CRAIN'S
PETROPHYSICAL
HANDBOOK
c. 1978 - 2008 E. R. (Ross) Crain, P.Eng.
Rocky Mountain House, Alberta Canada T4T 2A2
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Updated 7 July 2005
CHAPTER
THIRTY-FOUR:
STRATIGRAPHIC ANALYSIS
2 Table
of Contents Guest
Chapter Continue to Chapter Thirty-Five Publication History: This Chapter formed part of Chapter Seven of Volume Two of The Log Analysis Handbook, a series of course notes published by the author in 1978. Revised 1985 and 1993. Revised and re-organized for this electronic edition Oct 2002. CHAPTER
THIRTY-FOUR:
STRATIGRAPHIC
ANALYSIS
2 34.00
Introduction To This Chapter There are only four basic kinds of stratigraphic traps: unconformities, porosity permeability pinchouts, reefs, and drape structures. However, within the permeability pinchout category, there are many different types. Knowing which type is crucial to understanding how to explore for, and develop, these reservoirs. The methods used to identify stratigraphic traps from logs involve curve shape analysis for grain size and environment, analysis of dipmeter data for definition of bedding, and conventional log analysis calculations for porosity and lithology. In addition, the use of formation microscanner images to assess detailed stratigraphy is becoming more common. The end result of the analysis is a description of the rock facies and a three dimensional view of the sedimentary structure. This will include the type of structure, thickness, reservoir quality, and if possible, its shape and probable extent. As with any log analysis technique, calibration and control by using core and sample descriptions is very beneficial. In addition, well to well correlation and mapping can be used to help confirm stratigraphic interpretation made from dipmeter and curve shape analysis. 34.01
Dipmeter Patterns in Sedimentary Structures Standard high density computed dipmeters also display patterns of dip change which may be associated with smaller structures. Increasing dip with increasing depth, over short intervals, (RED patterns) may be related to faults, bars, channels, or unconformities. Patterns of decreasing dips with increasing depths, over short intervals, (BLUE patterns) may be related to faults, current bedding, and unconformities. To be related to stratigraphy, these patterns usually do not cross major lithologic boundaries. However, this rule may be broken if it is known that sediment type changed during a constant sedimentation cycle. Sometimes, the dipmeter pattern is the first clue that this might be possible. A review of sample, core, and palynology is in order if this is suspected. The GEODIP and DUALDIP techniques, and their equivalents from other service companies, reveal such patterns on a much finer scale than the usual HDT or CLUSTER programs. Details of these programs are given in Chapter Twenty-Six. With the increased number of dip determinations, it is possible to relate small scale patterns of dip variations with detailed internal structures of sedimentary bodies. In many cases, stratigraphic analysis can still be done on older HDT data, but the processing and resolution will not provide the same quality of results as more modern techniques. Because we are stuck with what already exists in well files, we will illustrate some examples from the older style logs. Figure 34.01 illustrates how easily and accurately changes in dips in very thin beds can be detected on the GEODIP arrow plot. The two bracketed intervals of lengths 2 ft. and 5 ft., have a southwest dip, deviating abruptly from the west northwest structural dip of the rest of the section. The southwest dip is assumed to represent current direction for those two units.
Such breaks in a geological column, as well as other patterns of sedimentary dips, can be analyzed, along with available information, in terms of lithology, sequential evolution, and depositional environment. This works best in shaly sand series, where scatter in dip magnitude, spread of azimuth variations, and constancy of a preferential direction indicate various types of internal cross-bedding of thin or thick layers. In turn, these features show either an intermittent and rapid deposition, or a continuous one with variable rate, or reworked sediments. The display of both resistivity and gamma ray curves curve on an arrow plot permits the analyst to relate sedimentary dip with lithologic changes revealed by resistivity or shale volume contrasts. Non-planar boundaries between formations signify a break in the sequence of deposition. When such breaks happen within the unit, and not at its limits, turbidite, deep sea fan, or similar facies may be considered. Non-planar dips are indicated when several dips are found at the same depth with a wide spread in dip angle. 34.02
Analyzing Dipmeter Patterns Dip patterns fit one of five general classifications: 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, or differential compaction. BLUE Patterns: decreasing dip with depth, representing current bedding. BLACK Patterns: abrupt changes or breaks in dip and/or direction, representing unconformities, or erosional boundaries between stratigraphic units. YELLOW (RANDOM) Patterns: caused by poor hole condition or random stratigraphic events, such as pre-depositional burrows and cracks. 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 the left side of Figure 34.02. Variations of the basic patterns, called features on the illustration, are given on the right hand side.
