BROSURE about
Applications and implications of combined low-temperature
thermochronological and geometer analyses and geological
observations for
- the identification and quantification of thermal
changes in a basin-orogen system
- oil & gas prospect
1. Introduction
6
2. Low-Temperature thermochronology
7
- Definitions
- The different thermochronometers
- Resetting
- Thermal modelling
7
8
8
3. Applications & implications for oil & gas prospect
9
3.1.
“Direct” thermochronology
9
3.1.1. Thermal modelling in a basin – Oil and Gas prospect (Amazon basin)
9
3.1.2. Differential exhumation across fault systems.
12
3.1.3. Changes in exhumation rates compared to sedimentation rates (Amazon) 13
3.1.4. Inversion of the Western High-Atlas, thrust propagation
15
3.1.5. Determination of Peak Temperature and its age, shale gas
16
3.1.6. Fault gouge/sealing dating.
19
3.2.
Detrital or indirect thermochronology
20
4.
Conclusions
21
Geoffrey Ruiz, GeoLogin 3G, madeleine 28, 1800 Vevey, Switzerland. Tel.: +41 78 847 43 68, email:
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1
5.
Modus operandi
1.
Introduction
21
You will find in the second section some brief descriptions and definitions on lowtemperature methods with accurate references if you wish to go deeper into the geochemistry
of these methods. In the third section is described the potential of these methods in any
geological context. I illustrate all this with some examples from our previous studies in
different orogens-basin systems (Andes, Amazon, Zagros, Atlas, Alps, Caucasus, South
Africa, Tunisia, Balkans).
Since 12 years I have used low-temperature thermochronology to quantify vertical
movements within the upper 10 kilometres of the crust. The low-temperature
thermochronometre with the lowest temperature of closure investigates the most recent
geodynamical evolution. Further, I am one of the few who use low-temperature
thermochronology apart from its common use to identify, quantify and simplify the
characterization of any vertical movement for any feature, morpho-tectonic domain of a
geodynamic setting.
Here is a brief list of what is detailed in section 2
1) A direct thermochronological approach permits
- Thermal modelling of basin fill series and hence determines residence time within the oil
and gas rich shale temperature windows.
- More recently, I combined Raman Spectroscopy (“graphitization”) to low-temperature
thermochronology analyses. This approach is in development but it has a high potential for
gas and oil prospect (see below).
- With colleagues of the University of Grenoble (France) and Lausanne (Switzerland) we are
dating fault gouge using Ar/Ar thermochronology on illite and microprobe analyses to
determine the temperature of equilibrium for phyllosilicates, chlorite, mica and smectite.
- To point active faulting, its age and to quantify differential movements across faults, and
finally fault and fold propagation.
- Vertical sampling allows identification of changes in denudation in the hinterland
2) Detrital analyses on past and present-erosion products of the orogen that allows past and
recent denudation to be traced in the hinterland
Some of the applications for low-temperature thermochronology I have listed above and note
that they are of major interest for hydrocarbon industry because they fill a void of data from
the “source” towards the sink aspect. The potential of our approach is important for oil and
gas industry because it is based on the combination of knowledge in isotope geochemistry,
structural geology, sedimentology, but also geomorphology. It necessitates a perfect
comprehension of low-thermochronological methods, their limits, fields of application and
some innovation.
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2
2.
Low-temperature thermochronology
- Definitions
Temperature increases with depth in the Earth and, at high temperatures, noble gases
emitted in radioactive decay diffuse away. Defects (tracks) in crystals, produced by the
charged particles expelled in nuclear fission, anneal. By measuring the concentrations of such
gases or tracks, one can date when the sample cooled below a temperature at which diffusion
or annealing becomes very slow. The cooling ages/paths provide an estimate of the average
exhumation rate if combined with a geothermal gradient, which in these cases equals the rate
at which material above the sampled rock has been eroded, and this is the reverse for
heating/burial. Such estimates apply to periods as short as 500,000 years to as long as several
100 million years. But in all cases they span several glacial and interglacial cycles, smoothing
out the effects of large climatic changes (Molnar 2003).
- The different thermochronometers
Figure 1: four of six
low-temperature
themochro -nometre,
i.e. AHe, AFT, ZHe,
and ZFT. Unstable
indicates that within
this
temperature
range fission, or
degasing
(He)
occurs. Metastable
correspond to the
partial
annealing
zone
discussed
below.
