1.01 Objectives
1.02 Strategies/Methodologies of Sedimentary
Simulation
1.03 Empirical Basin Simulations
1.04 Use of Sedimentary Simulations
1.05 Advantages of Sedimentary Simulations
1.06 Advantages of Using SEDPAK
1.07 Assumptions Used in SEDPAK Algorithm
1.08 Major Controls on Sedimentary Fill of
Basins
1.09 General Description of the Software
1.10 General Description of the Basin Model
1.11 Getting Started: The Computer Environment
1.12 Getting Started: Some Practical Advice
The purpose of this manual is to provide an introduction to the conceptual
framework for modeling the sedimentary fill of basins and to describe the basic
operations of SEDPAK. Exercises are provided in Chapter
6 to help learn how to operate the program and experiment with the various
input values and movies of some of the output of the program can be viewed in
Chapter 7. Once the exercises of Chapter
6 have been performed, it should be easier to apply SEDPAK simulations to solve
exploration/exploitation problems.
1.02 Strategies/Methodologies of Sedimentary Simulation
Deposition and accumulation of sediments and strata are controlled by a complex set of variables, many of which are not fully understood. However, by simplifying the model for sediment accumulation and identifying the critical variables, it is possible to simulate the essential stratal relations, which then provide a check on the stratigraphic interpretation and aid in the integration of other geological data. Depositional modeling describes the processes and products of sedimentation at a range of scales, from grains to basin fill. Two major classes of depositional simulations are process driven and empirical:
Process driven simulations
Empirical simulations
Stratigraphic simulations can be used for both exploration and exploitation,
because they help earth scientists understand the distribution of reservoir
and seal facies and improve their ability to predict those facies relationships
from well logs and seismic data. Exploration examples of sedimentary simulations
include the prediction of reservoir depositional processes in submarine
fans (Ross et al., 1994) and the prediction of basin wide stratal geometries
(Lawrence et al., 1990). Some exploitation examples include the prediction
of carbonate platform reservoir geometries (Borer and Harris, 1991; Harris
and Borer, 1992).
1.03 Empirical Basin Simulations
SEDPAK (Kendall et al., 1986; 1989a; 1989b; 1991a; 1991b; 1992; Strobel et al., 1987; 1989a; 1989b; Helland-Hansen et al., 1988; 1989; Nakayama and Kendall, 1989; Scaturo et al., 1989; Tang et al., 1989; Eberli et al., 1994) is one of several empirical sedimentary simulations used to visualize and analyze different sequence stratigraphic geometries expressed by sediment deposition in response to eustasy, tectonic events or climatic change. Others include:
Arco (Thorn and Swift 1991)
Colorado School of Mines (Lessenger, 1992)
Exxon (Reference in AAPG Schroeder 199?)
Fuzzim (Ulf Nordland 1995)
Kansas Geological Survey (Rankey et al., 1994)
Lamont (Steckler et al 1995)
Marathon (Ross et al., 1994; Watts et al., 1994)
PHIL (Bowman et al., 1993; Armentrout et al., 1994)
Shell (Aigner et al., 1987; 1989; Lawrence et al., 1989; 1990)
Strata (Flemings and Jordan 1989)
Strativarious (Frohlich and Matthews, 1991)
1.04 Use of Sedimentary Simulations
Sequence stratigraphy is a methodology widely used by oil companies in the exploration and exploitation of hydrocarbons. At the present time, this technology focuses on the use of paper media seismic and well cross sections, but the advent of computer based interpretation is changing this mode of analysis. The SEDPAK program is a forward sedimentary simulation which fills an important niche associated with these computer interpretations, enabling geologists to develop and test sequence stratigraphic models as they visualize their seismic and well data. In fact, sedimentary simulation is one of the seven suggested steps proposed by Vail 1987 for conducting an integrated seismic interpretation.
SEDPAK provides a conceptual framework for modeling the sedimentary fill of
basins by visualizing stratal geometries as they are produced between sequence
boundaries (Siregar 1995, Wong 1995, Prueser 1995). The simulation is used to
substantiate inferences drawn about the potential for hydrocarbon entrapment
and accumulation within a basin. It is designed to model and reconstruct clastic
and carbonate sediment geometries which are produced as a response to changing
rates of tectonic movement, eustasy, and sedimentation (Helland-Hansen et al.,
1988, 1989; Strobel et al., 1989a). The simulation enables the evolution of
the sedimentary fill of a basin to be tracked, defines the chronostratigraphic
framework for the deposition of these sediments, and illustrates the relationship
between sequences and systems tracts seen in cores, outcrop, and well and seismic
data.
SEDPAK has been used as the powerful tool to demonstrate the principles of Sequence
Stratigraphy. On the USC Sequence Stratigraphy
Web Site movies made from screen dumps of the SEDPAK ouput are used as the
backbone to much of the instruction provided.
