CHAPTER THIRTY-THREE: STRATIGRAPHIC ANALYSIS 1
Depositional Environment

Table of Contents
33.00 Introduction To This Chapter
33.01 Rock Facies - Origin and Depositional Environment
33.02 Classification of Depositional Environments
33.03 Sedimentary Structures
33.04 Genetic Units
33.05 Marine Transgressive Overlap - Fining Upward Sequence
33.06 Marine Regressive Overlap - Coarsening Upward Sequence
33.07 High Energy Marine Deposition - Cylindrical Sequence
33.08 Curve Shape Patterns in Continental Sequences
33.09.Stratigraphic Traps
33.10 Grain Size and Depositional Environment
33.11 Dip Spread and Depositional Environment
33.12 Current Bedding and Depositional Environment
33.13 Curve Shape Analysis and Depositional Environment
33.14 In Conclusion
33.15 Exercises for Chapter Thirty-Three
33.16 Bibliography for Chapter Thirty-Three

Continue to Chapter Thirty-Four

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-THREE: STRATIGRAPHIC ANALYSIS 1
Depositional Environment

33.00 Introduction To This Chapter
This Chapter covers evaluation of depositional environment, sedimentary models, and bedding patterns. Stratigraphic dipmeter patterns and case histories are covered in Chapter Thirty-Four. Interpretation of structure was covered beginning in Chapter Thirty-One. Dipmeter tool theory and data processing methods were described in Chapter Twenty-Six and will not be repeated here. However, analysis techniques which may be peculiar to sedimentary studies will be described where appropriate.

The sedimentary rock sequence can vary considerably in thickness, texture, grain size, and lithology from place to place. These differences create traps that will hold hydrocarbons which are called stratigraphic traps. Superimposed regional or local structure may also play a role in stratigraphic traps.

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.

33.01 Rock Facies - Origin and Depositional Environment
A description of a rock by its detailed type, origin, and depositional environment is usually called a facies description. It can be derived by observation of the rocks, or inferred from analysis and interpretation of well log data. To determine facies from well logs requires calibration to known rocks (cores, samples, or outcrops). Understanding the rock facies is the only way to reconstruct the paleogeography of a rock sequence, which in turn provides clues as to a potential reservoir's quality and lateral extent.

Facies description based on well logs is often called electrofacies analysis, because electrical logs are used. However, radioactive and acoustic data is also incorporated, so this handbook does not stress the term electrofacies, as it is slightly misleading.

The rock type can be derived from:
1. observation of samples
2. observation of cores
3. lithology analysis of an adequate log suite

If the world was perfect, all three sources of data would be available and would agree with each other. The data sources do not always agree, so the analyst must learn to compare, contrast, and possibly discard some data.

The origin of a rock can be inferred from its present depositional environment and a reconstruction of paleogeography. Both of these can, at least sometimes, be inferred from log data, especially from dipmeter data, which tells us about depositional energy and direction of transport, in conjunction with other log curves, which suggest the grain size of the rock. Log analysts usually concentrate on depositional environment and bedding patterns, along with dip direction and angle, and provide this information to geologists who make subsurface maps representing the analysis.

Geologists who do the whole job need to have special skills in open hole log analysis and should not rely entirely on the curve shapes of the raw logs. For example, the curve shapes on SP and gamma ray logs may be easy to interpret in a conventional shaly sand sequence, but could be very misleading in a complex sequence of anhydritic, dolomitic, shaly sands bounded by carbonates.

With this warning in mind, we will plunge into the standard topics.

33.02 Classification of Depositional Environments
The environmental classification (Figure 33.01) is:
1. continental
2. coastal or transitional
3. marine

FIGURE 33.01: Depositional environments

Most detrital sediments are continental or transitional, and most chemical sediments are marine.

33.03 Sedimentary Structures
The term sedimentary structures refers to stratigraphic features in the subsurface, created by erosion and deposition of sediments, as opposed to tectonic structures created by tension, compression, uplift, and subsidence.

