WO2020070534A1 - A method for computing a thermal parameter of a passive margin area and associated system - Google Patents

A method for computing a thermal parameter of a passive margin area and associated system

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Publication number
WO2020070534A1
WO2020070534A1 PCT/IB2018/001218 IB2018001218W WO2020070534A1 WO 2020070534 A1 WO2020070534 A1 WO 2020070534A1 IB 2018001218 W IB2018001218 W IB 2018001218W WO 2020070534 A1 WO2020070534 A1 WO 2020070534A1
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WO
WIPO (PCT)
Prior art keywords
margin area
computing
passive
passive margin
prism
Prior art date
Application number
PCT/IB2018/001218
Other languages
French (fr)
Inventor
Frank DESPINOIS
Jean-Noël FERRY
Gianreto MANATSCHAL
François SAPIN
Original Assignee
Total Sa
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Publication date
Application filed by Total Sa filed Critical Total Sa
Priority to PCT/IB2018/001218 priority Critical patent/WO2020070534A1/en
Publication of WO2020070534A1 publication Critical patent/WO2020070534A1/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V20/00—Geomodelling in general

Definitions

  • the present invention concerns a method for computing at least one thermal parameter of a passive margin area, the passive margin area being covered at least partially by a plurality of sediment layers forming a sedimentary prism, the plurality of sediments being deposited during a time period, the passive margin area comprising at least one calibration well crossing the plurality of sediment layers, the method being carried out by a system for computing at least one thermal parameter.
  • a passive margin area is the transition between oceanic and continental lithosphere. It is the area where continents have rifted apart to become separated by an ocean. This continental rift establishes due to stretching and thinning of the crust and lithosphere by tectonic plate movement. This is the beginning of the continental subsidence, i.e. the gradual downward settling of the continental crust. If the extension process continues, the phenomenon leads to a break-up of the continental crust and to the formation of a new oceanic crust at the mid-ocean ridge. The subsiding continental crust undergoes normal faulting. Then, crustal stretching ceases and the transitional crust (oceanic crust at the intersection between the continental domain and the oceanic domain) and lithosphere subsides as a result of cooling and thickening according to a phenomenon of thermal subsidence.
  • the plurality of sediment layers settle all along the passive margin area and during all the stages of the creation of the passive margin area: syn tectonic sediments at the early stage of the formation of passive margin area and post tectonic sediments at the late stage of the formation of the passive margin area.
  • thermal state vs. time evolution of the passive margin area is of high interest for petroleum exploration, since this process controls the location of potential oil and gas petroleum systems generation.
  • One aim of the invention is to provide an efficient and accurate method for evaluating a thermal parameter in the passive margin area and more particularly the time evolution of this thermal parameter and the geographical variations of the amplitude of this thermal parameter over the whole passive margin area.
  • the subject-matter of the invention is a method for computing at least one thermal parameter of a passive margin area as mentioned above, comprising the following steps:
  • the method according to the invention may comprise one or more of the following features, taken solely or according to any potential technical combination:
  • the current geometry of the sedimentary prism is obtained by processing and interpreting seismic data and/or well data;
  • the step for obtaining the time variation of the bathymetry at the calibration well comprises a biostratigraphic characterization of the plurality of sediment layers at the calibration well;
  • the step for computing the sedimentation rate of the passive margin area comprises decompacting the plurality of sediments layers in the passive margin area;
  • the step for computing the time variation of bathymetry throughout the time comprises a step for decompacting the plurality of sediments to take into account a successive compaction of the sediments layers during the time period;
  • the step for computing the subsidence rate of the passive margin area comprises a step for computing the eustatic sea level using a model
  • Rbathy mar9 ' n being the computed time variation of bathymetry, R sed mar9 ' n being the computed sedimentation rate and R eU static mar9 ' n being the eustatic sea level rate;
  • the thermal parameter is chosen among: a thermal flux or/and a temperature, the thermal flux and/or the temperature being computed at any point of the passive margin area;
  • the thermal parameter is computed as a function of time
  • the optimal model input parameters comprise at least one or a combination of beta factor, radiogenic flux and formations ages, the formations comprising at least one of : an oceanic crust, post-rift sediment layers, a lower continental crust, an upper continental crust, syn-rift sediment layers, late syn-rift to early post-rift sediment layers.
  • the invention further relates to a system for computing at least one thermal parameter of a passive margin area, according the described method, the system comprising:
  • the invention also relates to a computer program product comprising instructions, when the program is executed by a computer, cause the computer to carry out the method according to the method describes above.
  • FIG. 1 is a schematic cross-section of a passive margin area
  • FIG. 2 is a schematic representation of a system according to the invention
  • FIG. 3 is a schematic flowchart of a method according to the invention.
  • - figure 4 is an example of interpretation of a seismic line
  • FIG. 5 is graph representing porosity versus depth of burial
  • FIG. 6 is a schematic representation of a step of the method according to the invention.
  • FIG. 1 presents a schematic cross-section of a passive margin area 10.
  • the passive margin area 10 is formed by the juxtaposition of an oceanic domain 12, an ocean- continent transition (OCT) domain 14 and a continental domain 16.
  • OCT ocean- continent transition
  • the oceanic domain 12 comprises an oceanic crust 18 covered by post-rift sediment layers 20.
  • the continental domain 16 comprises a lower continental crust 22 and an upper continental crust 24 covered by syn-rift sediment layers 26 and post-rift sediment layers 20.
  • the oceanic domain 12 and the OCT domain 14 are covered by a body of water 28.
  • the continental domain 16 is partially covered by the body of water 28.
  • the body of water 28 is a sea or an ocean for example.
  • the passive margin area 10 comprises a sedimentary prism 30.
  • the sedimentary prism 30 is formed by a plurality of late syn-rift to early post-rift sediment layers 32. These sediment layers 32 extend in the OCT domain 14 and in a part of the oceanic domain 12 and of the continental domain 16.
  • the sedimentary prism 30 has substantially an inverted triangle shape in the cross section of figure 1 .
  • the plurality of sediments layers 32 forming the sedimentary prism 30 are deposited during a time period.
  • the time period is comprised between 5 million years (myr) and 20 myr, for example 10 myr.
  • the passive margin area 10 comprises at least one calibration well 34.
  • the calibration well 34 is bored through the sediments and crosses the plurality of sediment layers 32 of the sedimentary prism 30. Generally, the calibration well 34 is located in the proximal zone, i.e. in the continental domain 16.
  • the length of the calibration well 34 is for example comprised between 2000 m and 8000 m.
  • Figure 2 shows a system 100 for computing at least one thermal parameter of a passive margin area 10, according to the invention.
  • the system 100 comprises a calculator 1 10 for computing at least one thermal parameter of the passive margin area 10, a display unit 120 connected to the calculator 1 10 to display the results provided by the calculator 1 10 and a man-machine interface 130.
  • the calculator 1 10 comprises a database 140.
  • the database 140 is moreover able to store the results provided by the calculator
  • the database 140 is a local database comprised in the calculator 1 10.
