Welcome to this advanced sedimentology carbonate systems module! My name is Cedric john and I will be your instructor.
Behind me is the beautiful auditorium of the department of earth science and engineering at Imperial College London.
Here, we've been teaching traditional lectures for over a hundred years.
But this is not a traditional lecture, this is a flipped classroom.
It means we're using new technologies, it also means we have new opportunities.
Throughout my career I’ve been teaching carbonates in the field and conducting field research.
I’ve spent countless months in North America, the Middle East Europe, or at sea on research vessels.
I want to bring this experience straight to you! So the philosophy behind this course is that I will teach theoretical concept
straight at the outcrop. That way you can see in the rock how those theoretical concepts apply.
It also means that we'll have to travel the world are you ready for that? then let's go!
Look at that! It's spectacular, it's cold, it's because we are at 2000 meter altitude so
it's quite cold despite being in Oman. We're at the Shorfet Al-Alamin the summit of the Oman Mountains.
The reason I brought you here is because the rocks around us are carbonates and
they're absolutely spectacular, so it's a beautiful place to start with the introduction to our class.
So let's start with some learning objectives: what do I want you to know after you take my course?
The first thing is broadly speaking what I want you to have is a really broad understanding of carbonate systems.
And by this I mean really a full understanding of how carbonates fundamentally .work
And we have a whole week to work towards that
and that includes understanding the chemistry of carbonates because carbonates are primarily chemical sediments.
but despite what I just said that they're chemical sediments, it's also important to
understand that carbonates are mediated by biology. So that biology plays a big big role
in precipitating carbonate so it's important to understand ecology if you want to understand what controls carbonate.
And then of course carbonates form sediments so we need to understand sedimentology,
we need to understand sequence stratigraphy. and what's really interesting I
personally think with carbonates is that their history does not stop with death.
There is a post-death history and that's called diagenesis. Diagenesis are all of the processes that happen
to carbonate after deposition. And that's also something that we need to understand
if we want to be able to understand the carbonates that we see around us here at Shorfet Al-Alamin.
But you might wonder why should you care about carbonates? This is a fair question why do you
need to study carbonates? I'll give you two answers to that question:
The first thing is that half of all the reservoirs are carbonate sequences, so that means that half of the hydrothermal reservoirs,
half of the oil and gas reservoirs and half of the water reservoirs, are in carbonate sequences.
So carbonates are very important and very abundant in the rock record.
The second reason and this is specifically looking at oil and gas reservoirs is that all of the giant fields are in carbonate reservoirs.
And actually they're all in the Middle East so carbonate sequences are
extremely important for an understanding of the reservoirs. That's not where the story ends though
it's also important for you personally as young professionals to understand carbonates.
Because I think there's an opportunity and the opportunity is that carbonates are typically more heterogeneous.
So there is more heterogeneity laterally and vertically in carbonates.
Added to this there's also often more fractures in carbonate reservoirs.
Now this is of course a complication because it leads to variable recovery rates when you look at
field producing from carbonates. So this complication you can turn it on its head
and view it as an opportunity. Because if it's more complicated it means that we've not figured everything out,
so we need young brain power to look at these problems and typically in a company
if you are an expert in carbonates your pathway to promotion and success will be a tiny bit easier.
One thing I'd like to say and this is a good news! Is that you've learned already a
lot about sedimentology and you don't have to learn everything from scratch when you study carbonates.
A lot of what you've learned in clastic sedimentology is equally applicable
to carbonate sedimentology, because carbonates are sediments. So let me give you a couple of examples
you've learned in clastics that the more you transport a sediment the better sorted it becomes.
Well great news, it's also true for carbonates! The more you transport a carbonate sediment the more sorted it will be.
So the higher the hydrodynamic of the water, the system in which the sediment is
deposited the greater the sorting will become. It's also true that the greater the transport
the greater the rounding of the grains will be and that's both for clastic and for carbonates so the
point I am making is that carbonates are sediments. So do not forget your sedimentology because it's
completely applicable to carbonate settings. You will see bedforms, you will see cross-bedding,
you will see all the things that you're familiar with in carbonates and in clastics.
But there are of course differences and perhaps the best way to express this difference is to borrow the sentence
from Wolfgang Schlager, a famous carbonated sedimentologist who said "carbonates are born not made"
and that kind of sums it up it's because there is an influence from biology on carbonates
that they are different than classic sediments. Let me explain what this sentence truly means
and the first thing we can do is look at clastic systems. In this case I'm showing you the coast of
the Western United States, California and the point I want to make is that in a clastic system
like the one i'm showing you all the sediments are coming from the rivers.