To begin analysis, start at the top of the log (or somewhere above the zone of interest) and draw in the green, red, blue, and black patterns, in the order listed. Be careful not to cross a major change in dip direction with one of these patterns. Join arrows which are fairly close in depth. Use the gamma ray, SP, and resistivity curves as guides to formation boundaries. Stratigraphic units seldom cross obvious boundaries, but this rule may be broken, as discussed earlier. The end of a blue pattern can be the beginning of a red pattern and vice versa. Red and blue patterns should have roughly constant dip direction, or else they are not really patterns, merely random dips. In addition, red patterns must have a break at the base and blue patterns must have a break at the top of the pattern. Not all the results need to be included in every pattern. In the example in Figure 34.03, the top half of the log shows a trend of dips at 4 degrees downward to the south southwest - a GREEN pattern between "A" and "B". This is most evident in the left hand log, run with a long correlation interval to enhance regional and structural features. The horizontal line at "B" indicates a break in trend - a BLACK pattern. This is followed by stratigraphic BLUE patterns representing cross-bedding in a meandering stream point bar. This is best seen on the right hand log, run with a short correlation interval to emphasize stratigraphic features.
The scattered dips below the RED pattern represent festoon type bedding. This is followed by a BLUE pattern indicating foreset beds in the base of the sand, probably in a channel fill environment. This is followed by regional dip of 2 degrees to the west between "D" and "E". 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. However, some stratigraphic structures have a large spread in dip angle or direction or both, giving a solid clue to the structure's identity. In these cases, joining dips into patterns may be fruitless or impossible. Instead, an outer boundary may be drawn to reflect the spread. An azimuth frequency diagram will probably be useful in defining dip direction. Keep the scale of features in mind. Structural features (except faults) 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 several 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 blue patterns that are a reflection of faults and unconformities generally persist over much longer intervals.
FIGURE 34.04: Regional dip removal 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. 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. 34.04
Subtracting Regional Dip If, after drawing the patterns, you find regional dip to be greater than about 4 degrees in any zone, you should have the log redisplayed 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. 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 stratigraphic features. The true dip and direction of beds is still contained on the original log, and these are 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, as shown in Figure 34.05, 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.
34.05
Deciding What The Patterns Mean The second is to use a catalog or cookbook of typical patterns to compare your pattern with those already described. As mentioned earlier, regional dip removal can change a pattern, so the cookbook approach is not too useful unless dip removal has been done. Both methods require the use of gamma ray or SP curve shapes and energy level estimates, as described above, to distinguish between various models which may have similar patterns. To draw a sketch of dipmeter data, take a piece of graph paper, blank well log paper, or a photocopy of the dipmeter arrow plot itself. A log at 1 to 240 (5 inches per 100 feet) scale is suitable for stratigraphic analysis. To save effort later, it will be helpful to splice on a copy of the gamma ray or SP log if it does not already appear on the dipmeter. More detailed scales may be needed to analyze GEODIP or DUALDIP logs, such as 1:40 or 1:20. On a clear area of this montage, or on your graph paper, draw a vertical line to represent the wellbore. If the well is deviated, draw the line at this angle. Note that dip angles on a dipmeter are relative to vertical, so keep your dipping beds relative to the vertical, even if the well is deviated. Select the interval you wish to analyze and mark some depth lines to orient your data. Transfer the position of the black patterns to your sketch. These represent breaks in the geologic sequence, such as unconformities or sedimentary structures. Use the gamma ray curve or a computed lithology log and the well history data as guides to major erosional surfaces and the location of sedimentary structures. Next, choose regional dip in each major rock unit and draw short hash marks on the wellbore at an angle representing the actual dip shown on the log. Some vertical exaggeration may be appropriate. At this point you have to decide on the direction of cross section that your sketch will represent. For example, if regional dip is to the northeast, the cross section should run from southwest to northeast. Next position representative samples of the dip from the blue and red patterns onto your sketch. You are really creating your own stick plot. For stratigraphic analysis, it always helps to sketch the curve shape from the gamma ray log (if you are not working on a copy of the log itself) to define which of the three major sedimentary structures are present, regressive sands (funnel shaped - coarsening upward), transgressive sands (bell shaped - fining upward), or high energy (cylindrical - constant grain size). Now comes the hard part. Extend the hash marks to represent the bedding planes of a sedimentary structure, like the ones in Figures 34.06, 34.07, and 34.08. You are only dealing with a few sedimentary models, which are described below. Each model should be reviewed for its characteristic curve shapes and dipmeter patterns, then you can draw a rational interpretation of the dip patterns.
