- [Ali] The focus of this module here is barriers to fluid migration within pure silts classic systems. What you see in front is about three to four meters of shale, and this normally what we think about when we start talking about barriers, and yes indeed that really would act as a barrier to vertical fluid flow within the pure silts classic system, but not everything is as straight forward as a shale so we're gonna look at two more examples. So this example I showed you right there is from the Dad Sandstone in Wyoming. Now we're gonna look at the Muddy J, which produces hydrocarbons from Wyoming and Colorado. So if you look at this core here, you can see that when we focus, there are zones that very clearly show sedimentary structures; in this case, you can see cross stratification and on the laminate, all the cross-stratification, you can see some mud drapes; if you move lower again, you can see lower-angle cross-stratification, but then as soon as we move here, we can see those sedimentary structures are gone, so what happened? Well, it is highly unlikely to not get sedimentary structures in a shallow marine, silts classic setting, so where did those sedimentary structures go? If you look at this thing from a distance, you can see that entire zones within this core are now white, so what's happening? Well what's happened is, we've got some sort of cementation going on, and if we look closer we can find clues as to what that cementation type might be, so if you look here, we have got the stylolite, okay, and as you may know, stylolites normally develop in carbonates, so to find it in a pure sandstone is odd, okay. Here's another, so here's one stylolite right there, here's another stylolite right there, okay, so that may tell us that we're looking at carbonate cementation, but how do we verify? Do we just pour acid right on the face of the core? Well no, you don't do that, because if you do, you're going to ruin the core like someone has over here. So someone did want to check for cementation, they poured acid on the face, and now they've destroyed the face of the core. So if I did want to test this thing, instead of pouring acid on the face, all I have to do is pick up this piece, turn it around, and pour acid right there, and then I'm able to verify whether this is calcite cemented or not. So how is this problematic? It's problematic for two different reasons; when you're interpreting core, if there's no sedimentary structures, how do you tell what kind of environments deposition you have? Well, it's definitely a problem because if we zoom, if we zoom in, you could interpret this in a few different ways, you could be like, oh, you know what, sand here, these are all mixed up, these are typical salt-marsh facies right here, lots of root traces, and then inner gladed sand-beds, but if you look even closer, which is again hard to do because of all the cementation, this isn't exactly, they're not really roots. These are actually chondrites, if we focus there that's some paleophycus right there, so these may be storm beds that have been cemented, and now it's hard to determine what they were originally; so cementation can be a problem, not only for vertical fluid flow, it can also be problematic for interpretation of environments and deposition. You would want to take this into account, you would want to figure out whether the cementation is a continuous zone, or whether they are large nodules; as you can see in this photo from the UK, these outcrops show large calcite nodules. So it's important to figure out what the continuity is of your cemented zone. It's also important because if you're going to calculate recovery factor, one of the variables that goes into the calculation of the recovery factor is heterogeneity within the reservoir. So this is definitely going to quite play a part. The next example is actually an unslabbed core from the cretaceous fern sandstone in Wyoming, alright, sorry in Utah; it's one of the most well-studied deltaic successions in the world, I would strongly urge you to take a field trip to the fern sandstone, so this is an unslabbed core, not very useful for sedimentology and stratigraphy, probably get fracture data from it, but if you look in this zone right here, that's all coal, okay, so coal also can act as a barrier to fluid flow in a reservoir such as this, okay, so make sure you note the location of coals, shale breaks and cemented zones.