Discontinuity

Key Takeaways

• Seismic discontinuities are classified by their nature and scale: first-order discontinuities (such as the Moho, the core-mantle boundary at 2,891 kilometers depth, and the inner core-outer core boundary at 5,150 kilometers depth) are global features detectable by both reflection and refraction seismology at crustal and global scales; second-order discontinuities within the crust include the Conrad discontinuity (boundary between upper and lower crust at approximately 10 to 20 kilometers depth, marked by a compressional velocity increase from 6.0 to 6.5 km/s to 6.5 to 7.0 km/s) and various intra-crustal reflectors observed in deep seismic reflection profiles; in petroleum exploration, the relevant discontinuities are primarily the stratigraphic and diagenetic boundaries within the sedimentary section (depths of 0 to 6 kilometers) that create the reflection events visible on reflection seismic data, with each reflection event corresponding to an impedance contrast (density times velocity product changes) at the interface; the reflection coefficient at a discontinuity equals (Z2 - Z1) / (Z2 + Z1), where Z1 and Z2 are the acoustic impedances above and below the boundary, and the sign of the reflection coefficient (positive or negative) determines whether the reflected wave has the same polarity as the incident wave (hard kick, Z2 greater than Z1) or opposite polarity (soft kick, Z2 less than Z1).
• Stratigraphic unconformities are the most economically important class of discontinuities in petroleum exploration because they represent surfaces of erosion and non-deposition that create both the structural traps (truncation traps where porous reservoir rock is eroded at the unconformity surface and sealed by the overlying unconformable sequence) and the source rock-reservoir rock relationships that are fundamental to oil and gas accumulations: angular unconformities (where tilted beds are truncated by erosion and then overlain by subhorizontal beds) are visible as reflection terminations in seismic data (downlap of offlapping reflectors against the unconformity surface from below, onlap of transgressive reflectors against it from above), creating the seismic stratigraphic patterns used in sequence stratigraphy to reconstruct relative sea level history and identify reservoir-prone depositional systems; disconformities (where parallel beds have an erosional surface between them) are more subtle seismic features requiring amplitude or character analysis to detect; the distinction between a correlatable sequence boundary (a type 1 sequence boundary with forced regression and significant erosional discontinuity) and a parasequence boundary (a less pronounced flooding surface with minimal erosional relief) is a primary outcome of the seismic stratigraphic interpretation of discontinuities in the sedimentary section.
• Velocity discontinuities that do not correspond to major lithological contrasts are diagnostic of overpressure in the sedimentary section: in a normally pressured, normally compacted shale sequence, seismic velocity increases monotonically with depth as porosity is reduced by compaction; a velocity reversal (interval velocity decreasing with depth) within a shale or mudstone sequence indicates that the deeper section is undercompacted because abnormally high pore pressure has prevented normal compaction by supporting part of the overburden load that would otherwise be transferred to grain-to-grain contacts; this velocity discontinuity (transition from the normal velocity trend to the lower-velocity overpressured interval) is detectable on interval velocity maps derived from seismic data using the Dix formula, and its depth and magnitude are used in the Eaton pore pressure prediction method to compute the overpressure magnitude before drilling; the transition from normally pressured to overpressured sediments at this velocity discontinuity is often the most critical safety-relevant subsurface boundary in a pre-drill pressure prediction, determining the mud weight required to maintain wellbore stability without fracturing the formation.
• Electromagnetic discontinuities are detected by magnetotelluric (MT) and controlled-source electromagnetic (CSEM) surveys rather than seismic surveys, and include resistivity contrasts associated with hydrocarbon saturation (hydrocarbons are more resistive than brine-saturated rock by a factor of 10 to 1,000), conductive pathways through salt bodies (salt is highly resistive, creating a strong electromagnetic discontinuity at its boundaries), and basement topography (crystalline basement is typically much more resistive than overlying sediments); the electromagnetic discontinuity at the top of a hydrocarbon reservoir has been used in deepwater exploration as a complementary method to seismic amplitude anomalies (bright spots) for reducing exploration risk, because both methods are needed to distinguish a hydrocarbon-saturated reservoir from a brine-saturated sand with fortuitous amplitude anomaly; CSEM surveys detect the lateral extent of the resistive anomaly associated with the hydrocarbon contact, providing information complementary to the seismic reflection amplitude at the reservoir interface.
• Borehole measurements that detect subsurface discontinuities include the sonic log (which measures compressional and shear wave slowness, detecting velocity discontinuities with centimeter-scale vertical resolution), the density log (detecting density contrasts at lithological boundaries), the resistivity log (detecting fluid and lithology contrasts through the formation), and the formation microimager (detecting sub-millimeter physical discontinuities in the borehole wall, including fractures, bedding planes, and diagenetic boundaries); the calibration of seismic discontinuities to borehole measurements is the fundamental exercise of seismic-to-well tie and seismic inversion, where the acoustic impedance derived from density and sonic log measurements is compared to the seismic reflection amplitude at the corresponding depth to verify that the seismic event corresponds to the physical discontinuity identified on the borehole logs, ensuring that the seismic interpretation is grounded in physical reality rather than processing artifact.