Presentation on theme: "A North Sea processing & Interpretation story. The same well image was shown on both the before and after sections above. As you may have noted, it is."— Presentation transcript:
A North Sea processing & Interpretation story. The same well image was shown on both the before and after sections above. As you may have noted, it is difficult to see on the left. I repeat it here to show a pretty nice match on my final inverted and integrated result. Note the U1 regional shale identified as event B. It plays a major role in the series of fault slides that follows. Hopefully you will agree the strike slip faults I show on this cross line are supported by the in-lines that follow. While Nexen finally gave approval to display, they never seriously considered my results, and eventually gave up on the project. About a pioneering pro-bono effort that introduced the concept Before After of reservoirs being bounded by strike slip faults, and gave rise to my later work on coherent noise removal.
A brief explanation of the mess I had to work with. You see here a multi trace gather set where: A ABC A is the North Sea Chalk. B is the critical angle position, where the reflection begins cloning to a refraction. C.is where the pre-stack migration loses control, causing an energy explosion. The yellow line represents the deep mute I had to use to get any results. The trace at the far right is the stack of the raw data, proving that you will get something out no matter how much noise there is. Three major associated points call for your attention here: 1. Offshore work is not immune to coherent noise problems. With the advent of ridiculously long spreads, the type of cloning we see here is common. When it happens, all vertical travel is interrupted. Since nature abhors a vacuum, point source refractions penetrate from the outside of that travel cone. They are there for us to see if we really care to look. Looking back I should have gone even deeper with the mute, since the refractors work their way back towards the beginning of the spread. 2. This pre-stack migration was not able to handle refraction patterns. The exotic explosion of energy we see at the upper right is the result. When I was on my way out I was able to obtain some raw data that had no migration (or NMO). The set at the lower left is from the same zone. The A,B and C markers apply in the same way, and you can see there is no abrupt change in energy, proving the use of pre-stack migration to be at fault. Since that process mixes heavily in its attempt to draw in energy that has wandered due to strong dip, it has to really murder the fine detail on the fault breaks. 3. As a result of this phenomenon AVO on this data is virtually a joke. The energy explosion would have triggered the searcher, but this would not have had anything to do with the theory. Yet the major discussion theme when I came on the scene was whether to use inside, middle or outside trace combos. On the next slide I repeat what you see at the left so you can see how infinitely detailed the noise patterns are before being massacred by the migration. A B C
Same gathers, no prestack migration or NMO Notice the noise detail on the outside traces.
strike-slip fault tracking. This animated series will take you inside the Initial study I made on Nexen’s North Sea (Blackbird) data. This ADAPS output section is close to their discovery well. Their seismic interpretation at the time was on-lapping sands nestled against a major thrust? fault, as below. Hopefully you can remember how lousy the input was as we move along.. Click to see the main fault, then again for the ancillary detail. We’ve now introduced some ancillary faults. Hopefully you will have noticed the geologic picture getting clarified. With strike slip faults one side is “torn” from the other to accommodate deep plate movement. Normal and shear faults are easy to pick because of the relatively constant bed offsets. Strike slip faults are horizontal and we cannot depend on there being visible breaks. If the stratigraphy were completely regular and there was no vertical throw, we we would not see the (major) green fault so clearly. The simulated sonic log capability of ADAPS is crucial in this situation. Bed thicknesses are vitally important in cross- fault correlations, especially when pre- stack migration (and other “mixing” logic) blurs the breaks. Take the time to note how the picked faults separate partitions that make good geological sense. The next series of slides verifies the fault pattern and further proves that the bulk of what we are seeing represents real stratigraphic detail. The event marked by the four stars is a marker shale, identified at the well site. On this in-line it is easily correlated across the ancillary faults, with little vertical throw. As we move towards the discovery well, you will see the vertical throws changing. The first time through, click rapidly while you follow the stars. When you get to the end of the series, return here and study the faulting more carefully. Watch the bright spots, since I believe they represent probable reservoirs. Click anywhere to proceed.
We have moved 10 in-lines towards the discovery well. In my original study I followed the starred event on every in-line and can assure you I arrived at this correlation with no problem. The vital point I am trying to make with this series is that the final offset between the first and last stars is the result of faulting, rather than dip. As we go you will see the individual offset components changing Sonic log simulating is key to our visual correlation. Follow the sequence at left to the main green strike slip fault.
Pause here for a particularly good stratigraphic correlation showing shifts between the faults (up to the main one). There’s a temptation to jump across to the red star. If this turned out to be correct it would require a fairly drastic thinning in the correlation span I pointed out on the last slide. Of course this ties in with the idea of significant lateral movement. This is the same set of faults I’ve been following, although the pattern is a little different. I have jumped 20 in-lines this time. However. From a logical point of view, the fact that this general pattern independently fits, in-line after in-line, is apriori proof of resolution improvement. Note that fault A is taking on much of the vertical shift of the marker shale as we go on. A
A When an interpreter has spent years assuming faults are either normal or thrust, it’s tough to shift to the horizontal perspective, especially when horizontal faults are hard to see.. Shear, of course, is the biggest reason. When you slide one block against another the rough edges on both are smoothed (in the direction of the movement). Thus, on normal and shear faults we see a nice, linear lineup of the breaks on our seismic section. Compatible lithology is also a reason these traditionally accepted faults are easy to pick. In other words we can expect reasonable stratigraphic correlation across the breaks. On strike slip faults, compatible lithology works against us. If no vertical throw is involved, we have to pick breaks right through what looks like continuity. Obviously this precludes automatic picking and mapping. Complete detuning becomes more essential in the fault break picking process. Changes in bed thickness and amplitudes may be our only clues. While complete automation of these attributes might be possible in the future, visual interpretation is the obvious current fallback. Reservoir spotting. While we are on that subject, please take the time to note the various “hot spots” that appear in the series, noting that most coincide with obvious trapping circumstances. By the time we get to the discovery well I trust you will agree that particular site does not look too great.
3 things of interest here – 1.The green arrow points to where the pre-stack migration logic very likely is mistakenly assuming that a large fault break is really a steeply dipping event that it needs to handle. This is very common throughout this and other jobs.. 2.The black arrow points to a probable untapped reservoir. 3.The red arrow points to a probable “zero vertical throw” fault juncture. While this is highly subjective, such interpretive decisions require a lot of detailed visual examination.
Here we see a change of lithologic character across the same fault. This helps in the overall interpretation.
Another pause just before the well – I would like you to consider the rather good correlation between star X and star Y. X Y At least it seems good when one compares to the to the quality of the client’s stack.
. Repeat the fault series by clicking arrow Else press right arrow to proceed. Here’s the discovery well. The marker shale is at the top of the oval and the producing sands peak through at the bottom. Obviously the ADAPS results did not think much of this site. selection In summary, I have tried to show: 1.That the producing zone is cut by a number of strike slip faults, creating a complex set of potential reservoirs. 2.That more promising prospects can be seen using advanced detuning.