Structural Interpretation

(adsbygoogle = window.adsbygoogle || []).push({}); Learning Objectives After completing this topic "Structural Interpretation", you will be able to: Determine the location of favorable traps for hydrocarbon accumulations. Discuss how the change of thickness along a geological formation will cause the reflections to change the character of a wavelet. Outline the need for migration in complex structural geometries due to difficulties with reflector correlation. Explain the timing and the depositional mechanics of faulting and how this impacts oil and gas migration. State the different types of seismic effects that can be misleading during interpretation. Describe why we have no interest in traps which formed after the migration of hydrocarbons was complete. Introduction to Structural Interpretation The initial goal of seismic structural interpretation is to determine the location of favorable traps for hydrocarbon accumulations. The first step is to determine the most continuous reflectors. These are then color-coded and correlated to the other seismic lines in the grid. Intersecting lines should show the same reflectors at the same time, at least at the intersection points. These reflectors must be correlated to existing well logs and will be identified geologically, in terms of age and lithology. From sonic logs, synthetic seismograms can be constructed and these are used to predict the location of seismic horizons for interpretation. Initially, contour maps are produced in two-way reflection times. With the help of reflection surveys and sonic logs, velocities can be obtained, and then structural contour maps in depth can be produced. Lateral velocity variations due to lithological changes in the subsurface can produce discontinuous or uncorrelatable reflectors. In Figure 1, the continuity of the indicated horizon gap can be established by referring to the strength of the reflection, the dip above and below the incomplete area, and the parallelism of the top and bottom reflectors. Figure 1: Horizon continuity, reflection strength, dip and parallelism (adsbygoogle = window.adsbygoogle || []).push({}); Figure 2 shows an example of a natural discontinuity of sand lenses. A phantom horizon can be drawn to establish a common tendency of the discontinuous lenses. Figure 2: Discontinuity of sand lenses Another way of correlating reflections of doubtful continuity is to analyze the characteristics of the seismic trace. Figure 3 shows the shape of several seismic traces. Seismic traces are composed of wiggles made up of different combinations of peaks and troughs. A specific character that repeats laterally is normally an indication that they are generated by the same reflector. Figure 3: Seismic traces showing combinations of peaks and troughs Lateral variations, like the image shown in Figure 4, must be analyzed very carefully before giving a definitive interpretation. In effect, lateral lithological changes imply lateral velocity changes and consequently changes in the velocity contrasts at the top and bottom of the layer. As a result, in these examples the larger reflections will come from the top of the formation, and from the bottom alternatively, if the velocity contrasts are large enough. Figure 4: Lateral lithological changes (adsbygoogle = window.adsbygoogle || []).push({}); Another common case of difficult interpretation occurs with thinning horizons. The change of thickness along a geological formation will cause the reflections from the top and bottom of the layers intersecting at a point to overlap, resulting in the cancelling, reinforcing, or change of character of the reflections. Two examples of this are shown in Figure 5. Figure 5: Thinning horizons cause wavelets to overlap and change character Difficulties for correlation are also quite common in complex structural geometries. An additional step called migration must be performed in order to position reflections in the right location at depth. The difference of the reflection times of two reflectors can be contoured to produce isochronal maps, and, if the velocity between the two reflectors is known (interval velocity), then isopach maps can be produced. Following are useful guidelines for a sound structural interpretation: Determine if the seismic lines you want to interpret have been migrated. This is important to know early in the process because structures will shift between non-migrated and migrated seismic. Compare non-migrated lines with migrated lines to understand the effects introduced by migration. Remember that the interpretation of seismic lines is done on time sections (at least as a first approximation). This can be misleading because the deformation of the vertical scale is not usually linear. The time-depth relationship is not a direct one. Each geological formation has a different seismic velocity, which makes the time-depth conversion a difficult task. In the case of extensional geologic environments, it is recommended to start with the highlighting of the obvious faults. In the case of compressive geology, sequence stratigraphy interpretation should be conducted before structural interpretation. The interpreted stratigraphy will guide the reconnaissance of structures and the chronologic cataloging of events. For land surveys, it is important to review the location map of the survey and surface geology maps to understand the regional context. Frequently, bad data can be explained by surface conditions. Surface outcrops are certainly the best guide to understanding the subsurface, with