Direct Hydrocarbon Indicators
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Geology and whether this involvement has lasted for two years, or twenty years — has seen many adjustments in seismic expertise. It has certainly not been a static know-how; instead, it’s overwhelmingly dynamic. In actual fact, it’ll “leave you in the mud” if you do not keep up with it.
Let’s look at some latest developments in seismic expertise as they relate to direct hydrocarbon indicators.
1960s and 1970s: the brand new geophysicist
Within the late 1960s, the seismic methodology began to really feel the impression of the computer revolution. There were dramatic adjustments in acquisition strategies; seismic data processing developed; and customary depth point (CDP) know-how was born (Mayne, 1962). The late sixties also noticed the start of the seismic specialist. The place beforehand, the geophysicist stayed out in the field and was responsible for the entire product from acquisition by means of interpretation (we affectionately know him as a “doodlebugger”), the “new geophysicist” specialised in acquisition…or processing…or interpretation. From this time on, solely the acquisition skilled went out to the field; the processor stayed at the pc middle, and the interpreter stayed in the workplace.
In 1973, Tucker and Yorston released their classic work Pitfalls of Seismic Interpretation. This pamphlet introduced twenty-three examples describing a collection of problems that an unwary seismic interpreter may encounter to great regret. While the specific seismic knowledge high quality for the illustrations left one thing to be desired, the messages have been clear. The pamphlet introduced guidelines both for understanding every of the results and additional making a appropriate interpretation. Of particular curiosity was the fact that all of the interpretive difficulties had been placed into just three classes:
· recording and processing
This pamphlet locations the responsibility for addressing these pitfalls squarely on the interpreter, primarily based on his or her skills, expertise and intellect — it provides few “crutches” past doing careful work and gaining as a lot expertise as possible.
It’s interesting that only three years later, Neidell expressed a somewhat different interpretive philosophy, which the AAPG later documented in a set of revealed course notes (Neidell, 1984). Based on this work, we are able to set forth an interpretation procedure for handling seismic information that begins with a processed seismic section. For the second, we are going to suspend all questions or doubts concerning the effectiveness of the information processing, and interpret the section based on the following assumptions:
· Each trace of the part represents only major reflections from the subsurface, having areas immediately below the place we now have shown them to be plotted.
· Individual reflection occasions might be identified, and their amplitude is diagnostic of the change in acoustic impedance throughout the boundary inflicting the reflection.
In this perfect world, we could merely state the targets and procedures of both seismic and stratigraphic interpretation. Figure 1(relationship between lithology, propogating wavelet and seismic response) exhibits a portion of a lithology log and a corresponding acoustic impedance log.
Every contrast in acoustic impedance is marked by a mirrored image event having a easy waveform. The polarity or sense of the reflection and its measurement indicate the nature of the distinction. Individual reflection events for the model are shown, along with their summation within the ensuing seismic hint.
Interpretation begins, then, with the event from every hint of an acoustic impedance log or, equivalently, a reflectivity sequence. We correlate these outcomes hint-to-trace to provide the structural concerns, and correlate them also with whatever geologic info is available. Utilizing geological rules and insights appropriate to the area, we infer lithologic estimates, and from these estimates and the indicated hint-to-trace changes, geometry and depositional patterns, we interpret sequences and history.
The query of the wavelet’s shape is price considering, as Figure 2 demonstrates.
As we will see, the same sequence of lithology considered as an excellent normal incidence synthesized seismic hint is extraordinarily troublesome to interpret on the basis of untreated waveforms.
Motivation for this strategy followed from new developments in estimating seismic waveforms and changing them successfully to the simple zero-section symmetric type proven in Figure 3 .
The person superimposed waveforms are also shown to supply some cheap perspective for understanding the superposition.
As with many important developments, the control of seismic wavelets solely represented one added ingredient within the extraction abu dhabi oil refinery company takreer of subsurface data from seismic data. Actually, throughout that very same period, the position of seismic modeling as a quantitative bridge to subsurface parameters, and using seismic patterns for discerning lithology, depositional setting and different stratigraphic elements was launched in systematized type by Vail and the Exxon faculty of seismic stratigraphers (1977). In Determine 4, regional geology and borehole measurements are presented in relation to their expression as patterns which could also be seen in the seismic view.