There are four commonly used low-temperature thermochronometers, as
discussed in the introduction above. Temperatures of closure or Partial Annealing Zone or
metastable temperature range are different for each thermochronometres (Fig 1). They range
from 270 to 55°C. Analyses on titanite are also possible but not very common. They are
usually completed when zircon is absent. The presence and extraction of apatite and zircon
crystals is lithology dependant. Apatite and zircon rich lithologies are magmatic rocks,
conglomerates, sandstones, siltstones and present-day river sands.
Once apatite and zircon are extracted (Fig. 2), two mounts in epoxy and Teflon are
made. They are polished, etched and have a low uranium sheet of mica attached to the
surface. They are then sent for irradiation to determine their concentration in 235U and 238U.
Counting of tracks then determines the apparent age of the samples. Analyses for U-ThSm/He are easier because they necessitate fewer crystals, i.e. three to five, crystals which
should be inclusion free. They are packed in platinum foils and are first degassed to obtain the
He concentration using a laser and a quadrupole mass spectrometer. The crystals are
subsequently dissolved to obtain U, Th, Sm concentrations using Laser ablation ICPMS
methodology.
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Figure 2: Apatite
and zircon crystals
in reflective light.
- Resetting:
A sample will be reset when it rests sufficient time in the metastable and/or
unstable temperature range (Fig. 1). All its previous low-temperature
information is deleted and the clock put back to 0 Ma. For a sediment, resetting
homogenizes the different age population into a single initial 0 Ma age (see 3.2).
- Thermal modelling:
Inverse low-temperature thermochronometric modelling is the end result of high quality
datasets. Results from modelling are time -temperature envelopes that yield the
long-term history of the rock being studied. Thermal models are constrained within and by
the partial annealing zones of both 1) Fission track and 2) (U-Th)/He on the single apatite
crystals, i.e. 120-55°C (Fig. 1).
Thermal models quantify rates of cooling and heating from rock either exposed
today at the surface or from a well (higher present-day temperature). Using a calculated
geothermal gradient, it is possible to change 1) cooling and 2) heating paths/phases into 1)
denudation/exhumation and 2) sedimentary/tectonic burial. In addition, thermal models image
the residence time of the modelled sample within the 120-55°C isotherms (Fig. 3).
Thermal models require external constraints to be completed, which are illustrated in
figure 3 as an unconformity (geological constraint) and a presence at a certain temperature
(250°C & 185°C for this example) at a certain time (130 and 125 Ma) thanks to
thermochronometres with higher temperature of closure.
(For Additional information please read Reiners, P.W., Ehlers, T.A and Zeitler, P.K. Reviews in
Mineralogy & Geochemistry Vol. 58, pp. 1-18, 2005).
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3.
Applications and implications for oil & gas prospect
Figure 3. Sampling approach used by GeoLogin 3G to trace vertical movements using lowtemperature-thermochronology methods. Circles 1 are conventional & direct approaches whereas
circles 2 are either indirect and/or unconventional approaches. The end product is to identify and
quantify vertical movements anywhere and at any time using rates of cooling or heating. Temperatures
of closure and/or partial annealing zone see figure 1.
There are two ways to tackle vertical movements in orogen-basin settings, the 'direct'
and the 'indirect' or detrital approaches (Fig. 3).
The first one aims to 'directly' constrain vertical movements from bedrocks of the
orogen through low-temperature thermochronology on both apatite and zircon minerals.
These methods allow an immediate inspection of vertical movements for the upper crust.
Examples discussed below arise from past projects. With a multiple approach, it is possible
to investigate the short and long-term phases of orogenic growth, and burial in the basin..
‘Indirect’-detrital thermochronology is based on the fact that the denudation record of the
orogen is systematically deposited into the adjacent basins. Such approach is implemented,
when a direct approach is impossible, on the basin fill series and even present-day erosion
products (sands). It traces denudation records that are no longer present in the orogen since
erosion has often removed the record of earlier stages.
The oil and gas temperature window range within the bounds described by the
thermochronometers in figure 1. Thermal modelling is rigorous within the 120 and 55°C
isotherms with additional constraints are for example a near-surface presence for the AptianAlbian and/or additional low-temperature ages outside the 120-55°C bounds (Fig. 2). Such
thermal models 1) yield time-temperature paths of sedimentary sequences, 2) the residence
time for example of source or reservoir rocks within the oil & gas window and 3) phases of
uplift in the basins and orogens (Fig. 2).
3.1.
Direct thermochronology
3.1.1. Thermal modelling in a basin – Oil and Gas prospect
The Andean Amazon basin of Ecuador is a perfect example to illustrate how it is possible to
constrain the thermal history of a basin and hence investigate its oil & gas potentials (Fig. 4).
