Geomodel Facies: Methods and Madness

Luke is back to discuss one of the trickiest elements of the geologist's workflow: geomodel facies.

For a detailed view of this week's whiteboard - with added colours! - check out the still shot below.

TRANSCRIPTION

Geomodel Facies: Methods and Madness

 

Hello! Welcome back to the Cognitive Whiteboard. My name's Luke, and today, we're starting a series of videos around facies modelling.

Why do we do facies modelling?

So when we do facies modelling, we're normally trying to achieve one or two of these kinds of effects. We're trying to represent particular rock-flow behaviours, so there might correlative relationships between things like porosity and permeability, or relative permeability differences that might be on bin-like behaviour, or we're trying to represent geobody shapes. Usually, we see an outcrop like this psychedelic representation of a fluvial system. Now there's a lot of complexity in those geobodies out there, and facies are a very useful way of getting that into your model.

Different meanings for different folks


Now, when we do our facies modelling, we have to be careful because there are a lot of cats that like to use that term. The sedimentologist, the petrophysicist, the seismic interpreter, all may use the term "facies" for their own meaning. The sedimentologist is perhaps the most traditional way of thinking about it. They're trying to represent depositional systems, and they get to see all the way down to the millimetre scale textural relationships that are available essentially only to the naked eye and that can help them see quite a lot of character in the rocks.

By the time the petrophysicist gets to see most of the information, most of their logs are at the 10 centimetre to meter type resolution, and a lot of the particularly older wells lack image logs that can give them some of the same textural information that the sedimentologist sees. So, realistically, the petrophysicist is dealing with mineralogical effects. And of course, the seismic interpreter does their best to try to extrapolate that in 3D but they're working from a meter-plus vertical resolution and what they get to see with that acoustic response is orders of magnitude different to what the sedimentologist can do. So it's important that we get everyone around the table and understand how they're linked together because, particularly from seismic all the way down to sedimentology, there is a pretty difficult choice sometimes in trying to bring those two sciences together.

And when we do, we come then into the geomodel's realm. The geomodeller gets the choice of how they're going to try to distribute those properties, and they're not all that easy to do. The traditional object-based and pixel-based methods are still out there and still in use and still add lots of value. The pixel-based methods are very good at incorporating external trends so, say, seismic data or map-based behaviours that you want to instil upon your model, that could help you get those spatial relationships done very, very well - very good at honouring lots of different probabilities.

Object-based vs pixel-based methodologies


But the object-based models are perhaps more powerful than the pixel-based methods at preserving some of those geobody shapes. And that can be particularly useful in say, channelized bodies. But you can also see that some of the choices you get in creating an object-based model don't necessarily very well reflect what we see in the outcrop. So it's important to remember to model what's deposited and preserved, not what's in an active, modern system.

But the two came together with multi-point statistics where we used an object-based model and a pixel-based methodology to try to give us both the geobody shapes and the external trends all in the one kind of a solution. And in many regards, it's probably one of the most powerful methods that's out there in the industry today. It's preferred by a lot of the super majors. I've been using it for a long time. I did feel like a bit of a dunce when I started. It's very complex to do, and it takes a lot of learning. But if you understand the principles of what goes into it, it can be a very powerful tool to add to the arsenal.

We will talk all about these methods in the upcoming videos, so we'll go into a little bit more detail on how we can get these to sing and dance in the way that you want them to.

But, at the end, it's important that we have a good set of quality control checks to make sure we're getting what we want out of our model. We want to make sure that we have the consistency of the scale that we had at this conversation preserved inside our modelling methods. We also want to make sure that we've got all of the spatial relationships that we want to instil upon the map-based trends, the seismic-type trends, are coming into our modelling systems, and we want to make sure these internal architectures that we're interested in are preserved inside that.

So in the coming videos, we'll talk a little bit more about how we can bring all of those things together.

Thanks very much.