Over the years, I have found that only a rare few individuals have the gift to remember the patterns without aids, such as the service company catalogs of patterns, or the descriptions contained in this Handbook. Be sure to be familiar with the regional geology, the well history data, sample descriptions, and known sedimentary structures in the areas before proceeding. Much has been done in the last 20 years to improve both the dipmeter tool and the data processing capability to provide more detailed descriptions of bedding, lithofacies, and depositional environment. For example, Schlumberger's GEODIP or DUALDIP programs, followed by the SYNDIP program, can use dip data taken at the rate of 60 samples per foot from the Stratigraphic High Resolution Dipmeter (SHDT) and output 1.2 inch results showing dip angle and azimuth, bedding plane linearity, depositional environment, and interpreted lithology. This is done by creating synthetic logs, using principal component analysis, from such things as dip frequency, dip density, dipmeter resistivity curve activity, the ratio of the thickness of positive peaks to negative peaks, and sharpness of the curves. Comparison of these new techniques with standard high resolution dipmeter data is startling; the enormous detail available is almost overwhelming and boggles the mind of most mortals. An example is shown in Figure 34.09. Note that the depth lines are 0.4 meters (a little more than a foot) and that rational red and blue patterns can be seen spanning distances of less than 6 inches! To display this much information, depth scales of 1 to 40 (30 inches per 100 feet) or 1 to 24 (50 inches per 100 feet) are recommended, reminiscent of the 1 to 48 scales used in the distant past for micrologs and microlaterologs. Dip frequency azimuth plots from such data give much stronger statistical evidence of stratigraphic features.
34.06
Sedimentary Models Subaerial,
continental deposits, dominated by ice, gravity, and wind: Fluvial
dominated deposits, mainly rivers and streams: Delta deposits are a complicated environment in which one environment grades into another both vertically and laterally. There are three major zones. The delta plain contains distributary channel, lacrustine delta fill, bay fill, and abandoned channel deposits. The delta front is a high energy zone with tidal and wave dominated activity. Features include delta front sheet sands, distributary mouth bars, tidal channel deposits, nearshore and longshore barrier bars. The prodelta is the transition between the delta front and the marine shelf deposits. These are discussed below in: River
dominated: Tide
dominated: Wave
dominated: Marine
dominated deposits are low energy environments in deeper water: Analysis rules of this type are amenable to processing by an expert system, a computer programming style which allows a reasoned dialog to take place between a user and the computer. Such a system was described by Katherine B. Krystinik of USGS, in "An Example Expert System For Computer Interpretation Of Depositional Environments", USGS Open Report #85-30. The responses to previous questions dictate which questions will be asked next by the system. As you can see, the structured question and answer approach is modeled after the thought processes of an experienced sedimentologist. In using the sedimentary models presented below, you should attempt to create a similar logical sequence of questions and conclusions, eliminating as many possible solutions as you can, until only one answer remains.
Additional hints on interpretation techniques are listed in Figure 34.10, taken from Ed Bigelow's masterful paper "Making More Intelligent Use of Dipmeter Data", The Log Analyst, Jan 1985. Examples and quality control methods are also covered in that paper, which is highly recommended reading.
Readers interested in more detail than can be presented in this Chapter may wish to study "Sedimentary Environments From Wireline Logs" by Oberto Serra, published by Schlumberger, 1985; a 211 page review of the subject. This book is packed with examples and amplification of the sedimentary models presented here. Some of the following sections were condensed from this reference. 34.07
Glacial Deposits Varves are very thin, roughly horizontal layers deposited in a glacial lake bottom from debris rafted out by the ice. At the edge of the lake, distributary channels deliver sediment to delta type deposits with typical delta front foreset beds. Dipmeter patterns look like delta front distributary mouth bar patterns, but bedding planes are very close together. These occur only behind a receding ice front. Advancing ice will scour away any evidence of the lacustrine delta. Figure 11. shows a schematic drawing of a glacial lake environment and a typical dipmeter in a glacial environment.
Alluvial fans are long in the down slope direction and very narrow. Thickness varies from a few hundred to several thousand meters. Dip angle is high and scattered. Dip direction is not a good indicator of body geometry. The lower reaches of the fan may grade into a braided stream environment. A cross section of a scree slope and a typical dipmeter are seen in Figure 34.12.