There are four basic kinds of stratigraphic traps: unconformities, porosity or permeability pinchouts, reefs, and drape structures. River channels, beaches, bars, and deltas are sedimentary structures, usually associated with porosity pinchout traps. Drape structures over these may form additional traps.

Nearly one-third of the important oil fields of the United States are stratigraphic traps and many were discovered by random drilling rather than by scientific exploration methods. This indicates that strat traps are fairly common in the subsurface and make up a tremendous potential oil and gas resource. Today, 3-D seismic and sequence stratigraphy have evolved to the point where start traps can be defined quite accurately and even very small targets are drilled on purpose instead of by accident.

The analysis of sedimentary structures from logs, augmented by core, sample, and seismic data, is somewhat complex. There are, however, only a few major types of sedimentation patterns. Most of these patterns can be represented by a set of models which serve as a basis for interpretation and comparison by log analysts. The methods used 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, followed by geological mapping.

The difficulties in identifying sedimentary structures, and hence their associated facies descriptions, include the following:
1. interpretation is based on multiple lines of evidence (eg logs, cores, inferred geometry, fossils, mineralogy) obtained concurrently or in no special order.

2. there may be no unique solution even if all possible data were available.

3. interpretation is based on the preponderance of evidence, no single item will conclusively prove a hypothesis.

4. absence of a feature is common, so such absences do not help the analysis.

These points should be seriously considered when presenting results of a geological analysis of log data.

Sedimentary structures can be subdivided into predepositional, syndepositional, and postdepositional sedimentary features, which aid in describing the sequence of events which created the structure.

Predepositional sedimentary structures are those observed on the underside of a bed. These include erosional features, scour marks, flute marks, ripple marks, mud cracks, worm burrowings, grooves, and channel cutting. Of these, only channel cutting may sometimes be recognized on the dipmeter by the log analyst, although the smaller events may be seen on Formation Microscanner images.

Postdepositional sedimentary structures are those observed on the top side of a bed. These include load casts, quicksand structures, and movement by slump or creep. Drape due to differential compaction, and its counterpart, sag, can be measured by the dipmeter and can be diagnostic of certain types of sedimentary structures.

Syndepositional sedimentary structures are those occurring within the bed and take the form of cross bedding or current bedding. We are usually interested in the magnitude of current bedding angles, their characteristics such as whether the current beds are planar, wedge shaped, or festoon type, and their variations versus depth. These factors provide clues to the depositional mechanism, which in turn define the significance of the structure as a potential source of hydrocarbons.

33.04 Sequence Stratigraphy and Genetic Units
Sequence stratigraphy is a phrase used to indicate a method for describing the depositional environment of a sequence of rocks. The terms stratigraphic unit, genetic unit, or genetic increment of strata (GIS) are used to describe the presence of a sedimentary structure. A genetic unit encompasses the structure and its surroundings, usually the interval between an upper and a lower marker bed or lower erosional surface, with the sedimentary structure sandwiched in between. The marker beds are usually shales. The more massive shale beds are called maximum flooding surfaces and more minor shales are called local flooding surfaces. The name suggests that the breaks between successive genetic units are caused by inundation which stops this particular depositional cycle.

In this way, the context of the structure in its surroundings is used to help define the structure. Figure 33.02 illustrates some typical genetic units.

FIGURE 33.02: Sequence stratigraphy and genetic units

A GIS is made up of a series of depositional sequences, each being a further series of conformable strata or beds, as shown in Figure 33.03. The bedding planes within the depositional sequence are useful to us, because their dip angle and direction can tell us something about the arrangement and possible extent of the beds. The structure of these genetically related beds determines whether or not a stratigraphic trap is formed.

FIGURE 33.03: Bedding planes define genetic units

A genetic sequence of strata (GSS) is a group or series of GIS's, laid down with reasonable continuity, ie., there are no major unconformities or major changes in depositional environment. Thus, a GIS may be repeated several times in vertical succession. A GSS corresponds roughly to a formation and a GIS to a unit or member of the formation.