  • the database 140 is a remote database connected to the calculator 1 10 by a network.
  • the calculator 1 10 comprises a processor 150 and a memory 160 receiving software modules.
  • the processor 150 is able to execute the software modules received in the memory 160 to carry out the method according to the invention.
  • the memory 160 contains a prism geometry module 170 for obtaining a current geometry of the sedimentary prism 30.
  • the current geometry of the sedimentary prism 30 is obtained by processing and interpretation of seismic data and/or of wells.
  • the current geometry of the sedimentary prism 30 is advantageously stored in the database 140, and loaded when required.
  • the memory 160 comprises a calibration bathymetry module 180 for obtaining a time variation of bathymetry F ⁇ bathy we " throughout the time period at the calibration well 34.
  • the variation of bathymetry throughout the time period is the rate at which the bathymetry, i.e. the depth of water, increases or decreases through time.
  • the variation of bathymetry R bat hy we " throughout the time period at the calibration well 34 is stored in the database 140.
  • the memory 160 comprises a sedimentation module 190 for computing a sedimentation rate R seci mar9in throughout the time period of the passive margin area 10 using geological information from the calibration well 34 and the obtained current geometry of the sedimentary prism 30.
  • the sedimentation rate is the rate at which the sediments settle on the passive margin area 10.
  • the sedimentation rate R se d mar9i n throughout the time period of the passive margin area 10 is typically stored in the database 140.
  • the memory 160 also contains a bathymetry module 200 for computing a time variation of bathymetry R bathy mar9 ' n throughout the time period of the passive margin area 10 using the obtained time variation of bathymetry R bathy we " at the calibration well 34 and based on the obtained current geometry of the sedimentary prism 30.
  • the variation of bathymetry R bathy mar9 ' n throughout the time period of the passive margin area 10 is stored in the database 140.
  • the memory 160 further contains a subsidence module 210 for computing a subsidence rate R sub mar9 ' n of the passive margin area 10 throughout the time period, using the computed sedimentation rate R se d mar9in of the passive margin area 10 computed by the sedimentation module 190, the computed time variation of bathymetry R ath y mar9 ' n of the passive margin area 10 computed by the bathymetry module 200, and a eustatic sea level rate R e ustatic mar9 ' n of the passive margin area 10.
  • the eustatic sea level rate is the rate at which the sea level increases or decreases at a global scale in respect to the current sea level.
  • the subsidence rate R sub mar9 ' n of the passive margin area 10 throughout the time period and the eustatic sea level rate R eU static mar9 ' n of the passive margin area 10 are typically stored in the database 140.
  • the memory 160 also comprises a modelling module 220 for computing a modelled subsidence rate R sub model of the passive margin area 1 0 using a geodynamic basin model, the geodynamic basin model having a plurality of model input parameters.
  • the subsidence rate is the true rate at which the margin creates vertical space (also known as accommodation space).
  • the memory 160 contains an optimization module 230 for determining at least one set of optimal model input parameters minimizing the difference between the modelled subsidence rate R sub model and the computed subsidence rate R S u b mar9 ' n computed by subsidence module 210.
  • the memory 160 also comprises a thermal parameter module 240 for computing at least one thermal parameter of the passive margin area 10 using the optimal set of model input parameters determined by the optimization module 230 using the geodynamic basin model.
  • the display unit 120 is for example able to display maps of the thermal parameter computed by the thermal parameter module 240.
  • the display unit 120 is a standard computer screen.
  • the man-machine interface 130 typically comprises a keyboard, a mouse and/or a touch screen to allow the user to activate the calculator 1 10 and the various software modules contained in the memory 160 to be processed by the processor 150.
  • a flow chart of a method 500 for computing at least one thermal parameter of a passive margin area 10, according to the invention, carried out with a system 100 as described above is shown in figure 3.
  • the method 500 comprises a step 510 for obtaining a current geometry of the sedimentary prism 30.
  • the current geometry of the sedimentary prism 30 is obtained by processing and interpreting seismic data from at least one seismic survey performed in the passive margin area 10 and/or well data located in the passive margin area 10.
  • the current geometry of the sedimentary prism 30 is obtained by processing a plurality of parallel or sub-parallel two-dimension (2D) seismic lines located over the passive margin area 10.
  • Figure 4 shows an example of an interpretation of a 2D seismic line.
  • the seismic lines are for example interpreted using classical seismic interpretation software such as Sismage®, Petrel®, Gocad® or Voxelgeo®.
  • the interpretation comprises a global structural analysis of the passive margin area 10 in order to define the various domains of the passive margin area 10 and their limits such as the oceanic domain 12, the ocean-continent transition 14 and the continental domain 16.
  • This structural analysis is made by picking the seismic reflectors on each seismic line.
  • the continent ocean boundary COB
  • the continentward limit of the OCT 14 is defined by picking the last occurrence of the continental Moho.
  • the interpretation also comprises mapping the sedimentary prism 30 over the passive margin area 10. This mapping is also made by picking the seismic reflectors on each seismic line.
  • the seismic reflectors are more or less clear reflectors that are fan shaped with a common convergence point continentward, situated on the top surface of the syn-rift sediment sequence, which corresponds to a pivot point or inflection point (denoted IP in figures 1 and 4). All these reflectors downlap on the bottom surface of the sedimentary prism 30, which is not necessary the top basement surface.
  • This downlapping surface defines the length of the sedimentary prism 30. It extends along all the domains of the passive margin area 10 across the OCT 14, through oceanic crust. Creation of vertical and horizontal space during the early post-rift history of the margin, correlated with the sedimentation rate will modify the whole geometry of the sedimentary prism 30.
  • the sedimentary prism 30 is a substantially inverted triangle shape body defined by three main points, from the continent to the ocean (figures 1 and 4):
  • IP inflection point
  • each generation of prism 30 has a different OP;
  • CP continentward point
  • the sedimentary prism 30 comprises three sides (figure 1 ). Two of the three sides are both isochrones and the last one is a structural boundary. From the IP to the OP, the side is defined by an isochron. It corresponds to the global downlap surface mentioned above, i.e. the last sedimentary deposit layer of the considered generation of prism 30. From the IP to the CP, another isochron defines the deepest and landward side of the prism 30. This isochron is basically the first deposit composing the sedimentary prism 30, overlaying the syn-rift sequence of sediments and ending on the basement. The third side, from CP to OP, is a structural boundary which marks the boundary between the basement, oceanic, OCT and the overlaying sedimentary cover.
  • This line may be drawn by picking all the downlapping seismic reflectors within the sedimentary prism 30 between the CP and the OP.
  • This boundary is a simple surface or a thicker layer depending on the morphology of the oceanic basement and then of the spreading rate.
  • the last parameter to characterize the sedimentary prism 30 is the angle between the sea level and the IP to OP line. This angle is very useful to estimate the length of the sedimentary prism 30 when the OP is too far and not imaged on the seismic line.
  • the seismic data are 3D seismic data or the 2D seismic data are processed directly in 3D.