34.09
Sand Dunes Wind blown dune deposits are often difficult to distinguish from those laid down by water. The mechanics of both processes are quite similar. Although we normally think of dunes as occurring in a desert environment, they often form on beaches and barrier bars, as well as on continental deposits exposed to the air, as in present day Saskatchewan and many parts of North Africa and China. Thus some sediments may go through a wind phase before being finally deposited by water. The dune portion of a barrier bar will be eroded at the top by wave action; this may be repeated many times. Generally, eolian sands are better sorted than aqueous ones, leading to uniformly high porosity and permeability. This makes them excellent reservoirs if they come in contact with source rocks. Some prolific North Sea oil pools are sand dunes. As dunes migrate, sand grains are carried up the windward slope and then roll down the slip face. This results in cross-bedding with enormous set heights. As seen at a borehole, cross-bedding is tabular, high angle, and consistent in magnitude through the height of almost the entire dune. Consistent cross-bedding for intervals of up to 100 feet is common and this distinguishes eolian sands from all others. It is also characteristic that individual dunes, as seen in a borehole, bear no relationship to one another nor to the paleoslope. Wind direction is in the direction of steepest dip and sand body elongation is at right angles to wind direction. Curve shapes are normally cylindrical with funnel shaped bases, the latter being the wadi facies of a dune complex. Figure 34.13 shows the curve shapes and dip patterns on a series of dune deposits.
34.10
Braided Stream Channels The process is similar to alluvial fan deposition, but occurs on flatter ground and gravity falls of debris are not usual. Braided stream channels cover a large depositional area. They are straighter than meandering streams and rivers, which are formed on flatter terrain. The illustration in Figure 34.14 shows a typical braided stream environment.
Braided
stream alluvium is composed of moderately sorted sand and gravel
deposits to the exclusion of silts and clays. When sediments are
well sorted, braided stream deposits show little variation either
vertically or laterally. Both porosity and permeability are high,
forming excellent reservoirs. This textural sequence gives rise
to cylindrical curve shapes when no silt is present and serrated
shapes when silt is present. A fining upward sequence at the top
of each depositional cycle is common. As a result, curve shapes
may show numerous individual patterns which are not correlatable
between wells. This feature is shown schematically in Figure 34.15.
Silt is generally deposited in abandoned channels, giving rise to obvious dipmeter channel patterns which are, unfortunately, not very good reservoirs. The shale content indicators show this without difficulty. Water flow during deposition is highly turbulent, resulting in trough or festoon type current bedding. Dipmeter results are erratic in both dip angle and direction because of non-planar bedding surfaces and incomplete depositional sequences. Dip angle varies between zero and 35 degrees while direction may vary up to 180 degrees, but usually remains within a 90 degree arc, which reflects downstream direction and the direction of elongation of the sand body. Planarity rating of dip results are low. GEODIP and SYNDIP presentation assist in recognizing the non-planar beds.
The Mississippi River is a modern example of such a stream. Point bar deposits are also called lateral accretion deposits. At the base is an erosional surface overlain by pebbles and a sequence of sands with an upward decrease in grain size. Coarse festoon cross-bedded sands grade up into tabular cross-bedded sands of diminishing set height. These in turn grade into flat bedded fine sands and then into silts. This sequence gives rise to bell shaped curves. The reservoirs do not take the form of the meander channel but rather that of curved, tabular wedges of sand occupying a large portion of the meander belt. These may be separated from each other by abandoned channel or oxbow lake facies, filled with silt. The meander belt can be up to 20 times the width of the stream. Repeated reworking of the deposits within the meander belt winnows the fine grained material and results in a progressive downstream decrease in grain size. It also results in interrupted sequences and stacking of several of the basal coarse grained parts. Repeated fining upwards patterns (bell shaped) with a coarse grained zone of variable thickness at the base of each cycle is common. Dip magnitude will be erratic and high angled at the base in the festoon bedded sands, progressively becoming more consistent and flatter upwards in the tabular beds. Because of the wide swing in the direction of the depositing currents and the type of current bedding, a variation in dip direction of 180 degrees is normal. The average dip direction should reflect the overall downstream direction of the meander belt and the trend of the separate reservoirs. 34.12
Channel Cut and Fill Drape over the top of these sand bodies is not usually present. The channel fill itself, however, may drape or sag towards the axis of the valley. The drape within the channel, depicted schematically in Figure 34.17, should not be confused with drape ABOVE reefs or bars. Check the curve shape or lithology log to verify that the drape is inside the channel.