Oil is encountered in the Cretaceous basin of the Andean Amazon basin of Ecuador and
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source rocks vary in the Andean Amazon Basin: they can be Palaeozoic formations and black
shales from the Middle Cretaceous. Reservoir rocks vary as well ranging from early
Cretaceous to Eocene sandstones.
Figure 4: Geological map of the Eastern Cordillera (EC) and Sub-Andean Zone (SAZ) of northern
Ecuador (modified from Litherland et al., 1994). Basin fill encompass the Aptian-Albian Hollin Fm. to
the Quaternary. Substratum formations and units are indicated, i.e. the Misahualli Fm., Abitagua
Batholith, Pumbuiza Fm. and Paradalarga Unit. Above is indicated the position of cross-section of
Figure 6. SF, CF, AF, SAF, QF, RF, CH: Subandean Fault, Cosanga Fault, Abitagua Fault, SubAndean Front, Quijos Fault and Reventador Fault, Cordillera de Huacamoyos.
We sampled the complete Jurassic to present-day basin fill series in the Sub-Andean Zone.
None yielded any apatite, with the exception of Jurassic volcanics but many zircons because
apatite crystals are fragile and the depositional environments were of too high energy. This 1)
eluded AFT and AHe analyses and as a result time-temperature modelling of the postCretaceous series themselves and 2) forced to develop a novel approach in detrital
thermochronology (see above and 4).
However, we found an alternative. The Jurassic substratum and the Hollin Fm. are in
unconformable contact in the Sub-Andean Zone (Fig. 5). Hence they underwent a common
thermal history since the Aptian-Albian. Exact phases of heating, and cooling are identified
thanks to thermal modelling using input parameters that are Fission-Track and U-Th-Sm/He
analyses + some external constraints, i.e. geology (unconformity) and timing (other lowtemperature thermochronometers) – figure 5. Finally time of residence in the oil and gas
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window temperature can be estimated. Heating and cooling phases are interpreted as phases
of burial and exhumation using a geothermal gradient. All these discoveries are later utilised
to constrain oil and gas generation.
We can observe different phases from left to right that are:
1) The eruption of the volcanics that began in the early Jurassic.
2) A phase of burial that was related to the deposition of volcanics and volcano-sediments
still from the Jurassic volcanic arc
3) A phase of exhumation that was concomitant with the end of arc activity and related to the
accretion of a terrane along the Ecuadorian margin to the west. This phase most likely
generated the hiatus we dated.
4) A phase of burial that correspond to the development of a Cretaceous to Late Eocene backarc basin.
5) An ultimate phase of exhumation associated to oil migration in the Oligocene-early
Miocene generating the Sub-Andean Zone
Figure 5: Inverse thermal modelling for substratum of the Cretaceous Andean Amazon basin within the
120-55°C bounds that correspond to the partial annealing zones of the (U-Th)/He and Fission-Track
systems on apatite. AHe & ZHe: U-Th-Sm/He analyses on apatite and zircon. AFT and ZFT. fissiontrack on apatite and zircon Thick black lines: constrained paths. Dashed black lined represents
unconstrained paths but uses the local geological (unconformity) and additional thermochronometric
constraints. The substratum is composed of early Jurassic volcanics that are in unconformable (60 my
hiatus) contact with the overlying sedimentary reservoir Hollin Fm of Aptian-Albian age. AFT and
ZFT ages range between 180-175 Ma. The Hollin Fm. is composed of a succession of cross-bedded
sandstones with some conglomerates. Black: oil-bitumen dripping from the sandstones.
Because the maximum burial temperature was reached before the ultimate phase of cooling
towards the surface cooling (Fig. 5) it is possible to estimate the missing sedimentary pile
above this outcrop. We use a geothermal gradient ranging between 25 and 20°C/km, a
temperature difference of 50°C between the maximum temperature reached of 70°C and a
surface temperature of 20°C: 50°C/22°/km= 2.2 kilometres. Such value is in agreement with
1) reported stratigraphic thickness from a nearby well drilled by Petro-Ecuador and 2) values
from the proximal Amazon basin.
The thermal modelling of the substratum of any basin can be completed using bedrock and/or
core sample to determine with precision the thermal history but also the time of residency of
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source rocks within the 120-55°C temperature window. Such temperatures correspond to the
temperature range of the maturation of organic material. NB: substratum does not always
imply crystalline, volcanics or metamorphics, it can be a sedimentary formation that has been
fully reset (see above, i.e. section 2).