due considerations of scaling factors. Interpretation should start in the undisturbed areas and then move into the more complex structures. Attention should be given to the cause of the unconnected reflectors that are identified by divergent reflections. Faults are always associated to some type of detached reflectors. Faults are characterized by displaced reflections that need migration. It is important not to extrapolate the fault in noisy areas of the seismic section as this can lead to incorrect conclusions. Normally faults end in some detachment surface where no more displacement is visible. Attention should be given to different types of seismic effects that can be misleading for the interpretation. These might include multiples, migration effects, diffractions, velocity effects like "pull-ups," reflections from out of the seismic plane, etcetera. In cases of complex geology, it is advisable to produce different versions of the interpretation that may account for plausible, interpretation scenarios. Then additional information (structure maps at different horizons, thickness maps, etcetera) should be produced to identify the interpretation that best fits the available data. (adsbygoogle = window.adsbygoogle || []).push({}); Faults and Sedimentary Accumulations https://youtu.be/Y28ZeoytLxk Seismic Faults Interpretation (adsbygoogle = window.adsbygoogle || []).push({}); Figure 1 shows reflection character (in particular, the identifiable band of reflections in mid-section) that allows us to correlate across the indicated faults. Fault pattern recognition in interpretation is important as these structures can create leaks or traps for hydrocarbons. In this module, we will explore a number of different fault examples and examine their possible geological meaning. Figure 1: Seismic line showing reflection character with faults The age of the faulting may be specified, in terms of the age of the upper layers, by noting at what level the fault is no longer apparent. Figure 2 and Figure 3 provide examples of two faults that form a graben. In Figure 2, the two faults appear to be of the same age; the graben developed as a single feature. Figure 2: Graben as a single feature In Figure 3, the graben may have originally formed symmetrically, but, if so, the right-hand fault has certainly been rejuvenated. This example highlights the importance of correlating the complete sequence of reflections across the fault if the throw of the fault causes ambiguity between the black peaks on the two sides. Figure 3: Graben with rejuvenated fault (right) (adsbygoogle = window.adsbygoogle || []).push({}); Figure 4 is an example of a complex pattern of simple faults probably nine of them. This image emphasizes that we cannot have the necessary information to understand the full picture when we have only two-dimensional sections. Faults are surfaces, not lines. Figure 4: Complex fault pattern Sometimes faulting tells us something about the brittleness of the rocks. Thus, it is common to see faults totally absorbed in a thick salt layer, or in young or mobile shale. The faults in Figure 4 reduce to flexures in the topmost sediments, which were young and plastic at the time of faulting. In contrast, the extreme sharpness of the faults in the strong faulted reflection of Figure 5 seems to suggest brittleness, perhaps that of a carbonate layer. Figure 5: Sharp faults suggest brittleness and possibly a carbonate layer Some faults reveal the depositional environment. Figure 6 shows a fault in which the throw increases with depth this is a growth fault. The depositional message indicated by the fault is that clastic sediments were being transported along the sea floor across the fault. The water velocity was sufficient to limit deposition on the upthrown side, but the reduction in velocity as the water deepened across the fault caused more sediments to settle. A growth fault occurs during deposition; it is a syndepositional fault. This means that correlation of reflections across a growth fault may not be easy, and that there is danger in forcing such a correlation—the layers may be genuinely different on the two sides of the fault. In Figure 6, we can see good correlations in the shallow reflections, but increasing uncertainty at depth. Figure 6: Growth fault (syndepositional); throw increases with depth (adsbygoogle = window.adsbygoogle || []).push({}); When post-depositional faulting extends to the surface, erosion of the upthrown block deposits a talus of sediments on the downthrown block. Figure 7 shows three stair-step faults in ancient rocks, originally caused by rifting. We see the accumulation of erosional detritus on the first step-down (at right), and the overflow of this detritus down the steps (yellow lines indicate reflections). We would expect the sediments on the first step-down to be chaotic, and to include much coarse material. After the time represented by our pick, we can guess that a rising relative sea level caused the onlap visible above the pick; thereafter we see that the middle fault, and to a lesser degree the left-hand fault, were rejuvenated. Figure 7: Three stair-step faults caused by rifting The plentiful supply of sediments associated with the erosion of an upthrown fault block yields several possibilities for sediment accumulation. In Figure 8, we see one of these a sediment wedge, thickest at the fault, thinning and downlapping onto the depositional floor. During this process, the additional weight of sediments may contribute to the rejuvenation of the fault, causing the