So the rivers bring clastic sediment from land into the ocean and they're called point source system.
The point of the source is the river once the sediment get into the margin, into the continental margin
they are then transported by current, so in this case it is t the longshore currents that flow
around the California coast that transports the sediments so from north to south following
following the Ekman transport and those sediments eventually end up in the deep sea
once they intersect canyons. There's a number of deeply cut canyon dating from the last glacial
maximum that shunt the sediment deeper into the system so that's how a classic system works.
You can predict where the source is by knowing where the continent is and where the river mouth is,
it's your point source. And then everything is transported further and further away from
the source and of course that means better sorted, better rounded, etc.
For carbonate it's very different because carbonates are mediated by biology. So that means carbonates will grow
wherever ecological conditions are favorable! So here I'm showing you an example of atolls:
Atolls are carbonate islands in the middle of the ocean. They can be really in the middle of The Pacific,
they can be pretty much anywhere. In this case we're looking at the Maldives.
And the Maldives grow where they are because the ecological conditions are ideal for them.
The middle of the Pacific Ocean is a desert in terms of nutrients, we will see that this is important for carbonates
and they have found shallow water conditions which are absolutely essential for autotroph corals.
So these are great conditions for corals and great conditions for carbonates to grow.
And you can see the influence of ecology on where your source of carbonate is.
And the source of carbonate is termed a "Carbonate Factory". And we'll talk more about this in a few classes.
Another point to make about carbonates being biological sediments is that the shape of
the grains is controlled by their function. So clastic sediments can be easily approximated as beads of glass.
that would be the typical shape for a clastic grain. For carbonates it's not the case.
You see three examples here on the left: you have an Acropora coral you can see that we have
a metric size grain which is effectively the structure of the skeleton of the coral.
In the middle: we have a brachiopod, you can see we have two beautiful valves that are quite intricate,
the shape of these grains is is very intricate and on the right we have a crinoid.
And that crinoid also has a skeleton that is an articulated skeleton and is made up of hundreds of grains
of carbonates that have a very specific shape. So the fact that biology needs a skeleton or needs
a structure dictate the shape and the size of those particles and these particles are formed in situ.
They're not formed on the continent and transported for hundreds of miles into the ocean,
they can grow wherever those ecological conditions are favorable. So again that's a massive difference
and we will see that the shape of the grain has implications for the pore network.
So it impacts porosity and permeability of carbonate rocks. There's another aspect of carbonate that's quite
different from clastic and that is the chemical reactivity of the minerals that form carbonates.
So let's talk about siliciclastic sand, beautiful siliciclastic sand is pretty inert when it
comes to diagenesis, certainly at earth's surface conditions. So that means that early in the burial
process or at the sea floor there's very little that will happen to clastic sand, very little
diagenetic transformations, cementation, dissolution, etc. is not so common. Deeper in the reservoir at maybe four or five kilometers
things can change, but not at the surface. Carbonates are completely different
because their minerals are very reactive. So that means that you'll have a lot of early
transformation of carbonate minerals , typically in the ocean where water are supersaturated in
calcium carbonates it means you can have early cementation. The implication of early cementation is tremendous.
You're in fact you're looking at this right at this right now in this image.
You can see that we have a reef with a flat top but also a reef with a steep sided geometry
the only way you can pile up sediments to reach that type of steep sided geometry is if you cement
them early otherwise they would not be stable. And so we have lots of evidence for early cementation
of reefs which leads to early fracturing but also steep sided geometries.
Conversely it's also very easy to dissolve carbonates early if the fluid is undersaturated with respect to calcium carbonate.
And this is what you typically see in karstic system, in caves which I show you a nice example right here
So the potential for porosity creation but also for porosity destruction
early in the diagenetic history of the carbonate is tremendous. And this is why diagenesis is such
an important part of our class. Okay, so let's sum up what we've learned in this short class!
first we know that the deposition of carbonates is typically mediated by ecology.
So ecology is something important that we will need to learn. we've also seen that the shape and the size of
the grains the carbonate grains can be quite variable and that's important to keep in mind.
then we talked about early lithification the fact that the carbonates can
lithify very early in their history but we also talked about dissolution
and we know that dissolution can happen early or late in the history of the carbonates,
just like lithification can happen early and late but typically will be early.
And finally we've said that all of these have a tremendous impact on the porosity and the permeability of the rocks.
In other words has a tremendous impact on the petrophysical characteristics of the carbonate
so that's why we need to understand them. Okay, well that's it for this class now you're ready
to start in earnest and the next chapter that we will look at is the chemistry of carbonates rocks.