E-Modul

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Topic overview
1 (Interpretation depth conversion)
2 Geohistory
3 Isostasy
4 Tectonics
5 Temperature
6 Source rock matuartion
(7 Hydro carbon migration)
Geohistory Movie
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What is Basin Modeling

• The aim of basin modeling is to quantify the
  mechanisms that is forming sedimentary basins,
  and the generation of hydrocarbons.
• Basin modeling is the quantitative integrated
  study of sedimentary basins. It is of
  multidisciplinary nature, and includes
  disciplines like geophysics, sedimentology,
  structural geology and geochemistry.
• A knowledge of the behaviour of the lithosphere
  is essential if we are to understand the
  initiation and development of sedimentary basins.
  
• This module is focused on extensional basins. The
  formative mechanisms of
• sedimentary basins fall into three classes
• a) loading on the lithosphere causes deflection,
  and therefore subsidence
• b) thinning of the lithosphere by mechanical
  stretching is accompanied by fault-controlled
  subsidence
• c) purely thermal mechanisms, such as heat
  conduction.

Click on figure for enlarging
More What can basin modeling tell us
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What can basin modelling tell us?

• Basin modeling is well suited for
• 1) geohistory analysis
• 2) modelling of isostatic response to
  sedimentation, erosion and fault movements
• 3) estimating tectonic subsidence (amount and
  timing of of stretching)
• 4) model the palaeo heatflow into the basin
• 5) predicting area, timing, duration and rate of
  source rock maturation, together with timing of
  fault movements and prospective trap formation
• 6) evaluation of the effect of faulting on
  maturation timing and distribution
• 7) reconstructed fault geometries give insight
  into possible role of faults as conduits or
  barriers between mature sources and potential
  reservoir through time
• 8) burial and temperature history can give
  insight into possible diagenetic effects on
  porosity (e.g. quartz cementation).

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Work path of basin modeling
The first step in a basin analysis study is to
build the input geological section. This step
involves transferring the seismic profile into
the modelling tool. The next step will be to
simulate a geohistory for the geological section.
It is necessary to make a complete geohistory
before moving on to any of the other
tasks. During the subsidence analysis you will
draw the work you completed in the geohistory
reconstruction to constrain the model.The purpose
of this task is to calculate the palaeo heatflow
over the section. The temperature modelling
depends on the calculated heat flow history.
The final step in this module is the source
rock maturation modelling. This depends on the
temperature history of the basin, and on the
time. Possible HC migration modelling follows,
when the maturity is calculated. Please note
the dependencies in the modelling tasks, and that
the uncertainty increases during the modelling
tasks.
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Section 1 Interpretation depth conversion

• To do basin modelling information on
  stratigraphy, lithofacies distribution, and major
  structural features of the basin is required.
• As input an interpreted seismic section is the
  best starting point for building a geological
  model. This includes geometries and ages of the
  various horizons.
• In addition you need information about the
• 1) time to depth conversion factors. If the
  section is in seismic two way travel time, it
  has to be converted to depth. In this case, you
  need conversion factors.
• 2) palaeo water depths.
• 3) eroded or non-depositional surfaces. This
  includes information on the magnitude of erosion
  and the time-span of the non-deposition.
• 4) lithological boundaries and lithology types.
  Many important input parameters are tied
  directly to lithofacies, e.g. porosity-depth
  trends.

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Section 2 Geohistory

• Geohistorical analysis is the reconstruction in
  time and space of the sedimentary basin
  development. This can incorporate a high
  resolution sequence stratigraphic framework, and
  structural reconstruction of normal and reverse
  faults.
• The geohistory reconstruction provides the basis
  for all additional geological modelling on the
  section. It is thus important to do the modelling
  as correct and realistic as possible. The aim of
  the geohistorical analysis is to get the basin
  geometry through time correct.
• It is of special importance to restore the faults
  in a proper manner. If this cannot be done, the
  basin geometries will be significantly wrong.

Geohistory movies (2)
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Geohistory Input Data

• The data we need to make reliable modeling
• From the seismic section
• lithologies
• definition of faults
• We must know about
• erosion and non-deposition
• porosity depth functions
• palaeo water depths

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Decompaction
Present-day stratigraphic thicknesses in a basin
are a product of cumulative compaction through
time. Geohistory reconstruction relies primarily
on the decompaction of the stratigraphic units to
their correct thickness at the various times in
the evolution - in addition to fault restoration
and corrections due to palaeo water depth
variations. The decompaction of stratigraphic
units requires the variation of porosity with
depth to be known. Estimates of porosity from
boreholes suggest that normally pressured
sediments exhibit an exponential relationship
between porosity and depth. It is given on the
form f foe-cy where f is the porosity at
any depth y, fo is the surface porosity and c is
a coefficient that is dependent on lithology and
describe the rate at which the exponential
decrease in porosity takes place with depth.
The decompaction technique seeks to remove
progressive effects of rock volume change with
time and depth. One by one layer is removed, and
the layers underneath are decompacted. Any
compaction history is likely to be complex, being
affected by lithology, overpressure, diagenesis
and other factors. Consequently, what are needed
are some general porosity-depth relationship
which hold good over large depth ranges.
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Click image to watch animation
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Fault Restoration

