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Ashley Harris - Re-evaluating the Relationship Between Relative Sea Level and Sediment Distribution Using Numerical Stratigraphic Forward Models

AAPG Distinguished Lecture Series, 2018-19 Season

AAPG Distinguished Lecture Series, 2018-19 Season

Summary

A Distinguished Lecture talk given by Ashley Harris during 2018-19 AAPG DL Season. Click here for abstract.

Full Transcript

Hello, my name is Ashley Harris. And I work at Chevron in the Energy Technology Company. Today, I will present some interesting results of a study we've been working on for a few years. The title of my presentation is "Re-evaluating the Relationship Between Relative Sea Level and Sediment Distribution Using Numerical Stratigraphic Forward Models." So let's start with the summary of the whole entire study.

The first is that a process-based approach to the stratigraphic record will help better characterize or assess hydrocarbon reservoirs that we all explore or do asset development work in. The other is that autogenic processes and allogenic processes can produce similar results, autogenic processes being internal dynamics of sedimentary systems, including channel migration, avulsion, and slope failure. Allogenic processes include the processes external to the sedimentary system, such as tectonism or sea level. And lastly, the magnitude of deep-water sand delivery is similar for large and small sea-level changes, but it correlates poorly with rate of relative sea-level change and relative sea level. And we'll discuss why.

So what is the motivation? The stratigraphic record is the result of the complex interaction of multiple processes, including sea level, autogenic processes, compaction, diagenesis, and many others. This is important to understand these processes because they create the record that is the hydrocarbon systems that we all explore or do asset development in. But more often than not, we tend to focus on one aspect of the control in the stratigraphic record, and that is sea level.

So what is the reason for focusing on sea level? Well, it's rooted in our geologic thinking. And it goes back to the 1800s. And those ideas became even more popular with the publication of AAPG Memoir 26 in the 1970s. In that memoir, it was thought that eustasy controls stratal architecture and sediment distribution, eustasy being global sea level. Later on, it was thought that eustasy and tectonism, the combined effects of those, actually had the control on sediment distribution architecture. And we call that relative sea level.

So with that said, there are three main questions that we wanted to ask ourselves in reviewing the role of sea level and deep-water sand delivery, or sediment distribution in general, and the first being the timing of deep-water sand delivery-- when does that occur within a change in sea level-- the other being the magnitude of deep-water sand delivery, and finally, the volume of sand delivered to deep water. And of course, we're talking at a basin scale, not necessary at a small temporal-spatial scale.

So what is the state of the art? The current thinking is that during a sea-level fall, during the inflection or the most rapid rate of fall, that is when the coarse grain material is delivered over to shelf edge or to a canyon mouth. And from that inflection of the fall to its minima is when you would have most of your coarse grain material deposited. And what we're doing is going to test those ideas and see how well that holds up. So how we test those ideas is using a numerical stratigraphic model.

What is the benefit of using a forward numerical stratigraphic model? Well, it captures all the dynamics of a sedimentary system, including sea-level change, tectonism, and the autogenic processes I discussed earlier. To do that, we set up a model, or series of models, that is of a typical passive margin setting. In fact, the profile you see here is taken directly from the modern-day New Jersey margin. What we did was impose constant subsidence as well as constant sediment and water flux in the proximal part of the basin on the shelf. And the constant water sediment flux is typical of a moderate- or medium-sized fluvial system.

And then, finally, we have both the eustatic curves that we imposed on those models, so two models in particular we're going to look at, one with no eustatic variation in the orange, and in the blue one with eustatic variation that was derived from the New Jersey margin USA, from the Kominz et al. 2008 publication. So here are the results of those models. On the top are the results of the no-eustatic variation model. And on the bottom are the results of the eustatic variation model that we will refer to as the Kominz model. And each panel is showing a time series, or a time slice, of the model.

And in blue, we have erosion, and in red is deposition. And what you can see between the two models is that the pattern of erosion and deposition are similar. This is intriguing because one model has no eustatic variations, and the other does. But let's take a closer look at the no-eustatic variation model. Here, with a movie, we can see with the no-eustatic variation model that the delta progrades to the shelf edge, delivers sediment over the shelf edge. And as sediment accumulates on the slope and on the outer shelf, the fluvio-deltaic system aggrades and avulses. And this repeatedly happens throughout the model.

