hi and welcome to this talk on reconstructing thermal history using two novel geochemical
proxies so as an introduction you know i'm gonna assume that everybody is already convinced that
thermal histories are important because they give us some information some indication on
the diagenetic condition the diagenetic history but also of course the history of
cracking of organic matter oil emplacement so really something essentially fundamental
for oil and gas so what i'm going to do here is i'm going to try to convince you that in
carbonate settings we have a new tool in our arsenal it's one where we combine effectively
for the timing of the carbonate uranium/lead with laser ablation and for the temperature component
of that system we combine effectively a clumped isotopes with with with uranium lead right and if
you don't have the patience to watch the whole 20 minutes of the talk i'm going to leave you
already with a key message which is to say that clumped isotopes and the clumped isotope signature
of the recrystallized limestone recrystallization is very very important and very useful although
it is not the only thing that we can measure is really offering us a new paradigm shift a new
powerful tool to reconstruct thermal histories in in carbonate reservoirs so very briefly
the methods what is clumped isotopes if you start with a carbonate you can actually look at its
composition at its molecular composition and of course you will see that it's made up of a metal
calcium often with a carbonate ion now the carbonate ion is what we're interested in so
we're going to look more closely at the carbonate ion and you can see that it is composed of oxygen
and carbon of different isotopes C13 C12 O18 O16 and that's what's interesting here
more specifically for clumped isotope we are interested in the pairing of the rare heavy
isotopes O18 C13 in a molecule and we measured this relative to a stochastic distribution and
we call this cap (Δ)47. Δ47 this is a figure here from Petersen et al in 2019 it's a compilation
of all of the different uh calibrations for clumped isotopes that we have put together
and you can see that Δ47 varies with temperature it's only temperature dependent which is the
interest of this proxy and at low temperature Δ47 is heavy at high temperature Δ47 becomes lighter
so we have less and less uh difference between stochastic and the the what we measure in our
carbonates which makes sense now the other method we're going to use is icp ms laser ablation icp ms
as you can see on this figure here what that allows us to do is to use a laser beam and
get very very precise very very small points that we can then measure for uranium lead composition
and for this we use an icpms and when you do this you get a range of different points
and these points have various degrees of mixing with common lead and you can regress through
those points to obtain a an original or a clean uranium-led composition which we can
then translate into an age so where did that story begin that story of trying to do thermal history
in our research group for clumped using clump dye stops for carbonate well actually it began
a while ago in oman where we did a lot of field work and we noticed that in this setting
we had some clay packages with nice well preserved at least superficially well preserved oysters
a quick look at the thin section however revealed that although some of the oysters were relatively
well preserved quite a few of the oysters were recrystallized actually most of the oysters were
recrystallized and some of them had also some evidence of uh specification so so a much more
complicated digentic history and i thought it'd be interesting to do those measurements
and see what we get we also had dolomite in the region and one of my other phd students did some
measurements on the dolomite so i'm showing you here the results of both the oysters in red and
the dolomite in green and i'll start by pointing out on the vertical axis you have temperature
on the horizontal axis it's the back calculated fluid composition i'll talk about this later in
the in the presentation but what i want to point out is the well-preserved oyster comes up with a
temperature of about 37 degree which is a bit on the high side frankly so there's probably a bit of
diagenesis but it's closed from depositional and actually the composition of the fluid is about
one per ml so again maybe the genetic a little bit but also potentially indicating some hyper saline
conditions so it's not an unrealistic temperature for this type of setting however most of the
oysters and the dolomite points to temperatures of um in order of you know 60 degrees celsius
this is well above um earth's surface conditions so so we know this is not earth surface condition
for certain so what's our observation here our observation is that the fine-grained dolomite and
the recrystallized textures both yield similar temperature and i think this is really exciting
because it made me think that there was a potential application
so the the application is potentially we could use fine-grained texture and recrystallize carbonates
to you know derive a thermal history or thermal record for these carbonates because if we
take this example in oman and we look at the temperature we obtain it suggests a burial
of 1 to 1.2 kilometers depending on your estimate of the of the geothermal gradient
so now fast forward to the phd of clairv who has focused on the dolomite of the marion plateau
we decided for her to work in this particular setup because we had some good odp record so
we had access to a lot of material we had shallow water dolomitization that was clearly evidenced by