depositional floor to dip into the fault. Figure 8: A sediment wedge thinning and downlapping onto the depositional floor In Figure 9 we see an initial accumulation of the same type of sediments, but this is followed by a different sediment type (yellow reflections). The sediments are building outward as well as building upward; they are prograding. Characteristically this process imparts a sigmoid shape (like a stretched-out letter “S”) to the reflections from within the accumulation. In this case, also, we suspect that the deposition was much calmer out in the down lap region, far from the sediment source, which is where we are expecting the finer-grained sediments. Figure 9: Prograding sediment wedge building outward and upward (adsbygoogle = window.adsbygoogle || []).push({}); Figure 10 shows another variation of progradation. Within the reflection picks (yellow lines), we can see a sediment accumulation in which the bedding planes (though somewhat obscure) seem to rise to the upper surface of the accumulation at an oblique angle. The sediments are certainly prograding from right to left, but there is much more building outward than building upward. This suggests sediment transport across the upper surface of the accumulation (without much deposition of sediment on that surface), and then the dumping of a large mass of sediment over the edge. The most important example of this situation is a river delta wherever we see oblique progradation, we interpret the upper surface of the accumulation is a deltaic plain, rich in sandstones. Figure 10: Oblique progradation of a sediment wedge Prograding units are a frequent sight on seismic sections from marine basins. Often the whole shelf may be seen to prograde over considerable distances usually sigmoid, and occasionally oblique where the seismic line happens to cross an ancient delta. Figure 11 illustrates a sigmoid prograding shelf of unusually large proportions. This section indicates that if we are looking for find sandstones, we should be looking for local occurrences of obliquity in the shallow deposition on the shelf, or possibly in turbidite mounds in the deep deposition at the foot of the shelf slope. The rest of the sediments are likely to be rich in shales. Figure 11: Sigmoid prograding shelf Figure 12 illustrates the contrast between a lower zone representing calm deposition in fairly deep water, and an upper zone representing shallower and more active water. We call the upper zone hummocky deposition. Figure 12: Contrast between calm deposition (deep water) and upper hummocky zone (adsbygoogle = window.adsbygoogle || []).push({}); Timing of Structural Movements (adsbygoogle = window.adsbygoogle || []).push({}); Structural movements are important to understand in terms of how and when they occur. What causes the movement gives clues as to what is happening beneath the surface of the earth in terms of crustal movement (compressional, extensional, or intrusive, for instance) and types of faulting. Timing is important for fluid migration and maturation. Figure 1 is a marine section crossing two salt pillows. The section represents 27 km (17 miles) of line. Note how the vertical exaggeration (about 3:1) makes it easy to see the thickening and thinning of different intervals. An interpreter has picked the section at several levels. On the basis of correlations between the section and the wells drilled in the area, the interpreter has identified the intervals between the picks as: Lower Tertiary (TL) Upper and Lower Cretaceous (KU, KL) Lower Jurassic (JL) Upper and Lower Triassic (TRU, TRL) Lower Permian (PL) Zechstein salt (Ze) The interpreter has also suggested several faults (Figure 1). We see that, as usual, the deep faults are absorbed in the thick and mobile salt section. The Upper and Lower Triassic maintain substantially constant thickness over and between the salt pillows. The same parallelism seems to be maintained in the lowest part of the Lower Jurassic. We can interpret, therefore, that there was no significant movement of salt until sometime in the Lower Jurassic, at the earliest. Figure 1: Marine seismic line crossing two salt pillows Now we’ll examine what we believe occurred in the subsurface to lead to the image we see in Figure 1. Depositional and Structural Changes Over Time (adsbygoogle = window.adsbygoogle || []).push({}); During the Lower Jurassic time, the section would have appeared as in Figure 2A. The whole depositional system remained remarkably stable, with the rate of subsidence generally well matched to the rate of sediment deposition. First we’ll look at the evolution of the left salt pillow, which may have been triggered by rejuvenated faulting in the deep zone below the salt. We do not know (from this one line) what thickness of Jurassic and Lower Cretaceous sediments were deposited, because any Upper Jurassic and Lower Cretaceous sediments which were present have been eroded. However, the thinning of the remaining Lower Jurassic sediments over the left pillow suggests that the salt had moved, and the left pillow had started to form, before this erosion was complete. Further, the parallelism between the erosional surface and the lowest reflections in the Upper Cretaceous indicates that the erosional surface was substantially horizontal, and the salt had ceased moving, by the early Upper Cretaceous. This situation is shown in Figure 2B. Figure 2: Lower Jurassic structure (A) and early Upper Cretaceous structure (B) We turn now to the right pillow. Several interpretations are possible; we will discuss