1.05 Advantages of Sedimentary Simulations
1.06 Advantages of Using SEDPAK:
1.07 Assumptions Used in SEDPAK Algorithm
1.08 Major Controls on Sedimentary Fill of Basins
The sedimentary fill of basins is controlled by three major variables (Figure 1.8.1):
Subsidence:
"The thermal and mechanical properties of the lithosphere exert important
controls on the formation of sedimentary basins" (Steckler, 1990). Thermal
subsidence rates and the magnitude and distribution of subsidence due to
loading vary in basins of different tectonic settings (Steckler and Watts,
1978; Stephenson, 1990).
Eustasy (global sea level):
Eustasy refers to sea level relative to a fixed datum, such as the
center of the earth. Global sea level variations result from changes in
either oceanic basin volume or water volume. Eustasy combined with subsidence
results in relative sea level variations, which control accommodation for
sediment deposition (Posamentier et al., 1988; Posamentier and Vail, 1988).
Sediment Supply:
"The role of sediment supply in transgressions and regressions is a
fundamental one..." (Schlager, 1994). When the rate of sediment supply
is greater than the rate of relative sea level rise, accommodation space
will be filled.
1.09 General Description of the Software
SEDPAK is the University of South Carolina Development Foundation's
basin fill simulation program which executes on UNIX workstations using
X11R5 and Motif 1.2. The software, developed by the Stratigraphic Modeling
Group of the Departments of Geological Sciences and Computer Science, provides
a simulation tool that models the geometry of the generalized lithofacies
of a basin, resulting from the interaction between the major geological
processes listed above in Section 1.8:
SEDPAK constructs empirical models of sedimentary geometry. These sedimentary geometries are created by the infilling of a two dimensional basin from both sides with a combination of clastic sediment and in situ carbonate growth. The simulation is designed for an engineering workstation environment. Data entry is accomplished by using a graphical user interface. Values are entered for the initial basin configuration and, as a function of time, the following variables may be specified: local tectonic behavior, sea level behavior, amount and direction of clastic deposition, accumulation rates of carbonates both as a function of water depth and pelagic accumulation. The model traces the evolving geometries of clastic and carbonate sediments through time, responding to the depositional processes previously itemized. Sediment geometries are plotted as they are computed, so the results are viewed immediately. Based upon these observations, parameters can be changed interactively and the program rerun until the resultant geometries are satisfactory. SEDPAK is a powerful tool which can be used to interpret sequence stratigraphy from seismic and well cross sections. It allows the interactive testing of all the basin fill variables (including subsidence and uplift, sea level, and rates of sedimentation) against interpreted seismic or stratigraphic sections, magnification of a segment of simulation geometries by zooming in on the output, and comparison with interpreted seismic and stratigraphic data.
SEDPAK requires user-specified information about various minor parameters (e.g., repose angle) which affect the major process variables in the model. In return, SEDPAK provides the graphic and tabular outputs which describe the accumulated sediments deposited by the model over a specified geologic time period. These outputs may have several uses. In particular, SEDPAK enables explorationists and production geoscientists to extend and "complete" observed data, specifically seismic and/or well based cross sections, and compare model outputs with this data in an attempt to substantiate and verify inferences drawn about the hydrocarbon entrapment potential of a basin. Similarly SEDPAK offers the researcher an opportunity to investigate the sensitivity of a particular lithofacies model to such parameters as sea level behavior, tectonics, and sedimentary processes. In this way, a catalog of simulations that represent both promising and implausible zones for hydrocarbon entrapment can be acquired.
The simulation can be used to test hypotheses proposed to explain geometries seen in stratigraphic sequences, as response to sea level variation, sediment accumulation and tectonic movement. Though SEDPAK does not directly model such algorithms as crustal rigidity and thermal subsidence, it is possible to determine the direction and rate of tectonic movement from seismic data or from burial and crustal subsidence curves. These values may be input directly to the simulation, just as a sea level curve is specified. For instance, the Haq et al. (1987) sea level curve has been used successfully as prescribed inputs to model Mesozoic and Tertiary sections.
1.10 General Description of the Basin Model
Within SEDPAK, clastic geometries are primarily influenced by the volume (expressed as cross sectional areas) of shale and/or sand delivered to and redistributed within a basin. It is assumed that the source of these clastics is not located within the boundaries of the region of the simulation, and that good estimates exist for both the volume of clastics involved, and the distance that each type of clastic is transported into the basin during the time intervals modeled. In addition, the program simulates deposition of non-marine as well as marine clastic sediments. Clastic modeling includes sedimentary bypass, erosion, winnowing, and sedimentation in alluvial and coastal plains, marine shelf, basin slope, and basin floor settings. Included are mechanisms for infilling of topographic depressions, development of sediment fans, draped fill over topography, and procedures to ensure that clastic sediments are transported a minimum distance into the basin. The volume and area distribution of the sediments being deposited are monitored. The distance and volume (cross sectional area in the simulation) that sand or shale is transported into the basin can differ, although at each time step, they are treated as ratios of the total clastic sediment deposited. No attempt is made to recreate the interbedded relationships between the sand and shale, though these can be inferred and displayed as facies.
