6.26 Overburden

Read Section 3.17 to become familiar with the Overburden parameters which are accessible from the SEDPAK EDIT panel.


Keep in mind that the Mantle Density is usually turned off and the position of the initial basin surface is determined by subsidence history prescribed for it with the Subsidence plotter and datasheet. The Last Stage Burial represents an additional load of sediment that is added to the simulation after the last time step has been completed. The user specifies the thickness of the sediment for this parameter. The simulation then performs a final compaction based on this thickness of additional overburden.

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6.27 Time Boundaries

Read Section 3.18 on Time Boundaries. Times which are entered for the Time Boundaries option are used to define sequences, to mark the sequence boundaries with their ages, and to produce plots at these specified times during the simulation run. Sequences and system tracts can be viewed on the simulation output by selecting Basin-Sequence on the Display Modes option menu and Sea Level "on" the View pull-down menu from the SEDPAK EXEC panel (Section 4.03). VailClast.db (Figure 6.11.1) is a good example for illustrating sequences. Note that the time boundaries have been selected to delimit specific portions of the sea level curve.
Try defining different sequences by changing the time boundaries. Don't forget to use Save As... to copy the file. A quick first pass at delimiting Time Boundaries can be achieved by using the Create Range dialog.

Section 6.28

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6.28 Pseudo Well

Read Section 3.19 to become familiar with the SEDPAK Pseudo Well option on the SEDPAK EDIT menu.

A file with Pseudo wells turned on is seen in Figure 6.28.1.




Figure 6.28.1 Simulation output for Ans_Pseudowell.db.

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6.29 Out of Plane

Read Section 3.20 to become familiar with the SEDPAK Out of Plane option on the SEDPAK EDIT menu.

The results can be seen in Figure 6.29.1.


Figure 6.29.1 Simulation output for Ans_OutofPlane.db.

 

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6.30 Two Sided Basin Simulation

Note how clastics no longer fill from the right of the basin.

NB: Clastic Supply must be specified for both directions, and Depositional Distance must be specified for sand and/or shale for both directions to match Figure 6.30.1.



Figure 6.30.1. Final output for Ex_TwoSided.db.

Conclusions: The simulation of Ex_TwoSided.db starts off with a mixed clastic/carbonate setting responding to a fall in sea level. The volume of clastics increases, masking carbonate production. Next, the sea level rises and the clastic supply is drastically reduced, with clastic sedimentation occurring only near shore. This enables the formation of a carbonate buildup (or reef) offshore. Uplift of the right side of the basin commences, a slight rise in sea level occurs, and the clastic supply volume increases, covering the reef with clastic material. This is followed by another drastic reduction in the clastic sediment volume, enabling carbonates to dominate the basin once again. Meanwhile, the right side of the basin has undergoes a significant uplift. Finally, towards the end of the simulation, clastics dominate the basin again, with a most of the of clastics coming into the basin from the right side.

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6.31 Maturation Modeling - Thermal Gradient and Surface Temperature

Read Sections 3.21, 3.22 and 3.23 to become familiar with the Temperature Gradient, Surface Temperature and Maturation Parameters which can be accessed from SEDPAK EDIT menu and Section 4.04 to see how the different Display Modes are chosen from the SEDPAK EXEC menu.

Hydrocarbon maturation is determined from duration of burial and thermal history. This thermal effect is controlled by the heating history of the basin, surface conditions and stratigraphy. Using matur1.db and Ex_SurTemp.db the evolving stratigraphy and the effect of thermal history maturation can be examined.

€

-Open Ex_matur1.db.
-Click Thermal Gradient on the EDIT menu.
-Open the data sheet. Create a thermal gradient curve at location 0 and 20 as follows:

    Time Gradient
    -11.000 0.1
    -7.000 0.1
    0.000 0.1

    -Where is the oil window at location 10?
    -Where is the gas window at location 5?
    -Increase the thermal gradient to 0.2 at location 0.
    -Save and execute the program.
    -Where is the oil window now at location 10?
    -Where is the gas window at location 15?
    -See Figure 6.31.1. for the result.


    Figure 6.31.1 Modeling a section of the Pannonian Basin of Hungary with Ans_matur1.db.

    The matur1.db file has constant temperature gradient and surface temperature values. In this example it is possible to see how stratigraphy can modify the distribution of maturation, when the thermal parameters remain constant. At the beginning of the simulation, a prograding sedimentary wedge forms the right side of the basin. After a sea level drop at -5.7 Ma, another progradational cycle forms on the left side of the basin. Note that the right side of the section enters the oil window first. Here the sediments in the deepest troughs become mature. Then, as time passes, the maturation extends toward the left side of the basin, paralleling the prograding sediments (Figure 6.31.1). The sea level drop stops this process, so that the right side of the basin represents a relatively more mature zone, but the left side becomes more and more mature under the thickening lowstand sediments. The new progradational wedge from the left of the section causes intense maturation below it (Figure 6.31.1). Note that the younger outbuilding of sediments has more impact on maturation than the older ones because the sediments on the bottom of the section are older now. Thus, burial and temperature histories are important during late stages of basin development.

    € Purpose: To understand the relationship between surface temperature and hydrocarbon maturity.

    € Exercise: -
    Open Ex_SurTemp.db-
    Create a curve at location 0 and 25 km as follows:

                 Time    Temperature
               
     -11.5         20
                       0           20

- Save and execute the simulation.
-Where is the oil window at location 10?
-Where is the gas window at location 15?
-Decrease the surface temperature to 15šC
-Run the program.
-Where is the oil window at location 10?
-Where is the gas window at location 15?
-What is the relationship of surface temperature and the hydrocarbon maturity?

See Figure 6.31.2. for the result.




Figure 6.31.2. Modeling same section From the Pannonian Basin of Hungary with Ans_SurTemp.db.



Conclusion: Using the SEDPAK simulation it is possible to model how thermal maturation progresses through time and space in a studied geologic section, determine when and where the oil window formed, and also when and where hydrocarbons may have been expelled. This model helps understand how stratigraphic evolution and different thermal histories can affect the organic maturation history of a basin.


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6.32 Exercising the SEDPAK FACIES Menu

Read Chapter 5 to become familiar with the Facies option which is accessible from the SEDPAK LAUNCH menu. The Facies option is an extremely powerful way for the user to give an interpretive bias to the simulation output (Figure 5.1.1). Facies may be defined on the basis of the following attributes: lithology, distance from shore, depth of water, porosity, and distance from a reef position.

These files created in the previous versions of SEDPAK.

Copy the file VailMix.db (Figure 6.7.2) using Save As... and try defining new facies according to depth, lithology, distance from shore, and porosity, as well as combinations of attributes. Take some time to produce several interpretations using different attributes and facies definitions. Remember that if more than 45 facies are defined, some colors will be repeated. This could make the simulation difficult to interpret. Therefore it is necessary to take care when assigning duplicate colors. Also, remember that different facies may have overlapping attributes and it is necessary to set the Priority to display the facies in a particular order. A higher Priority takes presendence over a lower one, and the facies with the higher Priority will be plotted over the facies with a lower rank.


Chapter 8

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