  • the interpretation comprises picking seismic surfaces corresponding to the sedimentary layers 32 using the same kind of software mentioned above.
  • the method 500 comprises a step 520 for obtaining a time variation of bathymetry R ba thy we " throughout the time period at the calibration well 34.
  • This step 520 comprises a biostratigraphic characterization of the plurality of sediment layers 32 at the calibration well 34.
  • the biostratigraphic characterization is used to estimate, during the syn-rift and post-rift periods, the paleo-depositional environments and therefore the paleo-bathymetry, using for example foraminifera, nannofossils and palynomorphs.
  • biostratigraphic characterization may be found for example in Dodd and Stanton, 1981 , Paleoecology, concepts and applications: New York, John Wiley.
  • the calibration well 34 also allows dating the various seismic reflectors corresponding to the various sediment layers 32 forming the sedimentary prism 30.
  • the method 500 then comprises a step 530 computing a sedimentation rate R se d mar9i n throughout the time period for the passive margin area 1 0 using geological information from the calibration well 34 and the current geometry of the sedimentary prism 30.
  • the step 530 for computing the sedimentation rate in the passive margin area 10 comprises a step 535 for decompacting the plurality of sediments layers 32 in the passive margin area 10.
  • the step 535 comprises obtaining the lithological content of the plurality of sediment layers 32 at the calibration well 34 determined from samples of the calibration well 34.
  • the step 535 for decompacting allows taking into account the successive compaction of the sediments layers 32, each time a new sediment layer covers a previous sediment layer throughout the deposit of the plurality of sediment layers 32.
  • the decompacting step 535 comprises obtaining the porosity of the sedimentary prism 30.
  • a lithological log in the calibration well 34 is obtained, for example, through an electro-facies study.
  • Electro-facies study is based on the correlation between gamma-ray log and rock resistivity log. By combining the two logs, a first order lithologic log may be obtained.
  • the method divides the drilled rock column into four different major lithologies: sand, sandy shale, marl and limestone. The division is very brief but sufficiently accurate to perform a decompaction of the sedimentary prism 30.
  • Depth of burial versus porosity curves for the four different lithologies composing the prism are used. These curves can be determined empirically. They are simple exponential relationship.
  • Figure 5 shows the depth of burial versus porosity for sand 700, sandy shale 710, marl 720 and limestone 730.
  • the porosity of each sediment layer 32 may be computed using the following equation:
  • c sand c sandy shale c mari c iimestone are t e re
  • the original thickness of the sediment layer is related to the current thickness by the following equation:
  • T deposit T current y _ f deposit
  • ⁇ current is the current thickness of the sediment layer, advantageously obtained by the current geometry of the sedimentary prism 30.
  • ⁇ j> curreni and ⁇ $> de P° sit are respectively the current porosity and the original porosity of the sediment layer 32, computed with the previous equation. Thanks to this method, the sedimentation rate R se d mar9i n is computed for the whole passive margin area.
  • the step 535 for decompacting the sediments described below is a first order method.
  • a more precise method may be used for decompacting the sediments. For example, it may be improved by dividing the prims into more layers and decompacting one layer after each other one from the top to the base.
  • the method 500 comprises a step 540 for computing a time variation of bathymetry R bathy mar9 ' n throughout the time period for the passive margin area 10 using the time variation of bathymetry R bathy we " at the calibration well 34 and the current geometry of the sedimentary prism 30.
  • the step 540 is performed on each seismic line and for each layer of sediments 32 forming the sedimentary prism 30.
  • the bathymetry at the calibration well 34 is known.
  • the compacted/decompacted accumulation of sediments layers 32 is computed according to the previous described method.
  • the average dipping angle of deposit of the corresponding sediment layer 32 between the calibration well 34 location and the OP is fixed.
  • the average dipping angle is comprised between 0.5° and 1 ° .
  • the average dipping angle depends on the corresponding sediment dynamics and could be higher with the evidence of turbidites sediments for instance.
  • Figure 6 shows an example of the method applied on a seismic line with an average dipping angle of 1 ° .
  • Figure 6A shows the current geometry of the sedimentary prism 30 with the current bathymetry.
  • Figure 6B shows the bathymetry restored at an early stage of the time period.
  • the bathymetry at the calibration well 34 obtained by the method described above, is equal to 30 m.
  • the bathymetry of the OP is computed and equal to 1 147 m.
  • the intermediate bathymetries along the line may be obtained by interpolation.
  • the method 500 comprises a step 550 for computing a subsidence rate R SU b mar9 ' n for the passive margin area 10 throughout the time period, using the computed sedimentation rate R se d mar9i n , the computed time variation of bathymetry R b ath y mar9 ' n , and an eustatic sea level rate R e ustatic mar9 ' n ⁇
  • the eustatic sea level rate R eU static mar9 ' n may be positive or negative depending on the time period.
  • the step 550 comprises a step 555 for computing the eustatic sea level using a model.
  • the eustatic sea level R eU static mar9 ' n is for example computed using eustatic sea level model from Haq et al., 1987,“Chronology of fluctuating sea levels since the Triassic : Science, v. 235.
  • Other information regarding the modeling of eustatic sea level may be found in Kominz, 2013,“Sea level variations over geologic time”, in Reference Module in Earth Systems and Environmental Sciences.
  • the eustatic sea level rate R eU static mar9 ' n is for example considered constant during the time period and for the whole passive margin area 10.
  • the subsidence rate RTM 3in ⁇ s for example determined by the following equation:
  • the method 500 according to the invention further comprises a step 560 for computing a modelled subsidence rate R SU b model for the passive margin area 1 0 using a geodynamic basin model, the geodynamic basin model having a plurality of model input parameters.
  • the geodynamic basin model is for example the model proposed by Stein and Stein, 1992, in“A model for the global variation in oceanic depth and heat flow with lithospheric age”, Nature, v. 359.
  • Such a model allows modelling the subsidence of the oceanic crust from onset of formation to 150 myr.
  • Such a model is based on the thermal relaxation of the oceanic crust.
  • the modelled subsidence rate R SU b model is computed for the whole passive margin area.
  • basin model may be used such as TemisFlow package or PetroMod.
  • the input parameters of the basin model are for example isopachs, ages of formation and beta factor, bulk rock properties such as compaction curves, thermal conductivity, density, heat capacity and radiogenic condition, including radiogenic flux, erosional sections, fluid properties, sources rock properties, bottom boundary condition such as heatflow, top boundary such as temperature and calibration data...
  • Beta factor characterizes the stretching of the crust. It corresponds to a ratio between the thickness of the stretched crust and the initial thickness of the crust.
  • the method 500 comprises a step 570 for determining at least one set of optimal model input parameters minimizing the difference between the modelled subsidence rate R SU b model and the computed subsidence rate R SU b mar9 ' n ⁇
  • the method 500 comprises calculating and optimizing a cost function.
  • This cost function is a measure of the discrepancies between the modelled subsidence rate R SU b model and the subsidence rate Rsub mar9m computed in step 550.