3.1.2. Differential exhumation across fault systems.
Samples from the Sub-Andean Thrust Belt are located between the Subandean Fault and
Cosanga Fault (Fig. 6). They are 1) volcano-sediments of Jurassic age, and 2) Jurassic pelites
(Ju). AFT and ZFT ages from the SATB yielded similar ages to those from the Eastern
Cordillera ranging between 6.4 and 2.4 Ma, and 55-45 Ma respectively (Fig. 6). East of the
Cosanga Fault AFT and ZFT ages are much older and can reach 180 Ma (Fig. 6) as illustrated
with the sample modelled in figure 5.
Figure 6: Cosanga section (see Fig. 4 for location) with both apatite and zircon fission-track ages,
respectively AFT and ZFT. Thin black line: topography. Left-axis: altitude in meters. Right-axis: age
(Ma). X-axis: distance in kilometres. Light grey background: the Eastern Cordillera. Darker grey
background: extension of the Eastern Cordillera proposed in this text according to the numerous new
thermochronological data. Lighter grey with white crosses correspond to an Early Jurassic batholith
or Cordillera de Huacamoyos (CH; Fig. 4). Mid grey thick line: trend for Apatite Fission-Track ages;
dark grey thick line: trend for Zircon Fission-Track ages. EC, SATB, DSAF, SF, CF, SAF: Eastern
Cordillera, Sub-Andean Thrust Belt, Distal Sub-Andean Zone, Subandean Fault, Cosanga Fault, SubAndean Front.
Our dataset demonstrate that:
- The Subandean Fault (SF) is no longer active because the fission-track ages are identical
across it since at least since 55-45 Ma because identical cooling ages across the SF indicates
coeval exhumation from temperatures around 270°C (ZFT; Fig. 1) towards the surface.
Hence, the Sub-Andean Thrust Belt (SATB) is part of the EC since at least the Early Eocene.
- The Cosanga Fault does accommodate differential movement as suggested by the major
contrast in cooling ages across it. Cooling ages are “young” in the hanging whereas they are
much older in the footwall. It is thus the active fault system that localizes deformation of the
Eastern Cordillera onto the Sub-Andean Zone since the Early Eocene.
- Differential movements along the CF can be quantified using a geothermal gradient of
22°C/km. The AFT ages range between 6 and 2 Ma (so 4 Ma) for the hanging wall with a
temperature of closure of 110°C and a surface temperature of 20°C. The hanging wall of the
Cosanga Fault is thus exhuming a rate of ((110-20°C)/4Ma)/(22°C/km)=1.0 km/my.
In the footwall, AFT ages are much older and the thermal model of figure 5 is used to
estimate recent exhumation. Thermal model shows that cooling started roughly 30 Ma from a
temperature of 70°C ((70-20°C)/30)/(25C°/km)=0.075 km/my that is more than 10 time lower
than for the hanging wall.
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3.1.3. Changes in denudation/exhumation rates compared to sedimentation rates Ex.
SE Peru Amazon basin
The southeastern Peruvian Andes include the Eastern Cordillera and Sub-Andean Zone (Fig.
6) that act today as a topographic front for the Amazon precipitation. However knowledge of
changes in denudation is required to differentiate between tectonic and climatic stimulus that
shapes the 1) morphology of this flank of the Andes and 2) infilling of the Andean Amazon
Basin.
Figure 7: Left: DEM from South America (Cornell University) with Nazca–South American plate
convergence showing the extent of the studied area. Right: geological map of the studied area with
sample locations (source: 1:100 000 sheets maps INGEMMET). AAB, FT, ITS, AF, Maz., MP: Andean
Amazon Basin, Frontal Thrust, Inambari Thrust System, Andean Front, town of Mazuko and Macusani
Plateau. Black and white dashed line: cross-section illustrated in figure 7.
I present the first zircon U-Th/He (ZHe) and AFT ages from a transect against the strike of
the Andes (Figs. 7 & 8).
Fig. 8. Schematic non-balanced geological cross section (see figure 6 for locality) and
thermochronological ages projected onto it. The duplexes to the west of the section (SATB) are only
interpretation. AFTA, ZHe, QB, AF, SATB, ITS, DSAZ, FT, AAB, PS: Apatite fission track age, zircon
U-Th/He, Quincemil Basin, Andean Front, Sub-Andean Thrust Belt, Inambari Thrust System, Distal
Sub-Andean Zone, Frontal Thrust, Andean Amazon Basin, Punquiri Syncline.