In contrast to clastics, the source for carbonate sediments is assumed to be located within the boundaries of the simulation. SEDPAK treats carbonate geometries as being primarily influenced by the amount of in situ carbonate accumulation and the volume of transported carbonate talus and turbidite that accumulates during that same time interval. Carbonate accumulation is modeled as in situ production using a depth/time function, and as a background pelagic rain. Carbonate modeling can simulate progradation, development of hardgrounds, downslope aprons, keep up, catch up, backstep, drowned reefs, as well as response of lagoonal and epeiric sediments to eustasy as described by Scaturo et al. (1987), Strobel et al. (1988a), Strobel et al. (1988b), and Scaturo et al. (1989).
In SEDPAK, as in natural systems, the initial and evolving shape of the basin surface directly affects sediment geometries as deposition occurs. SEDPAK models additional influences on sediment geometries, including the amount of regional tectonic movement, faulting in the basin, water depth, erosion of previously deposited sediments, submarine slumping, compaction of sediments, and the isostatic response of a basin to sediment loading. Effects such as crustal rigidity, isostatic response to sediment load, etc., are modeled by prescribing rates of tectonic movement across the basin.
Figure 1.10.1 depicts how the evolving cross sectional sedimentary geometry of a basin is characterized by SEDPAK. The program is iterative and deposits sediments layer by layer over the number of time steps specified in the program inputs. Each time step corresponds to a specific number of years. The cross section shown as the xy plane in Figure 1.10.1 is referred to as the basin cross section. The horizontal x axis is taken to be (roughly) parallel to the earth's surface and at some reference elevation, e.g., an arbitrary datum. The vertical -y axis indicates negative elevation into the earth, measured downwards from an arbitrary datum towards the earth's center. The upper, lower, left and right boundaries of the cross section correspond to the basin top, bottom (or basement), beginning and end respectively.
Within each time step, SEDPAK continues to deposit sediments upon the
current surface, column by column, until either:
1) all the clastic sediment, that has been supplied to the program
during initialization, is exhausted and/or no more carbonates can accumulate,
or
2) all time steps set up by the input have been completed. Various
minor constraints may override either of these basic control structures
and will be described later in this manual.
At the termination of a SEDPAK run, the output has one of several user-specified
forms:
1) a list of numerical output parameters which have been computed by
the simulation, or
2) graphical output similar in form to that depicted in Figure 1.10.2,
and further illustrated in Chapters 6 and
7 of this manual.
The latter may be displayed on the workstation, and/or printed. Also,
the output of the simulation may be saved to a file for future reference,
one or more input variables may be reset for another SEDPAK simulation
run, or the session may be ended.
1.11 Getting Started: The Computer Environment
SEDPAK is delivered as an executable code that is compiled for a specific type of workstation. The engineering workstation that hosts your copy of SEDPAK must contain, or have access to, the following minimal set of hardware and software:
1.12 Getting Started: Some Practical Advice
It is not necessary to read this manual in order to use SEDPAK. However, if any problems do occur, please refer to this manual as well as the on-line Help system. The pull-down menus under Help on the SEDPAK EDIT and SEDPAK EXEC menus provide descriptions on all the items listed on the EDIT and EXEC menus (Section 2.09). Help on Text is a powerful option that activates the "Context Sensitive Help System" built into SEDPAK. When this menu item is selected, the cursor becomes a "?" which can be clicked on any menu item displayed on the SEDPAK menu interface from which Help was activated.
Although many SEDPAK users may want to try out the program without reading the rest of the manual, it is suggested that, at a minimum, they run the examples provided in Chapter 6.
When creating new models, it is strongly advised that you start with an existing data file that is close to the geological example you are modeling. You should modify this file to meet the specifications of the current model. It has been found from experience that although SEDPAK is intuitive to use and the values of most of the variables are obvious, many users do not remember to set all of the program parameters. The best approach is to work from an existing data set and modify it.
As the file is changed, it should be continually saved and the simulation should be rerun to test the effects of these changes. It has been found that the best way to work with a basin simulation is to turn off clastic and carbonate sedimentation (Section 3.02) and then: 1) establish the shape of the original basin surface (Section 3.04); 2) establish the eustatic sea level curve for the simulation (Section 3.05); 3) determine the subsidence history of the original basin surface (Section 3.06). It is only after this that the sedimentary fill of the basin should be modeled (Sections 3.07 through 3.16). This enables the position of sea level to be tracked with respect to the initial basin surface. It also helps determine whether reasonable subsidence and sea level histories have been input to the simulation. Once subsidence and sea level behavior (accommodation space) have been established, it is much easier to determine the controls on the character of the geometry of the sedimentary fill of the basin.
The final output of the simulation is often better viewed in zoomed
mode (Section 4.01). The user also
should remember when matching the simulation results to seismic or well
log cross sections: 1.) note the scale of the output and the vertical exaggeration;
2) keep track of the timing of the last time step - the simulation may
not be run to the present; 3) establish the frequency of the time steps
versus the rate of change of sea level, subsidence, and sedimentation in
order to avoid the effects of aliasing (see Section 8.02).