• Fault restoration capabilities are important for
  several reasons. Without fault restoration the
  basin geometry through time will, in many cases,
  be incorrect. This will affect the estimated
  temperature regime of the basin, and thus the
  predicted maturation history. Not less important
  is the insight into the geometry of possible
  hydrocarbon migrition pathways and traps through
  time.
• There are several different methods for fault
  restoration. We are using vertical simple shear.
  It is found that this method give very good
  results, appropriate for basin modelling
  purposes.
• The method is called vertical shear method of the
  following reason If you think of a fault block
  as consisting of a deck of cards, the cards
  remain vertical throughout the fault restoration
  process. During the reconstruction, the cards are
  translated up the fault system until the top
  timeline is continuous across the fault surface.
  Once the bars have been moved horizontally, their
  vertical position are determined by drawing
  upwards from their new positions along the fault
  plane.
• The resulting displacement has significant
  lateral as well as vertical translation.

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Decompaction/fault restoration
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When reconstructing the basin evolution, one by
one layer is removed, and the layers underneath
are decompacted acording to the porosity-depth
relationship. The faults blocks are also
translated up the fault system until the top
timeline is continuous across the fault surface.

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Geohistory
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The movie starts from 250 M years ago and
progress to present time
Click image to start movies
Next movie
Animation of the basin evolution of a section
over Sørvestlandshøgda over geological time.
Different colours indicate sediments of different
age. Note the time scale in the lower part of the
figure.
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Geohistory
Previous movie
The movie starts from 52 M years ago and progress
to present time
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Animation of the basin evolution of a section
over Sørvestlandshøgda over Tertiary time
(detailed view of the previous movie). Different
colours indicate sediments of different age. Note
the time scale in the lower part of the figure.
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Section 3 Isostasy

• The sediments accumulating in a basin represent a
  load on the lithosphere. Isostasy is the
  principle of Archimedes applied on the earths
  upper layers. It is one of the main processes
  operating in basin formation.
• The theoretical isostatic deflections are
  calculated due to the loading/unloading of
  sediments and water through time. Isostatic
  movements are often calculated using an Airy
  approximation. This assumes that the compensation
  takes place locally and instantaneously over
  geological time scales.
• More realistic models incorporate the effects of
  the elastic stiffness and the viscous flow that
  can occur in two dimensions.
• Elastic and viscous models each requires various
  earth parameters.

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Isostatic parameters
Sediments - matrix density - pore water
density - porosity
Moho
Lithospheric thickness - Elastic parameters
- Viscous parameters
Mantle lithosphere - Astenospheric
density
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Astenosphere
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Airy Model
rs 2.8 g/cm3

• Local instant response
• subsidence (rs / rm ) x h
• In this case
• subsidence 0.85 x h

h
subs.
rm 3.3 g/cm3
Illustration of the Airy model. This assumes that
the compensation takes place locally and
instantaneously over geological time scales. The
earth is reacting to loads as if it was
floating on a fluid mantle. The Airy model can
overestimate isostatic subsidence leading to
underestimated heat flow.
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Elastic model
The earths response to loading show that the
lithosphere acts as an elastic shell. If a load
is applied to the elastic lithosphere, part of
the applied load will be supported by the
lithosphere, and part by buoyant forces of the
mantle underneath, acting through the
lithosphere.
Sediments
Crust
Instant response
Effect of Elastic Lithospheric Thickness on
Isostatic Subsidence
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Viscous Effect on Isostatic Subsidence

• It is also known that the lithosphere has a
  viscosity which varies strongly with depth.
  However, the viscosity is large enough to act as
  an elastic plate over short time periods. Over
  long time spans the applied loads will start to
  subside into the lithosphere. Isostatic
  equilibrium will be achieved over hundred s of
  millions of years.

Sediments
Sediments
Crust
Crust
Viscous response over time
Instant elastic response
Will approximate the Airy model with time
Tickhnessm
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Subsidencem
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Compositional division of the earth

• There are three main compositional units of the
  earth the crust, mantle and core.
• Crust The crust is an outer shell of relatively
  low density rocks. The oceanic crust is thin,
• ranging from approximately 4 to 20 km in
  thickness, and with an average density of 2900 kg
  m-3.
• The continental crust is thicker, ranging from 10
  to 70 km, and with an average thickness of
  around 35km.
• Information on the density of crustal rocks has
  been obtained largely by observations on
  seismograms,
• coupled with laboratory experiments. The
  existence of a low velocity crust was discovered
  by the geophysisist
• Mohorovicic shortly after the turn of the
  century. The boundary between the crust and
  mantle is called Moho.
• The Moho can vary in depth considerably over
  relatively short distances.
• Mantle The mantle is divided into 2 layers, the
  upper and the lower mantle. The upper mantle
  extends to about 650-700 km. The lower mantle
  extends to the outer coreat 2900 km.