FIGURE 34.18: Dipmeter in channel cut and fill environment The red patterns still point to the center of the channel, and blue patterns point downstream. Pattern frequency azimuth plots are useful for sorting out these directions. Curve shapes are cylindrical or serrated cylindrical, depending on silt or shale deposition. Bars and channels can be mistaken for each other on logs and cross sections. In the case of a stream channel, the cross section of the sand deposit has it's greatest width at the top and a base that is convex downward. The sand bar has a cross section that is widest at the base, relatively flat at the bottom, and a top that is convex upward. Dipmeter data will usually resolve the two cases. The channel may have drape within the sand body; the bar may have drape above the sand body. 34.13
Delta Distributary Channels
FIGURE 34.19: Dipmeter in delta distributary front environment
When drape occurs in the channel, the dips point at right angles to the strike. These relatively low angle dips, when observable, are due to channel cutting, and arise from the channel base changing in a series of progressively shallower concave surfaces as infilling proceeds. Unlike braided or meandering stream deposits, which are quite wide due to lateral migration of the channels, distributary channel fills will produce long, narrow reservoirs, often with very thick sections. Distributary channels sit within a delta front sequence (described in the next section), and the channel fill curve shapes will be contiguous with delta front shapes immediately below. The distributary front is characterized by foreset beds (blue patterns) below the base of the channel. A reworked sand may separate the two, defined by random dips. Two examples are shown in Figures 34.20 and 34.21. 34.14
Delta Front Distributary Mouth Bars
There
are three main types of deltas: Figure 34.23 illustrates these three common forms of deltas. The arcuate delta appears to be the most important as a potential oil trap. Several Pennsylvanian sand fields in Oklahoma are deltaic in origin, as are many of the offshore Gulf Coast fields.
The form developed by a delta depends on the sediment load and the relative strengths of fluvial and marine processes. Where river currents clearly dominate, a highly constructive bird's foot delta, such as the modern Mississippi, will form. These are characterized by elongate bar fingers containing the distributary channels. Where marine processes such as longshore currents are more powerful, a cuspate type delta will develop. An example is the modern Baram delta of Brunei-Sarawak. This type of delta has few distributaries and grows by pro-grading wave generated beaches. A more balanced situation results in a lobate delta form such as the modern Niger. A cross section of a pro-grading delta front in a highly constructive situation is shown in the bottom half of Figure 34.23. This shows the relative positions and the lithologies of delta front deposits. A highly constructive delta is most favorable to the formation of distributary mouth bars or delta bar fingers. These are sands and silts dropped in front of the mouths of distributary channels which suffer little or no reworking by wave motion. The river currents are the principal factor in determining sand body geometry. Sand bodies usually take the form of elongate or lobate masses extending outward from the river mouth.
Distributary mouth bar sands are relatively fine grained and moderately sorted. However, curve shapes reflect a general coarsening upward in a highly serrated funnel type configuration. The serrations arise from thin shale layers laid down in times of low water flow. A typical example can be found in Figure 34.24. Current bedding is normally tabular and dips in the seaward direction, perpendicular to the strand line, unless deflected by longshore currents. The current bedding dips in the direction of sand elongation. The cross-bedding angle is steepest at the top of the sand unit and decreases downward (a blue pattern). Individual sand units are normally relatively thin. It is not uncommon to have a distributary channel cutting through the top of a distributary mouth bar (a red pattern on top of a blue pattern), shown in Figure 34.25.
Distributary fronts vary in shape and size due to differences in transport speed and volume, and the interference of the ocean. Long narrow fronts, as may be found in a bird's foot delta, have a relatively high spread in the dip angles of the foreset beds, usually greater than 10 degrees. More compact or fan shaped fronts have dips in their foreset beds of less than 10 degrees (Figure 34.26).
Narrow estuaries develop elongate sand bodies with characteristics similar to those of distributary channel fills except that cross-bedding may be bimodal, that is, cross-beds dip both toward and away from the sea in alternating layers. On the other hand, very wide estuaries create tidal flats which contain some sand, but are often predominately mud. Deposits formed in wide tidal estuaries tend to be a grouping of roughly parallel elongate sand bars amid silts and muds, as shown in Figure 34.27. In cross section, the profile shows coarse sands at the base, grading erratically upwards into shales, with a serrated bell curve shape. A typical example is given in Figure 34.28.
