8.01 Shore Error
8.02 Aliasing
8.03 No Color Fill
8.04 Boundary Conditions/ Edge Effects
8.05 Inactive Menu Options (greyed out buttons)

8.01 Shore Error

One of the most common problems users have in running a new data set for the first time is inadvertantly causing a Shore Error during the deposition of clastic sediments while the program is executing. SEDPAK will notify you with a Search for Shore failed message when a Shore Error occurs (Figure 3.3.4) (For further discussion see Sections 3.04 and 3.05). To avoid a Shore Error during deposition of clastics, sea level must intersect the basin surface to form a shoreline. Also sea level must be at an elevation above the sediment surface to accommodate clastic sediment deposition during that time step. The user is advised to turn off both the Clastic Source and Include Carbonates in the Constants dialog for first run of the program, and set the Display Controls to Plot every time step (see Section 3.02). This enables the user to track the position of sea level with respect to the basin surface. If sea level rises above or drops below the basin surface, the user can either change the position of the sea level curve by: shifting the Digitized Curve initiated from Sea Level of the EDIT menu (see Section 3.05); using Sea Level Offset in the Setup dialog; or increasing the dimensions of the Basin Surface by raising its sides or lowering its base (see Section 3.03, Figure 3.3.1). It is possible to modify the basin surface with the Deus ex Machina function during a simulation by eroding previously deposited sediment, even the initial basin margins can be removed (see Section 3.05). If an initial basin margin is removed in this way, and clastics are deposited when this removal occurs, a Shore Error will result. Remember that a Shore Error is produced during clastic deposition when sea level drops below the sediment surface and there is no accommodation space, or when clastic sediment enters from the side of the basin that drops below sea level. If the clastic deposition is reduced to zero for either the time interval that the basin surface drops below sea level, or the time interval for which there is no accommodation space, the Shore Error will not occur.

8.02 Aliasing

A second common problem is aliasing. Aliasing can occur if changes in the rate of change in sea level position, clastic sediment input, or carbonate accumulation occurs at finer time intervals than that of the time step or if the distances specified in Depositional Distance are smaller than the width of a column. When parameters are being changed using the EDIT menu, SEDPAK will notify the user when aliasing occurs (Figure 3.3.2), detailing its potential origin. The input parameters can be changed to avoid this problem, or the message can be ignored. Aliasing is avoided by defining the rate of change for either times or distances to be greater than or equal to the time intervals or distances set in the SetUp dialog.

The most common aliasing problem occurs when an insufficient number of time steps is used. If, for example, a section is modeled using 50 time steps, then no matter how many sea level points are entered for the sea level curve, SEDPAK will consider only 50 sea level points for that simulation run, interpolating between the data points and recognizing values only where the line of the interpolated intersects time or column boundaries. Thus higher frequency eustatic fluctuations may be skipped or may be applied for longer time intervals than was intended. The result is, for instance, a smoothing of the sea level history. The way around time-aliasing is to increase the number of time steps for the simulation or change the specified times for the offending parameter. NB: It is important to remember that SEDPAK takes the input values for all parameters and performs linear interpolations between them, selecting either the parameter value which is prescribed for that time step or column boundary, or will select the interpolated value crossing that boundary.

8.03 No Color Fill

A third problem occurs when:

• € The Pseudo Wells button is turned on from View controls pull-down menu on the EXEC menu, but no Pseudo Wells have been defined with the EDIT menu (see Section 3.20, Pseudo Wells). There will be no color fill between the time layers of the Basin, Sequence and Facies Cross sections. This can be very confusing to the first time user. To correct this, either turn off Pseudo Wells on the EXEC menu or define some Pseudo Wells using the EDIT menu;
• € Clastics are turned on in the Constants dialog and a rate of Clastic Supply is set but no Depositional Distance is input. In this case, clastics will be shown on sea level plot on the SEDPAK Simulation Window but no sediment fill is shown in the basin cross sections. This effect is removed by inputting Deposition Distance values.

8.04 Boundary Conditions/ Edge Effects

A fourth problem is the influence of boundaries, or edge effects, on the simulation and the sedimentary geometries produced by the simulation. It should be recognized that all sedimentary simulations have edge effects with respect to both time and/or space. This can be seen when viewing the basin edges of the SEDPAK simulation output (Figure 7.4.1). Here the physical laws or the empirical relationships that were used successfully within the center portions of the simulation do not produce realistic results. For instance, on the side, or sides, from which sediment enters the simulation, a shore must be provided. At the same time, because the user may not wish to extend the basin much beyond the area of interest, a nearly vertical margin is defined. The result may be unrealistic geometries close to this margin. Similarly, on sides which lie below sea level and over which sediments bypass from the simulation, the geometries of the bypassing sediments close to the edge can become an unrealistic series of "en echelon" parallel beds. In order to reduce these edge effects, the simulation is extended by a "phantom column" on both sides of the basin. These "phantom columns" provide a horizontal extension of the basin surface and are unseen on the output. Sediment deposition takes place within these columns, but beyond them, sediment is lost from the simulation. Despite the use of these columns, unrealistic geometries still result, for no sedimentary simulation can escape the inherent edge effects related to sediment entering the simulation or exiting from it.

The simplest solution to avoiding these boundary effects is to concentrate observations within the center of the simulation window, away from its edges. This is achieved by zooming the area of interest. In this area, sedimentary geometries are more likely to be realistic and the best effects of the simulation can be observed. This approach ignores results associated with the simulation edge, but accepts results for the center portion of the simulation. Therefore, when creating a simulation model, the basin should be made slightly larger than the actual area of interest (on the order of 50%; 25% on each side).

8.05 Inactive Menu Options (greyed out buttons)

When menu options are grey, and so inactive, this indicates that some other menu item should be activated first. For instance during execution most of the the menu options on the EXEC menu are inactive until the simulation is paused or completed. The Select Facies Definition menu item on the EXEC View pull-down menu is only activated when the Basin-Facies button on the Display Modes option list has been selected. Similarly the Oil Peak-Kinetic Maturity is only activated on the Display Modes option list when the Kinetic model has been selected from the View pull-down menu on the EXEC panel.

Appendix A. Technical Enhancements

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