Presentation on theme: "North Sea Strike Slip Fault Study – The ultimate before and after. To the left you see the best the client could do on a “well-cutting” cross line. To."— Presentation transcript:
North Sea Strike Slip Fault Study – The ultimate before and after. To the left you see the best the client could do on a “well-cutting” cross line. To the right is the ADAPS simulated sonic log section on the same data. The well log image is superimposed on both. As you click through this first series of comments, I will point out important differences. In the meantime just look at the display comparison and get a general feel for the quality of their stack. I have drawn two strike slip faults on the ADAPS side. If you continue through the entire series you will se these faults justified. An important goal of this show is to convince the reader that almost everything you see to the right is coming from real reflection events, whereas what you see on the left is mostly noise. This confidence is vitally important when we come to the task of lining up the strike slip fault breaks. First we look at zone A on the original stack. The ADAPS pre-stack processing has essentially removed this noise event. B (on both sides) is a regional marker shale, identified at the well. On the ADAPS output we show an uplift across the interpreted fault..Later we will use this market to illustrate how the system has enabled fault picking. Please note that I have not tried to pick all the faults I see here. Looking at point C on the left, one can see that the ADAPS processing has broken the previous continuity where we figured the one fault went. The same is true all the way up the picked fault. This is the essence of how the system improves visual interpretation. There is a lot to learn from this pair, so take your time, then click anywhere to continue. A B B C B
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.
This is a 200+ trace, pre migration, Blackbird gather. The strong event marked with the yellow arrow is undoubtedly the chalk. Very early it is cloning into a reverberating refraction. Energy from this phenomenon can be seen making its way back to the inside. In any case all the outside traces are essentially garbage. One can spend hours studying the various noise ramifications here. This whole subject is discussed fully in the main series. The point here is that the fact that ADAPS treated this generic problem on a pre-stack basis was one factor behind the remarkable improvement. Before ADAPS no one was looking at gathers, and all attention was on “near, middle and far trace stacks”. My “industry icon” certainly fits here. Then there is the general information display problem. This presentation visually displays the results of my interpretation effort. Passing the final ADAPS product to today’s automated mapping tools completely loses most of the important information that has been developed. One of my biggest arguments with Ikon Science (during development days) was their disregard of the importance of PowerPoint like reports in client presentations. Clients would come back with relatively meaningless time or horizon slices, missing the point of the enhanced lithology. In this project, where the potential reservoirs are split into fault blocks, trying to map reservoirs automatically (without a lot of preliminary analysis) is a big problem. This show is a essentially a subset of the original. Its main purpose is to illustrate how the powerful de-tuning capabilities of ADAPS were utilized in a comprehensive interpretation effort. Of course this is the product I am touting. Click here to start over - Click here to see the original Nexen show. Or here to go to the associated noise show. Or here to go to the ADAPS router. Remember these are PowerPoints and are slow to load!