A major increase in denudation is evidenced from the Eastern Cordillera in southern Peru at
4-3 Ma (Fig. 9) that can be correlated with a synchronous increase in the sedimentary
accumulation rates from the Amazon Basin from outcrops and well data but also as far as the
Amazon fan (Harris and Mix, 2002). On the basis of these results, I argue for the first order
importance of a Pliocene climatic change towards more erosive conditions along the eastern
slope of the Andes.
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Figure 9. Right and Bottom:
AFT and Z/He age-elevation
plot from the Eastern
Cordillera of SE Peru. Left
and Top: calculated sediment
accumulation rates in the
proximal AAB. Stratigraphic
correlations in the Eastern
Cordillera and Andean
Amazon Basin with
sedimentary thicknesses
taken out of Hermoza 2004.
(1) (2) & (3): Radiometric
ages from intercalated tuff
layer. AFT, ZHe: see figure
7.
In conclusion:
Age-altitude profiles allow recognition of changes in denudation on the hinterland and
question the stimulus of these changes. In the Atlas of SW Morocco it is evident that the
acceleration of denudation in the Oligocene was tectonically controlled (see 3.1.4) whereas
the one in the Eastern Cordillera of SE Peru has to be related to climate change.
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3.1.4. Inversion of the Western High-Atlas, thrust propagation (Sub-Atlas Zone)
I completed a robust U-Th/He dataset on paired apatite and zircon minerals to examine phases
of uplift. The U-Th/He dataset of zircon suggests that denudation was of the order of 0.03
km/my from 90 to 30 Ma whereas the apatite dataset clearly indicates that Alpine tectonic
inversion occurred since the Oligocene at an exhumation rate of 0.3 km/my (Fig. 10)
generating the present-day topography of the High-Atlas mountains.
Figure 10: A) Geological map of the southern flank of the Atlas system with sample location, and UTh/He thermochronological ages on apatite and zircon minerals (AHe, ZHe and maximum peak
temperatures. Map and stratigraphic column modified from Stets (pers. com) with Palaeozoic
substratum series. Cooling rates were transformed into denudation rates using the estimated
geothermal gradient. TNTF, SSAZ, Alt.: Tizi N'Test fault zone, Southern Sub-Atlas Zone, Altitude. B)
Age-altitude relationship for samples from the Axial Zone.
Thermochronometers with the lowest temperatures of closure, i.e. fission-track and (U-ThSm)/He analyses on apatite are better to trace recent changes, i.e. exhumation, fault and fold
development. We dated to 4.5 and 0.5 Ma (Fig. 10) the core of the frontal anticlines in the
Southern-Sub-Atlas Zone (SSAZ) using U-Th-Sm/He thermochronometry (80-55°C) on
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apatite. Hence the SSAZ undergoes in sequence thrust propagation since the Late Miocenetill today. It is locus of present-day deformation in the whole Atlas system.
3.1.5. Determination of Peak Temperature and its age, shale gas
Figure 11: Schematic sketch of natural gas resources (modified from Wikipedia). A: gas associated to
oil reservoirs; B: conventional gas; C: coal gas; D: gas encountered in ultra-compacted reservoir; E:
shale gas
A new geothermometre was recently developed in 2002 and upgraded in 2010. The principle
is simple and briefly detailed at this section.
Carbonaceous material is getting organized with temperature increasing. This process
is irreversible and permits to determine the maximum temperature reached by any rock that
hosting carbonate material. This geothermometre encompassed temperatures between 600°C
and as low as the oil and gas window, i.e. 150°C. We immediately jumped onto this method
when we realized its potential 1) once combined with low-temperature thermochronology and
but also 2) for shale gas prospect. Shale gas is usually encountered at depths of 3000-4000 km
(Fig. 11), which fit perfectly temperatures investigated by the Apatite Fission-Track (AFT;
120-60°C; Fig. 1) and U-Th-Sm/He analyses on zircon (200-160°C; Fig. 1) low-temperature
thermochronometers depending on the geothermal gradient. Once its maturity temperature
determined, it is possible using the accurate low-temperature thermochronometer to
determine when it occurred and if this level kept or not its kerosene’s potential.
We produced an important dataset for graphitization or peak/maximum temperature along a
transect against the strike of the Atlas of Morocco (Fig. 12). The very low-grade metaPalaeozoic (170-200°C) sediments belong to the Southern-Sub-Atlas Zone (Fig. 10). They are
organic rich with a clear smell of bitumen. Still these temperatures are too high for gas
prospect in southern Morocco and indicate that the potential Palaeozoic has been
“overcooked”. Maximum temperatures were in the range of U-Th-Sm/He analyses on zircon.
Results indicate that they were reached 260-240 Ma.
