Transcription
So far in this course, we've talked mostly about two minerals: aragonite and calcite. And although these two minerals constitute most of the carbonate rocks, there's another one, very important in volume and especially important for reservoirs, that we've not really mentioned, and that's dolomite. And it's a process of dolomitization, so the transformation of limestone into dolomite, that will be the topic of the next class.
Wadi Muaydin is famous in Oman for containing a record, a stratigraphic record of a near complete deposition from the Permian all the way to the Lower Cretaceous. So if you come to this wadi, you can see for yourself the carbonates and the tiny bit of clastic that were deposited in that time frame on the margin of the Arabian Plate. But the reason I brought you here is not necessarily for the stratigraphy, but it has more to do with the rocks behind me. This is the Permo-Triassic of Oman, and it's completely dolomitized. So don't get me wrong, these rocks were deposited as limestone, but there's a process that took place later: dolomitization, that transformed those limestone into dolomite. So let's explore what dolomite and dolomitization are.
So you may remember from your first class that dolomite is a calcium magnesium carbonate. So that means you have, for every calcium in the mineral structure, you've replaced it with a magnesium. And of course, you have to have two carbonate ions to equilibrate this equation. Now, you can precipitate dolomite straight from a fluid, and that's known as a dolomite cement. But the vast majority, so the biggest volume of dolomite that you find in a rock record, was not precipitated straight out of a fluid. Instead, it's a replacement of the calcite in a limestone into dolomite. So effectively, you dissolve the calcite and locally you reprecipitate the calcite magnesium ion. So a source of magnesium is important for dolomitization. If you do not have the magnesium, you cannot dolomitize. That is clear.
What's less clear is what the kinetics of dolomitization at low temperatures are. In the rock record, we find vast amounts of early dolomite, the low-temperature dolomite, and the example behind me is one of them. You know, you have maybe 600 meters, 800 meters of limestone completely dolomitized, and that's not a rare instance. But in the lab, we're still unable to precipitate or replace calcite by dolomite at temperatures below 60 degrees Celsius. And that's a problem, and it points out that we don't understand the kinetics, the steps that are required to transform calcite into dolomite.
But why do we worry about dolomites so much? Well, if you look at the volume of reservoirs of hydrocarbon reservoirs, you can see that about half of them are clastic, and the other half are carbonates. And within carbonates, half of those reservoirs are dolomite reservoirs. And especially the deeply buried reservoirs tend to be more commonly in dolomite because dolomite seems to resist pressure, so the burial pressure better and preserve better porosity and permeability. So dolomite and dolomitization is a very important topic in carbonate reservoirs.
So now let's review a little bit what types of fluid you would need to transform a limestone into a dolomite. So seawater is potentially a fluid that can dolomitize because there is a high concentration of magnesium in seawater. So provided that you have a diagenetic pump, so you can move even more fluid than required for calcification through your system, if you have this pump, you can dolomitize because you can bring a lot of the seawater magnesium in contact with the limestone.
All right, the other option is to have hypersaline water, so evaporated waters. And we'll have an opportunity to look at this later in the next class. Because if you have hypersaline conditions, you concentrate the magnesium, which means that this fluid is even more potent for dolomitization, even more efficient than seawater. So that's the second option.
The third option is to have subsurface fluid or hydrothermal fluids. And the reason why these are good fluids for dolomitization is because they typically contain concentrated magnesium, magnesium sorts of different lithologies for a long time. But also, they're also usually at high temperature, and remember that even in the lab, it's not tricky or complicated to precipitate dolomite above 60 or 80 degrees Celsius. It's much easier at high temperature. So those are really good conditions for dolomitization. But of course, that would be late dolomitization, not early diagenetic dolomitization. But these are three fluids that are potentially good for dolomitization.
How about meteoric water? How about rainwater, lakes, etc.? Well, in this case, not really, because these are chemically dilute fluids. So that means there is not a lot of magnesium in these waters, and dolomitization is not possible. Now, there has been some dolomite reported in lakes, but all of that dolomite is of very low volume, so it's a few microns of dolomite per, you know, a certain volume of sediments. And also, it's thought to be associated with bacterial processes, which is another topic altogether, a fascinating topic, but not one that explains the large volumes of dolomites we find in the geological record.