1

12. VERTICAL-DISPLACEMENT STRUCTURES ASSOCIATED WITH SALT AND OVERPRESSURED SHALE

Figure 1. Vertical structural family. Topics Habitat Mechanics Material properties Displacements Small Extension Style Classical Salt Tectonics: Vertical Evolution Flank traps Crestal traps Restoration and restoration pitfalls Salt Sheets Pseudoextension Roho fault system Stepped counter regional fault system Big Extension Style

2

Habitat Salt deposits are typically formed in the early stages of rifting. Salt and overpressured shale structures are predominately located on passive continental margins (figs. 2-3) with some examples located elsewhere including on the craton.

Figure 2. Passive plate margins.

Figure 3. Major salt deposits of the world (Martinez, 1974).

3

Mechanics Material properties The key factors that cause salt and overpressured shale tectonics to be very different from other structural styles is that both salt and overpressured shale are very deformable, buoyant relative to lithified sediments, and able to flow under the differential stress caused by differential gravitational loading (fig. 4).

a b

c Figure 4. Key mechanical factors influencing deformation of salt and overpressured shale. . a Soft and ductile material deforms easily. b. Buoyant oil rising into more dense syrup (Ramberg, 1981). c. Stress caused by differential load. Displacements The displacements leading to the deformation of salt, overpressured shale, and their cover sequences range from extension to vertical, to compressional. Here we are concerned with primarily with vertical displacements, but certain extensional styles are covered too. Flow of salt or shale toward a round or elliptical region produces vertical movement in the cover, resulting in a dome or diapir. Movement within a diapir is vertical. By definition, a diapir pierces its overburden. Piercement is defined as a high angle contact between the core material and the country rock. An active diapir forms by arching and fracturing the cover sequence (fig. 5). Either a full graben or a half graben above the dome can accommodate the extension.

4

a 1 unit

1

2

3

R

0%2.6%

b 1 unit

1

2

3

R

4.4%1.8%

Figure 5. Alternative models for the deformation in the arch above a hemispherical intrusion. Bedding is approximately constant in thickness on the flanks of the uplift. Layer-parallel requisite extensional strains are shown. a. Full graben. b. Half graben

Stratigraphic units that intersect the flank of a diapir without evidence of uplift (no increase in dip adjacent to the diapir) imply passive growth. In a passive diapir it is inferred that the top of the diapir remained at or close to the depositional interface while sediments were deposited around it. Much of the history of tall diapers appears to be passive growth (fig. 6).

Figure 6. Stratigraphic relationships showing a dome beginning as active/reactive, then becoming passive, then active at the last stage, schematic after Belle Isle dome, Louisiana (O.Neill, 1973, in Yin, 2003).

5

Pseudoextension produces a normal fault by rotation of the hangingwall and lateral displacement of the ductile substrate. Although a major normal fault is produced, there is no net horizontal displacement of the hangingwall. Steep dips in the hangingwall imply material transfer from below the subsiding half graben into an adjacent uplift (fig. 7).

Figure 7. Large rotations associated with lateral movement of shale (Dailly, 1976). A reactive diapir forms as the isostatic response to extension of the cover (fig. 8). The identifying characteristic is subsidence of the cover sequence adjacent to the diapir. Reactive diapirs formed in response to regional extension should be linear, not round. Round active or passive diapirs might be triggered by extension, however (Vendeville and Jackson, 1992). Structures produced by large regional extension involving salt or shale will be treated in this chapter because the structural styles in the overburden are more like vertical salt tectonics than the thin-skinned extension treated in Chapter 11.

Figure 8. Reactive diapir formed in response to horizontal extension (Nilsen et al., 1996).

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Small Extension Style The continental margin sequence above a salt or overpressured shale layer may glide or spread toward deep water, producing a variety of structures (fig. 9). Commonest are down to basin faults with salt-cored footwall uplifts known as rollers (fig. 10). A primary salt weld represents a horizon where the autochthonous salt is completely gone. The North Choctaw Ridge oil field, formed near the updip limit of salt in the Gulf of Mexico, is an example (fig. 11).

Figure 9. Structures related to slip on autochthonous salt (Rowan et al., 1999).

Figure 10. Salt rollers on autochthonous salt (Rowan, 1999, after Bally, 1981).

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Figure. 11. North Choctaw Ridge oil field, Alabama, USA (Qi et al, 1998). This field has produced about 5 million barrels of oil.

