Systems tracts and trap types

Each systems tract—highstand, lowstand, and transgressive—has a different trapping potential based on the vertical and lateral distribution of lithofacies deposited within specific depositional environments. White presents an excellent review of trap types within facies-cycle wedges, which are related to transgressive-regressive cycles and can be related most specifically to the transgressive systems tract and the highstand systems tract. In White's classification, prograding lithofacies of the lowstand systems tract might occur as subunconformity traps or might be mistakenly identified as highstand systems tract deposits. Gravity-flow deposits of slope and basin-floor fan systems are most often placed into the lowstand systems tract because they are deposited basinward of the shelf/slope inflection.

White discusses both siliciclastic and carbonate systems. Sarg provides an excellent discussion of carbonate systems. Only siliciclastic systems, similar to those of the Cenozoic of the central and western Gulf of Mexico, are discussed here.

Lowstand systems tract traps

Lowstand gravity-flow, sand-prone reservoirs occur in basin-floor and slope systems. They are most often encased within marine hemipelagic mudstones, which serve as seal and sometimes potential source rock. Traps are often stratigraphic, but postdepositional deformation that places the gravity-flow sand deposit in a structurally high position enhances the potential for focused migration of hydrocarbon fluids to the reservoir facies.

Lowstand prograding complex traps

Siliciclastic lowstand prograding complexes, imaged on seismic reflection profiles as clinoforms, are often fluvial-deltaic complexes with abundant sand in the depositional topsets (Figures 4-19 and 4-21). As the relative sea level cycle turns around from low to rising, the coarse-grained sediment supply decreases. The fine-grained sediments of the transgressive systems tract overlie the lowstand systems tract–prograding complex sand-prone facies, providing excellent top seal to the underlying sandy reservoir. If the transgressive shales are organic rich and buried in the thermal regime for kerogen cracking, hydrocarbons will be generated. If the lithofacies forming the preceding shelf edge can provide lateral seal, the prograding complex reservoir facies may become charged with hydrocarbons even without structural enhancement of the trap.

Transgressive systems tract traps

Transgressive systems tracts step toward the basin margin, with mud-prone facies overlying most of the sand-prone deposits, providing good top seal and often potential source rock to the underlying sands. However, because of the landward-stepping character of this systems tract, the sand-prone depositional facies are not likely to be very thick, resulting in volumetrically smaller reservoirs. Traps can be purely stratigraphic or enhanced with postdepositional structuring that focuses migration. However, the landward-stepping sand-prone facies may prevent adequate lateral seal for stratigraphic traps.

Highstand systems tract traps

Highstand systems tracts step toward the basin center and often prograde at the expense of the preceding parasequence due to erosion during relative fall of sea level. The falling sea level also decreases the space into which the sediment can accumulate, resulting in potentially rapid lateral shifting of the prograding deltaic lobes. This results in relatively thin but widespread sand-prone facies. An effective top seal for such a highstand system would require a very major transgression well landward of the updip end of the sandy facies of the prograding coastal plain. Such a transgression could be eustatic or tectonic in nature, as in a rapidly subsiding foreland basin setting. Postdepositional deformation forming anticlines enhances the potential for entrapping hydrocarbons in sheet-like highstand systems tract reservoirs.

Shelf margin systems tract traps

Shelf-margin systems tracts are the lowstand deposits of a type 2 sequence. Type 2 sequences are deposited when relative sea level falls but not below the preceding depositional inflection. Type 1 sequences are deposited when relative sea level falls below the preceding depositional systems tract.. The subsequent transgression may provide effective top seal, but the lateral seal of shelf margin systems tracts shares the same limitations as the highstand systems tract.

Early sequence stratigraphic studies, based largely on seismic reflection profiles, used the shelf edge as the depositional inflection reference point. Subsequent work on outcrops and high-resolution seismic reflection profiles redefined the deposition inflection as the shoreline break The shoreline break is generally coincident with the seaward end of the stream mouth bar in a delta or the upper shoreface in a beach environment. This change in depositional inflection scale results in nearly all seismically recognized lowstand deposits being attributed to type 1 sequences. Because the GOM basin analysis relies largely on seismic reflection profiles, all lowstand deposits are referenced to shelf-edge inflection points.

Systems tracts with greatest trapping potential

White) compiles data on the depositional setting of more than 2000 major oil and gas fields in 200 transgressive and regressive wedges within 80 basins. With clearly stated qualifications, White shows that most hydrocarbons found in siliciclastic reservoirs occur in the base to middle of the wedge in generally lowstand to transgressive depositional facies. This can be attributed to the greater probability of effective top seal in contrast to the highstand systems tract. By using the stratal stacking pattern, supplemented by lithofacies and biofacies data, depositional environments can be properly identified and paleogeographic maps constructed for each systems tract to predict between and beyond data points.

See also

• Sea level cycle phase
• Determining sea level cycle order
• Sea level cycle phase and systems tracts
• Systems tracts identification
• Identifying sea level cycle phase with biostratigraphy
• Biofacies and changing sea level
• Constructing age model charts
• Superimposed sea level cycles
• Sequence stratigraphy

External links

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