So let's sum up the sequence stratigraphic evolution of The Guadalupian in Texas.
So at first we have previous to The Guadalupian: two carbonate formation the Yeso and Victorio Peak formation,
so carbonates.
And you can see they formed a gentle ramp. And then at the top of this comes a base level lowering.
So we have a lowstand characterized by sand again both in the basin and on the platform.
And then after this of course base level rises, and that gives rise to the Lower San Andreas Formation.
Notice that there are cutoffs that are formed at lowstand
that represent erosion and the position of clastics. So if we look at this evolution,
we see that we have a regression of the shoreline so a progradation of the carbonates during the Yeso and Victoria Peak
but the San Andreas Formation represents a transgression of the shoreline,
So a retrogradation of the carbonate. So the carbonates during the Lower San Andreas
do not keep up with accommodation space. However, this changes after the Lower San Andreas
because then we have evidence for a maximum flooding surface and now the San Andreas Formation starts to prograde.
So the the production of carbonate outpaces the production of accommodation.
And of course you still have lower order sea level changes and every time you have a lowstand
you have accumulation of sand in the basin and that keeps on going but the San Andreas clearly is progradational.
And at the end of the San Andreas, we have a significant pile of sand deposited in the basin.
That's one of the reservoirs in this systems.
And then above the San Andreas Formation comes The Capitan Formation also associated with the
Sseven Rivers Formation in the in the back. So you can see that we have evidence for progradation
again here of the carbonates but also evidence for a steepening of the geometry of that system.
And that keeps on going and during lowstand of course we have sand, during high stand we have carbonates
and really we have good evidence that this system is completely prograding into the basin
and filling this, not completely feeling the basin but prograding into the basin
And filling some of the accommodation available in this Delaware Basin.
If you were to look at some logs through that system this is what you would see:
you would see in the back of the system some nice clastic sands
that could potentially be good reservoir. And they are characterized by, you know, high gamma ray.
And you also see the sands in the gamma ray that are associated with the slope and the basin.
In between in the reef very little sand. So the the reef is not where you'll have most of the sand.
So now at the end of the deposition of The Guadalupian Carbonates, we have evidence for increased evaporation
probably because we had increased isolation of the basin
and deposition of a formation known as the Castile Formation which is an evaporite that is deposited not just
in the salinas on the platform but also in the basin. So we feel this whole thing with an evaporative layer
and this is like a an ideal top seal.
If we sum up the evolution of this system, we've seen already that we have a regression during the
HST of the Yeso and Victoria Peak Formations, followed by a transgression so retrogradation
of the Lower San Andreas and then from the Upper San Andreas all the way to the Upper Guadalupian,
the Upper Capitan Formation, we have evidence for a regression of the shoreline.
The carbonates are completely filling accommodation. So this is a very large scale HST, if you want.
Although in between we have of course smaller scale HST, LST, if you look at high order sea level variations.
Now the whole system is tilted during the formation of the basin and range mountain
and so now the whole system in the subsurface has a tectonic dip to it.
And this is important to understand the emplacement of oil and gas because in the basin we have organic rich limestone
that can produce oil. And oil will then come and follow the bedding
all the way to, in the first instance: The Upper San Andreas Formation where it ends up being trapped by the Grayburg Formation
because the Grayburg Formation forms a seal. It doesn't have good permeability and porosity.
The second system that we have: is when the oil comes from the basin
it can follow the sand of the basin, follow the reef because the reef we see is fractured
and go straight into the back roof of the El Capitan Formation and then ends up you know
forming oil accumulation right at this location. And so this is a stratigraphic
trap again because the the lithologies beyond this are those salina lithologies
characterized by mudstone that are maybe not very permeable but also by the deposition of gypsum that acts as a trap.
And of course on top of all of this we have the Castile Formation.
And one thing I've not mentioned is that a lot of these limestones are dolmitized at the back
of the reef and we'll see what the implication for porosity and permeability is when we look at diagenesis.
So let's sum up what we have seen here in this example from the Guadalupian of Texas.
We talked about the concept of reciprocal sedimentation when you have clastic during lowstand
in both the shelf and the platform, sorry, the shelf and the basin.
We also said that karst is not common but can exist in this system. High aridity leads to lots of evaporation
and so potentially evaporative deposits in the back reef or if you push the system too far
you could also completely evaporate the basin and cap the carbonates by evaporites which are great seal
and that's exactly what happens in the Permian Basin here. We've seen that we have a
steepening of geometry over time in the Guadalupian. And this is very similar to what we've seen in
The Bahamas previously so we tend to have steepening of the geometries. And I think really I cannot
insist enough that the Permian Basin is a good example of why you should beware of using modern analogues.
By all means use modern analogues but you need to understand the ecology of the particular
system you're studying yourself. So in the next class we'll talk about 'M' factory sequence stratigraphy.