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Classical Salt Tectonics: Vertical The Paradox basin of the western U.S. provides an example of the early development of active piercement structures (figs. 12-13). The salt first migrates into anticlines which are then pierced at a few locations along their lengths to form domes. The termination of younger units at the diapir (fig. 13) indicates passive growth. Vertical salt movement is likely to begin where the salt is thickest, such as over the edge of the deep part of a half graben (fig. 13), a correlation seen elsewhere (e.g. Jenyon, 1986; Krzywiec, 2004).

a b Figure 12. Paradox basin, western U.S. a. Sinbad Valley, a salt piercement structure. b. Tectonic map of the salt structures in the basin (Elston and Shoemaker, 1963).

Figure 13. Sequential evolution of salt anticlines of the Paradox Basin (Elston and Shoemaker, 1963).

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The arching and extension of the cover make the Chinguetti dome an example of an active diapir (fig. 14). The turbidite reservoir unit is not deflected around the dome (fig. 14a), indicating that doming occurred after deposition.

a

b Figure. 14. Chinguetti field, offshore Mauritania, west Africa (Vear, 2005). The reserves are estimated to be 120 million barrels of oil. a. RMS amplitude extraction highlighting the turbidite sandstone reservoir facies. b. Profile along line A-A’, optical stack of 20 traces. Arrows indicate oil-water contacts.

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Evolution The structure evolves from a flat-lying bed into a pillow or anticline as salt/shale moves laterally. Stratigraphic thinning occurs over the crest and thickening into the area from which the salt has been evacuated to form the primary rim syncline. When enough material accumulates in the pillow or anticline, the overburden breaks and a diapir grows upward. The pillow/anticline collapses and the sediment fill forms the secondary rim syncline. The timing of dome growth is given by the relative ages of the primary and secondary rim synclines (figs. 15-17).

Figure 15. Sequential evolution of an active salt piercement structure (Sannemann, 1968)

Figure 16. Relative timing of dome growth dated by superposition of primary and secondary rim synclines (Sannemann, 1968).

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Figure 17. Relative ages of dome growth (Sannemann, 1968). The evolution from pillow to diapir may include erosional unconformities (fig. 18-19).

Figure 18. Seismic line across a North Sea salt dome with erosion in the pillow stage (Jenyon, 1986).

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Figure 19. Evolution of the salt dome in previous figure (Jenyon, 1986). Progressive salt withdrawal in multiple directions from beneath a primary rim syncline leads to the formation of a turtle structure (fig. 20). The turtle is a depositional syncline that is inverted to form an anticline after the salt has migrated away. The Bryan field is a turtle anticline oil field (fig. 21).

Figure 20. Formation of a turtle structure (Woodbury et al., 1980).

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a

b Figure. 21. Bryan field turtle structure, Mississippi interior salt basin, USA. Approximately 25 million barrels of oil have been produced from Cretaceous reservoirs. a. Schematic cross section (Oxley and Herlihy, 1972). b. Structure contours on top Sligo reservoir (Davis and Lambert, 1963).

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Flank Traps Beds close to the salt margin are usually poorly imaged on seismic reflection profiles (fig. 22), but abundant well data and some outcrop data are available to illustrate the structures. Many traps relate to passive growth phases (fig. 23a), but very steep dips suggest episodes of active diapirism (figs. 23b-24). Many stratigraphic traps are found on the flanks of diapers (fig. 25a). The ultimate piercement leaves the salt detached from the source bed (fig. 25b). The stage of evolution attained depends on the amount of salt present (fig. 26). Shale can produce all the same structures (fig. 27).

Figure 22. Piercement diapir, characteristically poorly imaged on seismic (Rowan, 1995).

a b Figure 23. Oil traps on salt piercements (Halbouty, 1967). a. Passive piercement, Cote Blanche dome, Louisiana. 138 million barrels of oil ultimate recovery, 10 bcf gas reserves. b. Active piercement, Nash dome, Texas. 3.3 million barrels oil from 4 separate traps.

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Figure 24. 2-D seismic line across flank of diapir in Precaspian Basin, Kazakhstan (Rowan et al., 2003).

a b Figure 25. Salt piercement structures (Halbouty, 1967). a. South Liberty dome, Texas. 88 million barrels of oil, 1.2 million barrels condensate and 32 tcf gas from 5 reservoirs. b. Eilte dome,Germany.

Figure 26. Salt geometry related to original salt thickness (Trusheim, 1960).