Within the 1970s, we realized that (1) the pitfalls which could be encountered might have analytic remedies by way of seismic data processing or special area practices, and (2) that more data is likely to be recoverable from seismic information than beforehand appreciated. The usage of seismic patterns for interpretive purposes and the lessons from seismic modeling testified for the second aim.
As noted earlier, a respect for the information contained in the seismic response developed. When particularly discovered, the value of seismic amplitude and that retention of accurate amplitude information is a should. Relatively than arbitrarily growing the amplitude to maximize the structural content, we realized that maintaining “true” amplitudes (also referred to as “relative” amplitude processing or RAP) may tell us a lot about sure qualities of the subsurface. And we discovered that once we did this, we noticed anomalously excessive amplitudes, or “vivid spots,” that we equated to qualities of oil and/or gasoline.
1980s – A critique
In the 1970s, we noticed the importance of bright spots and constructed a nicely-outlined hyperlink between seismic interpretation and the position of information processing (and, to a lesser extent, acquisition). The 1980s saw elevated efforts to excellent seismic imaging and extract as much subsurface abu dhabi oil refinery company takreer information as possible. It turned clear, as an example, that in an appropriate geologic setting, a porous, fuel-stuffed Pleistocene or Pliocene sand on a correctly processed and imaged seismic part could be readily recognizable. For example, we can establish an offshore Gulf Coast sand in Determine 5 .
The outstanding trough denoting the sand prime is labeled as is the gas-water contact and indicated by the primarily flat sequence of sturdy peaks.
For this similar data, Dedman, Lindsey and Schramm (1975) interpreted a lot of the sands within the subsurface column between 1.Zero and 1.8 seconds. Similarly, sands were identified by the use of effectively log measurements, and plotted on a two-approach travel time scale for direct comparison with the seismic knowledge. The outcomes are proven in Figure 6 .
In assessing the remarkable settlement, it’s important to notice that the top of a sand corresponds in each case to a trough, unfavorable amplitude or “white occasion”, while the base is outlined by a peak, positive amplitude or “black occasion.”
Hence, the waveforms inherent in our seismic data are extra amenable to interpretation as soon as we manipulate and rework them to simpler waveforms. We will now accomplish these transformations, in principle, for all reflection seismic information, and attain correlations between seismic knowledge and geological inputs to provide more definitive outcomes.
Determine 7 demonstrates the improved correlations of seismic photos with subsurface data, this time utilizing an elementary model, the artificial seismogram.
A seismic part over a North Sea oil area has been separated at the well location, and a number of other repetitions of the synthetic seismic signature computed from the velocity and density logs have been inserted. A standard waveform is used within the precise processed knowledge and the synthetic traces. Such agreements clearly enable correlations to be made which allow one to work away from the effectively management with a great degree of confidence.
Let’s now look at a processed seismic part from the offshore Texas Miocene ( Figure eight).
On this case, the Miocene sands are related to peak reflections, as distinct from the earlier association of reflections from sands as troughs. The truth that sand reflections could also be signaled by each peaks or troughs isn’t talked about within the literature describing seismic expertise (Rutherford and Williams (1989), Neidell and Berry (1989), and Neidell and Lefler (1992)). It actually was not mentioned in any respect prior to 1989. Obviously, such a fundamental matter deserves to play a task in our considering relating to seismic data, since some 50 % of world hydrocarbon manufacturing pertains to sands and sandstones.
The distinguished GulfCoast gasoline sand of Figure 5 could also have been confirmed through seismic modeling as a companion to the log correlations performed by Dedman et al (1975), which had been additionally previously proven in Determine 6 . For such a examine, we may apply two-dimensional seismic modeling.
In Determine 9(artificial seismograms creating a two-dimensional mannequin of partially gas gilled sand anticline – band pass zero phase symmetric wavelet versus actual contractor wavelets A and B),
the mannequin shows a mildly structurally closed sand unit of relatively uniform thickness. The upper 60 ft [18 m] of this 120 ft [36 m] sand is gas-saturated. Area seismic data exhibited a “shiny spot” about the amplitude relief shown in the figure. This model was originally computed with theoretically derived rock velocities and densities for the sand and shale values from data calibrated to nearby well logs. An amplitude improve of solely 25% was obtained using these values, and the mannequin was altered to have the values proven within the determine. These have been derived by assigning a .04 reflection coefficient to the shale-water sand interface and adjusting the gasoline sand velocity to provide the amplitude enhance shown on seismic data. Densities have been included within the reflection calculation and customarily comply with Gardner’s equation. For a detailed dialogue of Gardner’s investigations, confer with Carter and Siraki (1993).