33.05 Marine Transgressive Overlap - Fining Upward Sequence
A sedimentary structure is generally initiated by the accumulation of sediment on an old erosion surface. Deposition does not occur everywhere at once. Sediment will be deposited gradually at a location close to the source of sediments and will be spread outward from the source. This process is called overlap since the new sediments gradually overlap onto the older sediments.

Two kinds of overlap are recognized. Transgressive overlap occurs when the sea is advancing, or transgressing, upon a low, relatively flat land mass. In this case the land mass is subsiding relative to sea level. At the shore line, sand and gravel accumulate. Away from the land, beyond the beach sands, mud is laid down, and beyond that there may be organic oozes deposited on the sea floor. See top half of Figure 33.04.

FIGURE 33.04: Transgressive overlap - fining upward sequence (top)
Regressive overlap - coarsening upward sequence (bottom)

The sand, mud, and lime phases extend along the coast in three roughly parallel belts. As the sea encroaches on the land, the three types of deposits remain in the same relative position to each other, but they shift landward. The beach sand and pebbles are laid down on the old erosional surface. The mud from this phase overlies the sand deposited earlier, and the ooze overlies the older mud layer. Thus mud overlaps the sand and ooze overlaps the mud. This is marine transgressive overlap, and grain size becomes finer in the upward direction.

Thus, a vertical section through the formation shows finer grained rocks lying over coarser rocks, or deep water sediments over shallow water sediments. These sands are described as "fining upward". The gamma ray and SP curve shapes are as shown in the top of Figure 33.05, and are called bell shaped curves, due to the visual effect generated by a pair of regular and mirror image curves. Typical transgressive sedimentary structures are alluvial point bars in meandering rivers and tidal channels. Delta distributary channel fill can also show transgressive curve shapes, but more show regressive or high energy shapes.

FIGURE 33.05: Fining upward, coarsening upward, and cylindrical curve shapes

A serrated or saw tooth curve shape may be superimposed on a bell shaped curve; this is caused by interbeds of shale between the sand layers. Such interbeds represent short periods of deeper water deposition, possibly caused by floods or erratic transport of sediments.

33.06 Marine Regressive Overlap - Coarsening Upward Sequence
Regressive overlap is produced when the sea recedes from the land, as when the land mass is uplifted or the sea bed subsides. By this process, the beach zone migrates out over earlier offshore muds. See Figure 33.04, bottom half. At the seaward end, mud beds overlap the older ooze. A vertical section shows coarser material overlying finer material, or shallow water sediments overlying deep water sediments. These sands are described as "coarsening upward" and have a gamma ray or SP curve shape as shown in the center of Figure 33.05. These are called funnel shaped curves and would be indicative of barrier bars, or the edge of delta fronts. These curve shapes may also be serrated or saw tooth shaped.

Sea level dropping relative to the land is not necessary for marine regressive overlap. The same regressive sequence is produced if the sea level is stationary and there is sufficient supply of sediments, or even if the sea level is rising, provided the rate of supply exceeds the rate of rise of the ocean.

Different types of overlap often combine in the same stratigraphic section. Thus the sea may advance on the land and then recede, so that regressive overlap is found above transgression. This alternation may be repeated many times.

33.07 High Energy Marine Deposition - Cylindrical Sequence
Distributary channels in delta sequences represent high energy deposition. At flood stage, these deposits will fill old channels while new ones are formed. The sand bodies will be quite uniform, with large grain size and little visible bedding. These form cylindrical curve shapes, similar to those in the bottom illustrations on Figure 33.05. If flooding is erratic, the curve shape may be serrated.

Similar curve shapes can be formed in deeper water as sand slides, or fans, formed at the end of submerged canyons. Turbidite deposits, formed in deep water by slumping of unconsolidated slopes or by rapid movement of heavy, silt laden water, form serrated cylindrical curve shapes, often covering hundreds or thousands of feet of vertical section.