multiple evidence as being a a purely marine early dolomite of myosin age and the maximum burial we
had in the setting was only 600 meters and above this there is no overburden it's just water about
200 meters of water depth so we knew that this was a simple setting and we always like to start
looking at simple setting first because before we go into more complicated
uh settings so what do we see actually um here what what's um happening here well we see that
despite it being relatively simple as a setting we do notice a lot of different dolomite textures
from you know almost um almost well-preserved texture um in in the dolomite to loss of complete
loss of sedimentary texture and the deposition of effectively a a very large dolomite of several
hundreds of micron even with dissolution of its cores and long story short when um claire
did the measurement for clumped ice dopes what she noticed which was a bit of a of a shock is
even if you look at the first 80 meters of the record we see a change in the clumped isotope
temperature of of those dolomite it starts in the order of you know temperatures in order
of 15 18 degrees so compatible with current day temperature of precipitation if you want on the
seafloor but already at a few hundred meters you are looking at 40 degrees of uh of um clumped ice
top composition now this is actually suggestive of a of essentially a fully recrystallized um setting
so that was really really exciting because it does mean once again that these fine grained dolomite
are very susceptible to recrystallization and potentially if we could pair this with an age
we could come up with some some interesting thermal history so now i would like to
introduce you our third case study which is the resolution gaio odp leg 143 and what's what i
find really exciting about resolution gaiu is that it is completely isolated in the middle of
the pacific ocean it's a very simple cretaceous carbonate buildup right it's composed of rudists
and some sponges a few corals very classical carbonate for this time period very similar
to reservoirs in the middle east but the the fact that it's isolated is exciting for the application
of uranium lead because it means that presumably we'll have less detrital lead so a little bit of
an easier time here dating our carbonates and this is the reason why this particular setting
was selected the other thing is the sedimentary column at odp leg 143 is pretty thick we have
lower cretaceous carbonate of a thousand and six hundred meter in thickness so that's quite a thick
um deposit so we can actually really go and look at the detail of this thermal and sedimentology um
history and marijuan my former phd student did exactly that she sampled a number of things
and what she found in this gyro is that at the base she had iron rich brown dolomite
that was sometimes mixed with a little bit of what we call the white dolomite which
is a coarser dolomite that is not iron rich this white dolomite is also found a bit higher up in a
stratigraphy in a 50 meter window that is composed only of white dolomite so iron poor dolomite
and above this uh this dormitized section we enter a section that is characterized by limestone
uh in which we find actually a number of interesting target for clumped eyestop
including vogs recrystallized fossils single crystals of calcite and of course
microright or the limestone matrix and maria looked at all of these different textures
one thing to be aware of is that even though we have 1600 meters of sediment all of these
sediments are lower cretaceous and on top of this is seawater because the gaio is now sitting at
1300 meters below wave base it has been drowned at the end of the early cretaceous so there's
again no overburden on this so that's why you know the the thermal history is of particular interest
so let's look at some results and the first thing you can see in the results is we have a number of
these different textures the temperature at first appeared to be quite scattered we have some cold
temperatures in the the order of 20 degrees and some warmer temperatures in the order of you know
50 60 degrees so there's a range of temperature but on top of this we can plot the the current
geothermal gradient and because that gaio is at 1300 meters the the temperature at the surface
of the gyro are at about 10 degrees and the geothermal gradient that i plotted here in green
shows that our cold our cold textures match actually the coldest modern geothermal
gradient if we were to shift that geothermal gradient and push it to what it would have been in
the cretaceous we obtained this red line and again what's interesting is that this red line forms the
envelope of the warmest of our sedimentary texture so between these two geothermal gradient we find
all of our clumped isotope temperature which to me indicates that basically we've recorded
throughout a history of recrystallization and cement precipitation a thermal history
for this particular atom so now the the game of course is to pair this with uranium lead dating
so that not only we know the temperature but we also know when that temperature was recorded and
here you have it on the horizontal axis you have the h 120 million years is on your right and then
60 million years is on your left so the time goes from right to left and notice that i've indicated
also in a dashed line the time when the atoll was drowned okay so you can see that even though we
record some texture that are pre-drowning a lot of what we record happen actually post drowning okay
so what can we tell about this temperature well at first we seem to have a trend of temperature
around 30 to 33 degrees very compatible with let's say cretaceous surface water in the tropic
