Topic overview

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

3. 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

4. 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). Back

5. 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. Back

6. 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.

7. 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)

8. 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 Back

9. 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. Back Click image to watch animation

10. 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. Back

11. Decompaction/fault restoration Back 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.

12. Geohistory Back 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.

13. Geohistory Previous movie The movie starts from 52 M years ago and progress to present time Back 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.

14. Section 3: Isostasy • The sediments accumulating in a basin represent a load on thelithosphere. Isostasy is the principle of Archimedes applied on the earth’s 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.

15. Isostatic parameters Sediments - matrix density - pore water density - porosity Moho Lithospheric thickness - Elastic parameters - Viscous parameters Mantle lithosphere - Astenospheric density Back Astenosphere

16. 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. Back

17. Elastic model The earth’s 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: Back

18. 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 Tickhness[m] Back Subsidence[m]

19. 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 Back

20. 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 thicknessof the elastic lithosphere varies around the globe. In our area the thickness is estimated to 1 to 40 km. Back

21. 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. Back The animation shows how the istostatic movements are affected by sedimentation, erosion, fault movements and variation in the palaeo water depth over time.

22. 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

23. Total Subsidence Total subsidence • Total (Geohistory) Subsidence =Isostatic + Tectonic Subsidence Time Isostatic subsidence Tectonic subsidence Subsidence Back

24. 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 Back

25. Geohistory Reconstruction Present day geometry Load history Isostatic Subs. Tectonic Subs. Compare isost. + tect. subs. with geohistory basement subsidence Paleo Heat Flow No Subs. OK? Thermal Model Yes Flow diagram Back

26. Heatflow Back 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.

27. 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

28. Thermal reconstruction Back Animation showing how the temperature regime in the basin changes over time due to sedimentation, erosion and heat flow history.

29. 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

30. Source rock maturation Back Animation showing the deposition of source rock and the transformation from organic matter to hydrocarbons in the source rock.

31. 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.

32. Developers

33. 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.

34. BMT Back