Structural play geology
| Exploring for Oil and Gas Traps | |
| |
| Series | Treatise in Petroleum Geology |
|---|---|
| Part | Predicting the occurrence of oil and gas traps |
| Chapter | Exploring for structural traps |
| Author | R.A. Nelson, T.L. Patton, S. Serra |
| Link | Web page |
| Store | AAPG Store |
Structural traps are the most prolific and varied of all trap types; they account for most of the world's hydrocarbon reserves. They range from very large [e.g., Ghawar, Saudi Arabia (560,000 ac)] to small [Major County, Oklahoma, U.S.A. (160 ac or less)].[1] To effectively prospect at all scales in this size continuum, we must apply a wide variety of techniques, tools, and approaches.
Deformation, including sedimentary (diagenetic) processes of compaction, creates folds and faults, which can result in structural traps, including anticlines and fault closures.
Structural elements
To build a structural play and create the necessary factual and interpretive displays, we must analyze four structural elements of the play:
- The structural geometry of the play in three dimensions, including relative attitudes of formation and fault surfaces
- Deformation or diagenesis of reservoirs and seals (trap integrity)
- Timing of structural development and trap formation, and its relation to important petroleum system events
- Trap genesis in terms of structural process and/or tectonic context
Too often we focus only on structural geometry and ignore the other three elements. Timing, seal, reservoir, and process are what relate structural geometry to the petroleum system.
Unraveling structural geometry
To describe adequately the structural geometry of the subsurface trap, we must integrate subsurface data into a cohesive whole. Data include well logs, 2-D and 3-D seismic images (in both time and depth), gravity surveys, magnetics, and surface geology. These data are integrated with our understanding of the geometric possibilities for the structural style expected or demonstrated to exist in the area.
A structural style is a group of structures that often occur together in a particular tectonic setting. The following table from Harding and Lowell[2] lists the characteristics of the primary structural styles. Figure 1 illustrates schematic cross sections of hydrocarbon traps (black areas) most commonly associated with the major structural styles.
| Structural Style | Dominant Deformational Force | Typical Transport Mode | Primary plate-tectonic habitats | Secondary plate-tectonic habitats |
|---|---|---|---|---|
| BASEMENT INVOLVED | ||||
| Wrench-fault assemblages | Couple | Strike slip of subregional to regional plates | Transform boundaries | Convergent boundaries:
Divergent boundaries: |
| Compressive fault blocks and basement thrusts | Compression | High to low-angle convergent dip slip of blocks, slabs, and sheets | Convergent boundaries:
|
Transform boundaries (with component of convergence) |
| Extensional fault blocks | Extension | High to low-angle divergent dip slip of blocks and slabs | Divergent boundaries:
|
Convergent boundaries:
Transform boundaries:
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| Basement warps: arches, domes, sags | Multiple deep-seated processes (thermal events, flowage, isostasy, etc.) | Subvertical uplift and subsidence of solitary undulations | Plate interiors | Divergent, convergent, and transform boundaries |
| DETACHED | ||||
| Decollement thrust-fold assemblages | Compression | Subhorizontal to high-angle convergent dip slip of sedimentary cover in sheets and slabs | Convergent boundaries:
|
Transform boundaries (with component of convergence) |
| Detached normal fault assemblages (“growth faults” and others) | Extension | Subhorizontal to high-angle divergent dip slip of sedimentary cover in sheets, wedges, and lobes | Passive boundaries (details) | |
| Salt structures | Density contrast Differential loading | Vertical and horizontal flow of mobile evaporites with arching and/or piercement of sedimentary cover | Divergent boundaries:
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Regions of intense deformation containing mobile evaporite sequence |
| Shale structures | Density contrast Differential loading | Dominantly vertical flow of mobile shales with arching and/or piercement of sedimentary cover | Passive boundaries (deltas) | Regions of intense deformation containing mobile shale sequence |
Creating a concept
Using these data, we create a concept of the structural geometry of the play, following the steps in the table below.
| Step | Task | Why |
|---|---|---|
| 1 | Through stratigraphic correlation, determine or delineate “structural tops” for several mappable horizons from well and/or seismic data. The number of horizons depends on the quality of the data and the complexity of the structural style. | Changes in structural form with depth vary with structural style, mode of origin, and the operative deformational mechanisms. |
| 2 | Determine the relative attitude and thickness of units on fold limbs (dip panels) and/or units within fault blocks. | Given a deformational style, limb angles and thicknesses can be used to estimate fault and axial plane dip, and vice versa. |
| 3 | Determine the tightness of fold hinges with depth and the 3-D orientation of axial surfaces. | These features vary substantially with fold origin and are critical to predicting well paths. |
| 4 | Determine the position and offsets (throw, heave, separation, etc.) on faults and map their variation along the fault surface(s) (contour integration). | In any structural trap where faults play an important part in closure, the fault surface(s) must be contoured in order to accurately contour the top of the reservoir near the fault trace on that reservoir. |
| 5 | Determine closure (dip and/or fault) in all 2-D map directions. | Closure is the key element in all structural plays and must be evaluated at all appropriate horizons to look for vertical continuity and variation. |
Timing structural development
In the petroleum systems approach to exploration, the relative timing of major events, such as trap formation, is critical. Timing of structural trap development is difficult to determine and usually must be inferred. The techniques for determining timing are often integrated with one another using sequential restored sections (by hand or computer) that either back-strip the sedimentary layers by “flattening” to their depositional surface or palinspastically restore them to geometries prior to deformation by removing displacement on the faults and unfolding the folds.[3] In simplified structural settings, isopach maps of successive stratigraphic units may be regarded as paleostructural maps.