Here, the numerical models affords us the ability to look at the actual volume that's being delivered, quantifying that volume of sediment and sand that's being delivered to deep water, and deep water being over the shelf edge. What I'd like you to focus on is the purple line, which represents deep-water sand delivery. And you can see that deep-water sand delivery is very intermittent throughout the model. And this is intriguing given that we have no eustatic changes in the model.

But the question is, is there a precedent for this, or has this been observed in other areas or experiments? And the answer is yes. Here, we look towards physical tank experiments. These physical tank experiments are that of a deltaic system that progrades to the shelf edge. And the proximal position sediment and water fluxes input into the experiment, just like in our numerical model. And there's a shelf edge and a deep-water portion of the experiment.

One of the things that was observed in that experiment is that as the deltaic system prograded to the shelf edge, the channelized system avulsed. Once it reached the shelf edge, the fluvial system actually began to become net-bypass, meaning sediment was directly delivered to deep water. As sediment accumulated at the terminus of the system, the entire system aggraded and then avulsed.

Here is a conceptual model of what I just said. Once again, the deltaic system prograded to the shelf edge, became a net-bypass system delivering sediment to deep water. It is likely that the channelized systems that are attached to this sediment coming over the shelf edge aggrade and eventually avulse. And that's how the intermittent deep-water sand delivery we observe in our models are similar to that observed in these physical tank experiments.

And now, what about the eustatic variation model? One thing we observe is that the patterns of deposition are very similar to the no-eustatic variation model. In fact, sediment is delivered to the deep water through the delta prograding to the shelf edge, delivering sediment over the shelf edge, the slope aggrades, and then the deltaic system eventually avulses. So we have autogenic processes at work in this model. But also, each of those events do correspond to a change in sea level.

So now, let's look at results. Let's focus on the purple curve. And the purple curve is the sand delivery to deep water. And you'll note that each of those changes, more or less, do correspond to a drop in sea-level change. One thing we observe is that large and small sea-level falls produce similar results. Note the sea-level change at 40 million years. It has a peak in deep-water sand delivery that's similar to smaller sea-level changes observed between 10 and 20 million years. This is an interesting result, suggesting that large and small sea-level changes can produce similar depositional system responses.

Another thing we observe is that the peaks in deep-water sand delivery correspond not only to the falling part of sea level but the minima and even the rise. Another way to look at the data is to detrend deep-water sand delivery and the eustatic curve and do a comparison between the two. What we're doing here is comparing the degree of similarity. And one thing we observe, once again, is that the peaks correspond to both the falling, the minima, and the rise of sea level. And that's evidenced by the gray correlation lines that are bent down, across, and up, suggesting that deep-water sand delivery can occur in any part of the sea-level fall. But what's also interesting is that the peaks for large and smalls falls are very similar.

So now, let's do a model comparison. Focus on the purple curves. And you will observe that the peaks in deep-water sand delivery are similar for both models. What this suggests is that autogenic processes, such as those in the no-eustatic variation model where there was channel migration and avulsion, are similar to those driven by sea-level changes. This is an intriguing result, although we should note that the sea level-driven model, or the Kominz model, also had autogenic processes, meaning that those processes were interacting with the allogenic processes.

So what does this mean in terms of the total volume of sediment delivered to deep water? And we find minimal differences. On the y-axis is time, and on the x-axis is the cumulative sand volume. And note, there are differences in the character of those curves, but in general they're pretty similar. This suggests that autogenic processes can produce results that are similar to allogenic processes.

But let's dig a little bit deeper. One of the things we discussed is that it was the rate of change that controlled deep-water sand delivery, or that was thought in the conceptual model. And the rate of change is the eustatic rate of change minus the tectonic rate of change. And what that does is controls the rate in which accommodation is available on the shelf. So removing the accommodation very quickly on the shelf can cause sediment to be delivered to deep water.

To take a closer look at that, we ran numerous models. One of the benefits of running forward stratigraphic models is that you can exploit a parameter space. You can run many, many models. And one of the things we observe from running multiple models is that there is a poor correlation between the magnitude of deep-water sand delivery and the rate of relative sea-level change and relative sea-level change. However, we found a better relationship with relative sea level, where nearly 50% of the variance was explained by relative sea level. This suggests that the rate of relative sea level is likely not a good metric for describing the control on sediment distribution.

But why is that? Well, one of the things that has been known is that rate of relative sea level, and even relative sea level, is a 1-D measurement. This measurement is very inadequate for describing four-dimensional processes that happen in sedimentary systems. So what happens is we have a mismatch between the processes that are occurring in the sedimentary system versus our measurement of that system. Sedimentary systems operate in the four-dimensional world, which is XYZ and time. And given this mismatch in dimensional space, we are challenged to adequately characterize, say, the 50% variance that we discussed earlier.