  • R SU b model and R SU b mar9 ' n are considered to be due to thermal anomalies in the passive margin area 10 during the formation of the sedimentary prism 30.
  • the cost function to be minimized is
  • minimization algorithms may be used such as for example algorithms based on gradient or conjugate gradient.
  • the optimal model input parameters comprises for example at least one, or a combination of beta factor, radiogenic flux and formations ages...
  • the formations comprise at least one of : an oceanic crust 18, post-rift sediment layers 20, a lower continental crust 22, an upper continental crust 24, syn-rift sediment layers 26, late syn-rift to early post-rift sediment layers 32.
  • the method 500 comprises a step 580 for computing at least one thermal parameter of the passive margin area 10 using the optimal set of model input parameters in the geodynamic basin model.
  • the thermal parameter is computed using the geodynamic basin model with the optimal set of input parameters.
  • the thermal parameter is for example a thermal flux or a temperature in any point of the passive margin area 10, for example in the sediment layers 32.
  • the thermal parameter is preferably computed as a function of time. Therefore, the thermal evolution of the passive margin area 10 is obtained, including for example the amplitude and the time of the thermal peak induced by the upwelling of the lithosphere and/or the post-rift relaxation. Then, four-dimension maps (space and time) may be generated. Such kind of maps are useful for targeting of potential oil and gas reservoirs and for placing exploration wells and/or complementary seismic surveys for mapping the potential oil and gas reservoirs.
  • the passive margin area 10 comprises a plurality of calibration wells 34. Therefore, the step 520 comprises obtaining a time variation of bathymetry R ba thy we " throughout the time period for each calibration well 34. The time variations of bathymetry are then averaged.
  • the step 535 comprises obtaining the lithological content of the plurality of sediment layers 32 for each calibration well 34. Then, the determined values of lithological content are averaged for each sediment layer 32.

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Abstract

The invention deals with a method (500) for computing at least one thermal parameter of a passive margin comprising a sedimentary prism. The passive margin area comprises a calibration well crossing the sedimentary prism. The method comprises: - obtaining (51 0) a current geometry of the sedimentary prism, - obtaining (520) a time variation of the bathymetry (Rbathy well at the calibration well, - computing (530) a sedimentation rate (Rsed margin) of the passive margin area, - computing (540) a time variation of bathymetry (Rbathy margin) of the passive margin area, - computing (550) a subsidence rate (Rsub margin) of the passive margin area, - computing (560) a modelled subsidence rate (Rsub model) for the passive margin area using a geodynamic basin model, - determining (570) at least one set of optimal model input parameters of the geodynamic basin model, - computing (580) at least one thermal parameter of the passive margin area using the optimal set of model input parameters.

Description

A method for computing a thermal parameter of a passive margin area and
associated system
The present invention concerns a method for computing at least one thermal parameter of a passive margin area, the passive margin area being covered at least partially by a plurality of sediment layers forming a sedimentary prism, the plurality of sediments being deposited during a time period, the passive margin area comprising at least one calibration well crossing the plurality of sediment layers, the method being carried out by a system for computing at least one thermal parameter.
A passive margin area is the transition between oceanic and continental lithosphere. It is the area where continents have rifted apart to become separated by an ocean. This continental rift establishes due to stretching and thinning of the crust and lithosphere by tectonic plate movement. This is the beginning of the continental subsidence, i.e. the gradual downward settling of the continental crust. If the extension process continues, the phenomenon leads to a break-up of the continental crust and to the formation of a new oceanic crust at the mid-ocean ridge. The subsiding continental crust undergoes normal faulting. Then, crustal stretching ceases and the transitional crust (oceanic crust at the intersection between the continental domain and the oceanic domain) and lithosphere subsides as a result of cooling and thickening according to a phenomenon of thermal subsidence.
The plurality of sediment layers settle all along the passive margin area and during all the stages of the creation of the passive margin area: syn tectonic sediments at the early stage of the formation of passive margin area and post tectonic sediments at the late stage of the formation of the passive margin area.
Passive margin areas are important exploration targets for oil and gas industry since they are associated with favorable conditions for accumulation and maturation of organic matter contained in the sediments layers, which lead to the development of oil and gas accumulations.
In that context, the thermal state vs. time evolution of the passive margin area is of high interest for petroleum exploration, since this process controls the location of potential oil and gas petroleum systems generation.
One aim of the invention is to provide an efficient and accurate method for evaluating a thermal parameter in the passive margin area and more particularly the time evolution of this thermal parameter and the geographical variations of the amplitude of this thermal parameter over the whole passive margin area. To this aim, the subject-matter of the invention is a method for computing at least one thermal parameter of a passive margin area as mentioned above, comprising the following steps:
- obtaining a current geometry of the sedimentary prism,
- obtaining a time variation of the bathymetry throughout the time period at the calibration well,
- computing a sedimentation rate throughout the time period of the passive margin area using geological information from the calibration well and the current geometry of the sedimentary prism,
- computing a time variation of bathymetry throughout the time period of the passive margin area using the time variation of bathymetry at the calibration well and using the current geometry of the sedimentary prism,
- computing a subsidence rate of the passive margin area throughout the time period, using the computed sedimentation rate of the passive margin area, the computed time variation of bathymetry of the passive margin area, and an eustatic sea level rate of the passive margin area,
- computing a modelled subsidence rate for the passive margin area using a geodynamic basin model, the geodynamic basin model having a plurality of model input parameters,
- determining at least one set of optimal model input parameters minimizing the difference between the modelled subsidence rate and the computed subsidence rate,
- computing at least one thermal parameter of the passive margin area using the optimal set of model input parameters in the geodynamic basin model.
Thanks to the computation of this at least one thermal parameter, areas presenting thermal anomalies, for example a higher thermal flux during a predetermined time period, are mapped and may represent relevant exploration targets for oil and gas industry.
The method according to the invention may comprise one or more of the following features, taken solely or according to any potential technical combination:
- the current geometry of the sedimentary prism is obtained by processing and interpreting seismic data and/or well data;
- the step for obtaining the time variation of the bathymetry at the calibration well comprises a biostratigraphic characterization of the plurality of sediment layers at the calibration well;
- the step for computing the sedimentation rate of the passive margin area comprises decompacting the plurality of sediments layers in the passive margin area; - the step for computing the time variation of bathymetry throughout the time comprises a step for decompacting the plurality of sediments to take into account a successive compaction of the sediments layers during the time period;
- the step for computing the subsidence rate of the passive margin area comprises a step for computing the eustatic sea level using a model;
- the subsidence rate of the passive margin area throughout the time period is determined by the equation:
R margin _ p margin p margin p margin
sub — ■ 'bathy " '"'eustatic + ■ 'sed
Rbathymar9'n being the computed time variation of bathymetry, Rsed mar9'n being the computed sedimentation rate and ReUstaticmar9'n being the eustatic sea level rate;
- the thermal parameter is chosen among: a thermal flux or/and a temperature, the thermal flux and/or the temperature being computed at any point of the passive margin area;
- the thermal parameter is computed as a function of time;
- the optimal model input parameters comprise at least one or a combination of beta factor, radiogenic flux and formations ages, the formations comprising at least one of : an oceanic crust, post-rift sediment layers, a lower continental crust, an upper continental crust, syn-rift sediment layers, late syn-rift to early post-rift sediment layers.
