3.07 Clastic Supply
Inputs: Sediment Areas
Discussion: Clastic Supply
For this discussion refer to Section 3.08 on Depositional Distance.
3.08 Depositional Distance
Inputs: Depositional Distance
Discussion: Clastic Supply and Depositional Distance
SEDPAK derives the amount of clastic sediment to be deposited over the entire basin from the cross-sectional area of a series of right angle triangles. One triangle for sand and another for shale represent the area (a) of sediment available for deposition in a given time interval (Figure 3.8.1). The amount of sediment to be deposited over a series of time steps is linearly interpolated between the input values. If no amount has been specified for the start or end times, it is extrapolated from the intermediate input values. The length of the triangle base matches the width of the offshore sediment wedge seaward of the shoreline. This is the maximum distance (d) that sediment is transported into the basin from its margin (Figures 3.8.2 and 3.8.3). As with the sediment triangles, the distances for a series of time steps are linearly interpolated between the input values and are extrapolated from the intermediate input values if the start or end times are undefined.
Figure 3.8.1. Sediment areas.
Sediments are deposited column by column. Each column is filled by marine
sediments up to sea level. Alluvial sediments are deposited up to a surface
determined by an alluvial angle that is projected landward from the shore
to its intersection with the previous surface. Each time the area representing
the sediment column is subtracted from the triangles, the triangle heights
are reduced correspondingly. This process is repeated until the triangle heights
match the height of sea level above the sediment surface, at which time the
remaining area of the sediment triangle is deposited seaward as a single wedge
of offshore sediments. Deposition continues until the sediment area within
the triangle becomes zero.
Figure 3.8.2. Distance of sediment transport (penetration).
As illustrated by column A in Figure 3.8.3, sediment deposition on the "left side" of the basin begins with the first column on the left that lies below sea level and progresses to the right towards column B. Similarly, deposition from the right margin would start in the rightmost column that is below sea level, and proceed leftward. The direction can be set to activate clastic deposition from either side, both sides, or to turn clastics off (see Section 3.2, Constants).
It is necessary to define areas (a) and distances of transport (d) for sand and shale from the left, and sand and shale from the right. The density of each sediment type also can be defined (see Section 3.17). The density of the sediment affects the amount of compaction that occurs during the simulation. This consequently affects the accommodation space available for deposition, thereby controlling the distance that the sediment progrades into the basin.
The data sources for clastic sediment deposition are seismic cross sections, well data, outcrops, or a combination of these sources. Perhaps the best approach is to use a seismic interpretation tied to at least one well to determine the cross sectional area of sediment, its distance of transport, and the timing of its deposition.
Figure 3.8.3. Clastic sediment deposition during each time step.
Figure 3.8.4. Flow diagram for clastic sediment deposition during a time
step.
It is also important to note the effects of turning off clastic sedimentation during a simulation run. By turning off the clastic sediment supply (see Constants, Section 3.02), a Shore Error can be avoided if one or both of the basin sides drops below sea level. It is necessary to turn clastics off for the side or sides of the basin during the entire time interval that sea level is above the basin edge(s). Clastic sedimentation can be resumed when a shoreline is present.
In addition, erosion is dependent on clastic sedimentation. In order for
erosion to occur within a particular time interval, clastic sedimentation
must be turned on during this time. There also must be some measurable amount
of sediment entering the basin. The Clastic Supply cannot be set to zero. This has important implications with respect to
carbonate erosion. Erosion of carbonates only occurs if clastic sedimentation
is turned on, and no erosion will occur during the time interval with clastics
turned off (see Section 3.10 for a discussion of clastic and carbonate erosion).
Figure 3.8.5. Plotter for Clastic Supply.
Figure 3.8.6. Data sheet for Clastic Supply.
The amount of clastic sediment deposited during the simulation is entered
by selecting Clastic Supply from the SEDPAK EDIT menu. This command provides access to the Clastic Supply plotter and data sheet (Figures 3.8.5 and 3.8.6). The rates of clastic
sediment supply as a function of time are entered as individual time/rate
pairs on the plotter or data sheet. If a cyclic pattern of rates is required,
as is illustrated in Figure 3.8.5, this can entered by first entering a
range of times into the data sheet (see Section 2.11) and then using the Cyclic Description portion of the calculator to impose a cyclic character to the rates (see Section 2.12). The distance that this sediment can penetrate into the basin is entered
by selecting Depositional Distance from the SEDPAK EDIT menu. This command provides access to the Depositional Distance plotter and data sheet (Figures 3.8.7 and 3.8.8). Remember if no values
are entered for depositional distance then no clastic sediment is deposited.
Figure 3.8.7. Plotter for Depositional Distance.
Figure 3.8.8. Data sheet for Depositional Distance.
Clastic Supply values are entered in the plotter or data sheet for the desired sediment type (sand or shale) and direction (left or right) as a rate-time pair (m2/ka or ft2/ka). The rate of clastic supply is linearly interpolated between the input values. It is extrapolated from the input values to the start and end times of the simulation, if the rates are undefined for these times. Remember that time-aliasing can occur if the defined times are too close together and lie within a single time step (refer to Section 1.11).
The Clastic Supply table (Figure 3.8.9) summarizes the data values entered in the Clastic Supply plotter and data sheet. It has a column for time and four additional columns:
sand and shale from the left, and sand and shale from the right. The table
cannot be edited.
Figure 3.8.9. Table for Clastic Supply.
Depostional Distance values are entered in the plotter or data sheet for the desired sediment type (sand or shale) and direction (left or right) as a distance-time pair (distance in km or miles). The depositional distance is linearly interpolated between the input values. It is extrapolated from the input values to the start and end times of the simulation, if the distances are undefined for these times.
The depositional distance defines how far from the shoreline the clastics are deposited into the basin. The actual distance that the sediment is transported may vary, however, because the depositional angle may prevent the accumulation of sediment and cause it to bypass (refer to Depositional Parameters, Section 3.10). If the surface slope is steeper than the angles set for the Shallow and Deep depositional angles (Section 3.10), the sediment will be deposited further out into the basin than prescribed by Depositional Distance. Refer to the exercises in Section 6.15, 6.16, and 6.17 for some examples of the effect of depositional angle on penetration distance. Once again, remember that time-aliasing can occur if the defined times are too close together and lie within a single time step (refer to Section 1.11).
The Depositional Distance table (Figure 3.8.10) summarizes the data values entered in the Depostional Distance plotter and data sheet. It has a column for time and four additional columns: sand and shale from the left, and sand and shale from the right. The table cannot be edited.
Figure 3.8.10. Table for Depositional Distance.