3.20 Out of Plane Deposition

This section describes the SEDPAK Out of Plane Deposition option. It is used to control deposition of sediments from out of the plane of the simulated cross section.

Discussion: Out of Plane Deposition

Out of Plane Deposition is accomplished by a combination of actions with the SEDPAK EXEC and the SEDPAK EDIT menus. First, the simulation is run to the time step at which out of plane deposition should be started. The simulation is Paused and the Surface Snapshot button is pressed from the SEDPAK EXEC menu. This records the current basin surface of the Paused SEDPAK simulation window. It also activates a file dialog (see Section 2.06) to which the selected surface is saved with the suffix .sur. A file of the lithology thickness and location is created in the directory in which SEDPAK data files are located (see Option command on EXEC menu).

Pressing Out of Plane Deposition on the SEDPAK EDIT menu activates a plotter entitled Deposition from out of plane (Figure 3.20.1). If no data is shown on this sheet, it is loaded by pressing the Load button. A dialog box for selecting files from current directories is displayed (see Section 2.06, Figure 2.6.1) and the required surface snapshot file with the suffix .sur file is retrieved. While loading, a dialog is activated which asks "Surface loaded, should file be erased?". Yes or No buttons are provided. If the Snapshot file is required later, press No. Once this action is completed, the file is loaded onto the plotter (Figure 3.20.1) for editing. There are usually so many data points associated with the imported surface from Surface Snapshot that large numbers of these have to be deleted before data points can be selected and dragged. The Simplify Current Curve from the curves pull-down menu can be activated and used to delete some of the extra points (see Section 2.10 and Figure 2.10.3). After simplifying the data on the plotter, the data sheet should be activated from the options pull-down menu. This is because despite simplifying the data displayed on the plotter, editing the out of plane surface is difficult. Further unnecessary groups of data points may thus need to be removed by using the data sheet to select and delete them (see Section 2.11).

Scan down the data sheet to the locations at which sediment is to be added. The lithology thicknesses should be increased to match those required for the Out of Plane Deposition. OK is clicked on the data sheet and the plotter, and the file is Saved. Load and Restart the file from the EXEC panel. The result of the modifications to the surface can be seen and the Out of Plane Deposition verified. If the results look incorrect, the plotter and data sheet are activated from the EDIT menu again and reedited. The program should be run and the files edited iteratively until the required shapes and areas of sediment are achieved. This process has to be repeated for each time step at which sediment is brought in from out of the plane of section. It is advisable to start at the beginning of the simulation and work to the top of the section when producing a series of these out of plane events.

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3.21 Thermal Gradients

SEDPAK uses the different models for maturation calculations, the Lopatin-Waples TTI (time temperature index) model (Waples 1980), and the Arrhenius equation-based kinetic model (Tissot et al. 1987) (see Section 3.23 for a fuller description of this). Temperature Gradients, Surface Temperatures and Maturation Parameters are required to calculate thermal maturation within SEDPAK. Each temperature gradient is a function of location and time. Read Sections 3.22 and 3.23 to become familiar with Surface Temperatures and Maturation Parameters accessed from the SEDPAK EDIT menu and Section 4.04 to see how the different Display Modes are chosen from the SEDPAK EXEC menu.

Inputs: Thermal Gradients

Discussion: Thermal Gradients

In SEDPAK, the temperature distribution for maturation calculations are approximated by linear functions with depth. The surface temperature (which is a boundary condition (see Section 3.22), and the temperature gradient determine the temperature-depth functions for each location at each time step. The temperature gradient may vary as a function of location and time (x,t). SEDPAK fills every address in the (x,t) grid over all time and space steps by linearly interpolating between the input values, extrapolating from the intermediate input values, if the start or end times are undefined.

The input parameters used in SEDPAK for thermal maturation calculations are time and temperature. The burial time is derived from the simulation, while temperature is based on user provided values, input as a linear curve in the form of a temperature-depth function. This linear function is defined by the Temperature Gradient, a constant value at depth, and the temperature value at zero burial depth (Surface Temperature). This function gives the temperatures for depths calculated from the input Surface Temperature and Temperature Gradient.

SEDPAK computes temperatures at each time step and at different temperature gradients and surface temperatures for different locations and times, enabling the temperature distribution to vary both through time and along lateral distances. For example, thermal heating and cooling in time can be input by using an increasing Temperature Gradient followed by decreasing Temperature Gradient values. Warmer and cooler portions of the simulation cross section can be modeled if different Temperature Gradients are input along the section. For example, a rifted (faulted) part of the section can be modeled with higher temperatures than that of an adjacent uplifted horst block (see Section 5.24, Maturation modeling).

To enter temperature gradient data, select the Temperature Gradients parameter on the SEDPAK EDIT menu and the Temperature Gradient Plotter will be invoked (Figure 3.21.1). Select a curve by location (Section 3.6), initiate the data sheet using the Options menu, and enter the data manually. Fill the Time column and the Temperature Gradient column. Remember, the program linearly interpolates between data points. Linear interpolation gives temperature gradient data for each grid node, both through time steps and x-steps. On the plot, the horizontal axis is time, and the vertical axis is temperature gradient. The temperature gradients should have positive values.

In Figure 3.21.1 the locations of a number of temperature gradients are indicated. For instance temperature gradients behave differently at each location. At location 52.0 and location 63.0 it is constant through time, while at locations 54.0 and 61.0 it changes through time. At location 33 the temperature gradient increases from -36 MY to -28 MY and decreases from -28 MY, to -27 MY, to -25 MY with different rates (Figure 3.21.1 and Figure 3.21.2).