First, we computed model responses for two completely different however documented marine fundamental wavelets. These might be considered coming from two totally different contractors. It’s not our position to choose between these two wavelets. It is apparent that the seismic sections look quite totally different, and in the event that they have been members of a grid of data on the identical prospect, it can be troublesome to tie them.
For conventionally processed knowledge, it is generally unreliable to attempt picking the highest and backside of the fuel-sand for thickness estimates. However, it can be attainable to detect the probable presence of the sand and to put it on the map. Wavelet B would possibly leave us questioning if now we have one sand or two, whereas wavelet A causes us to wish for some greater frequencies in the hopes extra element as to the exact stratigraphy might be seen.
Both wavelets A and B have a bandwidth roughly equal to the 8-32 Hz response shown within the figure. Thus, this model response is what every of the other two might have been converted to, with acceptable processing. Underground No problems of tying information between the two data sets would then exist.
We should always in reality have grow to be quite suspect at this point, in that even this straightforward application of modeling required us to drive parameter values to realize abu dhabi oil refinery company takreer an acceptable fit. Strictly speaking, our model should even have included multiples, as the North Sea artificial seismogram ought to have ( Figure 7 ). Experience has taught us that including all multiples in mannequin calculations (of any type) degrades our matches considerably, though sometimes we will doc the presence of a multiple or two.
Sadly, our quest for added data from seismic information pressured us to face circumstances through which the out there instruments could not remedy the problem. Determine 10and Determine eleven clearly illustrate this level.
These figures have been developed from the Rocky Mountain space, and exemplify the issue of distinguishing between a coal seam and a partially gasoline-stuffed sand encased in a sandy shale.
Simulated time sections (with and with out noise, utilizing a fairly typical bandwidth) recommend that in sensible terms, it is not going that this distinction could be reliably achieved utilizing regular seismic displays and strategies.
The 1980s saw the introduction of new forms of HCIs. Within the early eighties, seismic inversions turned a preferred technique of straight figuring out subsurface hydrocarbons. In this technique, the seismic hint is converted to a synthetic impedance nicely log. Once we apply inversion to a sequence of adjoining seismic traces, we then produce a synthetic impedance part. We interpret the impedance part as if we had a sequence of intently spaced acoustic, or sonic nicely logs.
The late 1980s also noticed the use of amplitude information develop in its place. Utilizing equations of power partitioning, we began relating adjustments in amplitude as a operate of distance from source to receiver (or offset) to modifications in fluid type. Relying on the circumstances, we noticed that seismic amplitudes either increase or decrease in response to the fluid content. The sort of HCI is called amplitude versus offset, or just AVO. Utilizing this technique, the issues of discrimination shown in Figure 10 and Figure eleven could presumably be resolved.
View of the 1990s and beyond: the 3-D explosion
We might now consider making “ties” between seismic information and the subsurface as we comprehend it from wellbore observations and measurements. There are lots of features to such an goal. One apparent one is the initial situation of imaging seismic knowledge most successfully. We must also look at how we might best use such shows. At the moment, we are experiencing an explosion in seismic know-how within the type of three-D information. This new technology, together with advances in computer workstations, is forcing us to learn new methods to operate, course of and interpret seismic info. It is important to clearly appreciate the relationship of 3-D know-how to 2-D methods. Fortunately, this is a straightforward matter, and includes solely a simple extension of 2-D technology.
At the same time, we must clearly define (even on a global basis) the geological relationships between lithology and their characteristic reflections. Also, we must rigorously scrutinize the fashions we employ and the theoretical equations on which we rely. We’ve got already seen major discrepancies between such models and the behavior they predict (for instance the prediction of the presence or absence of a number of reflections and the power to predict seismic amplitude ranges). If there’s to be real progress, we can’t tolerate blind spots of such fundamental significance.
Just because the early nineties introduced us to new excessive decision within the horizontal area within the type of 3-D seismic, we are going to see new revolutions within the vertical domain within the late 1990s. At that time, not will a 2 ms pattern rate and 20 ft resolution be the norm, but we’ll routinely see 0.1 ms information and 1 ft vertical resolution.
Have you ever ever recorded any knowledge utilizing seismograms
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