only the simplest. The right pillow did not start to form at the same time as the left pillow (Figure 2B). Everything was stable through the period of Jurassic and Lower Cretaceous deposition and erosion; at the end of the erosion a topographic high remained at the right of the line. The salt started to move quite early in the Upper Cretaceous, and continued until about three-quarters of the Upper Cretaceous was in place, then it stopped. We can see the clear pattern of onlap reflections in the Upper Cretaceous during the period of uplift. The uplift to the right pillow, in distinction from the left pillow, caused some drop-down faulting over the crest. The parallelism of reflections continues through the Upper Cretaceous and into the Lower Tertiary indicating there was no movement of the left pillow during this time (Figure 3A). Then, sometime in the Lower Tertiary, the big uplift started. Looking at the Tertiary reflections just to the right of the left uplift, we see thinning toward the left pillow structure, then a zone of parallelism, and finally renewed thinning. From this, we infer that the salt started to move, stopped again, and then surged upwards (Figure 3B). Figure 3: Late Upper Cretaceous/early Lower Tertiary structure (A) and Lower Tertiary structure (B) (adsbygoogle = window.adsbygoogle || []).push({}); Everything then remained stable until early in the Lower Tertiary (Figure 3A). The right pillow was stationary while the left pillow initially started to move, and then the final movement started on both. During this period the withdrawal of salt from between the pillows caused significant subsidence in the middle of the line (Figure 3B). Subsequent erosion of the Tertiary sediments led to the situation of Figure 4A. No movement of the left pillow has occurred since that time, and sedimentation is proceeding uneventfully at present (Figure 4B). Figure 4: Middle Tertiary structure (A) and Upper Tertiary structure (B) During the final erosion, the collapse faulting over the right pillow hastened the erosion. A valley also developed, and broadened to the left of this pillow (Figure 4A). Since then, the valley has been filled, though differential compaction of these young sediments has caused some draping into the valley (Figure 4B). Figure 5 provides a different salt example. This shows a salt wall, some 7 km (4.4 mi) in width, which once broke to the surface. This dramatic example gives us an excellent record of the salt movement. The figure shows seismic with a velocity overlay; salt velocity is in yellow. Figure 5: Salt wall movement through time (adsbygoogle = window.adsbygoogle || []).push({}); Figure 6 illustrates complete withdrawal of salt from the area between two pillows. Extensional subsidence in this area has ruptured the Triassic rocks, and salt has filled the space between them. The right pillow has created a fault of very considerable throw. Figure 6: Complete withdrawal of salt and extensional subsidence These examples are dramatic, brought about by the extreme mobility and low density of salt. In some areas, similar effects can be caused by mobile shale. However, the techniques which we learn from these dramatic examples are an excellent guide in establishing the order of events in geologic time. The importance of these events to the interpreter is significant. Salt movement is only one type of structural movement we might encounter during interpretation. Let’s look at another type of structural movement. In Animation 1, we see a sedimentary series with the potential for petroleum accumulation. As the subsurface changes over time, and fluid migration occurs from the source rock to the reservoir rock, we see the importance of the timing of trap formations. https://youtu.be/WeDKVGCnuuE Sedimentary rock layers with significant potential for petroleum accumulation Animation 1 This animation shows why we have no interest in traps which formed after the migration of hydrocarbons was complete. As an area develops, some structures contain oil or gas, and others do not. The interpretation task, then, is to relate the drilling success to the timing of structures, and to propose drilling locations only on those structures which are of sufficient age. This is a very important skill. (adsbygoogle = window.adsbygoogle || []).push({}); Assessment Structural Interpretation (adsbygoogle = window.adsbygoogle || []).push({}); 1. Interpretation should start in the undisturbed areas of reflections and then move into the more ________. 2. For land surveys, what are the best guides to understanding the subsurface, with due considerations of scaling factors? A .TopographyB .River patternsC .Surface outcropsD .Surface fault patterns 3. Which of the following listed seismic effects can be misleading for interpretation? (Select all that apply.) A .Pull-upsB .MultiplesC .DiffractionsD .Reflections from out of the seismic plane 4. In complex geological geometries, an additional step called ________ must be performed in order to position reflections in the right location at depth. 5. How much interest should an interpreter show in traps which formed after the migration of hydrocarbons was complete? A .No interestB .Much interestC .Some interest 6. The difference of the reflection times of two reflectors can be contoured to produce what kind of maps? A .IsopachB .IsochronalC .DepthD .Velocity Recommended for You Interpretation with Seismic Attributes Seismic Interpretation Methods Seismic Stratigraphy Introduction to Seismic Interpretation (adsbygoogle = window.adsbygoogle || []).push({});