The invention further relates to a system for computing at least one thermal parameter of a passive margin area, according the described method, the system comprising:
- a module for obtaining a current geometry of the sedimentary prism,
- a module for obtaining a time variation of bathymetry throughout the time period at the calibration well,
- a module for computing a sedimentation rate throughout the time period of the passive margin area using geological information from the calibration well and using the current geometry of the sedimentary prism,
- a module for computing a time variation of bathymetry throughout the time period of the passive margin area using the obtained time variation of bathymetry at the calibration well and using the current geometry of the sedimentary prism,
- a module for computing a subsidence rate of the passive margin area throughout the time period, using the computed sedimentation rate of the passive margin area, the computed time variation of bathymetry of the passive margin area, and an eustatic sea level rate of the passive margin area, - a module for computing a modelled subsidence rate for the passive margin area using a geodynamic basin model, the geodynamic basin model having a plurality of model input parameters,
- a module for determining at least one set of optimal model input parameters minimizing the difference between the modelled subsidence rate and the computed subsidence rate,
- a module for computing at least one thermal parameter of the passive margin area using the optimal set of model input parameters in the geodynamic basin model.
The invention also relates to a computer program product comprising instructions, when the program is executed by a computer, cause the computer to carry out the method according to the method describes above.
The invention will be better understood, based on the following description, given solely as an example, and made in reference to the following drawings, in which:
- figure 1 is a schematic cross-section of a passive margin area,
- figure 2 is a schematic representation of a system according to the invention,
- figure 3 is a schematic flowchart of a method according to the invention,
- figure 4 is an example of interpretation of a seismic line,
- figure 5 is graph representing porosity versus depth of burial, and
- figure 6 is a schematic representation of a step of the method according to the invention.
Figure 1 presents a schematic cross-section of a passive margin area 10. The passive margin area 10 is formed by the juxtaposition of an oceanic domain 12, an ocean- continent transition (OCT) domain 14 and a continental domain 16.
The oceanic domain 12 comprises an oceanic crust 18 covered by post-rift sediment layers 20.
The continental domain 16 comprises a lower continental crust 22 and an upper continental crust 24 covered by syn-rift sediment layers 26 and post-rift sediment layers 20.
The oceanic domain 12 and the OCT domain 14 are covered by a body of water 28. The continental domain 16 is partially covered by the body of water 28. The body of water 28 is a sea or an ocean for example.
The passive margin area 10 comprises a sedimentary prism 30. The sedimentary prism 30 is formed by a plurality of late syn-rift to early post-rift sediment layers 32. These sediment layers 32 extend in the OCT domain 14 and in a part of the oceanic domain 12 and of the continental domain 16. The sedimentary prism 30 has substantially an inverted triangle shape in the cross section of figure 1 .
The plurality of sediments layers 32 forming the sedimentary prism 30 are deposited during a time period. For example, the time period is comprised between 5 million years (myr) and 20 myr, for example 10 myr.
The passive margin area 10 comprises at least one calibration well 34. The calibration well 34 is bored through the sediments and crosses the plurality of sediment layers 32 of the sedimentary prism 30. Generally, the calibration well 34 is located in the proximal zone, i.e. in the continental domain 16. The length of the calibration well 34 is for example comprised between 2000 m and 8000 m.
Figure 2 shows a system 100 for computing at least one thermal parameter of a passive margin area 10, according to the invention.
The system 100 comprises a calculator 1 10 for computing at least one thermal parameter of the passive margin area 10, a display unit 120 connected to the calculator 1 10 to display the results provided by the calculator 1 10 and a man-machine interface 130.
The calculator 1 10 comprises a database 140.
The database 140 is moreover able to store the results provided by the calculator
1 10.
In the example of figure 2, the database 140 is a local database comprised in the calculator 1 10. In a variant (not shown), the database 140 is a remote database connected to the calculator 1 10 by a network.
The calculator 1 10 comprises a processor 150 and a memory 160 receiving software modules. The processor 150 is able to execute the software modules received in the memory 160 to carry out the method according to the invention.
The memory 160 contains a prism geometry module 170 for obtaining a current geometry of the sedimentary prism 30.
Typically, as explained in details later, the current geometry of the sedimentary prism 30 is obtained by processing and interpretation of seismic data and/or of wells. The current geometry of the sedimentary prism 30 is advantageously stored in the database 140, and loaded when required.
The memory 160 comprises a calibration bathymetry module 180 for obtaining a time variation of bathymetry F{bathy we" throughout the time period at the calibration well 34.
The variation of bathymetry throughout the time period is the rate at which the bathymetry, i.e. the depth of water, increases or decreases through time. Advantageously, the variation of bathymetry Rbathywe" throughout the time period at the calibration well 34 is stored in the database 140.
The memory 160 comprises a sedimentation module 190 for computing a sedimentation rate Rseci mar9in throughout the time period of the passive margin area 10 using geological information from the calibration well 34 and the obtained current geometry of the sedimentary prism 30.
The sedimentation rate is the rate at which the sediments settle on the passive margin area 10.
The sedimentation rate Rsedmar9i n throughout the time period of the passive margin area 10 is typically stored in the database 140.
The memory 160 also contains a bathymetry module 200 for computing a time variation of bathymetry Rbathy mar9'n throughout the time period of the passive margin area 10 using the obtained time variation of bathymetry Rbathy we" at the calibration well 34 and based on the obtained current geometry of the sedimentary prism 30.
The variation of bathymetry Rbathy mar9'n throughout the time period of the passive margin area 10 is stored in the database 140.
The memory 160 further contains a subsidence module 210 for computing a subsidence rate Rsub mar9'n of the passive margin area 10 throughout the time period, using the computed sedimentation rate Rsedmar9in of the passive margin area 10 computed by the sedimentation module 190, the computed time variation of bathymetry R athymar9'n of the passive margin area 10 computed by the bathymetry module 200, and a eustatic sea level rate Reustaticmar9'n of the passive margin area 10.
The eustatic sea level rate is the rate at which the sea level increases or decreases at a global scale in respect to the current sea level.
The subsidence rate Rsub mar9'n of the passive margin area 10 throughout the time period and the eustatic sea level rate ReUstaticmar9'n of the passive margin area 10 are typically stored in the database 140.
The memory 160 also comprises a modelling module 220 for computing a modelled subsidence rate Rsub model of the passive margin area 1 0 using a geodynamic basin model, the geodynamic basin model having a plurality of model input parameters.
The subsidence rate is the true rate at which the margin creates vertical space (also known as accommodation space).
The memory 160 contains an optimization module 230 for determining at least one set of optimal model input parameters minimizing the difference between the modelled subsidence rate Rsub model and the computed subsidence rate RSub mar9'n computed by subsidence module 210. The memory 160 also comprises a thermal parameter module 240 for computing at least one thermal parameter of the passive margin area 10 using the optimal set of model input parameters determined by the optimization module 230 using the geodynamic basin model.
