So how about non-fabric selective porosities? Well, we have fractures as a potential non-fabric
selective porosity. And of course non-fabric selective porosity fracture being a good example
of this: always are secondary they are never present here at time of deposition. So that's why we
don't distinguish between primary and secondary for not fabric selective porosity.
So here we have fractures. This is an outcrop scale fracture but you can have them also at the thin
section scale. We can also have channels which are effectively fractured that were dissolved.
So here's an example of a channel. We can have vugs, vugs are an important type of porosity now
a vug is a dissolution feature like the moldic porosity was. It's also a dissolution
feature. But the difference is a vug cross cuts the fabric of the rock. So here you can see:
that the vug in this case is cross cutting the ooids. It's not following the the pattern
of the ooids. So that's an important distinction. And finally we can have cavern porosity.
And cavern porosity essentially designate a man's size or larger porosity of a channel or vug shape.
Okay. So that's a very large dissolution feature that is termed a cavern porosity. Now that brings
us to an interesting topic. There are porosities that you can either qualify as fabric selective
or you can qualify as as not fabric selective depending on your point of view. Let me show you
the first one that we encounter is breccia porosity so breccia porosity is the porosity
between the breccia fragments. Now this is a a fabric selective porosity if
you consider that the fabric of the rock is the breccia itself. In that case it's fabric selective.
But if you consider that the fabric of the rock is the fabric of the component of the rock.
So the limestone blocks within the breccia, then that porosity between those primary components
becomes a secondary porosity. So we can't really determine which one of the two it is. Another type
of porosity that can be either fabric selective or not are boring or burrows. It can be considered as
fabric selective because they're here at time of deposition and so they form their own fabric
within the rock. But you could consider that the sediment or the rock had to be here before the
burrowers or before the boring organisms. So then it becomes a not fabric selective
porosity because it cross cuts the fabric of the rock. So again either one is correct.
And finally shrinkage porosity or mud cracks can also be considered as either primary or not.
It's fabric selective if you consider that it was
here at time of deposition or it's not fabric selective if you consider that the mud themselves
represent the fabric of the rock. And here's a nice example of one of those mud cracks.
One thing that is not at all accounted for in this Choquette and Pray classification is the so-called microporosity.
Because Choquette and Pray has been based on thin section and you cannot see
microporosity on thin sections. But here you can see
that in carbonate grains we can have tremendous amounts of porosities.
So for instance, if you look at the benthic foram. This is a an Eocene nummulites here.
You can put this under an SEM and you will see that within the framework of the shell itself,
you have a lot of really really really small porosity. You know in the order of a few micron here. Maybe
50 micron or less. And even if you look at mud at micrite. Micrite is usually made of aragonite
and these aragonite are rod shaped crystals. And in between these crystals you can have micron
scale porosity so carbonates can have a lot of porosity. And that micro porosity is not present in
the classification i've shown you. But it is a very important porosity for some reservoirs, especially
in the Middle East. The problem of course is how do you extract fluids from such a small porosity.
And that that's a challenge and something that you know is beyond the scope of this class today.
So that brings me to my summary. What have we seen in this class? Well first of all, we use the
Choquette and Pray classification as our standard classification. This is an important one to know.
It is a genetic classification which means that from this classification you can understand how
the porosity was formed. The key is to determine whether the porosity was fabric selective or not.
The problem though is that even though this is a great classification to talk about the
history of diagenesis or the history of porosity or to understand a mature field, it's very difficult
to predict permeability based on Choquette and Pray. In fact you can't predict permeability
based on just Choquette and Pray. And that's a challenge that we will tackle in our next class.