The temperature gradient is in degrees Celsius/meter, or Fahrenheit/feet. Values should usually be below 0.2šC/m (and greater than zero). A gradient of 0.1šC/m will produce a temperature of 100šC (212 Fahrenheit) at a depth of 1000m (1km), which is representative of a basin with high heat flow.

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3.22 Surface Temperatures

SEDPAK uses two different models for maturation calculations, the Lopatin-Waples TTI (time temperature index) model (Waples 1980), and the Arrhenius equation-based kinetic model (Tissot et al. 1987) (see Section 3.23 for a fuller description of this). Temperature Gradients, Surface Temperatures and Maturation Parameters are required to calculate thermal maturation within SEDPAK. Each temperature gradient is a function of location and time. Read Sections 3.21 and 3.23 to become familiar with Temperature Gradients and Maturation Parameters, accessed from the SEDPAK EDIT menu.

Inputs: Surface Temperatures
Temperature gradients and surface temperatures are required to calculate thermal maturation within SEDPAK.

Inputs: Thermal Gradients

Discussion: Surface Temperatures

The surface temperature can vary in space and time. Surface temperatures in SEDPAK are represented by a two dimensional array of location and time. For this reason the user can set up any temperature history for a variety of locations. Local and temporal heating or cooling events can be included according to the user's model. SEDPAK provides the resulting maturation history, evolving through space and time.

The Surface Temperatures parameter is activated from the SEDPAK EDIT menu. The Surface Temperature can vary with time and space (Figure 3.22.1 and 3.22.2). Different Surface Temperature values can be input for lateral x-steps at each time step. Note that SEDPAK linearly interpolates between parameter values entered at different time steps and at different locations; the interpolation works both for time and space.

The Surface Temperature needed is that of the depositional surface, either below or above sea level. If a sigmoidal progradational geometry is formed and there is a constant Temperature Gradient, data are entered for each x-step and sigmoidal temperature lines will be produced along the section parallel to the depositional surface. If a temperature distribution is not parallel to the stratigraphic geometry, different Surface Temperature data and/or different Temperature Gradient data should be input laterally for the required x-steps. If smaller rates of increase in temperature are required for a portion of the section, lower Temperature Gradient values should be entered. If the same rate of increase is needed but for lower temperature values, lower Surface Temperature values should be entered. If the maturation modeling involves a cold temperature at the bottom of a deep sea (4šC) and a hot temperature on land, this can be done by entering different Surface Temperature values along the section (x-steps) at different times.

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3.23 Maturation Parameters

SEDPAK uses the different models for maturation calculations, the Lopatin-Waples TTI (time temperature index) model (Waples 1980), and the Arrhenius equation-based kinetic model (Tissot et al. 1987) (see Section 3.23 for a fuller description of this). Temperature Gradients, Surface Temperatures and Maturation Parameters are required to calculate thermal maturation within SEDPAK. Each temperature gradient is a function of location and time. Read Sections 3.21 and 3.22 to become familiar with Temperature Gradients and Surface Temperatures, accessed from the SEDPAK EDIT menu and Section 4.04 to see how the different Display Modes are chosen from the SEDPAK EXEC menu.

Discussion: activation energy and frequency factor

The Activation Energy and Frequency Factor in the Maturation parameters menu are input values to the basic kinetic maturation equation (see Section 5.24).

The Activation Energy (E) is a measure of the energy required for a reaction to proceed for a given temperature and a given frequency factor. Its value is dependent on the age, organic matter composition, pore fluid composition, mineral composition, and sediment facies (Waples 1984). The Activation Energy is an input parameter to the SEDPAK Maturation model, its values change for different basins and for different kerogen types. The default value is 50 000 cal/mol. A geologically reasonable range is 10 000 to 80 000 cal/mol (Tissot and Welte 1978).

When the activation energy is varied, larger values cause the maturation to be less for a given temperature (more energy is needed for some rocks to become mature). The effect of the frequency factor is opposite, an increase of its value makes the maturation higher. Changes in the values of these parameters on simulation runs without changes in the Surface Temperature and Temperature Gradient values produce the different maturation outputs as in the SEDPAK Simulation window.

SEDPAK uses two different models for maturation calculations, the Lopatin-Waples TTI (time-temperature index) model (Waples 1980), and the Arrhenius equation-based kinetic model (Tissot et al. 1987).

The TTI model calculates an integral using time of deposition and temperature history as inputs. TTI is used to track the maturation of organic matter as a function of burial and temperature history. The maturation is an irreversible process, and a maturation level developed earlier in the reaction cannot be diminished, even if the temperature decreases in response to uplift or thermal cooling.

Waples examined many hydrocarbon fields from around the world and established the appropriate TTI values for major maturation intervals as:

TTI maturation
15 oil onset
75 oil peak
160 oil end
1500 wet gas preservation deadline
65000 dry gas preservation deadline

The equivalence of these TTI values to maturation intervals vary from basin to basin and with different types of organic matter. However, the oil window generally matches TTI's of between 15 and 160.

In contrast the kinetic model in SEDPAK uses the first-order kinetic equation for decomposition reactions as they are applicable to thermal cracking of petroleum products:

where x = residual petroleum potential of the organic matter and k = a constant parameter for a given temperature.

The equation indicates that the rate of decomposition of organic matter (dx/dt) is negatively proportional with the amount of current residual material (x). The k factor is not constant during the reaction process, but it can be expressed by the Arrhenius equation:

The two maturation models in SEDPAK (TTI and Kinetic) offer the possibility to model the thermal maturation of organic matter in two different ways, and compare their results.

Chapter 4

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