The display unit 120 is for example able to display maps of the thermal parameter computed by the thermal parameter module 240.
Typically, the display unit 120 is a standard computer screen.
The man-machine interface 130 typically comprises a keyboard, a mouse and/or a touch screen to allow the user to activate the calculator 1 10 and the various software modules contained in the memory 160 to be processed by the processor 150.
A flow chart of a method 500 for computing at least one thermal parameter of a passive margin area 10, according to the invention, carried out with a system 100 as described above is shown in figure 3.
The method 500 comprises a step 510 for obtaining a current geometry of the sedimentary prism 30.
Preferably, the current geometry of the sedimentary prism 30 is obtained by processing and interpreting seismic data from at least one seismic survey performed in the passive margin area 10 and/or well data located in the passive margin area 10.
For example, the current geometry of the sedimentary prism 30 is obtained by processing a plurality of parallel or sub-parallel two-dimension (2D) seismic lines located over the passive margin area 10.
Figure 4 shows an example of an interpretation of a 2D seismic line.
The processing of the seismic lines is classical and will not be detailed here.
The seismic lines are for example interpreted using classical seismic interpretation software such as Sismage®, Petrel®, Gocad® or Voxelgeo®.
The interpretation comprises a global structural analysis of the passive margin area 10 in order to define the various domains of the passive margin area 10 and their limits such as the oceanic domain 12, the ocean-continent transition 14 and the continental domain 16. This structural analysis is made by picking the seismic reflectors on each seismic line. For example, the continent ocean boundary (COB) is defined by picking the oceanward first occurrence of the deepest strongest reflector which corresponds to the oceanic Moho. The continentward limit of the OCT 14 is defined by picking the last occurrence of the continental Moho.
The interpretation also comprises mapping the sedimentary prism 30 over the passive margin area 10. This mapping is also made by picking the seismic reflectors on each seismic line. The seismic reflectors are more or less clear reflectors that are fan shaped with a common convergence point continentward, situated on the top surface of the syn-rift sediment sequence, which corresponds to a pivot point or inflection point (denoted IP in figures 1 and 4). All these reflectors downlap on the bottom surface of the sedimentary prism 30, which is not necessary the top basement surface. This downlapping surface defines the length of the sedimentary prism 30. It extends along all the domains of the passive margin area 10 across the OCT 14, through oceanic crust. Creation of vertical and horizontal space during the early post-rift history of the margin, correlated with the sedimentation rate will modify the whole geometry of the sedimentary prism 30.
In cross-section, the sedimentary prism 30 is a substantially inverted triangle shape body defined by three main points, from the continent to the ocean (figures 1 and 4):
- the inflection point (IP) which is the continentward common convergence point at which all the seismic reflectors composing the sedimentary prism 30 converge. The shallowest reflector starting from the IP is a global downlap surface for the above reflectors. Other global downlap surfaces may be found into the sedimentary prism 30. Each surface marks different generation of the sedimentary prism 30. In the example of figure 4, two generations of the sedimentary prism 30 may be identified.
In the following, the method according to the invention will be applied taking into account only the first generation of sedimentary prism 30;
- the oceanward point (OP) which is the point at which the previous shallowest seismic reflector marking the global downlap surface ends by downlapping on the oceanic crust or proto-oceanic crust. As mentioned before, each generation of prism 30 has a different OP;
- the continentward point (CP) which is the point at which the deepest seismic reflector, starting from the IP, ends by downlapping on the basement surface of the prism 30. It is often located in the OCT but in some case in the proto-oceanic or oceanic domain.
In the cross-section, the sedimentary prism 30 comprises three sides (figure 1 ). Two of the three sides are both isochrones and the last one is a structural boundary. From the IP to the OP, the side is defined by an isochron. It corresponds to the global downlap surface mentioned above, i.e. the last sedimentary deposit layer of the considered generation of prism 30. From the IP to the CP, another isochron defines the deepest and landward side of the prism 30. This isochron is basically the first deposit composing the sedimentary prism 30, overlaying the syn-rift sequence of sediments and ending on the basement. The third side, from CP to OP, is a structural boundary which marks the boundary between the basement, oceanic, OCT and the overlaying sedimentary cover. This line may be drawn by picking all the downlapping seismic reflectors within the sedimentary prism 30 between the CP and the OP. This boundary is a simple surface or a thicker layer depending on the morphology of the oceanic basement and then of the spreading rate. The last parameter to characterize the sedimentary prism 30 is the angle between the sea level and the IP to OP line. This angle is very useful to estimate the length of the sedimentary prism 30 when the OP is too far and not imaged on the seismic line.
The interpretation is repeated for each seismic line. Then, three-dimension model of the sedimentary prism 30 is built by interpolating the thicknesses of sediment layers 32 measured on the each seismic line.
In a variant, the seismic data are 3D seismic data or the 2D seismic data are processed directly in 3D. Then, the interpretation comprises picking seismic surfaces corresponding to the sedimentary layers 32 using the same kind of software mentioned above.
The method 500 according to the invention comprises a step 520 for obtaining a time variation of bathymetry Rbathywe" throughout the time period at the calibration well 34.
This step 520 comprises a biostratigraphic characterization of the plurality of sediment layers 32 at the calibration well 34.
The biostratigraphic characterization is used to estimate, during the syn-rift and post-rift periods, the paleo-depositional environments and therefore the paleo-bathymetry, using for example foraminifera, nannofossils and palynomorphs.
Further information regarding the method of biostratigraphic characterization may be found for example in Dodd and Stanton, 1981 , Paleoecology, concepts and applications: New York, John Wiley.
The calibration well 34 also allows dating the various seismic reflectors corresponding to the various sediment layers 32 forming the sedimentary prism 30.
The method 500 according to the invention then comprises a step 530 computing a sedimentation rate Rsedmar9i n throughout the time period for the passive margin area 1 0 using geological information from the calibration well 34 and the current geometry of the sedimentary prism 30.
Preferably, the step 530 for computing the sedimentation rate in the passive margin area 10 comprises a step 535 for decompacting the plurality of sediments layers 32 in the passive margin area 10.
The step 535 comprises obtaining the lithological content of the plurality of sediment layers 32 at the calibration well 34 determined from samples of the calibration well 34. The step 535 for decompacting allows taking into account the successive compaction of the sediments layers 32, each time a new sediment layer covers a previous sediment layer throughout the deposit of the plurality of sediment layers 32.
For example, for the decompacting step 535, the method developed by Van Hinte, 1978, Geohistory Analysis-Application of Micropaleontology in exploration Geology, AAPG Bulletin 62(2), is applied.
More particularly, the decompacting step 535 comprises obtaining the porosity of the sedimentary prism 30. To determine the porosity, a lithological log in the calibration well 34 is obtained, for example, through an electro-facies study.