Read more about the mechanical division of the
earth
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Mechanical division of the earth

• The mechanical divisions of the interior of the
  Earth do not necessarily match the compositional
  zones. One of the mechanical zoneations of
  interest in basin studies is the diffrentiation
  between the lithosphere and asthenosphere. This
  is because the vertical motions in sedimentary
  basins are responses to deformations of this
  zone.
• Lithosphere is the rigid outer shell of the
  Earth, comprising the crust and upper part of the
  mantle. It is of particular interest to note the
  difference between the thermal and elastic
  thicknesses of the lithosphere.
• It is generally believed that the base of the
  lithosphere is represented by an isotherm of
  1100-1300 oC, at which mantle rocks approach
  their solid's temperature. This defines the
  thermal lithosphere.
• The rigidity of the lithosphere allows it to
  behave as a coherent plate, but only if the upper
  half of the lithosphere is sufficiently rigid to
  retain elastic stresses over geological time
  scales. This is the elastic lithosphere. The
  thickness of the elastic lithosphere varies
  around the globe. In our area the thickness is
  estimated to 1 to 40 km.

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Isostasy
The straight red line, are the position the
basin strata had 250 Ma. ( Surface level )
The varying level lines shows how the
strata subside non-linear downward in the crust.
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The animation shows how the istostatic movements
are affected by sedimentation, erosion, fault
movements and variation in the palaeo water depth
over time.
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Section 4 Tectonics

• The observed subsidence estimated by geohistory
  analysis is mainly due to two processes
  isostatic movements and tectonic movements due to
  lithospheric thinning.
• The tectonic subsidence is commonly deduced by
  the McKenzie model. McKenzie showed that
  sedimentary basins could form when the
  lithosphere is stretched, resulting in reduced
  crustal thickness and upwelling of hot mantle
  material. After the stretching
• event the surface will subside due to thermal
  contraction of the lithosphere (see next page).
• The sum of the isostatic calculations and the
  tectonic modelling will be compared with the
  observed subsidence (calculated by the
  geohistory analysis). The amount of stretching
  is then the tuning parameter. When the fit is
  acceptable, the amount of stretching over the
  basin is quantified. And simultaneously, the
  palaeo heat flow history is found. This is again
  input to the temperature modeling.

Look on a flow diagram visualising this prosess
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Total subsidence

• Total (Geohistory) Subsidence Isostatic
  Tectonic Subsidence

Time
Isostatic subsidence
Tectonic subsidence
Subsidence
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Schematic Illustration of Stretching
Before stretching
After stretching
C/bc
Crust
bc
C

• Subsidence due to
  thinning
• Thermal expansion

SC/bsc
Mantle lithosphere (or sub-crust)
SC
bsc
Upwelled Astenosphere
Thermal equilibrium

• Thermal subsidence

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Flow diagram
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Heatflow
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Here is shown how the heat flow history changes
over geological time, due to the amount of
lithospheric stretching over the basin. The
heatflow is to a certain degree also affected by
sedimentation and erosion.
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Section 5 Temperature
The temperature history of the basin is
calculated after the heatflow modeling is
finished. The temperature depends on 1) the
basin geometries calculated in the geohistory
analysis 2) the heat flow history from the
tectonic modelling 3) the palaeo surface
temperature 4) the thermal conductivity and heat
capacity structure of the sediments.
Temperatur development movie
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Thermal reconstruction
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Animation showing how the temperature regime in
the basin changes over time due to sedimentation,
erosion and heat flow history.
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Section 6 Source rock maturation
There is now a wealth of geochemical evidence
that petroleum is sourced from
biologically-derived organic matter buried in
sedimentary rocks. Organic-rich rocks capable of
expelling petroleum compounds are known as source
rocks. The parameters governing the formation of
petroleum are 1) temperature 2) time 3) organic
matter type Thus the reliability of the
prediction of oil and gas formation depends on
1) the reliability of the temperature history 2)
the reliability of the organic kinetic parameters
used in the maturation modelling
hydrocarbon maturation movie
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Source rock maturation
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Animation showing the deposition of source rock
and the transformation from organic matter to
hydrocarbons in the source rock.
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Hydrocarbon migration

• Hydro-carbon migration is not treated in this
  module, but is often the final modelling task in
  basin modelling. Hydrocarbon migration (also
  termed secondary migration) concentrates
  petroleum into specific sites (traps) where it
  may be commercially extracted.
• The mechanics of hydrocarbon migration from
  source to reservoir are well studied. The main
  driving forces behind the migration is buoyancy
  (caused by the density contrast between the
  petroleum and pore water), and pore pressure
  gradients which attempts to move all pore fluids
  (both water and petroleum)to areas of lower
  pressure.

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Developers
Module made by
Student Hege Anita Handeland Petroleum Technology Dept. Stavanger University College NORWAY Student Odd Egil Overskeid Petroleum Technology Dept. Stavanger University College NORWAY
Topic Author and Coordinator Dr. Willy Fjeldskaar Chief Scientist  Petroleum Technology - Research and Development Rogaland Research wf_at_rf.no NORWAY
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References

• The source for animations and movies are taken
  from BMT - Basin Modeling Toolbox, a trademark of
  RF-Rogaland Research, Stavanger, Norway. BMT is
  also marketed by Geologica as.
• Text provided by Willy Fjeldskaar.

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BMT
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