that seems to go to span from 120 maybe all the way to 90 million years so we record things that
are clearly sufficient water but that's of course not the the only part of the story we also see a
trend to colder water post 90 million years and in fact our our white dolomites this big white
iron poor dolomite shows pretty cold temperatures in order of 20 22 23 degrees so much much colder
than what we've expected in the cretaceous they're also dated as much younger by uranium lead in the
order of 70 um 75 million years also notice that we have some anomalies a few of our data points
plot at a relatively old age of 110 million years so pre-drowning or just after drowning
but with much warmer uh temperatures so now let's look at the calculated fluid composition because
we can calculate the fluid composition from the clumped isotopes and what you see here is
that the fluid composition through time shows largely a pre xenozoic composition of seawater
around -1 per ml so completely compatible with essentially marine settings however
there are a few points here that diverge from this purely marine trend and seem to indicate
much heavier fluids so you know at first at first thought you'd think maybe those are brines
and basin old rinds except that we don't really have a base in here so what's happening
well to understand what's happening i think we have to go back to that concept of
what a rock ratio what is high and what is low water rock ratio and in the case of a high water
rock ratio an open system delta outing of calcite and or or dolomite and cap 47 temperature are at
equilibrium so we back calculate the the actual composition of the fluid from which our sediments
were deposited when we are in a closed system or at very low water rock ratio
the delta routine does not change because the delta 18 is based on the bulk isop composition
of the fluid whereas the the the temperature recorded by cloned isotope because cloned
isotope is concerned with the arrangement of the atoms within the the molecule that actually um is
recording the actual temperature so the fact that the delta routine is still preserved as
the pristine original depositional temperature whereas the cap 47 records recrystallization leads
to a calculation of a fluid composition that is wrong and if the temperature has increased
it will give you a heavier delta 18 than what was really happening so i think that what our
records indicate is that we have some process that happened at high water rock ratio it's the green
arrow there and some process that happened in the closed system or if you prefer a low water rock
ratio so let's try to look at the dating of the different events so i've put here first of course
the limestone deposition and platform drowning which we've gathered from from previous studies
what we see is that the first thing the the oldest record we have the oldest phase we have is
this early dolomite that is iron rich and that's followed then by several different uh processes
um including the recrystallization of the brown dolomite but also infilling of bugs and fine
grained cementation that happened that seemed to straddle just the time when the atoll was growing
and then was was in the early stage of drowning and then we have a second phase of the genetic
transformation that uh happen later and also often are recording colder temperature and that's
basically the coarse grain calcite um and and low water rock ratio fossil recrystallization it's the
recrystallization of the iron rich dolomite into the white dolomite the iron poor dolomite i'm not
going to go into the detail of why we think this is happening here but we have evidence for this
and finally we have some porphyric crystals which are the youngest that we record at
up to 40 million years so eocene age we can now divide this into paragenesis we can recognize
one paragenesis that is uh phase one which which is sin depositional or early drowning and then
phase two which is actually uh occurring mostly at high water rock ratio and is post drowning
uh and is influenced by colder uh water as the atoll is sinking so based with this and with the
depth at which these different events occur we can come up with a thermal history for resolution grio
which you're you're seeing uh right there because we can reconstruct paleo geothermal gradients
we can super impose on this our diogenetic phase 1 you can see that this is happening in the top
warm tropical seas during deposition or during early drowning where we're still into this very
warm water environment and phase 2 digenetic phase 2 is happening later during drowning and starts to
record much colder temperature and it's just an effect of the drowning effectively that basically
means that the carbonates are now in much deeper warmer waters so that leads me to my conclusions
i think that and i hope i've convinced you that pairing clump dystope with
iranian lead is a very powerful method uh it gives you quantitative thermal and genetic histories
i think the benefit of the uranium lead lai cpms method is that you can do that at a very high
resolution its drawback is that not every sample is dateable the benefit of the clump tie stop
method is that it always works but the drawback is you need relatively large samples so pairing
both method gives you almost like the best of both world because you will always get some
form of information from your sample and finally i'll end up by saying that recrystallization is
your friend i think we can do a lot with recrystallized samples we can do a lot of
geothermal constraint in carbonate settings which have been traditionally very difficult
to reconstruct the terminal history from thank you for your attention