33.08 Curve Shape Patterns in Continental Sequences
Bell shaped curves are found in alluvial point bar sands, in meandering streams, and in drape inside a channel fill. Funnel shaped curves occur in foreset and crossbeds in channels. Cylindrical patterns are restricted to sand dunes and scree slopes, the latter being often serrated.

There are obvious ambiguities between marine and continental curve shapes, so curve shape alone will not suffice to uniquely determine environment. With all four sources of environmental data, namely curve shape, dip angle and spread, bedding type, and shale volume, it is usually possible to assess the environment and hence the sedimentary structure quite precisely. If one source is missing, especially the dip data, life becomes more difficult, but regional knowledge will usually fill in the gaps.

33.09 Petroleum Traps Formed By Stratigraphy
1. Unconformities
An unconformity is a hiatus in the normal geological sequence caused by a break in the process of deposition, by erosion, or by structural deformation. It results in a missing amount of sediments corresponding to a missing geological time as compared to the normal sequence. It is made of two different series of strata separated by a surface of unconformity.

Strictly speaking, there are four main types of unconformities:
1. nonconformity, in which sediments overlie igneous rocks.
2. paraunconformity, in which strata are parallel on both sides of the unconformity, but some of the rock sequence is missing, due to lack of deposition (not due to erosion).
3. disconformity, in which strata are also parallel on both sides, but there is an erosional surface as well as the missing section. Lack of deposition may also have occurred.
4. angular unconformity, in which the strata above and below are not parallel. Erosion has almost always taken place.

The last two mentioned are the only ones to concern us, and are illustrated in Figure 33.06.

FIGURE 33.06: Unconformities and disconformity

The contact between sedimentary layers and intrusive salt, gypsum, and shale domes is very similar to an angular unconformity, but the process is caused by compression and the traps are considered to be structural rather than stratigraphic.

Unconformities can be classified:

1. according to lateral extent as:
- regional, occurs across a large area or possibly the entire basin
- local, occurs over a small area

2. according to the amount of missing geological time as:
- major, where a long time sequence is missing
- minor, where a short time span is missing

Since there is no change in dip trend between the upper and lower strata of a paraunconformity or a disconformity, it may go completely unnoticed except for changes in microfossils. However, a disconformity may be detected by the following features:

- weathered zone, reflecting the effects of the work of ground water, such as solution vugs or caverns, brecchia, jumbled rock, or cross bedding occurring immediately above or below the the disconformity surface.

- local erosion, which can result in a local high or local low at the unconformity surface. When deposition resumes, this dipping surface is filled by the overlying sediments. The erosional surface is sometimes apparent on logs.

Disconformities and angular unconformities are relatively easy to spot on a dipmeter log; they are the so-called black patterns, or major changes in dip angle or direction. The dip of bedding planes above an angular unconformity differs from that of the bedding planes below. The underlying strata have been tilted during the period of non-deposition. Like faults, angular unconformities are characterized by a change of trend of dip (either dip angle or dip azimuth or both). In addition, erosion may cut a pre-existing structure and produce an irregular topography, characterized by varying dip angles and directions on the old surface.

There may be no significant curve shape anomaly at an unconformity, for example where one shale bed lies unconformably on another. There may be a resistivity or apparent porosity change due to differences in silt content or shale compaction. If the unconformity is at the top of a sandstone, the erosional surface may create a very sharp break at the top of a regressive sequence or at the top of a high energy sequence, but may go unnoticed at the top of a transgressive sequence.

It is easy to mistake a fault for an angular unconformity and vice versa, based on dip information only. In general, dip is steeper below the unconformity surface than above it, although more recent tilting may have altered this relationship. Weathering, local erosion, and change in rock quality may occur at an angular unconformity as well as at a disconformity, and gouge or brecchia may occur at a fault. Both cases produce erratic dips at the boundary.

Unconformities and disconformities do not always form traps, but if porous rock lies below and shale above the unconformity, the regional picture may provide the trapping mechanism, especially in the case of angular unconformities. The most familiar unconformity sand trap in the United States is the East Texas field; it has produced over 3.1 billion barrels of oil since its discovery.