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Figure. 27. Regional dip line across northern Port Isabel fold belt trend, western Gulf of Mexico (Camerlo et al., 2005).The Anahuac diapiric shale is shaded. The shale is not overpressured.

Crestal traps

The most obvious salt-dome trap is the dome itself (fig. 28). The structure of a dome being actively pierced usually includes numerous faults (fig. 30) which are difficult to image on seismic (fig. 31) and difficult to correlate between wells.

Figure 28. Dome above a salt diapir, Scoter field, UK North Sea (Hempton et al., 2005). The field is estimated to contain 200 bcf gas.

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Figure. 29. Reitbrook dome, Germany Schmitz and Flixeder, 1993).

Figure 30. Crestal area of diaper (Rowan et al., 2003).

Because of the three-dimensional complexity of the fault pattern over an active dome, it is important to recognize what fault systems are kinematically possible. Based on a review of low-relief domes over piercement diapers, it appears that the the initial fault pattern is either two or three crossing normal faults (figs. 31-32). The faults may either cut the top of salt or detach at that level (figs. 31-32). The styles are controlled by these two factors and the direction of the cross section with respect to the master faults (figs. 33-36).

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Figure 31. Characteristic features of circular domes (Yin and Groshong, 2007). a. X pattern, West Clara field dome, Mississippi (after Davis and Lambert, 1964). b. Y pattern, Clay Creek dome, Texas (after McDowell, 1951). c. Smooth salt top, Clay Creek dome, Texas (after McDowell, 1951). d. Faulted salt top, Heide salt dome, northern Germany (after Boigk, 1981).

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c Figure 32. Displacement on X and Y pattern faults (Yin and Groshong, 2003; 2007). a, b. Faults cut top of salt. c. Strained zone required if faults detach on top salt.

Figure 33. Map view of Y-pattern faults (Yin and Groshong, 2007). a. Faults cut top salt. b. Faults detach on top salt.

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Figure 34. Style as a function of direction where faults cut salt (Yin and Groshong, 2007). a-c. Cross sections parallel to a master fault. d-g. Cross sections perpendicular to a master fault.

Figure 35. Style as a function of direction where faults detach on top salt (Yin and Groshong, 2007). h-j. Cross sections parallel to a master fault. k-m. Cross sections perpendicular to a master fault.

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a

b Figure 36. SW-NE profile across South Pierce dome, Central Graben, North Sea (Davison et al., 2000). A map is in the following figure. The cross section style is a half graben although the fault pattern is radial, reminiscent of figs. 33a and 34e,e). a. Cross section adjusted to V.E. ~1:1. b. Seismic profile on same line as cross section.

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Figure. 37. Structure contour map on top of Paleocene reservoir, Pierce field, Central Graben, North Sea (Dennis et al., 2005). This field has reserves of 40 million barrels of oil and 202 bcf gas. The oil-water contact is tilted because of lateral pressure gradients in the aquifer.

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Restoration and restoration pitfalls Restorations of salt-related tectonics are frequently done using the constant bed length, flexural-slip technique (fig. 38). The methodology must be modified to retain the variable bed thicknesses attributed to growth during deposition (Groshong, 2006). Alternative methods are vertical or oblique simple shear. Any of these methods is doubtless just an approximation to the actual kinematics. Here we examine the quality of and pitfalls in the approximation. For deformation that is vertical at a scale greater than an individual dome, a vertical loose line off structure should restore to vertical. The good fit of the fault blocks but somewhat irregular loose line in fig. 38 indicate that the cross section is valid and the irregularities are probably the result of ignoring sub-resolution length and thickness changes. The restoration demonstrates active piercement beneath a substantial thickness of overburden.

Figure. 38. Flexural slip restoration of the Pool Creek salt dome, Mississippi (Jerry Bowling, 1998). A potential pitfall is doing a 2-D restoration of a 3-D deformation. This problem is examined by restoring constant-thickness piercement models with straight-line flexural slip (fig. 39). The biggest pitfall is apparent thickness changes caused by the dip of fault blocks being oblique to the line of section. This causes apparent growth in units that have constant thickness. The exceptionally thin block (fig. 39, profile d) is the result of sub-

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resolution 3-D strain in the central graben. When the apparent thicknesses are corrected for block dip using equation 1, the apparent growth disappears (fig. 40b).

t = tvcos δ, (1) where t = true thickness, tv = apparent thickness, δ = dip of bedding.

