3.15 Pelagic Deposition

Inputs: Pelagic Deposition

Discussion: Pelagic Deposition

In contrast to the carbonate accumulation rate which is defined as depth vs. rate at a given time, the Pelagic Deposition function is defined as time vs. rate, independent of depth. This allows carbonate deposition to be supplemented during specified time periods. Pelagic carbonate accumulation is defined (Figure 3.15.1) in feet or meters per 1000 years (rpi), at specified times (tpi). It is important to note that Pelagic Deposition deposits a uniform layer of carbonate over the entire basin. This enables carbonate to be deposited in areas below the zone of carbonate accumulation (which was specified with Carbonate Rates, Section 3.11). The areas within the zone of neritic accumulation will also receive this additional pelagic carbonate.

Figure 3.15.1. The rate of pelagic carbonate accumulation with time.

Varying the rates of pelagic and depth dependent carbonate accumulation, in combination with the Hardgrounds, Lagoonal Damping, and Wave Damping functions, enables the simulation to model the great range of processes which occur within carbonate margins.

Figure 3.15.2. Plotter for Pelagic Deposition.

To specify Pelagic Deposition rates, select Pelagic Deposition from the SEDPAK EDIT menu. The plotter for pelagic accumulation rates will appear (Figure 3.15.2). Data may be entered either directly on the plotter or in its associated data sheet (Figure 3.15.3). Pelagic Deposition is defined as time vs. rate, as opposed to Carbonate Rates, which are defined as depth vs. rate through time. Pelagic Deposition may be turned on and off (set to 0.0) as many times as desired. Remember that a time-aliasing error will occur if times are entered which are too close together (see Section 1.11). Also, remember that since Pelagic Deposition deposits carbonate over the entire basin below sea level at the specified times, there is only one pelagic curve. As with other SEDPAK parameters, the pelagic rate will be linearly interpolated between the input values, and extrapolated from the intermediate input values if the start or end times are undefined.

Figure 3.15.3. Data sheet for Pelagic Deposition.

Table of Contents

3.16 Carbonate Parameters

Inputs: Carbonate Parameters

The program limits the growth of reefs or build up crests to sea level. Excess carbonate accumulation, which would cause the build ups to rise above sea level, is stored as slope sediment, transported off the build up, and deposited as talus or turbidite (Figure 3.16.2). The percentage of the slope sediment to be transported off the carbonate platform into the basin (%cs) is specified. The remnants (100%-%cs) are transported as backreef facies into lagoons or onto an epeiric shelf.
The slope sediment (%cs and 100%-%cs) is deposited as talus (%ct) and turbidites (100%-%ct). The distance which the slope sediment extends away from the reef into the basin and/or lagoon is either dta for near reef debris talus or dtu for turbidite fans (Figure 3.16.2).

Figure 3.16.1. Dialog for Carbonate Parameters.

The amount entered for the Percent to sea in the Carbonate Parameters dialog determines how much of the excess carbonate from the "reef" is transported seaward into the basin or landward into the lagoon. The Percent talus parameter indicates how much of this carbonate is deposited as talus and how much is turbidite. The Talus penetration distance (in kilometers or miles) determines how far into the basin and/or lagoon the talus is deposited. The Turbidite penetration distance (in kilometers or miles) determines how far the turbidite is deposited into the basin and/or lagoon.

Carbonate deposition begins once the program determines that segments of the basin are open basin, carbonate build up crest (or reef), lagoon (or epeiric shelf), or a subaerial surface. Build ups are modeled first, using the carbonate depth-rate pairs. These build ups, or "reefs," can be suppressed by the presence of clastics in the water column. Damping of carbonate accumulation by clastics is modeled as the product of the percentage of space between sea level and the clastic sediment surface, the depth dependent carbonate accumulation function (Carbonate Rates, Section 3.15), and the Clastic suppression of carbonates parameter.

Figure 3.16.2. Excess carbonate production on reef bypassing into lagoon and basin.

Figure 3.16.3. Example Clastic suppression of carbonates curves.

The data sources for carbonate sediment deposition are seismic cross sections, well data, outcrops, or a combination of these sources. Using a seismic interpretation tied to at least one well, an estimate can be made for the rate of carbonate accumulation as a function of depth, and the distance of penetration of carbonate turbidites and aprons. Burial history curves are also a major source for accumulation rates. Other parameters can be determined by experiment and published examples.

Table of Contents

3.17 Overburden


Figure 3.17.1. The Overburden dialog.

Select Overburden from the SEDPAK EDIT panel in order to view the Overburden dialog (Figure 3.17.1). The parameters within this dialog have been assigned default values, but these may be changed as needed. The parameters include Sand Density(gm/cc), Shale Density(gm/cc), Carbonate Density(gm/cc), Mantle Density(gm/cc), and Last Stage Burial (ft or m) (which occurs after the final time step).

If the Airy Isostacy button is turned on in the Constants menu, then subsidence is handled by assuming that the sediments are isostatically compensated at some depth beneath the boundaries of the simulation. The weight of the newly deposited or eroded sediments is calculated, and the elevations of the sediments are adjusted according to the formula:

dz = ((Density of sediment * S * Height) + (Dpf * (1 - S) * Height)) / Dm

where dz is the change in elevation due to sediment deposition or erosion, S is the sediment solidity (1 - Porosity, where Porosity is a fractional amount between 0.0 and 1.0, rather than a percentage), Height is the height of sediment in each column, Dpf is the density of the pore fluid, and Dm is the density of the mantle (Baldwin and Butler, 1985). The densities specified for sand, shale, carbonate and mantle may be changed so that compaction can be handled differently. The porosity values which decrease with depth and used in SEDPAK were measured for the North Sea (Baldwin and Butler, 1985).

