Large patterns become predictive tools to define trends, reduce exploration risk

July 1, 1999
US Gulf syndepositional structures, tectonic provinces

In the last decade, as explorers have moved into new plays such as deepwater turbidites, subsalt and fold belts, there has been renewed industry interest in trying to understand Gulf of Mexico (GOM) geology on a regional scale as a means to mitigate exploration risk.

New regional work has been bolstered by regional 3D and long-record seismic. New insights have emerged about fundamental controls on the geology and distribution of hydrocarbons in the GOM. One such insight is in the concept of syndepositional structural systems and their distribution in tectonic provinces.

Shale-based detachment system. Regressive cycle A: Shelf progradation triggers formation of detachment system. Shelf progradation stalls along flexure due to rapid subsidence. Regressive cycle B: shelf progradation bypasses flexure A, which triggers the formation of detachment system B.
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Syndepositional structures form in response to sedimentary loading. Deltaic and turbidite fan complexes are on a scale of tens of miles, so logically the syndepositional structures must be of similar size. Hydrocarbon-trapping structures are actually quite small components of larger-scale syndepositional structural systems.

We define syndepositional structural systems as structural complexes comprised of a number of different but genetically related structures. These occur in generally repetitive and predictable patterns, linked causally with reservoir distribution and the unstable substrate being loaded. Because the individual components composing a structural system, are genetically related, they tend to occur within the system in an orderly and generally repetitive pattern. With a good understanding of the general patterns, one has a predictive tool to apply to the area or prospect being evaluated.

Within the northern GOM there are three principal types of structural systems:

  • Shale-based detachment faults
  • Salt-withdrawal minibasins
  • Salt-based detachment faults.

These systems are not distributed at random but are usually segregated into areas dominated by a single system. These areas, defined here as tectonic provinces, also have an organized pattern that reflects some of the fundamental controls on the development of the GOM basin.

Marine shales and turbidites pond in a withdrawal syncline (pillow stage). Prodelta shales initiate diapiric development, forming an asymmetric withdrawal basin and turtle structure (diapir stage). Salt "bottoms out" and the shelf bypasses the basin.
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Shale-based detachment

Shale-based detachment systems are gravity-slide phenomena driven by progradation of the shelf margin onto an unstable shale, resulting in a regional decollement. The updip zone of extension is characterized by listric splay and keystone faults and by the development of rollover structures and/or rotated fault blocks. Delta progradation stalls at the head of the system and sands tend to be stacked in the rollover, or are conveyor-belted downdip along the fault ramp to form rotated wedges. In map view, the linear region of splay and keystone fault development is the "flexure" zone.

The basinward edge of the flexure is a planar synthetic fault beyond which is a synclinal region. Tectonically, this is the true first-order structure of the gravity slide, and is filled with pro-delta slope facies. Further basinward is the undeformed slide block, typically seen as a horst, generally starved of coeval sedimentation and terminating in a compressional toe zone. The slide block and toe zone are often obscured or removed by the next younger system.

A salt sill forms along a salt withdrawal mini-basin. A shelf depositional system loads tubular salt, and stalls at the head of the system.
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Salt-withdrawal minibasin

Salt-withdrawal minibasin systems are circular to elliptical in map view. They are comprised of an asymmetric salt-withdrawal syncline with its coupled diapir(s), a north dipping "counter-regional" fault and secondary basin rimming hinge faults. When the counter-regional is stepped, there will also be a secondary turtle and/or a series of hinge faults. The overall geometry of the mini-basin, and the nature of the sedimentary fill within the basin, vary considerably, depending on whether the mini-basin was loaded in a shelf or slope environment.

Shelf-loaded minibasins were formed by loading of thick salt, initially in deepwater, then under prograding shelf loads. Development of the typical salt withdrawal system began in the Late Jurassic to Early Cretaceous with the formation of salt pillows. Source rocks and turbidites ponded within the developing syncline. Subsequent loading of the mini-basin by shelf sedimentation initiated rapid withdrawal of salt from the basin into diapirs, forming an asymmetric wedge of pro-deltaic shale filling the developing syncline.

As the axis of synclinal development shifted basinward, the original syncline overturned leaving a turtle structure anticline in the wake of the migrating syncline. Progradation of the shelf margin stalled along the basin-rimming hinge faults, resulting in a thick section of stacked deltaic sands and shales. In map view, shelf-loaded salt-withdrawal mini-basins are typically aligned, forming a depo-trough.

