, or minor stratigraphic events embedded in a larger structure. In
the example in Figure 31.14, the top half of the log shows a trend
of dips at 9 degrees downward to the east - a GREEN pattern. The
horizontal line at "A" indicates a break in trend -
a BLACK pattern. This is followed by another GREEN pattern, indicating
regional dip of 5 degrees to the west south west below an unconformity
at depth "A". This is followed by a RED pattern indicating
drag above a fault. The fault plane is at or slightly above point
"C". This is followed by the reverse drag on the down
thrown block - a BLUE pattern, lying above another unconformity
at point "D".
Although we have described a plausible interpretation in the above description, it may not be the only interpretation. However, it is not necessary or even practical to analyze the meaning of all the patterns at this stage - more than one interpretation is possible for all patterns. For example, the event at "C" could be a stratigraphic feature or another unconformity. We need to look at the open hole logs and other well data. For structural interpretation, you may have to ignore stratigraphic dips. This involves drawing the patterns through dissenting dips in the sandstone layers. This is called macro-colouring as opposed to micro-colouring, used in stratigraphic analysis. For stratigraphic work, do not join points across a dissenting dip. The dissenting dips are the clues to stratigraphic changes. Join arrows of about the same dip direction. The greater the dip magnitude, the more similar the azimuths should be. Conversely, when very small dips are considered, the azimuth can vary up to 90 degrees. Keep the scale of features in mind. Structural features may encompass hundreds or thousands of feet of data. Stratigraphic features may be superimposed on the structural patterns, and encompass only a few feet to a few hundred feet. However, drape over reefs and differential compaction may persist over several thousand feet, and these features are associated with stratigraphic traps. Red patterns associated with faults and unconformities tend to show greater variations in dip magnitude over smaller vertical intervals. Blue patterns associated with sedimentary structures are usually short (up to a few feet on the vertical scale), whereas the patterns that are a reflection of faults and unconformities generally persist over much longer intervals.
Regional dip may not be easy to find. In thick sandstones, there may be too many stratigraphic features, and in thick carbonates there may be no bedding or too many fractures. Therefore, shale sections should be preferred for the selection of structural dip. If there is not much shale, choose the minimum consistent dips in the sands. However, shale sections do not always exhibit a regular dip. The mode of deposition as well as post-depositional slumping or fracturing may induce erratic dips. Bad or rough hole will also cause dips in shale beds to be scattered. A statistical approach may then be needed to determine structural dip, and local experience is the best guide. Here is a case for using the modified Schmidt plot or the frequency azimuth plot to help find regional dip. It is essential to subtract the structural dip (by means of dip vector rotation), preferably by having a regional dip removed arrow plot created in the computer. Indeed, a blue pattern may become a red one and vice versa after subtraction of the general trend. The term "absolute red" and "absolute blue" are used to describe patterns which are distorted by regional dip. Examples were shown on the bottom half of Figure 31.13.
FIGURE 31.15: Effect of regional dip removal on pattern shape Figure 31.15 shows three dipmeter presentations of the same structure. Each has a different regional dip superimposed on the red pattern, which is only visible in one of the illustrations. This emphasizes the absolute necessity of working with dip removed data. You will have to re-analyze the dip removed log for red and blue patterns. You will use these ONLY to identify the nature of the structural features and the direction to thicker pay zones. The true dip and direction of beds is still contained on the original log, and it is still used for mapping structures and looking for the next drilling location. A dipmeter log should always be correlated with the rest of the open hole logs when the patterns are being drawn. A computed lithology log is especially helpful to prevent drawing silly patterns which cannot be supported by the obvious lithology. For instance, it would make little sense to unite in the same blue pattern two arrows belonging to different lithological units. A good well history and the formation tops should also be at hand, since most major unconformities will occur at one of these points. Some of the modern dipmeter presentations, such as SYNDIP, GEODIP. and STRATIM will make life much easier. However, they are only available on modern logs, and even then, may not have been run (to save money), or may not be in the well file or in the public domain. These logs are aids, not answers - be sure the aid is properly processed and makes sense in light of other known data. On older wells, you will have to create your own equivalent aids by laying out and possibly analyzing the other open hole logs. The minimum requirement is a gamma ray or SP log at the same depth scale as the dipmeter arrow plot.