FIGURES 34.27 and 34.28: Sedimentary model and dipmeter in tidal flat environment
Each sand is a separate reservoir, and several producing trends may develop. Such trends can extend for many miles with production confined to the regional noses or highs. The Cotton Valley sands in north Louisiana produce along a trend of over 100 miles. Where drilling is sparse, it may appear that the different beach sands are all the same sandstone body. After sufficient drilling, these apparently blanket sands usually break up into their respective components. Drilling and production decisions will be drastically altered if multiple sands are mistakenly identified as a single sand. Current bedding reflects the wave action showing gentle, tabular, unimodal cross-bedding, with lower angle dips at the base and steeper at the top of the sand. The direction of cross-bedding is seaward, normal to the direction of elongation of the sand body. Beaches cannot be distinguished easily from distributary mouth bars by dipmeter data. Stacking of several beaches is common and both regressive (funnel shaped), transgressive (bell shaped), and cylindrical curve shapes are possible. In rare cases, overlying shales may show some draping dip. This dip points in the direction of seaward pinchout, but in practice it probably occurs over surfaces which have been cut by erosion, and therefore may not indicate the coastline orientation. As mentioned, beaches generally give rise to long, narrow sand bodies. On the other hand, deltas dominated by longshore currents create beaches which tend to form to the side of, and between, distributary mouths. As the delta progrades, sheet sands are formed with regressive characteristics and low angle, tabular cross-bedding. These are sometimes broadly classified as delta front sands. Log curve shapes and dip data cannot distinguish between these sheet sands and narrow beach sands.
Basal unconformity sands have stronger cross-bedding angles, proportional to the topographic relief. In addition, there is usually draping dip due to differential compaction in the overlying beds. Draping dips point away from the local high on the erosional surface. Granite wash sands typically exhibit this behavior. These sands are composed of granite fragments, feldspar, quartz, and silt. They do not have very good curve shapes due to the radioactive elements affecting the gamma ray log and the impermeable granite affecting the SP. The density log curve shape or a well computed lithology log using natural gamma ray spectral and photoelectric data may do the job.
With
an adequate supply of sediment, a bar becomes emergent, forming
a barrier. Barrier bars have gentle dips on both sides and are
often associated with coal swamps or evaporitic lagoons on the
landward side. The barrier bar is formed by regression of the
shoreline, and contains three fundamental units: These units can usually be identified on logs by the coarsening upward (funnel) curve shape. If sequence is surrounded by shales, the shale may undergo more compaction than the sandstone of the barrier bar. This differential compaction, or drape, over the sand bar is noticeable on dipmeter data or on well to well correlations, and is a strong clue to following the bar for future drilling. An example of curve shapes and dip characteristics of a bar type deposit is shown in Figure 34.29.
34.19
Marine Shelf Sands (Blanket Sands) There is no good standard type section for marine shelf sands. The shelf sands themselves usually have a rounded base and rounded top on the gamma ray or SP log, but stacking of these sands, and reworking of the top into bars makes the patterns difficult to spot. Erratic dips at the top surface due to ripples, scouring, and animal burrows or tracks may be visible. Sorting may vary from good to poor. Curve shapes, on the average, will exhibit a serrated combination funnel-bell appearance. Current bedding is low angle and polymodal or random. An example is shown in Figure 34.31.
These true blanket sands should not be confused with beaches and bars (which are sometimes called blanket sands), as their curve shapes and dipmeter patterns are very different, not to mention the reservoir extent and quality.
FIGURE 34.32: Dipmeter in marine shelf (blanket carbonate) environment Bell and cylindrical curve shapes usually correlate to red patterns, and bell shaped to blue patterns. However the shapes are not taken from the usual gamma ray and SP logs. They are taken instead from the dipmeter microresistivity curves. Any one of the 4 or 8 curves, or the resistivity correlation curve, will do.
Present day reefs occur mainly in shallow tropical seas. Reef growth requires sunlight (clear water, shallow depth), oxygen (rough water), food supply, and a favorable temperature. Fringing reefs are linear and stretch parallel to the coast with no intervening lagoon. Barrier reefs are similar but a lagoon separates them from the land. Atolls are subcircular reefs enclosing a lagoon often built around a sinking volcano. The seaward or fore reef edge of a reef is normally quite steep and may form a talus slope of reef detritus at its base. Cross-bedding dips pointing away from the immediate reef high can be measured in this detrital wedge. The back reef, or quiet lagoonal side of the reef, is made up of very fine grained material, interbedded with calcareous mud and may show relatively flat beds or no bedding at all on the dipmeter. Figure 34.33 shows a cross section of a reef.
After reef organisms are killed due to some change in conditions, the reef mass may be buried in mud. Overburden weight causes compaction in the muds leading to sizeable draping dips. Compaction may be as great as 50%, generating draping dips as high as 30 degrees. Draping dips point down, away from the reef buildup.
