Structure Map

Key Takeaways

• Depth conversion from seismic two-way time to depth is the most uncertainty-prone step in building a structure map from seismic data — seismic reflection surveys record the time at which acoustic signals return to the surface after reflecting from subsurface boundaries, not the actual depth of those boundaries; converting time to depth requires knowledge of the seismic velocity of every layer above the reflector of interest, because depth = velocity × time / 2; seismic velocities vary laterally (due to lateral changes in rock type, compaction, and fluid content), with depth (increasing as compaction increases), and with temperature and pressure; the velocity model used for depth conversion is typically estimated from seismic velocity analysis (moveout velocities from normal moveout correction, which approximate interval velocities) and calibrated against well check shots or VSP (vertical seismic profile) measurements that measure the actual one-way travel time from the surface to a specific depth in the well; even with well calibration, depth conversion errors of 1-5% of depth (50-250 feet in a 5,000-foot reservoir) are common, and these errors directly translate into structure map contour errors that affect the predicted closure area, trap volume, and well locations; in complex velocity fields (beneath salt, in areas with strong lateral velocity gradients), depth conversion errors can be 10-15% without sophisticated velocity modeling.
• Fault interpretation on structure maps is essential for understanding trap geometry and reservoir compartmentalization — faults appear on structure maps as lines where the contours are discontinuous (on one side the contours are offset from the other, reflecting the displacement across the fault plane), and the throw of the fault (the vertical component of displacement between the same horizon on opposite sides of the fault) is read directly from the contour offset; normal faults (where the hanging wall moves down relative to the footwall) create down-to-basin structural geometry that can form fault-dependent traps where the fault itself acts as the updip seal for the hydrocarbon accumulation; reverse faults (where the hanging wall moves up) create structural elevation on the hanging wall that may be a structural crest; strike-slip faults create lateral displacement of contours without systematic vertical throw; interpreting fault geometry, throw distribution, and fault seal capacity (the ability of the fault plane to prevent fluid flow across it) from structure maps combined with seismic data is one of the most consequential and most uncertain parts of petroleum prospect evaluation, because fault seal failure is one of the most common reasons that structurally valid traps contain no commercial hydrocarbons.
• Volumetric calculations from structure maps determine the size of potential hydrocarbon accumulations — the gross rock volume (GRV) of a potential trap is calculated by measuring the area enclosed by the spill point contour (the structural contour at which hydrocarbons would spill out of the trap if the column were higher), multiplying by the net-to-gross ratio (the fraction of gross rock volume that is reservoir quality), and converting to volume; the uncertainty in this calculation is dominated by the uncertainty in the structure map itself (the position of the spill point contour, the depth conversion error, and the uncertainty in the structural geometry between control points) rather than by the uncertainty in reservoir parameters like porosity and net-to-gross; risk assessment in exploration uses probability distributions on the GRV calculation (P10, P50, P90 cases based on the range of plausible structural interpretations) to quantify the resource uncertainty, and the range of GRV across scenarios is often enormous — P90 volumes that are 10 times smaller than P10 volumes are not uncommon for poorly constrained structures in frontier exploration.
• Time-lapse (4D) structure mapping tracks reservoir depletion and fluid movement by comparing structure maps (specifically, amplitude maps on the reservoir reflector) between baseline and monitor seismic surveys — as oil or gas is produced from a reservoir and replaced by water encroachment or injection, the seismic properties of the reservoir change in ways that are detectable as amplitude, velocity, and phase changes on successive seismic surveys; these changes can be mapped on the structure map horizon to show where the reservoir has been swept (water or gas replacing oil), where bypassed oil remains, and whether reservoir compartments are communicating (if pressure changes propagate across fault boundaries, the fault seal is probably not maintaining pressure isolation); 4D seismic analysis in fields with active waterflood programs has directly identified infill well targets for bypassed oil that conventional production data analysis would not have located, adding hundreds of millions of barrels of incremental recovery in fields where 4D monitoring was applied to a strong baseline 3D survey.
• Structural uncertainty quantification from probabilistic structure mapping is increasingly standard practice in reservoir development decision-making — rather than presenting a single "best estimate" structure map as the basis for well location and development decisions, modern geoscience practice generates multiple plausible structure map realizations that honor the control data but represent different geological interpretations of the under-constrained areas; these realizations are used in decision tree analysis and Monte Carlo simulation to quantify the probability distribution of trap volumes, well locations, and development economics; the P10-P90 range of cumulative production forecasts derived from the full set of structural realizations provides a more honest characterization of development uncertainty than a single deterministic forecast based on the "most likely" structure map; presenting uncertainty explicitly is increasingly required by both regulatory bodies (for reserve disclosures) and company management (for capital investment decisions), making probabilistic structural mapping a professional standard rather than an optional enhancement.