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Atti Ticinensi di Scienze della Terra, S.S. 9 (2003)
THERMAL MODELLING OF SEDIMENTARY SUCCESSIONS
MODELLIZZAZIONE TERMICA DI SUCCESSIONI SEDIMENTARIE
D. GRIGO (1) & S. SCHMALHOLZ (2)
ABSTRACT
We present the thermo-tectonic modelling software, TECMOD2D, which
enables reconstructing the thermal history of sedimentary successions
deposited in extensional basins. The software includes a numerical forward
model, which simulates extensional basin formation and deposition of
sedimentary successions. The forward model is based on pure shear kinematics
and a set of stretching factors controls the velocity field during extension. The
forward model is coupled with an automatic inversion algorithm and
TECMOD2D detects the set of stretching factors, which generates modelled
sedimentary successions that are close to observed successions. The set of
stretching factors minimizing the misfit between observed and modelled
sedimentary successions are termed the optimal set. The thermal evolution
calculated with the forward model using the optimal set is then considered as
thermal history reconstruction. The main objective using TECMOD2D is the
assessment of the thermal history in frontier areas characterized by a lack of
boreholes and deep to ultra-deep waters. The application of this software and
of the relevant methodology to a deep-water passive margin demonstrated the
capability in the prognosis of the thermal profile of three wells drilled after.
RIASSUNTO
Presentiano il programma di ‘modelling’ termo-tettocnico, TECMOD2D,
che permette la ricostruzione della storia termica di una successione
sedimentaria in bacini distensivi. Il programma include un modello numerico
‘forward’, che simula la formazione di un bacino distensivo e la deposizione
di una seria sedimentaria. Il modello ‘forward’ e basato su di una cinematica
di ‘pure shear’, mentre una seria di fattori di ‘stretching’ controllano il campo
di velocità durante l’estensione. Il modello ‘forward’ è accoppiato con un
algoritmo di inversione automatica e TECMOD2D definisce le serie di fattori
‘stretching’ che generano succcessioni sedimentarie simulate che sono il più
vicine alle successioni ossservate. I fattori di ‘stretching’ che minimizzano
l’errore tra successioni sedimentarie simulate ed osservate sono definite come
ottimali. L’evoluzione termica calcolata col modello ‘forward’ utilizzando la
serie ottimale, viene quindi considerata per le ricostruzioni termiche. Nell’utilizzo di TECOMD2D l’obiettivo principale è la definizione della storia
termica in aree non ancora perforate, caratterizzate da assenza di dati di pozzo, e da aree di acque profonde. L’applicazione di tale software e della
metodologia relativa ad un bacino passivo in acque profonde ha dimostrato la
capacità di prevedere i profili termici di tre pozzi perforati successivamente.
KEY-WORDS: LITHOSPHERIC MODELLING, BASIN MODELLING, DEEP WATER
PAROLE CHIAVE: MODELLISTICA LITOSFERICA, MODELLISTICA DI BACINO, ACQUE PROFONDE
(1) ENI Exploration and Production Divison, Milano, Italy
(2) GeoModelling Solutions GmbH, Zurich, Switzerland
1. THE FORWARD MODEL
The forward model is based on pure shear kinematics
allowing for depth-dependent stretching and multiple rifting
events of finite duration [KOOI et al., 1992; MCKENZIE, 1978;
ROYDEN & KEEN, 1980]. The lithosphere is represented by a
series of vertical columns and each column is assigned a
stretching factor for the crust (d) and lithospheric mantle (b).
The velocity field resulting from kinematic stretching is used
to advect the temperature field. The evolution of the temperature field in 2D is determined by the equation
where r, c, T, t, vx, vz, k and A are the density, the specific
heat, the temperature, the time, the velocity in the x-direction,
the velocity in the z-direction, the thermal conductivity and
the volumetric heat production, respectively, and the
subscripted index i determines the model unit (e.g., i=4 for
the crust). Sediment deposition is controlled by a userspecified water-depth (variable in both space and time).
Sediments are compacted and included within the thermal
calculations, which enables calculating the insulating effect
of the sediments (“thermal blanketing”). The density changes
due to stretching and thermal alteration causes loads that
differ from the initial isostatic equilibrium and cause a
deflection of the crustal topography. Considering a flexural
strength of the lithosphere the deflection of the crust is
constrained by the equation [KOOI et al., 1992; WATTS et al.,
1982]
where D, w, rm, rin, g, S and q are the flexural rigidity, the
reversible flexural deflection, the average density of the
mantle, the average density of the basin infill, the acceleration
due to gravity, the irreversible deflection and the “dead” load.
