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)]. 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

Figure 1 Schematic cross sections of hydrocarbon traps (black areas) most commonly associated with the major structural styles. After Harding and Lowell.

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 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: Foreland basins Orogenic belts Arc massif Divergent boundaries: Offset spreading centers
Compressive fault blocks and basement thrusts	Compression	High to low-angle convergent dip slip of blocks, slabs, and sheets	Convergent boundaries: Foreland basins Orogenic belt cores Trench inner slopes and outer highs	Transform boundaries (with component of convergence)
Extensional fault blocks	Extension	High to low-angle divergent dip slip of blocks and slabs	Divergent boundaries: Completed rifts Aborted rifts; aulacogens Intraplate rifts	Convergent boundaries: Trench outer slope Arc massif Stable flank of foreland and fore-arc basins Back-arc marginal seas (with spreading) Transform boundaries: With component of divergence Stable flank of wrench basins
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 Passive boundaries
DETACHED
Decollement thrust-fold assemblages	Compression	Subhorizontal to high-angle convergent dip slip of sedimentary cover in sheets and slabs	Convergent boundaries: Mobile flank (orogenic belt) of forelands Trench inner slopes and outer highs	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: Completed rifts and their passive margin sags Aborted rifts; aulacogens	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. 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 Missing time and rock section Relations to eustacy Sequence stratigraphy Angular discontinuities	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 Up-section termination horizons Lower detachment planes	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 Rb-Sr, Ar-Ar, and K-Ar dating of fault zone material Electron spin resonance techniques Fracture fabric sequencing in 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: Mechanical tests Resistivity logs (in shales) Composition–porosity–grain size predictions	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

• Cross section
• Structural maps and cross sections
• Selling a structural play

External links

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