A rule-of-thumb has been established to determine the reef shape. In the first case, compaction contemporaneous with deposition is characterized by low resistivity shales exhibiting a gradual buildup of dip versus depth. Here, reef front angle equals maximum draping dip plus 10 degrees. If compaction is mainly after deposition, characterized by a fairly constant draping dip in the overlying shale, then reef front angle is about twice the maximum draping dip. In some areas of the world, such as northern Alberta, salt solution around the reef accentuates drape, and the above rules do not apply. Reef porosity is extremely variable and follows no particular pattern versus depth. The biolithic zones, or reef core, will normally be the most porous. All three usual rock types, biolithite, calcarenite (fore reef), and calcilutite (back reef) may develop porosity in the form of vugs, fractures, and dolomitization. Porosity curve shapes are therefore not very predictable, but the gamma ray will be cylindrical. The SP usually will not have a meaningful shape due to the effect of nearby impermeable carbonates. The percentage of dolomite can be determined by porosity log crossplots and calcarenite (limestone matrix) can be distinguished from calcilutite (quartz/chert matrix) in the same manner.
Turbidite deposits can be tabular, elongate, or fan shaped. Individual sand beds are poorly sorted, but the upward fining of grain size produces roughly bell shaped log curves for each cycle of deposition. Rhythmic alternation of graded beds with shales produces a stacking of numerous similar curve shapes. A characteristic of turbidites, in spite of being a high energy deposit, is the absence of appreciable cross-bedding. Dipmeter results will therefore show little variation from structural dip, and will not be very helpful in defining sand body geometry. Figure 34.36 shows LOCDIP results for a sand turbidite sequence and another from a carbonate turbidite.
Turbidites deposited in deep marine basins may be interbedded with muds which can be hydrocarbon source rocks. However turbidite sands do not generally make good reservoirs because poor sorting and clay matrix inhibit porosity and permeability. The thin bedding of turbidite sands and the intervening shales make reservoirs numerous, but thin and disconnected.
Additional patterns for structural features are included in Chapter Thirty-One. Since some structural patterns can be confused with some stratigraphic ones, you may need to review that Chapter before settling on the final interpretation. You
should study these patterns carefully, comparing patterns from
various structures to define differences and similarities. Previous
examples of both curve shapes and dipmeter patterns in this Chapter
are also worth reviewing.
Disconformity: This erosional surface will not be indicated by a dipmeter because the dip direction and magnitude do not change.
Angular
unconformities show up on a dipmeter as a marked change in dip
angle. The dip direction will probably change in addition.
Angular unconformity that may be the result of folding of the formations before erosion. Similar patterns can be produced by slumping of beds below an unconformity.
Salt
dome, a structural feature often found in the Gulf Coast area,
causes formations to bend appreciably. These forces compact and
often rupture the formations involved. The dip pattern can be
in a constant direction with high dip angles. The dip direction
will normally be away from the center of the dome and will occasionally
reflect a side profile of the dome.
Lenticular cross-bedding, erratic dips occurring in definite sets. These are the dips commonly referred to as cross-bedding in sands where the dips are in all directions. The angle can be up to 30 degrees above the structural dip impressed on the adjacent formations. This difference is algebraic and dips can vary from above regional to below regional, depending upon which direction the beds have been tilted subsequent to deposition.
Tabular
planar cross-bedding, most sets dip in the same direction but
at different angles. Each set of cross-beds has a consistent angle
and direction. Individual cross-bed sets can range in thickness
to a hundred feet or more depending upon the environment of deposition.
Wedge-shaped cross-bedding sets characteristic of aeolian deposits. Patterns of markedly different directions with good correlation and several arrows in each group; this is probably reflect wind deposition.
Foreset
bedding found frequently in deltaic deposits. The arrows indicate
the direction the current was flowing at the time of deposition
if the structural dip of the region is subtracted from the dips
recorded. The minimum dips at the base of each sequence frequently
reflect the structural dip of the area as these were nearly horizontal
at the time of deposition.
A sand bar can often be detected on the dipmeter if the well is drilled where the borehole passes through the steep side of the sand bar. The dip increases rapidly to the top of the bed boundary, then gradually decreases through the upper beds until structural dip is again evident. This pattern of increased dip with depth and then a return to the normal trend is also characteristic of a normal fault. Here again, the need for some lithology definition is important. The
channel sand shown is an ideal type and many channels are not
easily interpreted from a dipmeter. Many channel sand sequences
do not exhibit the increase of dip as the bottom of the channel
is approached. This increase of dip with depth as the bottom of
a channel is approached probably occurs only a small percentage
of the time. Many thick sequences of fluvial (water deposited)
sands show no indication of this phenomenon. Many people have
become disenchanted with the use of the dipmeter for interpreting
stratigraphic traps because its use in channel sands has been
considerably oversimplified. In deltaic sequence, the "classical"
pattern is frequently found, but many other channels are not interpretable
in any simple way from the dip log.