The second is to use a catalog or cook book of typical patterns to compare your pattern with those already described. As mentioned earlier, regional dip removal can change a pattern, so the cook book approach is not too useful unless dip removal has been done. Also, the cook book patterns presume that dip directions shown on logs are always parallel to your cross section direction. This is not always true so it becomes necessary to rotate dips to get the "best" patterns. Both transverse and longitudinal cross sections should be visualized when analyzing dip patterns. Before you start sketching patterns, review the basic structural features described earlier in this Chapter. Then draw a sketch of the dipmeter data. Take a piece of graph paper or blank well log paper and draw a vertical line to represent the wellbore. A log print with gamma ray, SP, and resistivity is also a good place to draw your diagram. Select the interval you wish to analyze and mark some depth lines to orient your data. For structural analysis, a compressed scale of 1 or 2 inches to 100 feet or smaller is appropriate. Transfer the position of the black patterns to your sketch. These represent breaks in the geologic sequence, such as unconformities or tops and bottoms of sedimentary structures. Use the gamma ray curve or a computed lithology log and the well history data as guides to major erosional surfaces.
Next, choose regional dip in each major rock unit. At this point you have to decide on the direction of cross section that your sketch will represent. Usually, for structural analysis, it is chosen to be the regional dip direction, although another sketch drawn at right angles to the first may be useful in many cases. For example, if regional dip is to the south east, the cross section should run from north west to south east. Draw short hash marks on the well bore at an angle representing the actual dip shown on the log. Some vertical exaggeration may be appropriate. An example of this simple case is shown in Figure 31.16.
Next, position representative samples of the dip from the blue and red patterns onto your sketch. You are really creating your own stick plot. It may be helpful to include a sketch of the dipmeter log itself on the same piece of paper, as shown in Figure 31.17. If the red and blue patterns are contained within a sand body, they are stratigraphic dips and should not be used in a structural interpretation.
Now comes the hard part. Extend the hash marks to represent the bedding planes of a structure. Basically you are only dealing with regional dip, anticlines, synclines, unconformities, and faults. To propose a fault, there should be some evidence from the well history, some scattered dips at the break between the red and blue patterns, a change in direction of dip, some missing or repeated section, or drag and rollover features. The red pattern is usually connected to the green pattern above it.
Reverse
faults and overturned anticlines can have similar patterns - the
anticline is distinguished by dips approaching 90 degrees (Figure
31.18), whereas reverse faults seldom do this (Figure 31.19). Normal faults, overthrust faults, channels, unconformities, and disconformities can have similar patterns - repeat section indicates the overthrust case. The lithology and dips indicating crossbedding help distinguish channels. Choose the model which suits the local geology the best. Most patterns can be interpreted without imposing a fault and the most common error in dipmeter analysis is the suggestion of too many faults.
Growth faults, contemporaneous with deposition, usually show rollover, which is a dip pointing toward the upthrown block. Post-depositional faults usually show drag, which is a dip pointing toward the downthrown block. Hybrid faults can exhibit complex patterns. Some faults show no rollover or drag. Remember the effect of regional dip on these patterns, emphasized again in Figure 31.20, right side. Drawing your own stick diagram and interpreting a plausible geologic section to match it takes practice, patience, and a good grasp of 3-D space. A good knowledge of geology doesn't hurt. The effect of hole deviation must also be considered. Although the dips presented on the log are true dips, with hole deviation taken into account, their position in space may not have been corrected for true vertical depth or the track of the borehole.
Trying to analyze a dipmeter without knowing which rocks are shales and which are reservoirs is pointless. Don't ignore the evidence from the other available logs. Don't try to analyze dipmeters in isolation with a blindfold hiding the other logs; use an integrated approach incorporating all available data. Additional cookbook patterns for stratigraphic features are included in Chapter Thirty-Three. Since some structural patterns can be confused with some stratigraphic ones, you may need to review the stratigraphic patterns before settling on the final interpretation. The
illustrations and text in this section are from "Diplog Interpretation",
published by Dresser Atlas (now Baker Atlas) in 1984.