We were surprised to read that a major oil company drilled 3 kilometres deep in the
Palaeozoic - 700 kms to the east of our zone of study. They did not find any gas, or of very
poor quality. To the contrary we were not surprised by this result but rather because we could
have predicted it and prevented drilling.
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Figure 12: Top-left: morphotectonic map of the SW Atlas system. Thick black dashed line. Topographic
section illustrated in grey lines elsewhere. Numbers 1 to 6 correspond to the different morpho-tectonic
domains that are in the other 3 rectangles labelled as HP, NSAZ, AZSSAZ, SP, AA: Haouz Plain,
Northern Sub-Atlasic Zone, Axial Zone, Southern Sub-Atlasic Zone, Souss Plain, Anti-Atlas. HA, AA:
High-Atlas, Anti-Atlas. Red lines: sections along which I sampled. Top right: U-Th-Sm/He analyses on
apatite. Left-axis: Altitude. X-axis: distance. Right-axis: age in Ma. Bottom right: U-Th-Sm/He
analyses on zircon. Axes are the same. Bottom Left: Peak temperature in °C (left-axis). Right axis:
altitude. X-axis distance.
------------------------------------------------------------------------------------------------------Details on the method Peak Temperature – thermal maturity
Introduction
The transformation (carbonization or coalification) of carbonaceous material (CM) has
been used as a proxy for studying the low-temperature evolution of rocks for quite some
time, applying methods like vitrinite reflectance or Rock-Eval pyrolysis. Although some of
these methods are quite reliable for studying thermal maturity in advanced diagenesis,
equivalent tracers for low-grade metamorphism are still lacking. In this temperature range
(from about 200 to 320°C), CM is a complex material in terms of chemistry and structure
Raman spectroscopy of CM (RSCM thermometry) is used as a quantitative
geothermometer for sediments in the range 330–650°C and indicates the peak temperature
of a metamorphic cycle. In this temperature range, graphitization results in the irreversible
polymerization and reorganization of the aromatic skeleton, for which Raman
microspectroscopy provides sensitive quantitative information. The Raman spectrum of
such CM is rather different from that of graphitic CM because of several additional 'defect'
bands, and there is no consensus on their attribution.
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Figure 13: Representative Raman spectra of carbonaceous material along 4 locations in the
Alps for calibration.
Conclusion
The qualitative changes in the spectra’s (Figure 13), as well as the quantitative ratios,
exhibit a significant and remarkably consistent evolution with metamorphic grade, which
may be used as a quantitative proxy f or metamorphic grade. Temperature is most likely
the key factor controlling the structure of low- grade metamorphic CM. However, further
work in various geological contexts is needed (and is in progress) to assess whether the
spectral evolution observed i n t h e v e r y l o w - g r a d e r o c k s ( < 1 5 0 ° C ) may be
generalized and to test whether it is possible to calibrate a general, empirical and
quantitative thermometer based on the Raman spectrum of CM in lower-grade rocks.
Samples: 0.5 kgs of bedrock or from well-core, an uncovered thin section, polished to 20
microns
Analyses: completed in Switzerland or France
Conclusion
The qualitative changes in the spectra’s (Figure 13), as well as the quantitative ratios,
exhibit a significant and remarkably consistent evolution with metamorphic grade, which
may be used as a quantitative proxy f or metamorphic grade. Temperature is most likely
the key factor controlling the structure of low- grade metamorphic CM. However, further
work in various geological contexts is needed (and is in progress) to assess whether the
spectral evolution observed i n t h e v e r y l o w - g r a d e r o c k s ( < 1 5 0 ° C ) may be
generalized and to test whether it is possible to calibrate a general, empirical and
quantitative thermometer based on the Raman spectrum of CM in lower-grade rocks.
Samples: 0.5 kgs of bedrock or well cores, uncovered thin section, polished to 20 microns
Analyses: completed in Switzerland or France
Geoffrey Ruiz, GeoLogin 3G, madeleine 28, 1800 Vevey, Switzerland. Tel.: +41 78 847 43 68, email:
geologin3g@gmail.com, http://www.geologin3g.com
15
3.1.6. Fault gouge/sealing dating. Ex. High-Atlas Morocco
Fault gouges can be dated using précised methodology and analyses. Once the sample
selected, a thin section is made that is analysed by X-ray to determine its composition and
percentage in phyllosilicates.