A similar unconformity in Canada, with far less reserves, is formed by the Mississippian unconformity, capped by Jurassic shales. Similar traps, not too far away, occur where the Jurassic sandstones pinch out under the Lower Cretaceous shales. A typical unconformity trap is illustrated in Figure 33.07.

FIGURE 33.07: Typical unconformity trap

2. Porosity Permeability Pinchouts
As a general rule, shallow water sandstone beds are likely to thin, and deeper water beds such as limestones are likely to thicken, away from the shoreline. If a rock layer continues to thin in a certain direction, it may finally pinchout or lense-out altogether. The beds above and below it will then become contiguous. This can happen to blanket sands, beaches, bars, and delta fronts and are called sand pinchouts. River channel fill and point bars in meandering streams and rivers thin toward their edges; this effect is also called a pinchout. The thin edge of any sand body can be described as a pinchout.

The large Pembina field in Alberta is a good example of this type of trap. These traps extend for many miles along a fairly narrow belt at the updip limit of the sand. Although sand pinchouts are stratigraphic traps, folding and faulting may be important in controlling production.

Frequently porous limestone or dolomite grades updip into a non-porous rock. These are called porosity permeability pinchouts and may be of local or regional importance. The Carthage gas field in east Texas is an excellent example of a permeability pinchout. The producing limestone grades updip into an impermeable limestone that is barren. Later arching of the sediments formed the Carthage pool that covers nearly 250,000 acres.

Typical examples are shown on Figure 33.08.

FIGURE 33.08: Sand pinchout (top) and Sand channel (bottom)

A porosity permeability trap is formed when the thin edge of the porosity is updip from the thicker part of the zone, as in the top half of Figure 33.09.

FIGURE 33.09: Permeability pinchout trap (shaly sand shown, similar traps are also formed in carbonates)

Obviously very complex combinations of deltas, cut by channels, and faced by offshore marine bars can exist. An example is shown in Figure 33.10. The same comment is true in river channel cut and fill situations where meandering streams can provide a complex depositional pattern, unfortunately seen only as isolated one dimensional views by the logs run in the well bore.

FIGURE 33.10: Complex stratigraphic traps

3. Reef Traps
Reefs are productive in many parts of the world. Many types exist, such as atolls, table reefs, pinnacle reefs, barrier reefs, fringing reefs, biostromes, and bioherms. They occur as small dome like features that may be reflected in the overlying sediments by drape. Drape is described in the next section of this Chapter. Some reef trends extend for hundreds of miles, such as the Leduc reef trend in Alberta. The size of a reef ranges from a few acres to several square miles. Seismic exploration is the best way to find reefs.

The reef core grows upward and usually outward as the sea level rises. Detrital reef material falls on the ocean side, forming the fore reef. The back reef is formed on the lagoon or quiet side by deposition of limestone and lime mud, illustrated in Figure 33.11.

FIGURE 33.11: Reef trap

If sea level rises too fast, the reef may drown and die. If water level drops it may begin to grow again, forming very complex structures. Some examples are shown in Figures 33.12.

FIGURE 33.12: Complex lithology of a Devonian reef

Reefs are usually easily identified by draping dips, often extending several hundred to a few thousand feet above the reef. Dips in the reef core and fore reef are erratic, and those in the back reef may be visible or nonexistent.

4. Drape Traps
Differential compaction causes drape over reefs and sand bodies and this can form traps. A sandstone or carbonate layer above the bar or reef can be bent in such a way as to have closure, that is, the ability to contain and trap hydrocarbons. The bending is caused by the fact that the reef or sand body does not compress to the same degree as the shales to either side of it. Therefore a topographic high can be propagated upward through the section for quite some distance.

FIGURE 33.13: Drape and sag

These traps look like folds in a cross section or on the dipmeter patterns. They were not formed by tectonic activity, but rather by the sedimentary process itself. Dips underneath the reef or bar will be regional, in contrast to the anticline. Drape is important in identifying sedimentary structures from dipmeter data, and is often overlooked as a trapping mechanism in the beds lying above the target formation.