Regional dip is indicated by the dips recorded in these shale sections. The sands are cross-bedded and the limestone fractured, giving readings which cannot be interpreted as regional dip. In many instances, only one of each 15 or 20 dips reflects actual structural dip. An
anticline well drilled through the crest of an anticline or the
trough of a syncline would exhibit low angle dips. these dips
can be low enough to give low angle dip scatter. Wells drilled
on either flank of this fold will indicate larger and more consistent
dips. These dips will reflect the structural dip at the point
where the well cuts the formation.
The direction of dip in this asymmetrical anticline decreases until the crestal plane of the structure is encountered then increases to nearly vertical as the well bore cuts the formations essentially parallel to the bedding planes. Other overturned anticlines will produce different Diplog patterns depending upon the amount of overturning present. Recumbent syncline bed "A" is the youngest bed. Beds "B", "C", "D", etc., become progressively older. A recumbent anticline would be the same except that bed "A" would be the oldest bed with the others becoming progressively younger.
Recumbent anticline dip increases to 90 degrees where the borehole crosses the vertical section of the fold. Below this the dips are reduced and would usually dip in a direction approximately 90 degrees to that above the axial plane of the fold. Normal
fault with no bedding plane distortion. Fault is not apparent
from Diplog and must be located by other methods. If only one
well is drilled in an area, this type of fault will normally not
be found. Bed "E" has been cut out where the borehole
crosses. If this is the zone of interest, the well must be sidetracked
or re-drilled to encounter the objective horizon.
Normal fault with fault plane dipping same direction as formation bedding planes and exhibiting a small drag zone near the fault plane. Normal
fault with fault plane dipping opposite to the dip of the formations
illustrating drag into the fault plane.
Normal fault with rollover. This is a pattern typical of growth faults which frequently exhibit reverse drag or rollover. These rollover anticlines are important hydrocarbon traps along the U.S. Gulf Coast. Reverse fault with no bedding plane deformation. Beds "D" and "E" ad parts of "C" and "F" have been repeated. Repetition of beds is good evidence that a fault is a reverse fault.
Reverse fault with fault plane dips in the same direction as the dip of the beds. Drag zone in both fault blocks. Reverse fault with fault plane dips opposite the dip of the formations. Drag zone in both fault blocks. Sandstone is assumed to be free from cross-bedding in this illustration.
Thrust fault with marked bedding plane distortion on both sides of the fault plane. This same pattern could be generated by a recumbent or overturned fold. In some areas, both thrust faulting and overturned folds are commonly encountered. Obviously, the correct interpretation of this arrow pattern depends to a considerable extent upon knowledge of the regional and local geology. Thrust
fault with drag on bottom block and little or no deformation of
the beds above the fault plane. This type of response is shown
by the Lewis overthrust which has formed Chief Mountain in Glacier
National Park, Montana. You should study these patterns carefully, comparing patterns from various structures to define differences and similarities.
Figure
31.31 Unconformity
Figure
31.31: Unconformity
Figure 31.35 Normal Fault with Drag and No Rollover
Figure
31.37: Normal Fault with Rotation Figure 31.38 and 31.39 Normal Fault with Correlation to Unfaulted Well In
Well X, the long correlation interval (12') shows clearly the
structural dip. It increases from near zero at 1,200 feet to 7
degrees at 4,600 feet,
Figure 31.39: Map and cross section for normal fault
Figure
31.41 Correlation in Thick Sand Sequence
Figure
31.42 Angular Unconformity
Figure
31.43 Growth Fault in Thick Sand Sequence
Figure
31.44 Overturned Anticline
31.11
In Conclusion Although dipmeter analysis can be ambiguous, sufficient geological constraints, local knowledge, and experience serve to improve skills and speed analysis. Modern computer processing, in particular dip removed arrow plots and stick plots, are essential ingredients. Image processing techniques, while relatively new, have proven useful because of their visual impact. However, the analysis of structure still depends on the basics: dip angle, dip direction, and a plausible model that fits the data.
2. Describe and illustrate the formation of mountains (10 marks). 3. Describe and illustrate the formation of geosynclines. (10 marks) 4. Describe and illustrate types of folds and faults. (20 marks) 5.
On the following illustrations, shade in the areas which could
form
6. Draw a vertical well on each structure. Draw a schematic dipmeter for each. Colour green, red, blue, and black patterns for each. (30 marks)
EXERCISE
31.03: Integrated Example (50 marks) 2. What is the lithology in the producing interval? 3. What is regional dip? 4.
