Critical Moment

Key Takeaways

• Timing risk assessment using the critical moment concept requires comparing the time of trap formation (when structural closure was established by folding, faulting, or stratigraphic pinchout) to the time of peak generation and migration from the source rock (when the source was at the depth and temperature of maximum petroleum expulsion): if the trap was structurally closed before generation peaked (early trap formation), the petroleum had a receptacle waiting when it migrated from the source, and the timing risk is low; if the trap formed after generation peaked (late trap formation), the early-generated petroleum migrated through the location of the future trap without being captured and was lost to the surface or to the next trap updip, and the risk of an empty or only partially charged trap is high; in compressional fold belts (thrust belts, foreland fold belts) like the Zagros fold belt of Iran and Iraq, the thrust-related anticlines that form the giant carbonate reservoir traps typically formed during or after the main generation pulse from the Jurassic and Cretaceous source rocks buried beneath the thrust sheets, creating a timing risk that was nevertheless overcome by the enormous source rock volumes that generated more than enough petroleum to fill the late-forming traps; in extensional rift basins (like the North Sea Viking Graben), normal faults formed during Jurassic extension created structural closures that were in place before the Kimmeridge Clay source rock reached peak generation in the Late Cretaceous and Paleocene, providing early traps that captured petroleum as soon as it migrated from the source.
• Basin modeling provides the quantitative framework for determining the critical moment by reconstructing the burial history, temperature history, and petroleum generation history of the source rock and comparing these to the structural and stratigraphic history of the trap: 1D basin models (at the source rock well location) compute the transformation ratio of kerogen to petroleum as a function of time, using the time-temperature integral and kinetic parameters for the specific kerogen type, to determine when generation began, when it peaked, and when it ended; 2D and 3D basin models extend this analysis to compute the migration pathways and timing of petroleum arrival at each potential trap, allowing the critical moment to be identified as the time when the trapped petroleum volume was maximized; the comparison of modeled critical moment timing with the geological record (structural timing from seismic stratigraphic analysis of growth faulting and fold development, erosion events from unconformity-based analysis, seal formation from diagenetic cementation timing) provides a risk assessment that can be quantified as a probability — the probability that the timing relationships were favorable enough to allow commercial accumulations to form and survive; petroleum systems models calibrated to known accumulations (fields with measured pressures, fluid compositions, and production histories that constrain the generation and migration model) can be used to predict the timing relationships in adjacent undrilled areas with much greater confidence than uncalibrated models.
• Preservation risk after the critical moment is a separate but equally important component of petroleum systems timing analysis: even if generation, migration, and trapping all occurred at the right moment, subsequent geological events can destroy or remobilize the accumulated petroleum; the most common post-critical-moment risks include tilting of the trap structure (by regional tectonic uplift, by differential subsidence in a compacting basin, or by salt withdrawal and salt movement in evaporite basins) that causes the oil-water contact to shift and spill oil from the trap updip; erosion of the cap rock by tectonic uplift and exposure at the surface (as occurred during the Laramide orogeny over parts of the Cretaceous Seaway reservoirs of the US Rocky Mountain region, where Paleocene erosion removed the cap rock from some structural traps); biodegradation of the oil at the trap location due to later influx of meteoric water containing oxygen and nutrients that enables bacterial degradation of the lighter hydrocarbon components; and fault reactivation that creates new fracture pathways through the cap rock seal, allowing vertical migration of petroleum from the trap to the surface; the petroleum system events chart captures the preservation window (the time from the critical moment to the present) as an explicit component of the petroleum system analysis, and basins with large post-critical-moment tectonic events have correspondingly higher preservation risk in their petroleum system assessment.
• Multiple critical moments in a single petroleum system occur when multiple phases of petroleum generation (from the same source rock at different burial depths, or from multiple source rocks of different ages) reach peak generation and migration at different times, filling or refilling the same trap through successive migration pulses at different geological ages: in the Reconcavo Basin of Brazil, Cretaceous lacustrine source rocks generated oil during Aptian burial, then experienced a second generation pulse during additional Tertiary subsidence of the basin, creating two distinct critical moments that both contributed to the oil columns in the structural traps of the basin; in the Central North Sea, the Jurassic Kimmeridge Clay source rock generated oil during Late Cretaceous to Paleocene burial in the deep parts of the Viking Graben, and the critical moment for the major oil fields of the Brent Province (Statfjord, Gullfaks) is typically assigned to the Paleocene, when the combination of peak generation, established migration pathways along carrier sands, and intact Cretaceous chalk and shale seals over the tilted fault block traps created the optimal configuration for oil accumulation and preservation; a second critical moment occurred in the Eocene for some fields as continued burial of the source in the graben deepened the generation window and provided a late charge of lighter oil that mixed with the earlier oil in some traps.
• Petroleum system nomenclature using the critical moment assigns a geological age designator to each petroleum system based on the age of the source rock, the age of the reservoir rock, and the age of the critical moment: the Magoon-Dow naming convention for petroleum systems uses the format "Source Rock Name(!)Reservoir Rock Name(!)!" where the ! indicates the certainty of the system identification (one ! for hypothetical systems based on geochemical inference, two !! for known systems confirmed by oil-source correlation); the critical moment age is included in the petroleum system description as the time during which the key elements were in the right configuration; in the Anadarko Basin of Oklahoma, the Woodford Shale(!)Morrow Sandstone(!)" petroleum system has a critical moment in the Late Pennsylvanian to Permian, when the deeply buried Devonian Woodford Shale was generating oil and expelling it into the Pennsylvanian Morrow sandstone reservoirs through the interconnected aquifer system of the basin; identifying all the petroleum systems in a basin and their critical moments provides a systematic framework for exploration strategy — understanding which source rocks were generating at the time each trap was available, and which traps were available when each generation pulse occurred, defines the exploration targets with the most favorable timing relationships and therefore the highest charge probability.