Drape is illustrated schematically in Figure 33.13 for both the reef and the sand bar case. Channel fill can also cause drape, again due to differential compaction of surrounding shale. Bedding inside the channel may be complex, but is usually regional under the channel. However, the mass of a reef or channel sand may compact the rock under the body, causing apparent sag below the base of the zone.

33.10 Grain Size and Depositional Environment
There are four primary ways to estimate depositional environment from well logs:
1. shale volume/grain size analysis
2. dip spread/water depth analysis
3. bedding angle/bedding type analysis
4. curve shape/depositional sequence analysis

All of the techniques rely on a strong correlation between depositional environment and the energy needed to produce certain characteristics that we can see on well logs over the rock sequence.

Depositional energy level correlates well to grain size, which in turn is usually proportional to shale volume. Thus the gamma ray or SP curve can augment environment estimates from dipmeter analysis. Low values of gamma ray (or high SP) indicate high energy, low shale content zones. These are inner shelf or upper continental slope in a marine environment, or alluvial or fluvial regimes on the continent.

Higher shale volume indicates lower energy deposition; that is, deeper water outer shelf, lower continental slope, continental lacrustine, or paludal environments.

Curve shape analysis depends almost entirely on the shape of the SP or GR curves versus depth, so the shale volume/grain size/depositional energy relationship is a strong component of our analysis method. The reconstructed resistivity curve from the dipmeter or a microresistivity curve can also be used as a grain size indicator in shaly sand sequences. An example is shown in Figure 33.14 from a SYNDIP presentation.

FIGURE 33.14: Grain size estimates from log curves (SYNDIP)

A combination of curve shape and bedding patterns are used to differentiate the ambiguity obvious in the above discussion. Grain size alone, as indicated by shale content, is not a sufficient criteria to determine the environment, but it does help distinguish high, medium, and low energy environments.

33.11 Dip Spread and Depositional Environment
For most situations, the spread in the dip angle values correlates to energy level, as shown in Figures 33.15 and 33.16.

FIGURE 33.15: Depositional environment, water depth, and dip scatter

FIGURE 33.16: Dip scatter and water depth

The continental slope and abyssal plain zones also have distinctive energy patterns, with very high energy at the upper slope, due to slumping and turbidity currents. Energy levels decrease rapidly with distance from the upper slope. This results in dip ranges of 60 degrees in the upper slope zone to a few degrees in the abyss, corresponding to Zones 4, 5, and 6 in Figure 33.15.

Dips on the continental zone range up to 20 degrees for fluvial deposits and 45 degrees for eolian and alluvial deposits.

Even though energy level, water depth, and grain size can be inferred from logs, this is still not enough information to segregate all sedimentary structures.

33.12 Current Bedding and Depositional Environment
The geometry of a sandstone unit is related to its internal structures, which are functions of its depositional environment. The internal structure is influenced mostly by current bedding. The direction of paleocurrents is indicated by the orientation of the current bedding, which can be measured by using dipmeter data after removal of structural dip. This direction is an aid to tracking the reservoir, but the correct model for the reservoir must be chosen before the direction information is of any use.

The term current bedding is used to describe the beds laid down in a channel parallel to the direction of current flow. The current beds will dip downward in the direction of the current flow and will be from a few inches to a few feet thick.

Crossbedding, although the term suggests otherwise, is also parallel to the direction of current flow. However, crossbeds do not often occur in river channels but usually on the front of deltas or shallow marine sand bars. Crossbeds dip considerably more steeply, but in the same direction, as the dips of the delta or sand bar surface.

Planar or tabular bedding, as the words suggest, involve flat layers of rock (maybe lying at an angle) laid down in streams, lakes, or in deltas. Festoon bedding creates layers which are convex top and bottom, and are usually laid down in braided streams. Wedge shaped or nonparallel bedding is planar bedding with concurrent erosion which has removed a portion of the bed, such as on the curve of a meandering river. Examples of these bedding patterns are given in Figure 33.17.