Slope-loaded mini-basins are similar to shelf mini-basins, but are at an earlier evolutionary stage of development and involve much more salt than the shelf systems. The slope-withdrawal basins are structurally simple, being flanked by salt walls with little fault rim development. They are typically obscured by salt wings, so that we see only glimpses of the primary structure through windows. As with the shelf-loaded mini-basins, formation of the slope-loaded mini-basins began during the Late Jurassic to Early Cretaceous with ponding of turbidites triggering the diapiric stage.

Because of the great thickness of autochthonous salt involved, turbidite ponding is more or less continuous, so long as there is salt withdrawing from the basin. If the depocenter shifts during the diapiric phase, during the hiatus, a wing forms which overflows the basin. This results in the formation of a salt canopy. Once the allochthonous salt has been evacuated, turbidite deposition bypasses the basin.

Salt-based detachment

Salt-based detachment systems, known as Roho, are combination gravity slides and salt withdrawal structures formed in response to the progradation of shelf sediments onto a salt wing. The updip zone of extension is marked by a series of nested highly listric faults, which in map view, display a characteristic horseshoe geometry. Deltaic sands and shales tend to be stacked in the rotated wedges associated with these faults.

The central portion of the structural system is typically manifest as a complicated zone of remnant salt, perched diapirs, salt-floored faults, and strike slip faults. Basinward of this region is the compressional toe, which consists of a melange of salt and deformed sediment. All faults sole into the evacuated remnant of the original salt tablet, which typically overlies a salt-withdrawal mini-basin. In map view, the overall geometry of the salt-based detachment system is constrained by the geometry of the salt tablet upon which it is forming.

The northern Gulf of Mexico basin showing arcuate tectonic provinces.
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System distribution

The northern GOM basin is divided into a number of tectonic provinces arranged in arcs around the basin center. Each province is characterized by the predominance of one type of structural system. At the largest scale, the most fundamental control of the provinces was the differentiation of the northern Gulf Basin into two regions with different rift basin geometry separated by a transform fault boundary.

The west side of the basin was a high standing rift platform, while the east had a plunging ramp profile, resulting in a thin salt veneer to the west and a basinward thickening wedge of salt to the east.

Because of originally thin Luann salt, the Texas onshore and shelf is characterized by shale-based detachment systems. Where seismic resolution allows, the detachments are observed to overlie a substructure of either horst blocks and compressional toe structures of Early Tertiary to Mesozoic-aged shale-based detachments, or aborted Mesozoic to early Tertiary salt-withdrawal mini-basins.

From updip to downdip, well-developed shale-based detachment systems are observed for the Wilcox, Vicksburg, Frio, Early Miocene, and the Middle Miocene. An associated but discontinuous belt of compressive folds and reverse faults lies along the down-dip end of the Detachment Province along the Texas shelf and upper slope.

The shale based systems terminate downdip but also to the east where original salt thickness was sufficient to maintain long term structural growth. Throughout the Houston Salt Embayment, and much of the Louisiana shelf, are shelf-loaded salt withdrawal mini-basins.

The mini-basins are seen bottoming into complex collapse structures at a regionally continuous zone of seismic reflectors at approximately 9.5 sec (~45,000 ft) that most likely is the autochthonous salt level. The size and the amplitude of the salt bodies associated with the mini-basins increases downdip, reflecting the thickening of the original Luann wedge.

Along the present outer shelf of offshore Louisiana, where the original Louann salt was sufficiently thick, a number of amalgamated tabular salt sheets formed during the Late Miocene along a series of slope-loaded salt withdrawal mini-basins. These were subsequently evacuated by progradation of the Plio-Peistocene shelf edge to form salt-based detachment-fault systems (Roho).

The thin remnant salt separates a hydropressured and mostly Plio-Pleistocene deltaic suprastructure from underlying geopressured slope- loaded mini-basin substructure of Miocene and older sediments. Seismic resolution sub-Roho is generally poor, but mini-basins and salt feeder systems rooting into the 9.5 sec reflector package can occasionally be observed. Within the Roho Province, a continuum from stepped counter-regional faults to Roho can be recognized and is related to original supply of salt.

An extensive salt canopy extends across the region of the present day slope. The canopy is a complicated amalgam of partially obscured primary slope-loaded mini-basins, secondary salt floored basins, and shallow allochthonous salt wings. The canopy is most extensive in the area of the central Sigsbee bulge. The salt wings become generally thinner, and the deep-seated salt windows become more prevalent and extensive both updip and laterally away from the Sigsbee bulge, particularly toward the eastern Gulf.

Flanking and extending wing-like from beneath the deepwater salt canopy are the Oligocene-Miocene folds and thrusts of the Perdido fold belt and the somewhat younger Miocene folds and thrusts of Mississippi Canyon fold belt. These compressive structures are an "edge effect", localized at the down-dip limit of original salt at the transition between attenuated continental and oceanic crust.