Carbonate reef interpretation from a diplog is fairly straightforward for reef facies, but becomes rather complex in back reef facies. Under favorable conditions, a reef will grow upward until its organisms die due to some extreme change in environment. The reef may then be enveloped by deposits of mud. The effect of overburden and compaction will shape this new shale around the reef. The shale will reflect a pronounced change on the flanks of the reef and the shale above will reflect a lesser, but distinct amount of dip. When the reef itself is encountered the dip pattern will usually be scattered and exhibit increased dip angles. These are usually measurement of vugs, fractures, joints, etc., and should not be interpreted as a structural picture of the reef. Graded-bedding is frequently found in beds deposited by turbidity currents. These beds are originally deposited nearly parallel with pre-existing surfaces, but may have large dips impressed upon them by orogenic movement which has taken place after deposition. The SP may not indicate the cyclic repetition if clay minerals are not more abundant in the finer grain segments of the turbidites.
Bimodal distribution of red and blue patterns indicates probable deposition in a stream channel to the East of the channel trough. The current flow is to the South. Trimodal
distribution of patterns. This pattern indicates probable deposition
in a stream channel near the trough and with the current within
the channel flowing to the South.
Unimodal distribution of red and blue patterns. Usually associated with barrier bar and other shoreline sediments where primary action is perpendicular to the trend of the sand body. The angles can be more dispersed if the waves hit at angles other than perpendicular or if longshore currents influence deposition to any extent. Bar trends from East to West and thins to the South. Bimodal distribution of dip patterns probably indicating a well drilled through the crest of a barrier bar type deposit. 34.24
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.
1. Sketch each of the following structures and draw a dipmeter arrow plot for each: (100 marks) a. Glacial Deposits b. Alluvial Fan and Scree Slope Deposits c. Sand Dune Deposits d. Braided Stream Deposits e. Meandering Stream Point Bars f. Channel Cut and Fill 9. Sketch each of the following structures and draw a dipmeter arrow plot for each: g. Delta Distributary Channels h. Delta Front Distributary Mouth Bars i. Tidal Channel Deposits j. Beach and Shoestring Sands k. Basal Unconformity Sands l. Offshore Bars and Barrier Bars m. Marine Shelf Sands (Blanket Sands) n. Marine Shelf Carbonates o. Reefs and Carbonate Banks p.
Turbidite Slumps 2. Draw an interpretation sketch on each of the illustrations on the following pages, (100 marks)
34.26
Bibliography for Chapter Thirty-Four 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 mid-continent; 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 14th Annual Logging Symposium, p. 5-18, 1973 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 non-uniform 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 |
FIGURE
34.14: Stratigraphic model for braided stream environment
Point
bars are formed in the inside of bends in rivers and streams,
where the current slows down and drops out some of its sediment
load (Figure 34.16). These bars are small, and difficult to find
due to the meandering nature of the original river. They are attractive
exploration targets because their reservoir characteristics are
usually good. 
FIGURE
34.16: Stratigraphic model and dipmeter in meandering stream environment
FIGURE
34.17: Stratigraphic model and dipmeter in channel cut and fill
environment. 
Foreset
beds within the channel, shown in Figure 34.18, mask the drape
effect.
By
blockage of the mouth or shifting of the stream above, stream
velocity may drop and deposition will occur. The depositing material
is coarse grained and well sorted. Normally more coarse material
is found at the base and there is a general fining upward. However,
in many cases, the entire channel becomes clogged with uniform
sands. These give rise to characteristic cylindrical curve shapes,
possibly grading into bell shaped at the top. 
FIGURE
34.24: Dipmeter in distributary mouth bar environment
In
subtidal marine carbonate detrital sections, detailed dipmeter
analysis is fruitful if the data is of good quality. Usually the
GEODIP program is necessary. Red and blue patterns have their
usual relationships to transgressive and regressive behavior,
but they will often be very short patterns due to the slow depositional
process this far offshore. Dip direction is unimodal, in the deepening
(usually thinning) direction. Direction of elongation is perpendicular
to dip direction. 
FIGURE
34.33: Sedimentary model in marine reef carbonate environment
Figure
34.37: Arrow Plot - Disconformity and Angular Unconformity
Figure
34.38: Arrow Plot - Angular Unconformity and Drape Over Salt Dome
Figure
34.39: Arrow Plot - Festoon or Lenticular Cross-bedding and Tabular
or Planar Cross-bedding
Figure
34.40: Arrow Plot - Nonparallel Cross-bedding and Foreset Cross-bedding
Figure
34.41: Arrow Plot - Sand Bar and Drape Inside Channel Sand
Figure
34.42: Arrow Plot - Drape Over Reef and Deep Water Turbidite