FIGURE 33.17: Bedding patterns

As described earlier, a strong correlation exists between depositional energy and grain size of the rocks. The larger the grain size, the greater the depositional energy. Therefore, steep current bedding, which can only be supported by large grains, is usually interpreted as high energy deposition. Flatter beds represent lower energy deposition. This rule usually holds when deposition occurs in a place away from the transportation artery, such as in a delta front or when deposition is associated with ocean wave energy.

However, this rule could be broken when deposition and transportation occur simultaneously, as in a channel, where the highest energy may produce the flattest, even reversed, current bedding dips. An example is torrential bedding, illustrated at the bottom of Figure 33.17.

Specific sedimentary environments give rise to characteristic patterns of current bedding dips versus depth. Such patterns, seen on the dipmeter plot, can be used to help identify the depositional environment. For example, most bar type deposits will exhibit a high dip angle in the upper part, decreasing to a low angle near the base.

This table is adapted from "Reservoir Delineation By Wireline Techniques" by J.F.Goetz, W.J.Prins, and J.F.Logan, published in The Log Analyst, June, 1977.

To evaluate current bedding, its characteristics (type, angle, pattern, spread) and its orientation (direction and scatter) are considered together. The above tables should be used in conjunction with the sedimentary model descriptions given later in this Chapter.

In case of a conflict between evidence supplied by the various approaches, current bedding patterns should overrule curve shapes, because the dipmeter has better resolution. This extends to the determination of the boundaries of genetic units. Sometimes the incoming material may change while the same depositional conditions persist, with the result that lithological unit boundaries may not match those of genetic units. One genetic unit may be made up of more than one lithological unit or vice versa. Interpretation involving sedimentary structures is based on genetic units and should not be too strongly influenced by lithology variations.

33.13 Curve Shape Analysis and Depositional Environment
Grain size and bedding both influence the overall curve shape of a log versus depth. There are four basic curve shapes:

1. straight line, indicating constant shale, evaporite, clean sand, or carbonate, caused by continuous deep water deposition

2. bell shaped, indicating a fining upward sequence, ie., lower energy at the end of a cycle

3. funnel shaped, indicating a coarsening upward sequence, ie higher energy at the end of a cycle

4. cylindrical shaped, indicating constant energy throughout the cycle

The last three are the usual patterns considered in an environment analysis. Variations exist. Serrated patterns are caused by abrupt changes in energy, resulting in layers of silt or shale interbedded in an otherwise regular pattern. Short patterns way be imbedded in longer ones. Thus, short coarsening upward patterns may contribute to a larger coarsening upward pattern. Patterns of all three kinds may be imbedded in one larger one.

Examples are illustrated in Figure 33.18.

FIGURE 33.18: Fining upward, coarsening upward, and cylindrical curve shape

These shapes are most obvious on gamma ray and SP curves, but may also be derived from resistivity (on a logarithmic scale), porosity, or a computed curve. In particular, the synthetic resistivity curve on the dipmeter arrow plot or SYNDIP presentation are widely used.

Combining the rules for grain size indicators, dip spread, current bedding and curve shape is a formidable task. Add to this the palynology and paleontology (micro and macro fossils), as well as the lithology descriptions, and you have an almost undecipherable problem to solve. A rule based expert system could be constructed from the tables given above. An example is shown in Figure 33.19, from a USGS program. A more elaborate program, Dipmeter Advisor, utilizing dipmeter patterns was described in Chapter Twenty-Six.

FIGURE 33.19: USGS expert system to determine depositional environment

33.14 In Conclusion
This chapter has reviewed the origin and classification of sedimentary and stratigraphic features. These structures often can be identified by characteristic log curve shapes and dipmeter patterns. Without dipmeter data, curve shape 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 for reducing ambiguity and error.

33.15 Exercises for Chapter Thirty-Three
Exercise 33.01: Stratigraphic Analysis Concepts

1. Define the terms: sedimentary structure, facies, and depositional environments. (10 marks)

2. Explain how grain size and depositional environment are related. (10 marks)