Later, a cartography of the thin section is made with a microprobe to determine which
minerals are in equilibrium. Equilibrium between micas and Illite indicate the pressure for a
given temperature that is indicated by chlorite. The estimated temperature for our sample is
250-225°C. There are always two families of illite in fault gouges, one in the range or smaller
than two microns and a second one in the range or greater than 14 microns in length. The
authigenic illite is the smallest, and was generated when the two blocks slipped. Ar/Ar
thermochronological analyses on this illite are finally being produced to determine the age of
the fault gouge.
Figure 14: Left: panorama of a giant recent landslide along a Triassic normal fault plane with massive
tilted colluviums. The white line is the fault gouge constituted of phyllosilicates and barite that can be
followed for many kilometres. Right: details of the fault plane and gouge. The footwall is a Precambrian
gneiss whereas the hanging is constituted of Cambrian carbonates and Triassic siltstones. The gouge has
an average width of 2 metres.
The example illustrated in figure 14 is a huge Triassic Fault plate with a two meters thick
gouge in the High-Atlas of Morocco. It is 1.2 km long here but can be followed many
kilometres to the north and south using the white line constituted of Barite. The road is at the
level of the river so we could not see the fault plane from the bottom. What stroked us and
made us stopping was both the size and tilting of the pile of colluviums. We followed the
procedure and analyses indicated above. Results indicate that the age of the fault gouge is still
Triassic being not affected by the second and very recent slip.
Geoffrey Ruiz, GeoLogin 3G, madeleine 28, 1800 Vevey, Switzerland. Tel.: +41 78 847 43 68, email:
geologin3g@gmail.com, http://www.geologin3g.com
16
3.2.
Detrital or indirect thermochronology.
Direct approaches on bedrocks from the hinterland are sometimes limited since erosion has
often removed the record of earlier stages of orogenic growth. To overcome this, we decided
to study orogenic sedimentary records through the use of detrital thermochronology and
heavy mineral analysis (Ruiz et al. 2004).
Any detrital study is based on the notion of the lagtime (Fig. 15) and applicable to
any low-temperature thermochronometer. The lagtime is detailed and explained in the figure
12. It brought a significant advance in thermochronology for the geoscientific community in
terms of methodology but also geodynamics notably of the Ecuadorian Andes. The
associated publication was awarded by the journal Basin Research as one of the best three of
the year.
Please note that transportation is almost always considered as negligible but this is
untrue for example when the hinterland progressively cannibalizes the proximal series of
a foreland basin. These series are not reset because they were not buried to temperatures
high enough to homogenize the different age population. These series contain multiple
detrital populations that are later put in the drainage basin. As a result, transport as
defined in figure 16 is far from being always negligible.
The detrital approach is not much applied because it requires more analyses and carefulness.
This did not prevent us to update this approach and to push forward applying this approach to
the recent, i.e. present-day-river. Sands are selected at the end of a drainage basin and
compare results with an age-altitude profile completed from the same catchment (see 3.1.3).
Our goals are multiple: 1) first we wanted to test if denudation records are identical and 2) if
valid to extrapolate analyses to modern sands in order to minimize the amount of analyses.
The AFT cooling ages from modern river sands but also from the vertical profile from the
Eastern Cordillera in SE Peru reveal a coincidence between rapid erosion that is focused
along the topographic barrier and climatic change from warm to ice-house for the Pliocene.
This dataset corroborates that tectonism through important rock uplift exerted first the
dominant control on the denudation pattern that was later overtaken by climate-driven erosion
denudation since the Late Miocene-Pliocene.
Figure 15: The notion of lagtime
in (a) a schematic cross- section
and (b) within time temperature
scales. The lagtime includes the
time taken for the mineral to
cool through the specific closure
temperature, Tc, to the surface
where the temperature is Ts and
transported into the basin. The
times of closure, exposure on the
surface and time of deposition
(stratigraphic age) are tc, te and
td, respectively. tc1 and tc2
represent two levels in the
hinterland carried into the basin
with stratigraphic ages are td1
and td2.
Sediment hosting apatite, or zircon crystals keeps its provenance signature if it was not reset
or partially reset, in another words heated to metastable or unstable temperature range (Fig.
1). If the sediment is reset, provenance information is deleted. As a result, it hosts the
exhumation pattern of its present-day location similarly to thermal modelling of the
substratum series of the basin (see 3.1.1).
