Pull-Up

Key Takeaways

• Salt bodies are the most commonly encountered and most challenging source of seismic pull-up in petroleum exploration, because salt has a P-wave velocity of approximately 14,500 feet per second (4,400 m/s) compared to 7,000-10,000 feet per second for the surrounding sediments, and the large velocity contrast combined with the often substantial thickness of salt creates travel time anomalies that pull up sub-salt reflectors by hundreds of milliseconds; the classic sub-salt pull-up scenario involves a salt dome that has pierced through a thick sequence of sediments — reflections from the sediments beneath the base of the salt appear as an apparent high-amplitude anticline in the time domain, suggesting a structural trap directly below the salt; without velocity correction, this apparent anticline could drive the decision to drill a well; after depth conversion accounting for the salt velocity, the same reflectors may resolve to a completely flat or even synclinal geometry, eliminating the apparent structural trap and the drilling rationale; the Gulf of Mexico sub-salt play required extensive 3D velocity model building and pre-stack depth migration to correctly image sub-salt structure and distinguish genuine traps from salt-induced velocity artifacts.
• Carbonate reefs are another prominent pull-up source in exploration basins where ancient barrier reefs are preserved as isolated high-velocity carbonate buildups surrounded by lower-velocity shale and mudstone — reef carbonates typically have velocities of 15,000-20,000 feet per second compared to 8,000-12,000 feet per second for the surrounding mudstones; at productive reef plays including the Devonian reefs of western Canada (the Rainbow and Kalamazoo pools in Alberta), the Silurian pinnacle reefs of the Michigan Basin, and the Cretaceous platform carbonates of the Middle East, seismic interpreters must carefully distinguish genuine reef-cored structural highs from the velocity pull-up artifact that the reef creates beneath it; a carbonate reef exploration workflow typically involves building a velocity model that explicitly includes the reef's high velocity, performing depth conversion with that velocity model, and verifying that the structural interpretation of the sub-reef section is consistent with the geology expected given the depth-converted geometry rather than the time-domain apparent position.
• Distinguishing pull-up from genuine structural uplift requires comparing the geometry of the apparent high with the geometry of the high-velocity body causing it — a velocity pull-up anomaly directly mirrors the geometry of the causative high-velocity body (appearing as an upward bulge in the reflectors beneath the body that is proportional to the velocity contrast and the body thickness), while a genuine structural high will affect reflectors at multiple levels in a way that is geometrically consistent with a fault or fold that exists independently of any velocity contrast; seismic attributes can help distinguish the two: genuine anticlines typically show consistent structural relief across multiple reflectors without corresponding amplitude anomalies at the top of the putative low-velocity body, while pull-up artifacts are associated with a high-velocity body that can be identified by its own distinctive seismic character (the bright reflection at the base of the salt, the distinctive dimming of amplitudes within the carbonate reef compared to surrounding shales); well control from offset wells is the definitive test — if the apparent structural high disappears in the well's depth-domain check shot velocity data after correcting for the local velocity, it was pull-up, not structure.
• Pre-stack depth migration (PSDM) is the processing technique that best addresses velocity pull-up and related velocity artifacts because it uses a detailed 3D velocity model to migrate seismic energy to its true subsurface position in the depth domain rather than in the time domain — conventional time migration, which uses a simplified velocity model, cannot accurately reposition energy that has traveled through strongly velocity-contrasting bodies like salt or large carbonates; PSDM requires building a velocity model that explicitly includes the geometry and velocity of all major anomalous bodies in the subsurface (salt bodies, carbonate buildups, high-pressure zones), and this velocity model building is an iterative process that uses reflection tomography, well check shot data, and geological constraints; the quality of the depth image is directly limited by the accuracy of the velocity model, and in complex velocity environments like the Gulf of Mexico deepwater or the Norwegian sub-salt basins, velocity model building for PSDM can consume more geoscience effort than any other single component of the seismic processing workflow.
• Gas hydrate layers create a specific form of pull-up in deep water and Arctic seismic data where methane hydrate-saturated sediments have higher velocity than the surrounding water-saturated sediments, causing the bottom-simulating reflector (BSR) that marks the base of the gas hydrate stability zone to generate pull-up in the deeper free-gas zone reflections beneath it — the BSR itself is a velocity discontinuity (high-velocity hydrate above, low-velocity free gas below) that generates a polarity-reversed reflection, and the free-gas zone below the BSR is both a velocity sag target and the potential hydrocarbon source for hydrate formation; in arctic and deepwater exploration the correct identification of genuine structural highs in the free-gas zone versus BSR-related velocity artifacts is important for both conventional gas exploration and for understanding gas hydrate dissociation risk in global warming scenarios; the BSR pull-up magnitude provides a direct estimate of the hydrate saturation contrast above and below the BSR, which is used in resource assessment models for gas hydrate deposits.