Parasequence Set

Key Takeaways

• The three fundamental parasequence set stacking patterns -- retrogradational, aggradational, and progradational -- are distinguished by comparing the lateral position of the shoreline (or its depositional equivalent) between successive parasequences within the set: retrogradational stacking (also called backstepping) occurs when each younger parasequence has its coastal facies deposited landward of the coastal facies in the underlying parasequence, indicating that the rate of accommodation creation exceeded the rate of sediment supply, causing the shoreline to migrate landward; aggradational stacking occurs when successive shorelines overlie each other vertically with no significant lateral migration, indicating that accommodation creation and sediment supply are approximately balanced; progradational stacking occurs when each younger parasequence has its coastal facies deposited basinward of the coastal facies in the underlying parasequence, indicating that sediment supply exceeded accommodation creation, causing the shoreline to migrate seaward; in subsurface well log cross-sections, these stacking patterns are recognized by the systematic upward-deepening (retrogradational: each parasequence cap is deeper-water facies), upward-shallowing (progradational: each parasequence is shallower and more sand-prone), or constant water depth (aggradational) trend in the gamma ray and porosity log response from parasequence to parasequence.
• The transgressive systems tract (TST) in sequence stratigraphy is defined by retrogradational parasequence sets deposited during the phase of sea level rise when accommodation is being created faster than sediment can fill it: the TST begins at the transgressive surface (or, in some sequence stratigraphic models, at the sequence boundary itself) and is bounded above by the maximum flooding surface (MFS), which represents the deepest water conditions of the sequence and is typically characterized by a condensed section of organic-rich, pelagic-dominated sediment (the starved basin facies where sediment supply is so overwhelmed by accommodation that only pelagic settling can keep up with rising sea level); the condensed section deposited at the MFS is often the source rock facies in petroleum systems (high organic carbon content from enhanced preservation under oxygen-deficient deep-water conditions) while the retrograding sand bodies of the TST parasequence set are the reservoir facies that the source rock charges during secondary migration up the dipping strata; this geometric and petroleum system relationship between TST parasequence set reservoirs and condensed section source rocks is one of the most predictive frameworks in exploration geology.
• The highstand systems tract (HST) is bounded below by the maximum flooding surface and above by the sequence boundary (the overlying unconformity), and contains progradational to aggradational parasequence sets deposited as sea level slows its rate of rise, reaches its highstand peak, and begins to fall: the early highstand parasequence sets are typically aggradational (balanced supply and accommodation) while the late highstand parasequence sets are progradational (supply exceeding accommodation as sea level rise rate decreases) and potentially forced regressive (supply plus sea level fall driving the shoreline basinward); the progradational highstand parasequence sets build deltaic and shoreface sands outward from the basin margin, creating the progradational wedge geometry visible on seismic data as downlapping reflectors that form some of the most prolific stratigraphic trap types (truncation pinch-outs, stratigraphic traps at the updip feather edge of highstand sand bodies) in petroleum exploration; the transition from aggradational to progradational stacking within a highstand systems tract is a predictive indicator of the maximum regression position (the basinward limit of the highstand progradation) and of the location of the sequence boundary unconformity that will eventually develop as relative sea level falls.
• Parasequence set boundaries (the major flooding surfaces that separate one parasequence set from the next within a systems tract) represent the most regionally correlatable surfaces within the sequence stratigraphic framework, because they correspond to the episodes of maximum flooding that covered the broadest areas with relatively deep water, depositing the most areally continuous fine-grained facies (the MFS of each parasequence set in the retrogradational TST, or the inter-parasequence flooding surfaces in the progradational HST) that appear as the sharp, high gamma ray spikes on well logs used as correlation markers; in practice, the distinction between a parasequence set boundary and a sequence boundary is one of scale and the degree of erosional truncation: sequence boundaries represent the longest-duration and most erosionally significant breaks (often associated with incised valley systems cut by rivers during lowstands when sea level fell and rivers adjusted to lower base levels), while parasequence set boundaries are significant deepening events but lack the evidence of subaerial exposure and erosion that characterizes sequence boundaries.
• Petroleum reservoir prediction using parasequence set analysis follows from the observation that sand distribution within a parasequence set is governed by the stacking pattern: retrogradational parasequence sets typically have isolated or poorly connected sand bodies (each parasequence is a discrete, landward-stepping sandy unit surrounded by marine shales on three sides and sealed by the next flooding surface above) that form stratigraphic traps with good vertical seals but limited lateral connectivity, while progradational parasequence sets typically have laterally connected sand bodies (each parasequence builds outward, connecting to the previous parasequence seaward) that form better-connected reservoir systems with higher aggregate permeability but potentially limited by the flooding surface seals at the top of each parasequence; the practical exploration application is that identifying the stacking pattern of parasequence sets in a basin from well log cross-sections predicts whether the reservoir sand bodies are likely to be isolated (retrogradational: individual accumulations requiring dense well spacing) or continuous (progradational: lower-risk reservoir connectivity, but potentially compromised sealing).