Electro-facies study is based on the correlation between gamma-ray log and rock resistivity log. By combining the two logs, a first order lithologic log may be obtained. The method divides the drilled rock column into four different major lithologies: sand, sandy shale, marl and limestone. The division is very brief but sufficiently accurate to perform a decompaction of the sedimentary prism 30.
For each sediment layer 32, the relative content for each four lithologies is obtained in the calibration well 34.
Depth of burial versus porosity curves for the four different lithologies composing the prism are used. These curves can be determined empirically. They are simple exponential relationship. Figure 5 shows the depth of burial versus porosity for sand 700, sandy shale 710, marl 720 and limestone 730.
The porosity of each sediment layer 32 may be computed using the following equation:
Figure imgf000011_0001
csand csandy shale cmari ciimestone are t e re|atjve content for the four lithologies and
Figure imgf000011_0002
determined With the depth Of burial versus porosity curves of Figure 5.
The original thickness of the sediment layer is related to the current thickness by the following equation:
y _ f current
T deposit = T current y _ f deposit
^current is the current thickness of the sediment layer, advantageously obtained by the current geometry of the sedimentary prism 30. <j>curreni and <$>deP°sit are respectively the current porosity and the original porosity of the sediment layer 32, computed with the previous equation. Thanks to this method, the sedimentation rate Rsedmar9i n is computed for the whole passive margin area.
The step 535 for decompacting the sediments described below is a first order method. In variant, a more precise method may be used for decompacting the sediments. For example, it may be improved by dividing the prims into more layers and decompacting one layer after each other one from the top to the base.
Then, the method 500 according to the invention comprises a step 540 for computing a time variation of bathymetry Rbathy mar9'n throughout the time period for the passive margin area 10 using the time variation of bathymetry Rbathy we" at the calibration well 34 and the current geometry of the sedimentary prism 30.
In the presented example, the step 540 is performed on each seismic line and for each layer of sediments 32 forming the sedimentary prism 30.
For each layer of sediments 32 and therefore for each stage of the time period, the bathymetry at the calibration well 34 is known. For each stage of the time period, the compacted/decompacted accumulation of sediments layers 32 is computed according to the previous described method. The average dipping angle of deposit of the corresponding sediment layer 32 between the calibration well 34 location and the OP is fixed.
For example, the average dipping angle is comprised between 0.5° and 1 ° . The average dipping angle depends on the corresponding sediment dynamics and could be higher with the evidence of turbidites sediments for instance.
Figure 6 shows an example of the method applied on a seismic line with an average dipping angle of 1 ° .
Figure 6A shows the current geometry of the sedimentary prism 30 with the current bathymetry. Figure 6B shows the bathymetry restored at an early stage of the time period. For example, at the considered stage, the bathymetry at the calibration well 34, obtained by the method described above, is equal to 30 m. By applying the above mentioned method, the bathymetry of the OP is computed and equal to 1 147 m. The intermediate bathymetries along the line may be obtained by interpolation.
This computation is repeated for each seismic line. The bathymetry and then the time variation of bathymetry Rbathy mar9'n throughout the time period of each point of the passive margin area 10 are obtained.
The computation of the time variation of bathymetry Rbathy mar9'n throughout the time period for the passive margin area 10 may also be made using other backstripping techniques. Further details may be found in Kenneth et al.,“An overview of basin and petroleum system modeling: Definitions and concepts, in“Basin modeling: New horizons in research and applications”, AAPG Hedberg Series n °4.
Then, the method 500 according to the invention comprises a step 550 for computing a subsidence rate RSUbmar9'n for the passive margin area 10 throughout the time period, using the computed sedimentation rate Rsedmar9i n , the computed time variation of bathymetry Rbathy mar9'n, and an eustatic sea level rate Reustaticmar9'n·
The eustatic sea level rate ReUstaticmar9'n may be positive or negative depending on the time period.
Preferably, the step 550 comprises a step 555 for computing the eustatic sea level using a model.
The eustatic sea level ReUstaticmar9'n is for example computed using eustatic sea level model from Haq et al., 1987,“Chronology of fluctuating sea levels since the Triassic : Science, v. 235. Other information regarding the modeling of eustatic sea level may be found in Kominz, 2013,“Sea level variations over geologic time”, in Reference Module in Earth Systems and Environmental Sciences.
The eustatic sea level rate ReUstaticmar9'n is for example considered constant during the time period and for the whole passive margin area 10.
The subsidence rate R™ 3in\s for example determined by the following equation:
^margin _ p margin _ p margin . p margin
^sub ~ ^bathy eustatic ' ^sed
a n > Sc ^d ^Gίp are scalar quantities as defined above.
The method 500 according to the invention further comprises a step 560 for computing a modelled subsidence rate RSUbmodel for the passive margin area 1 0 using a geodynamic basin model, the geodynamic basin model having a plurality of model input parameters.
The geodynamic basin model is for example the model proposed by Stein and Stein, 1992, in“A model for the global variation in oceanic depth and heat flow with lithospheric age”, Nature, v. 359.
Such a model allows modelling the subsidence of the oceanic crust from onset of formation to 150 myr. Such a model is based on the thermal relaxation of the oceanic crust.
The modelled subsidence rate RSUbmodel is computed for the whole passive margin area.
In variant, other basin model may be used such as TemisFlow package or PetroMod... Typically, the input parameters of the basin model are for example isopachs, ages of formation and beta factor, bulk rock properties such as compaction curves, thermal conductivity, density, heat capacity and radiogenic condition, including radiogenic flux, erosional sections, fluid properties, sources rock properties, bottom boundary condition such as heatflow, top boundary such as temperature and calibration data...
Beta factor characterizes the stretching of the crust. It corresponds to a ratio between the thickness of the stretched crust and the initial thickness of the crust.
The method 500 according to the invention comprises a step 570 for determining at least one set of optimal model input parameters minimizing the difference between the modelled subsidence rate RSUbmodel and the computed subsidence rate RSUbmar9'n·
Typically, to determine the set of optimal parameters, the method 500 comprises calculating and optimizing a cost function. This cost function is a measure of the discrepancies between the modelled subsidence rate RSUbmodel and the subsidence rate Rsubmar9m computed in step 550.
The discrepancies between RSUbmodel and RSUbmar9'n are considered to be due to thermal anomalies in the passive margin area 10 during the formation of the sedimentary prism 30.
For example, the cost function to be minimized is || RSUbmodel - Rsubmar9'nH -
Various minimization algorithms may be used such as for example algorithms based on gradient or conjugate gradient.
The optimal model input parameters comprises for example at least one, or a combination of beta factor, radiogenic flux and formations ages...
The formations comprise at least one of : an oceanic crust 18, post-rift sediment layers 20, a lower continental crust 22, an upper continental crust 24, syn-rift sediment layers 26, late syn-rift to early post-rift sediment layers 32.
Finally, the method 500 according to the invention comprises a step 580 for computing at least one thermal parameter of the passive margin area 10 using the optimal set of model input parameters in the geodynamic basin model.