What depth is the fault? What type of fault is it? What is the
fault strike 5. Are there any fractures present? 6. Is there a repeat section? 7. What is the borehole deviation? 8. Draw east - west and north - south cross section at the fault.
1: Application of dipmeter surveys; Stratton,E.F., Hamilton,R.G.; American Institute of Mining Metallurgical Engineers Meeting, 21 p., 1947 2: Application of the continuous dipmeter to reef study; Schlumberger; Schlumberger News, 9 p., 1961 3: Continuous dipmeter survey can be an important exploration tool; Thompson,J.D.; The Oil and Gas Journal, 4 p., 1961 4: Diplog; Hammack,G.W.; Dresser Atlas Tech Bulletin, 15 p., 1964 5: Determining true formation thickness from the dipmeter; Norman,J.L., Thibodaux,J.B.; Pan Geo Atlas Corporation, 6 p., 1964 6: Interpretation of continuous dipmeter surveys; Schlumberger; Schlumberger Training Aid, 26 p., 1964 7: Detailed stratigraphic control through dip computations; Gilreath,J.A., Maricelli,J.J.; American Association of Petroleum Geologists Bulletin, v. 48, no. 12, p. 1902-1910, 1964 8: How to compute dips quickly from dip log correlations; Walker,T., Robertson,E.; World Oil, p. 135-140, 1964 9: Stratigraphic interpretation of continuous dipmeter survey data; Schlumberger; Interpretation Bulletin, 6 p., 1966 10: Log interpretation in Bolivia; Salisch,J.A., Brown,H.D.; Society of Professional Well Log Analysts, 20 p., 1966 11: A review of log interpretation methods used in the Niger delta; Poupon,A., Strecker,I., Gartner,J.; 53 p., 1966 12: Applications of the continuous dipmeter in Western Canada; Goetz,J.; The Journal of Canadian Petroleum Technology, 5 p., 1966 13: Dipmeter - Middle East; WEC, 13 p., 1967 14: Logging programs in northeastern South America; Brown,H.D., Salisch,H.A.; Society of Professional Well Log Analysts, 27 p., 1967 15: The continuous dipmeter as a tool for studies in directional sedimentation and directional tectonics; Rodriquez,A.R., Pirson,S.J.; Society of Professional Well Log Analysts 9th Annual Logging Symposium, 25 p., 1968 16: Stratigraphic applications of dipmeter data in midcontinent; Campbell,R.L.,Jr.; American Association of Petroleum Geologists Bulletin, v. 52, no. 9, p. 1700-1719, Sept 1968 17: Interpretation of dipmeter data in the Devonian carbonates and evaporites of the Rainbow and Zama areas; Cox,J.W.; 19th Annual Technical Meeting of Petroleum Society of Canadian Institute of Mining, Paper No. 6820, 1968 18: Depositional environments defined by dipmeter interpretation; Gilreath,J.A., Healy,J.S., Yelverton,J.N.; Transactions - Gulf Coast Association of Geological Societies, v. 19, p. 101 111, 1969 19: Geological application of well logs; Fons,L.,Jr.; Society of Professional Well Log Analysts 10th Annual Logging Symposium, 44 p., 1969 20: East Cameron block 270, a Pleistocene field; Holland,D.S., Sutley,C.E., Bertlitz,R.E., Gilreath,J.A.; 24th Annual Symposium of Gulf Coast Association of Geological Societies Symposium, 9 p., 1970 21: Dipmeter - Libya; WEC, 13 p., 1970 22: Fundamentals of Dipmeter Interpretation; Schlumberger; Schlumberger Training Aid, 145 p., 1970 23: Distributary front deposits interpreted from dipmeter patterns; Gilreath,J.A., Stephens,R.W.; Transactions - Gulf Coast Association of Geological Societies, 8 p., 1971 24: Deficiencies of computer correlated dip logs; Robertson,J.M.; Society of Professional Well Log Analysts 13th Annual Logging Symposium, 15 p., 1972 25: Structural geologic considerations in diplog interpretation; Holt,O.R.; Society of Professional Well Log Analysts 13th Annual Logging Symposium, 30 p., 1972 26:
Geological aids; Schlumberger; Society of Professional Well Log