Equations (1) and (2) are solved with a conservative finite
difference scheme.
D. Grigo & S. Schmalholz
2. THE AUTOMATED INVERSION
ALGORITHM
Automated inversion (or reconstruction) of
extensional basin formation can be treated as
a constrained optimisation problem
[BELLINGHAM & WHITE, 2000; POPLAVSKII et
al., 2001]. The function to be minimized is
the misfit between observed and modelled
sedimentary successions. Constraints are
imposed because of modelling (e.g., d factor
must be larger than one) or geological reasons
(e.g., rifting lasts 15 million years). The
inversion consists in the iterative search for
the optimal set of d, b and palaeo-water depth
values, which yield the best fit between the
observed and modelled sedimentary
successions. The optimal set of d, b and
palaeo-water depth values minimizes the
chosen goal function, which is the misfit
between the observed and modelled presentday depths of stratigraphic horizons. The goal
function contains the misfits between every
observed and modelled stratigraphic horizon
for a chosen number of points on a cross
section, which makes the minimization procedure equally sensitive for all observed data.
The thermal evolution calculated with the
forward model using the optimal set of d, b
and palaeo-water depth values is considered
as thermal history reconstruction.
Fig. 1 - Modelled and observed sedimentary successions. Numbers in the legend are ages in
million years before present.
Sequenza sedimentaria osservata e modellata. I numeri in legenda si riferiscono alle età in
milioni di anni.
3. APPLICATION
The present-day sedimentary successions
of a real deep-water margin have been
reconstructed with TECMOD2D (Fig. 1).
The area was previously detected by well
only in the shallow water setting outlining a
Fig. 2 - The optimal set of crustal (d) and lithospheric mantle (b) thinning factors. The numbers
in the legend indicate the age of the horizon, which was used to fit the thinning factors.
thermal gradient distribution slightly
decreasing toward the present shelf break.
I parametri ottimali di assottigliamento della crosta (d) e del mantello litosferico (b). I numeri
in legenda indicano le età degli orizzonti usati per riprodurre i fattori di assottigliamento
The extrapolation of this trend together with
consideration about the proximity of oceanic
crust in the deep water settings supported in the past a model
magnetometric response in order to better constrain the depth
of a strong decrease of the thermal gradient passing from
model. As a matter of fact at those deeper level, rarely reached
shallow to deep water sectors. In the present exercise the
by well, the seismic velocity control in quite scarce. The
results coming from the TECMOD application will be
presence the shape and the magnitude of gravimetric and
checked against those outlined by the previous model, and
magnetometric anomalies can be also helpful in the definition
finally validated on the results of the exploratory campaign
of crustal features that are very important in the lithospheric
that followed.
modelling. The possibility to define the crust type and
To perform a Thermo-Tectonic or Lithosperic modelling
thickness, for example, is very important in the definition of
application a complete definition of basin geological model
the crust heat production along the modelled profile. The
has to be carried out, with particular emphasis to the aspect
identification of the presence of transfer zones could be also
affecting the thermal setting s ant it’s evolution trough time.
useful in the definition of a widespread thermal model. The
The definition of the present basement topography is the
last element can also be used in the selection of a proper
starting point for the reconstruction of basin evolution. This
profile direction in order to detect the thermal setting of a
can be obtained combining the seismic definition of it together
homogeneous crust portion or cell of the passive margin.
with detection and modelling of the gravimetric and
The seismic interpretation of the sedimentary fill has then
Università degli Studi di Pavia
70
Thermal modelling of sedimentary successions
Initial crustal thickness
Initial lithospheric thickness
Thermal conductivity lithospheric mantle
Thermal conductivity crust
Thermal conductivity sediments
Radiogenic heat production crust
Efold length heat production crust
Radiogenic heat production sediments
Thermal expansion coefficient mantle
Thermal expansion coefficient crust
Isotherm that defines effective elastic thickness
Depth of necking
Rifting event 1
Rifting event 2
to outline the main element affecting the passive margin
evolution. The total thickness of the syn-rift sediments
has to be carried out together with the presence of possible
intra syn-rift unconformities that can suggest the present
of multi-rift phases. During the interpretation of the
sequences deposited during the thermal cooling phase
particular care has to be applied not only at the definition
of the entire post-rift sequences thickness but, also in this
case, to the definition of sequence boundaries outlining
strong change in the sedimentation rates suggesting
changes in the thermal cooling rate and/or in the
relationship between sediments supply and
accommodation space.