And now, let's discuss the seemingly invariant response of the depositional system to sea level. One of the things we observed was that large and small sea-level changes cause similar magnitudes in deep-water sand delivery and that those peaks can happen in any part of the fall. We think part of that could be related to something called the shelf-regulated equilibrium regression. What this really means is that when the delta progrades to the shelf edge, it creates a slope such that subsequent progradational events can more easily reach the shelf edge. So as a result, you get this seemingly invariant response to sea-level changes.

Now, we'll go back to the questions that we started out with in the beginning of this presentation, the first being the timing of deep-water sand delivery. Deep-water sand delivery seems to occur during the fall, minima, and even during the rise in relative sea level. That's a pretty intriguing result and suggests that our current conceptual model can be refined. The other thing is that the peaks in deep-water sand delivery are similar for both large and small sea-level changes. And that's because of the shelf-edge equilibrium regression as well as some of the autogenic processes we described.

And finally, the volume of sand to deep water-- we find that there are very similar volumes of sand delivered to deep water. By taking a close look at that cumulative deep-water sand delivery curve, we observed there are different characteristics of this curve, which suggests sea level does have an influence on the deep-water sand delivery. However, the net effect is a very similar volume of sand delivery to deep water. And now, to summarize the results of the study-- autogenic and allogenic processes can produce similar results with respect to the magnitude of deep-water sand delivery and also the total volume. What this means is that it may be difficult to distinguish between autogenic and allogenic processes when interpreting the stratigraphic record, which presents a challenge to geologists.

The other thing is that the magnitude of deep-water sand delivery correlates poorly with the rate of relative sea-level change. And we think this is, in part, due to the fact of the shelf-edge delta equilibrium regression, where the delta progrades to the shelf edge and creates a slope that makes subsequent progradational events easier to deposit sediment into deep water. Also, we find that the magnitude or peaks in deep-water sand delivery occur during falls, minima, and the rise of sea level. This creates another challenge for describing the rock record.

And finally, what we observed is that the total volume of sediment that was delivered to deep water was similar for both models. This is interesting, and it means that there is a non-uniqueness on the controls of the stratigraphic record, meaning that different processes can have similar results and also that maybe exploration geologists have focused more on the volume of sediment and sand that's being delivered to a basin rather than sea-level changes. Of course, this is meant to describe long-term controls rather than the short-term controls. As we observed, there is some influence of sea level on deep-water sand delivery.

And now, to conclude this presentation, I have a couple of things that we should consider with respect to our investigation of the stratigraphic record and its interaction with sea level. In particular, our conceptual models often overgeneralize or maybe simplify the processes that we observe both in the numerical models and the tank experiments. And that's for good reason and definitely understandable. But the first that we need to consider is that sediment supply is often treated as constant. However, we know from geomorphology studies that sediment supply varies, at least on short spatio-temporal scales. We should consider how sediment supply varies on larger spatio-temporal scales. Of course, that's an ongoing effort by many of the source-to-sink studies that geoscientists and geomorphologists are conducting.

The second thing is that we often describe depositional systems as having a linear response to some forcing, the forcing being sea level in this case. However, we observed there isn't a linear response and that peaks in deep-water sand delivery could happen during the fall, minima, and even during the rise in sea level. Finally, we should also consider that autogenic processes possibly occur on larger spatio-temporal scales. Often, we consider them a very short spatio-temporal scales.

And I'll end with, we should work in the domain in which the problem lies. Often, our measures or characterization of the stratigraphic record are one dimensional. However, sedimentary systems operate in a four-dimensional space, XYZ and time. That dimensional mismatch causes us to not be able to fully characterize the rock record and would possibly explain why we could not explain 50% of the variance in one of our models with respect to deep-water sand delivery and relative sea level. That's why I'm an advocate for process-based models, physical tank experiments, and careful geologic observations.

Finally, I'll conclude with how I started, which is that a process-based approach to the stratigraphic record can help us better characterize, assess, and explore for the hydrocarbon systems that we are all working on. And that concludes my presentation. My name is Ashley Harris. I'm a member of the Energy Technology Company at Chevron. I'd like to thank AAPG for nominating me to be an AAPG Distinguished Lecturer. I'd also like to thank AAPG Foundation for their continued support. Finally, I'd like to thank Chevron for allowing me to present this research to the world. Thank you.

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