Figure 39. Flexural slip restoration of profiles b-d across the dome model in fig. 33a (Yin and Groshong, 2007) The second restoration pitfall is apparent length changes caused by out-of-plane displacements. When the pin lines are placed in their correct positions, significant gaps occur across the faults (fig. 40c). This is the result of the central fault block having a wedge shape in map view and having moved across the plane of the section. This is a general pitfall in the restoration of sections, restoring across a fault with displacement oblique to the line of section. The effect on a piercement dome is to make it falsely appear that shortening has occurred. The problem can also occur in map restoration (fig. 41). Vertical simple shear leaves wide gaps at the faults, again falsely implying shortening. The best restoration algorithm in 3-DMove (Midland Valley) is flexural slip of each block separately around their individual centerlines.

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Figure 40. Flexural-slip restorations of profile b across the dome model in fig. 33a (modified from Yin and Groshong, 2006). a. Uncorrected profile. b. Thickness corrected using eq. 1. c. Length corrected by moving pin line to original location, leaving gaps across the faults.

Figure 41. Map restoration (Yin and Groshong, 2006). a. Original 3-d model. b. Correct restoration without any length changes. c. Vertical simple shear restoration leaves large gaps across faults. d. Flexural-slip restoration parallel to the center line of each block separately.

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Salt Sheets If enough salt is present a salt diaper can extrude as glacier or as a very shallow sill (figs. 42-43). If enough salt is present it can spread out to form very large sheets under little or no overburden (fig. 43). Sheets can coalesce into salt canopies which appear to be new salt beds.

Figure 42. Centrifuge experiment with silicone putty diapir intruding overburden of painter’s putty (Ramberg, 1981). This diapir is driven by density inversion.

Figure 43. 3-D seismic evidence of a salt glacier having flowed out of diapir exposed during the Triassic in northern Germany (Mohr et al., 2007). a. Variance map 50 m below unconformity at base of the Cretaceous displays uppermost salt extrusion as fan-like structure inside the circle. B: Geological interpretation of A showing geometry of Late Triassic salt glacier. C, D: Enlarged seismic sections (2.5 ×) showing salt glacier generations and salt diapir, and accompanying sedimentary layers (location in A). Vertical dotted lines mark intersection of the two seismic profiles. Trace of well A is shown by dotted line in southern part of section D.

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Figure 44. Formation of allochthonous salt sheets from diapiric feeders (Hudec and Jackson, 2006). Detached salt sheets were in the past thought to represent either episodes of salt deposition or the tops of huge salt massifs. Until fairly recently the seismic penetration was poor and the deep structure was unknown. Modern seismic reveals sediments below the salt, creating a major new play (figs. 45-47).

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a b Figure 45. Location of Mitchell dome salt sheet in 4,350 ft of water (Shirley, 1991). a. Location of Mississippi Canyon Block 211. b. Surface topography.

Figure 46. Mitchell dome salt sheet (Simmons, 1991). The discovery well penetrated 3000 ft of salt. The trap is estimated to contain 100-200 million barrels of oil equivalent.

29

a

b Figure 47. The Princess field, a sub-salt discovery below allochthonous salt in the northern Gulf of Mexico (Bouma et al., 2006). a. Location map. b. 3D pre-stack depth migration done after the discovery based on a single 2-D line. This field is estimated to contain 175 million barrels of recoverable oil.

30

Salt sheets become the locus of a new round of salt tectonics. Sediments deposited over the top displace salt and subside to form mini-basins (figs. 48-49).

Figure 48. Salt nappe and developing minibasins (Rowan et al., 1999).

Figure 49. Sea floor topography in northern Gulf of Mexico minibasin province (Diegel et al., 1995).

31

Figure 50. Perched minibasin beginning to subside into salt (green) (Diegel et al., 1995). The minibasins move apart and are separated by normal faults (fig. 48). The salt uplift in the zone of extension (fig. 51) is a reactive diapir, being entirely the isostatic response to extension. The reactive nature of the diaper is indicated by a sag over the top, not an uplift as seen in an active diapir.

Figure 51. Dip section across fault zone between minibasins (Rowan et al., 1999). Subsidence above diapir indicates reactive origin.

32

Pseudoextension Pseudoextension is a term applied here to normal faults that form by rotation with little or no overall extension. The process was nicely described by Dailly (fig. 52) although he did not use this term. More recently the process has been recognized as being widespread and as applying to both salt and shale substrates.