Determining timing
Following are tables of primary, secondary, and tertiary techniques that can be applied to determine structural trap timing, with primary techniques being the most useful.
Primary techniques
The following techniques are the most useful in determining structural trap timing.
| Technique | Function |
|---|---|
| Isopachs of time-specific intervals | Isopach maps are a basic subsurface tool. Thicks and thins displayed in those maps are assumed to be depositional variations related to vertical components of structural relief and/or movement. |
Unconformity studies
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The ages of surfaces of erosion, nondeposition, condensed section, or angular discordance can be used to time the structural motion that caused them. |
| Facies/isolith distributions | Often structural motion or relief does not cause interval isopaching but does cause facies or environment of deposition changes due to subsidence rate differences or sediment pathways. |
Fault terminations
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Consistent vertical termination of faults within the section can help us bracket timing relative to the ages of the section they cut and do not cut. |
| Relative crosscutting relations of faults | The crosscutting nature of discrete fault sets can help us infer the relative timing of motion of those sets. |
| Subsidence profiles | Changes in subsidence rate as shown in time and thickness profiles imply times of uplift and subsidence. |
| Thermal maturity profiles | Inflections in curves of maturity vs. depth depict burial/uplift history and can help us model structural development. |
Secondary techniques
The following techniques are useful in determining structural trap timing.
| Technique | Function |
|---|---|
| Vertical and lateral distribution of depositional environments to document uplift and subsidence | Tectonic activity can cause changes in sediment source terranes, bathymetry, and depositional environment, resulting in structurally controlled facies variations. |
| Radiometric dates of crosscutting intrusives and capping volcanics | Absolute age dating of these units can help to constraint the age of deformation of the host sedimentary rocks. |
| Unroofing sequences/ clastic lithology studies | The age of deposits shed off erosional highs relative to the age of the rock(s) being eroded implies the time of uplift. |
| kinematic indicators]] | Tectonic fabrics showing crosscutting or overprinting relationships suggest the sequence of deformation events. |
Tertiary techniques
The following techniques are the least useful in determining structural trap timing.
| Technique | Function |
|---|---|
| Fission-track thermal history modeling | These data help us model the temperature history of a rock from the time it cooled below a threshold temperature, thereby helping to date uplift and erosional events. |
| Inflections in shale compaction curves and velocity profiles | Vertical changes in percent compaction in shales inferred from logs can document changes in depth and/or rate of burial. |
| Paleoseismic indicators due to fault motion | The presence of synsedimentary or soft sediment deformation may indicate paleoseismic activity and date the tectonic motions responsible, in a relative sense. |
Geochemical and geophysical investigations of fault zones
|
Fabric analysis and relative dating of fault zone diagenesis can be used in some cases to date periods of fault motion. |
Reservoir and seal deformation
The table below describes the procedure for determining the relative deformation of seals and reservoirs in a structural trap.
| Step | Task | Explanation |
|---|---|---|
| 1 | Based on outcrop studies and subsurface data, subdivide the stratigraphic section according to the relative mechanical strength of the units. | In all structural styles, the mechanical makeup of the stratigraphic package has a strong and often predictable effect on structure geometry. |
| 2 | Determine the mechanical properties (brittle vs. ductile) of the individual reservoir and seal rocks using the following:
|
These properties help predict the deformation mechanisms activated during deformation. In siliciclastic reservoirs, these mechanisms may result in deformation-induced dilatant or compactive changes which in turn may have a large impact on reservoir quality. |
| 3 | Interpret equivalent strain maps derived from curvature analysis, such as Gaussian curvature. | These maps determine possible compactive zones and predict fractured reservoir properties, such as fracture permeability. |
| 4 | Define deformation mechanisms (fracture, cataclasis, intracrystalline flow, pressure solution, etc.) in seal and reservoir rocks at appropriate depths, and relate them to capillary pressure for sealing capabilities. | These mechanisms help us predict deformation-related changes in seal and reservoir rock properties. |
| 5 | If needed, create equivalent plastic strain maps or sections (numerical mechanical modeling, e.g., boundary value problems and finite element modeling). | Numerical mechanical modeling can predict and map (1) deformation mechanisms and (2) reservoir and seal property changes related to deformation |
Reservoir and seal changes
Structural deformation changes the petrophysical properties of the reservoir and seal facies. This physical diagenesis of reservoirs and seals in structural traps can take the form of compaction (reduction in porosity, permeability, and/or pore size) or dilatancy (increase in permeability by fracturing). These deformation-related changes should be either documented or predicted to estimate and risk reservoir and seal properties accurately in a structural trap.
See also
References
- ↑ J. Coughlon, personal communication, 1996
- ↑ 2.0 2.1 Harding, T. P., and J. D. Lowell, 1983, Structural styles, their plate-tectonic habitats and hydrocarbon traps in petroleum provinces, in A. W. Bally, Seismic Expression of Structural Styles: AAPG Studies in Geology series 15, vol. 1, p. 1–24. Plate tectonic studies.
- ↑ Nelson, R. A., T. L . Patton, S. Serra, S., and P. A. Bentham, 1996, Delineating structural timing: Houston Geological Society Bulletin, v. 39, no. 1, p. 14–17. Timing techniques.
External links
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