The thermal parameter is computed using the geodynamic basin model with the optimal set of input parameters.
The thermal parameter is for example a thermal flux or a temperature in any point of the passive margin area 10, for example in the sediment layers 32.
The thermal parameter is preferably computed as a function of time. Therefore, the thermal evolution of the passive margin area 10 is obtained, including for example the amplitude and the time of the thermal peak induced by the upwelling of the lithosphere and/or the post-rift relaxation. Then, four-dimension maps (space and time) may be generated. Such kind of maps are useful for targeting of potential oil and gas reservoirs and for placing exploration wells and/or complementary seismic surveys for mapping the potential oil and gas reservoirs.
In variant, the passive margin area 10 comprises a plurality of calibration wells 34. Therefore, the step 520 comprises obtaining a time variation of bathymetry Rbathywe" throughout the time period for each calibration well 34. The time variations of bathymetry are then averaged.
Similarly, the step 535 comprises obtaining the lithological content of the plurality of sediment layers 32 for each calibration well 34. Then, the determined values of lithological content are averaged for each sediment layer 32.

Claims

1.- A method (500) for computing at least one thermal parameter of a passive margin area (10), the passive margin area (10) being covered at least partially by a plurality of sediment layers (32) forming a sedimentary prism (30), the plurality of sediments (32) being deposited during a time period, the passive margin area (10) comprising at least one calibration well (34) crossing the plurality of sediment layers (32), the method (500) being carried out by a system (100) for computing at least one thermal parameter, the method (500) comprising the following steps:
- obtaining (510) a current geometry of the sedimentary prism (30),
- obtaining (520) a time variation of the bathymetry (Rbathy we") throughout the time period at the calibration well (34),
- computing (530) a sedimentation rate (RSedmar9'n) throughout the time period of the passive margin area (10) using geological information from the calibration well (34) and the current geometry of the sedimentary prism (30),
- computing (540) a time variation of bathymetry ( Rbathymar9'n) throughout the time period of the passive margin area ( 1 0) using the time variation of bathymetry (R athy we") at the calibration well (34) and using the current geometry of the sedimentary prism (30),
- computing (550) a subsidence rate (RSUbmar9i n) of the passive margin area (10) throughout the time period, using the computed sedimentation rate (RSedmar9'n) of the passive margin area (10), the computed time variation of bathymetry (R athymar9i n) of the passive margin area (10), and an eustatic sea level rate (Reustaticmar9'n) of the passive margin area (10),
- computing (560) a modelled subsidence rate (RSub model) for the passive margin (10) area using a geodynamic basin model, the geodynamic basin model having a plurality of model input parameters,
- determining (570) at least one set of optimal model input parameters minimizing the difference between the modelled subsidence rate (RSUbmodel) and the computed subsidence rate (RSUbmar9'n) ,
- computing (580) at least one thermal parameter of the passive margin area (10) using the optimal set of model input parameters in the geodynamic basin model.
2.- The method (500) according to claim 1 , wherein the current geometry of the sedimentary prism (30) is obtained by processing and interpreting seismic data and/or well data.
3.- The method according to claim 1 or 2, wherein the step (520) for obtaining the time variation of the bathymetry (R athy we") at the calibration well (34) comprises a biostratigraphic characterization of the plurality of sediment layers (32) at the calibration well (34).
4 The method according to any one of the preceding claims, wherein the step (530) for computing the sedimentation rate ( RSedmar9'n) of the passive margin area (10) comprises decompacting (535) the plurality of sediments layers (32) in the passive margin area (10).
5.- The method according to any one of the preceding claims, wherein the step (540) for computing the time variation of bathymetry (R athy mar9'n) throughout the time comprises a step for decompacting (545) the plurality of sediments (32) to take into account a successive compaction of the sediments layers (32) during the time period.
6.- The method according to any one of the preceding claims, wherein the step (550) computing the subsidence rate (Rsub mar9in) of the passive margin area (10) comprises a step (555) for computing the eustatic sea level using a model.
7.- The method according to any one of the preceding claims, wherein the subsidence rate of the passive margin area (10) throughout the time period (Rsub mar9in) is determined by the equation:
R margin _ p margin p margin p margin
sub — ■ 'bathy " ■ 'eustatic + ■ 'sed
Rbathy mar9'n being the computed time variation of bathymetry, Rsedmar9i n being the computed sedimentation rate and ReUstaticmar9'n being the eustatic sea level rate.
8.- The method according to any one of the preceding claims, wherein the thermal parameter is chosen among: a thermal flux or/and a temperature, the thermal flux and/or the temperature being computed at any point of the passive margin area (10).
9.- The method according to claim 8, wherein the thermal parameter is computed as a function of time.
10.- The method according to any one of the preceding claims, wherein the optimal model input parameters comprise at least one or a combination of beta factor, radiogenic flux and formations ages, the formations comprising at least one of : an oceanic crust (18), post-rift sediment layers (20), a lower continental crust (22), an upper continental crust (24), syn-rift sediment layers (26), late syn-rift to early post-rift sediment layers (32).
1 1.- System (100) for computing at least one thermal parameter of a passive margin area (10), the passive margin area (10) being covered at least partially by a plurality of sediment layers (32) forming a sedimentary prism (30), the plurality of sediments (32) being deposited during a time period, the passive margin area (10) comprising at least one calibration well (34) crossing the plurality of sediments layers (32), the system (100) comprising:
- a module (170) for obtaining a current geometry of the sedimentary prism (30),
- a module (180) for obtaining a time variation of bathymetry (Rbathy we") throughout the time period at the calibration well (34),
- a module (190) for computing a sedimentation rate ( RSedmar9'n) throughout the time period of the passive margin area (10) using geological information from the calibration well (10) and using the current geometry of the sedimentary prism (30),
- a module (200) for computing a time variation of bathymetry (Rbathy mar9'n) throughout the time period of the passive margin area (10) using the obtained time variation of bathymetry
Figure imgf000018_0001
at the calibration well (34) and using the current geometry of the sedimentary prism (30),
- a module (210) for computing a subsidence rate ( RSUbmar9i n) of the passive margin area throughout the time period, using the computed sedimentation rate (RSedmar9'n) of the passive margin area (10), the computed time variation of bathymetry (R athymar9i n) of the passive margin area (10), and an eustatic sea level rate (Reustaticmar9'n) of the passive margin area,
- a module (220) for computing a modelled subsidence rate ( RSubmodel) for the passive margin area (10) using a geodynamic basin model, the geodynamic basin model having a plurality of model input parameters,
- a module (230) for determining at least one set of optimal model input parameters minimizing the difference between the modelled subsidence rate (RSUbmodel) and the computed subsidence rate (RSUbmar9'n) ,
- a module (240) for computing at least one thermal parameter of the passive margin area (10) using the optimal set of model input parameters in the geodynamic basin model.
12.- A computer program product comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method according to any one of claims 1 to 10.
PCT/IB2018/001218 2018-10-05 2018-10-05 A method for computing a thermal parameter of a passive margin area and associated system WO2020070534A1 (en)

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