Then as last step the Geological Model
developed has to be completed with crustal
parameter and rifting age as listed in Table 1.
Once the fitting of the stratigraphy has been
obtained, the optimal set of the crustal (d)
stretching factors show a rough profile,
because they are used to match the rough
basement topography during rifting (Fig. 2).
The maximal values of the lithospheric mantle
(b) stretching factors are located at positions
different to these of the crustal stretching
factors. The lithospheric mantle stretching
factors are also smoother than the crustal ones,
because they are used to fit the thermal postrift subsidence. The palaeo-bathymetry
evolution for the modelled section is also
computed and shown in figure 3. A strong
increase in sea water depth can be outlined in
the deep water settings during thermal cooling
due to the higher induced subsidence not
accompanied by sufficient sediment supply.
This is confirmed by sedimentation rate
decrease and open marine condition in the well
drilled on the shelf. The palaeo-heat flow at
the crust-sediment boundary, as presented in
figure 4, has a maximum of about 3.1 HFU
around 110 Ma between X-positions 0 and 80
km. The average present day heat flow along
the section is around 1.3 HFU and do not show
the strong decrease postulated by the previous
thermal model. The heat flow value computed
along the section was then extrapolated
accounting of the basin structural setting and
of the presence of transfer zones, obtaining a
set of heat flow maps to be used in a 3D Basin
Modelling application.
The exploration history of the area
determines the drilling of three wells along or
in the vicinity of the modelled section. The
temperature curves profile obtained from the
heat flow values computed by TECMOD2D
are compared in figure 5 with those coming
from the previous thermal model and against
the wells measures. In general the thermal
model coming from TECMOD application
match more properly the well data outlining
35 km
125 km
3.5 W/K/m
2.5 W/K/m
2.0 W/K/m
3e-6 W/m^3
7.5 km
1e-6 W/m^3
3.2e-5 1/K
2.4e-5 1/K
200°C
10 km
138 to 116 Ma
116 to 110 Ma
Tab. 1 - Parameter values used for the numerical simulation.
Parametri usati per la simulazione numerica
Fig. 3 - The palaeo-bathymetry of the numerical simulation.
Le paleobatimetrie della simulazione numerica
Fig. 4: - The palaeo-heat flow at the crust-sediment boundary.
I paleoflussi di calore al limite crosta sedimenti
71
Atti Ticinensi di Scienze della Terra - S.S. 9
D. Grigo & S. Schmalholz
Fig. 5 - Computed temperature
(in °C) compared among
different models with those
measured in wells after the
simulation.
Temperature calcolate (in °C)
comparate tra modelli diversi e
con quelle misurate in pozzo
dopo la simulazione.
Gravity Modelling With an Application to the Gulf of Lions Margin
(Se France), Journal of Geophysical Research-Solid Earth, 97
(B12), 17553-17571, 1992.
MCKENZIE, D., Some remarks on the development of sedimentary
basins, Earth Planet. Sci. Lett., 40, 25-32, 1978.
POPLAVSKII, K.N., Y.Y. PODLADCHIKOV & R.A. STEPHENSON, Twodimensional inverse modelling of sedimentary basin subsidence,
Journal of Geophysical Research-Solid Earth, 106 (B4), 66576671, 2001.
ROYDEN, L. & C.E. KEEN, Rifting processes and thermal evolution
of the continental margin of Eastern Canada determined from
subsidence curves, Earth Planet. Sci. Lett., 51, 343-361, 1980.
WATTS, A.B., G.D. KARNER & M.S. STECKLER, Lithosphere flexure
and the evolution of sedimentary basins, Philosophical
Transactions of the Royal Society of London, 305, 249-281, 1982.
the risk of temperature underestimation that was present in
the previous model. In this kind of settings the temperature
differences of the two models at wells can determine at the
depth of the source a dramatic change in the prevision of
maturity and HC volumes generated and Expelled.
REFERENCES
BELLINGHAM, P., & N. WHITE: A general inverse method for modelling
extensional sedimentary basins, Basin Research, 12 (3-4), 219226, 2000.
KOOI, H., S. CLOETINGH & J. BURRUS, Lithospheric Necking and
Regional Isostasy At Extensional Basins .1. Subsidence and
Manoscritto definitivo consegnato il 1 aprile 2003
Finito di stampare il 23 maggio 2003
Università degli Studi di Pavia
72