Geoffrey Ruiz, GeoLogin 3G, madeleine 28, 1800 Vevey, Switzerland. Tel.: +41 78 847 43 68, email:
geologin3g@gmail.com, http://www.geologin3g.com
17
The reverse, i.e. an unreset dataset traces exhumation in the hinterland as discussed
above. I do prefer to use the term substratum because it has no implication on the nature of
the substratum Attention, all this is dependent of the thermochronometer used. Sediments in a
basin are most likely reset for the AHe system than for the ZFT’s one due to the important
difference in temperature between the 2 unstable zones (Fig. 1). Two-three and ten kilometres
of overlying sedimentary pile are required to reset these two thermochronometers
respectively. These two values are considerably different; hence, using one
thermochronometer rather than the other one depends on what you are looking for.
Applications of detrital thermochronology for oil industry are many: we can tell from surface
or well data if a level reached or not a certain temperature. Second, if the sample was not
reset and kept its detrital signature and hence past denudation in the hinterland using the
different detrital age populations. If the sediment was reset the multiple age populations are
homogenized and put back to zero when thermal models, path, history of the basin could be
produced as if this level was part of the substratum.
4.
Conclusions
The expertise we presented you is of high importance for oil and gas prospection because it
reconstructs the past thermal history of a basin within the oil & gas windows, as in the
hinterland. Using the geothermal gradient phases of cooling and heating are interpreted and
quantified in terms of exhumation and burial. We decided to present you the existing or the
one we developed potential of these methods, i.e. fault dating, fault accommodation. Finally,
a novel approach is introduced. It combines a new geothermometer with lowthermochronology analyses to investigate gas shale. We believe in these methods as they are
well established in academia. We are convinced that this expertise (see the Laius in the
introduction) is important for Oil & Gas industry. Furthermore, we think that such study
should be completed before any phase of exploration because it can generate shortcuts, avoid
unnecessary investigations – in a way it will 1) probably save you money but also 2) force
you to have a different look at the geological object you target.
We aimed this booklet to any geoscientist, i.e. manager, petroleum geologist,
structural geologist, exploration geologist, exploration manager, consultant. As a geologist, I
would feel fulfilled with all these applications because I would be able to characterize and
quantify an orogen-basin system using established but also novel approaches. The
investigated temperatures reach 270°C so the 10 first kilometres of the crust that encompasses
the oil and gas window from which time-Temperature paths are extracted with exactitude.
5.
Modus operandi
a) Mineral separation
- Selection of key lithologies from key-regions under our supervision (the amount of
samples depends on the objectives)
- Accurate lithologies are all magmatic rocks (volcanics, plutons, breccias),
metamorphics ones if grain size is not too small, continental sediments (silt, slumps,
conglomerates & sandstone)… please avoid carbonates,
- Sample should weight 5-6 kgs and a thin section made
Geoffrey Ruiz, GeoLogin 3G, madeleine 28, 1800 Vevey, Switzerland. Tel.: +41 78 847 43 68, email:
geologin3g@gmail.com, http://www.geologin3g.com
18
- Sample is being crushed and the 355-50 microns fraction kept
- Mineral separation using a wilfley table (table utilized in mining industry)
- The Heavy fraction will further go in the heavy liquids separation (density of 3.1
and 3.3). Apatite mineral has a density of 3.10-3.15 whereas zircon exceeds 4.
- Iron rich aggregates or minerals will be removed with a magnet
- Apatite and zircon rich fractions will be cleaned thanks to hand picking if needed
and acid for zircon.
b) Fission-Track http://en.wikipedia.org/wiki/Fission_track_dating
- Apatites are mounted into epoxy, polished and chemically attacked with acid to
reveal the tracks due to the spontaneous decay of U238. Mounts are later covered by a
glass, or plastic, or mica Uranium free layer and send for irradiation by thermal
neutrons to generate the fission of U235 (more stable) in a nuclear reactor. Once back
the cover is chemically attacked generating tracks. Tracks are counted and an age is
produced. Length and width of the U238 tracks are measured to allow thermal
modelling.
- For Zircons the principle is the same except that there is no thermal modelling and
that the acids utilized are much more dangerous.
For Fission-Track on apatite and zircon up to 20 crystals should de dated to get an age
but more is better.
U-Th/He
From the Apatite and zircons separates, the most beautiful, i.e. inclusion free, size,
crystals are selected and packed for determining the He content thanks to a mass
quadrupole mass spectrometer whereas U and Th are determined using and ICPMS.
Up to 5 crystals should be analysed to produce an age.
c) After collecting the data,
they are first all put back into their geological context
(map, section) before producing age-altitude profiles, thermal modelling using existing
software (HeFty)
Geoffrey Ruiz, GeoLogin 3G, madeleine 28, 1800 Vevey, Switzerland. Tel.: +41 78 847 43 68, email:
geologin3g@gmail.com, http://www.geologin3g.com
19