Structural Analysis

Key Takeaways

• Seismic interpretation as the primary tool for structural analysis at basin and field scale relies on converting the two-way time of seismic reflections to depth using a velocity model, picking horizon surfaces that correspond to geological boundaries, and mapping faults from the offsets and discontinuities in the picked horizon surfaces: in 3D seismic surveys, the structural interpretation produces horizon depth maps (in time or depth) that show the three-dimensional shape of the reservoir top, the reservoir base, and key marker horizons above and below, revealing the structural closure (the area enclosed by the structural contour at the highest spill point — the deepest point through which hydrocarbons could leak from the trap before the closure is breached), the fault geometry (the dip, strike, and throw profile of bounding faults that may seal the trap laterally or provide migration pathways), and the structural complexity (whether the reservoir is a simple dome or a complex compartmentalized structure with multiple sub-closures separated by faults or saddles); the accuracy of the structural interpretation depends on the quality of the velocity model used for time-to-depth conversion (a 5% velocity error in a 3 km overburden translates into a 150-meter depth error that can dramatically change the interpreted closure area and column height), the resolution of the seismic data (which limits the precision of fault mapping to faults with throws greater than approximately half the dominant seismic wavelength, typically 10-30 meters in Tertiary basins and 30-80 meters in deeper, lower-frequency imaging conditions), and the skill of the interpreter in recognizing the structural style from incomplete and noisy seismic data.
• Balanced and restored cross-sections are the primary tool for testing the geometric consistency of a structural interpretation and for reconstructing the deformation history of complex fold-thrust belts, rift basins, and salt-influenced basins: a balanced section is a geological cross-section in which the interpreted structure is geometrically consistent with the conservation of line length (in 2D, assuming plane strain) or area (for section compaction accounting for pressure solution) during deformation — that is, the cross-section can be restored to an undeformed, geologically plausible pre-deformation state by mathematically reversing the mapped fault displacements and fold geometries; a cross-section that cannot be restored (a section in which beds stretch or compress non-systematically when the deformation is reversed) is geometrically inconsistent and indicates either a wrong interpretation of the fault geometry, a wrong assumption about the deformation kinematics, or genuine three-dimensional strain that is being incorrectly forced into a two-dimensional section; in fold-thrust belts (the Zagros, the Rockies, the Appalachians), balanced sections provide the quantitative constraint on shortening magnitude and thrust fault geometry that guides prospect mapping; in extensional basins (the North Sea, the Gulf of Mexico shelf, East African rifts), restored sections define the pre-rift geometry and the fault displacement required to produce the observed graben architecture, constraining the timing of trap formation relative to hydrocarbon generation and migration.
• Fault seal analysis is a critical component of structural analysis that evaluates whether a mapped fault will seal hydrocarbons against the fault plane or will allow hydrocarbons to migrate across it and leak from the trap: the two primary fault sealing mechanisms are juxtaposition sealing (where the reservoir is juxtaposed against a shale or low-permeability unit on the other side of the fault, preventing horizontal fluid flow across the fault) and membrane sealing (where the fault rock (gouge) itself has low enough permeability to prevent fluid flow across the fault even when permeable reservoir units are juxtaposed); juxtaposition sealing is evaluated from the Allan diagram (also called a juxtaposition diagram), which maps the lithological units on either side of the fault plane as a function of depth to identify reservoir-to-reservoir juxtapositions (potential leakage points) versus reservoir-to-shale juxtapositions (potential seals); membrane sealing is evaluated from the shale gouge ratio (SGR), a dimensionless parameter (0-1) that estimates the proportion of shale in the fault zone based on the clay content of the section that has moved past a point on the fault during slip, with high SGR (greater than 0.15-0.2) indicating sufficient clay incorporation in the fault gouge to create a membrane seal; hydrocarbon column heights supported by specific SGR values have been calibrated empirically from the analysis of faulted fields where the column height and the fault SGR are both known, providing a basis for predicting the maximum column the fault can seal in an undrilled prospect.
• Structural analysis of salt-influenced basins (the Gulf of Mexico, offshore Brazil, the North Sea Central Graben, the Permian Basin, offshore West Africa) requires specialized methods because salt flows plastically and creates structures (diapirs, salt sheets, salt welds, turtle structures) that have no analog in non-salt basins: salt diapirs rise buoyantly through overlying sediments and create traps by juxtaposing impermeable salt against reservoir units (salt-flank traps), by draping and folding overlying strata into rollover anticlines (salt-cored anticlines), and by withdrawal of salt from underlying sections (turtle structures where sediment subsides into the evacuated space and is subsequently inverted by the weight of the overlying sediments); seismic imaging below and around salt bodies is technically challenging because the high acoustic impedance contrast between salt (5.5 km/s P-wave velocity) and surrounding sediments (1.5-3.5 km/s) creates strong multiple reflections, shadow zones beneath the salt base, and complex refraction patterns that degrade the image of sub-salt targets; full-waveform inversion (FWI), reverse time migration (RTM), and least-squares migration are the processing methods that best resolve sub-salt images, but residual imaging problems remain even with these advanced methods, creating fundamental uncertainty in the structural geometry of sub-salt traps that is a major component of deepwater Gulf of Mexico and Bacia de Santos exploration risk.
• Structural uncertainty quantification transforms structural analysis from a deterministic interpretation exercise into a probabilistic evaluation that captures the range of structural geometries consistent with the available data, providing the input for risked resource calculations that honor the full range of possible geological outcomes: the primary structural uncertainties in a typical exploration prospect include velocity model uncertainty (the range of plausible depth conversions from the seismic time interpretation), fault position uncertainty (the range of fault locations within the seismic resolution limit), interpretation uncertainty (the range of plausible horizon picks within the noise level of the seismic data), and structural style uncertainty (the range of structural geometries that could produce the observed seismic pattern — for example, whether an amplitude anomaly represents a genuine closure or a velocity pull-up artifact); Monte Carlo simulation of structural parameters (sampling from distributions of each uncertain parameter and computing the resulting closure area and column height for each random draw from the joint parameter distribution) provides the probabilistic resource distribution (P10-P50-P90 range of unrisked resources) that quantifies the structural upside and downside relative to the base case estimate; this probabilistic resource range, combined with a geological chance of success (the probability that the trap actually contains hydrocarbons of the required quality) and a risk factor for each geological element (reservoir, seal, source, trap), produces the risked resource estimate that drives exploration investment decisions.