Analysts 27: Dipmeter outlines petroleum entrapment on flaks of diapiric shale done; Franke,M., Hepp,V.; Society of Professional Well Log Analysts 14 Annual Logging Symposium, 20 p., 1973 28: Dipmeter interpretation in the southern and northern North Sea basin; WEC, p. 56-59, 1974 29: Vermilion block 16 field: a study of irregular gas reservoir performance related to nonuniform sand deposition; Seal,W.L., Gilreath,J.A.; Transactions - Gulf Coast Association of Geological Societies, v. 29, 1975 30: Interpretation of log responses in a deltaic environment; Gilreath,J.A., Stephens,R.W.; American Association of Petroleum Geologists Marine Geology Workshop, 31 p., 1975 31: Quick interpretation of the high resolution dipmeter; Okitsu F.; Society of Professional Well Log Analysts 17th Annual Logging Symposium, 10 p., 1976 32: Reservoir delineation by wireline techniques; Goetz,J.F., Prins,W.J.; Society of Professional Well Log Analysts: The Log Analyst, p. 12-40, 1977 33: Preplatform exploration of High Island Blocks A-560 and A-561; Lund,J.W., King,J.S., Berlitz,R., Gilreath,J.A.; The Oil and Gas Journal, p. 254-273, 1979 34: Dipmeter Workbook; Schlumberger; Workbook, 72 p., 1980 35: Schlumberger dipmeter interpretation; Schlumberger; Manual, 58 p., 1981 36: Application of dip related measurements to a complex carbonate clastic depositional environment; Bigelow,E.L.; Society of Professional Well Log Analysts: The Log Analyst, p. 9-30, 1982 37: Diplog analysis and practical geology; Dresser Atlas; Manual, 57 p., 1983 38: Applications of the SHDT stratigraphic high resolution dipmeter to the study of depositional environments; Chauvel,Y., Seeburger,D.A., Orjuela,A.C.; Society of Professional Well Log Analysts 25th Annual Logging Symposium, 23 p., 1984 39: Geological evaluation of high resolution dipmeter data; Nurmi,R.D.; Society of Professional Well Log Analysts 15th Annual Logging Symposium, 24 p., 1984 40: Comparative results of quantitative laminated sand shale analysis in Gulf Coast wells using maximum diplog microresistivity information; Quinn,T.H., Sinha,A.K.; Society of Professional Well Log Analysts 26th Annual Logging Symposium , 25 p., 1985 41: The use of dipmeter synthetic data to determine rock texture and depositional environment; Standen,E.; 10th Canadian Well Logging Society, 12 p., 1985 42: A fundamental approach to dipmeter analysis; Enderlin,M.B., Hansen,D.K.T.; 10th Canadian Well Logging Society, 14 p., 1985 43: Making more intelligent use of log derived dip information; Bigelow,E.L.; Society of Professional Well Log Analysts: The Log Analyst, p. 26-42, 1985 44: Schlumberger sedimentary environments from wireline logs; Serra,O.; Manual, 210 p., 1985 45: Geologic interpretation of alternative dipmeter analyses from a permocarboniferous glaciogene sequence in the Fitzroy Graben Canning Basin, Western Australia; Golstein,B.A.; Society of Professional Well Log A |
Complex
faults are typical of salt domes. A salt dome field may produce
from 10 or 20 separate reservoirs, or even more because of the
faulting and geometry of the flanking sandstone layers. A simple
example is given in Figure 31.09. 
If,
after drawing the patterns, you find regional dip to be greater
than about 4 degrees in any zone, you should have the log re-displayed
with dip removed. Obviously a different amount of dip will have
to be removed from different geologic sections. Pick these values
for each section from your green patterns between each major break
defined by the black patterns.
FIGURE
31.16: Stick plot of regional dip
FIGURE
31.17: Stick diagram for a normal fault with drag
FIGURE
31.18: Stick diagram for an overturned anticline
FIGURE
31.19: Stick diagram for overthrust fault
Between
1,800 and 2,300 feet in Well Y, a reversal of dip direction is
evident, increasing to a high angle to the northwest at 2,300
feet. This is a strong red pattern. It is followed by a blue pattern
of short duration. This
R
R
R
R