The compaction of sediments in the simulation is determined by using an approach similar to that of Baldwin and Butler (1985), who describe several well defined compaction equations for shales, sandstones and carbonates. In SEDPAK, compaction is handled as a function of the height of the sediment column overlying the sediment that is compacting. The greater the overburden, the more the porosity of the sediments is reduced. The equation used for compaction of sands and carbonates is (Baldwin and Butler, 1985):

Burial Depth (in km) = 3.7 ln(0.49/(1-S))

where S is the sediment solidity. The equation used for the compaction of shales is (Baldwin and Butler, 1985):

In order to simulate the rigidity of the crust, it is necessary to input a value for the Mantle Density that is greater than that found in nature. Should a realistic value for the Mantle Density such as 3.4 gm/cc be used in the simulation, the crust loses its rigidity and collapses plastically and unrealistically under the weight of the overlying sediment. This situation occurs within the simulation because there is no connection between the behavior of one column and its neighbor. Therefore, it is necessary to damp the response to the overburden. A realistic crustal response can be modeled in the simulation by turning off the Airy Isostasy button on the Constants menu (or by setting the Mantle Density to an artificially high value such as 34.0 gm/cc), as well as defining the subsidence behavior inferred from geological and seismic sections using the Subsidence parameter (refer to Section 3.06).

After the last time step has been completed for the simulation (Section 3.03), a final compaction is performed. The amount of compaction is determined by the Last Stage Burial (zls). This parameter represents an additional load of sediment that is applied to the simulation after completion of the last time step. The thickness of the overburden is specified for this parameter and the final compaction is based on the thickness of this additional sediment load. The Last Stage Burial is used when sediment geometries in the simulation need to be modeled following the compaction expected for an additional overburden.

This is necessary to simulate actual geometries after burial to match more realistically those geometries found on seismic or in cross section. It is also effective to apply this when simulating porosity Facies. When this is done greater or less overburden can be used to match control porosity from wells.

Table of Contents

3.18 Time Boundaries

This section describes the SEDPAK Time Boundaries option. The times which are entered for this parameter are used to delineate sequences and system tracts, label the ages of the sequence boundaries, and produce SEDPAK output at specified times during the simulation run.

Selection of Time Boundaries from the SEDPAK EDIT menu invokes the data sheet for entering the times when SEDPAK will produce plots during the simulation run. This data sheet has two columns. The column to the left is used for entering the times for displaying the simulation plots (Figure 3.18.1).

When modeling sequences, the times entered should coincide with the ages of sequence boundaries. In Sequence Mode (Figure 4.3.3), SEDPAK will demarcate packages of sediment bounded by the times entered in the Time Boundaries data sheet (Figure 3.18.1). The sea level plot attached to the simulation window marks the sea boundaries with a large dark dot and the color of the line of the sea level curve matches the time equivalent colors of the sequence plot.

In addition to determining the times for which plots should be produced, the Time Boundaries data sheet can be used to create age markers for any surface within the simulation output. The bubble marker contains the age of the surface and has an attached arrow which points to the surface (Figure 4.3.1).

The age markers are created by entering the age in the left column of the Time Boundaries data sheet. The horizontal location of the marker in kilometers or miles from the left of the simulation plot is entered in the right column (Figure 3.18.1). The display of this bubble can be turned on or off from the pull-down or tear-off View menu of the SEDPAK EXEC menu (see Section 4.05).

Figure 3.18.1. Time boundaries data sheet.

Table of Contents

3.19 Pseudo Wells

This section describes the SEDPAK Pseudo Well option. This option is used to locate known, or proposed well locations on the SEDPAK simulation cross section so the user can compare the stratigraphy of wells (sand/shale/carbonate ratios, facies, or sequences) to the simulation cross section.

Inputs: Pseudo Wells

Discussion: Pseudo Wells

Selection of Pseudo Wells from the SEDPAK EDIT menu invokes the Pseudo Wells data sheet which is used to locate pseudo wells on the simulation cross section seen in the Lithologic Ratio, Sequence and Facies modes. The well name is specified in the label column on (see Figure 3.19.1). The horizontal location of the pseudo well in kilometers or miles from the left of the simulation plot is entered in the Distance column. Display Well (on/off) controls whether the well is visible on the display. The entry is set to On if the Pseudo Well is to be displayed and Off if the Pseudo Well is not to be displayed. Display label (on/off) controls the display of the well name. This entry is set to On if the name of the Pseudo Well is to be displayed and Off if the name of the Pseudo Well is not to be displayed. The Label Depth is used to specify the vertical position in meters or feet for the Pseudo Well Label on the SEDPAK output. In order to remove well locations that have been previously defined, entries in the entire row have to be blank for the appropriate well. This can be achieved by selecting the number in the first column.

The Pseudo Well(s) designated using the SEDPAK EDIT menu can be viewed by pausing the simulation and selecting the Pseudo Wells taggle button on the Display Controls of the SEDPAK EXEC menu (Figure 4.3.2). Remember if no Pseudo Wells are defined but the Pseudo Well button is on, then the Lithologic Ratio, Sequence and Facies modes will have no colors. This effect can be confusing particularly if the Pseudo Well button is left on by mistake.

Figure 3.19.1. Pseudo well data sheet.

Chapter 3, Section 20

Table of Contents