Figure 52. Progradational sand wedge deposited on undercompacted shale leads to rotational subsidence of the sand and vertical displacement of the shale (Dailly, 1976). The major pseudoextensional fault systems described here develop from allochthonous sheets of salt or shale (fig. 53a). A Roho, named for C.C. Roripaugh’s moho (Schuster, 1995), is a thin band of high-amplitude discontinuous reflectors that form the base of most of the faults and allochthonous salt features. In the Gulf of Mexico, a roho fault system (fig. 53b) is a major, listric, down-to-basin growth fault that soles into an intra-Tertiary salt evacuation surface (known as a tertiaty salt weld). A stepped counter-regional fault system (fig. 53c) consists of major, listric, up-to-basin growth “faults” that sole into salt evacuation surfaces (welds).

33

Figure 53. Major fault styles related to allochthonous salt (Schuster, 1995). a. Allochthonous salt tablet. b. Roho system. c. Stepped counter-regional system.

Roho fault system A roho system is, in effect, a large gravity glide toward deep water, accompanied by the evacuation of salt/sale from the glide horizon (figs. 54-59). The amount of strike slip along the margin of the area (fig. 58) must be small because the horizontal displacement of the cover is small.

Figure 54. Roho fault system (Rowan, 1999).

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Figure 55. Map of a roho system (Schuster, 1995). Salt is green, evacuated salt is tan, inferred salt feeders are red arrows, major growth faults are blue.

Figure 56. Dip section across roho system, location on previous figure (Schuster, 1995, fig. 5). Diagonal lines are salt, cross hatching is lower Pliocene.

35

Figure 57. Strike section across roho system, location on fig. 55 (Schuster, 1995, fig. 6). Diagonal lines are salt, cross hatching is lower Pliocene.

36

Figure 58. Strike-slip fault along margin of roho block (Rowan et al., 1999).

FF = fault family

37

Figure 59. Evolution of the roho cross section shown in fig. 56 (Schuster, 1995).

38

Stepped counter-regional fault system Major counter-regional fault systems (fig. 60) are the result of the evacuation of salt sills that cut obliquely up section in the downslope direction (figs. 61-62). Remnant salt along the ramp sections of the welds can form diapirs (figs. 63-67).

Figure 60. Structures associated with a stepped counter-regional fault system (Rowan et al., 1999).

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Figure 61. Counter-regional fault system between salt diapers (Rowan et al., 1999, his fig. 20 in next fig.). Time slice in next figure.

Figure 62. Time slice at 2.8 seconds across counter-regional fault system shown in previous figure (Rowan et al., 1999).

40

Figure 63. Salt domes of southeast Louisiana (Schuster, 1995).

Figure 64. Profile A, fig. 63. Seismic dip section across Bourbon dome, Louisiana (Schuster, 1995).

41

Figure 65. Profile B, fig. 63. Seismic dip section across counter-regional fault just west of Bourbon dome (Schuster, 1995).

Figure 66. Evolution of Bay Marchand-Terrebonne-Bourbon stepped counter-regional fault systems (Schuster, 1995). Profile B, figure 63.

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Figure 67. Summary of evolution of stepped counter-regional fault system (Rowan et al., 1995). An extremely large counter-regional fault system is developed on the Atlantic margin of Brazil (fig. 68-70).

Figure 68. Seismic profile across Brazilian continental margin south of Rio De Janeiro (Mohriak et al., 1995).

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Figure 69. Sechematic depth interpretation of seismic profile in previous figure (Mohriak et al., 1995). Apparent downlap caused by progressive salt evacuation.

Figure 70. Apparent extensional structure caused by lateral movement of salt and subsidence of overburden (Mohriak et al., 1995).

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Big Extension Large horizontal movement of the cover coupled to large vertical movemnets of a ductile substrate is what is here referred to as “big extension” in the context of vertical tectonics. The type example is the rafts on the Angola continental margin (figs. 71-73). Jackson (fig. 74). In such a feature, the lower unit is significantly younger than the stratigraphic age of the salt. The Lundin model (fig. 72) indicated that the faults bounding the two sides of the mock turtle are of different ages whereas the model of Jackson (fig. 74) has them the same age.

Figure 71. Progressive basinward graben development between large undeformed rafts (horsts) of the pre-tectonic cover sequence (Lundin, 1992).

Figure 72. Generalized cross section of salt withdrawal basin in the Gaivota Graben, Angola (Lundin, 1992). A = pre-salt sequence, B = raft sequence, C = growth sequence, C1 = core, C2 = landward-expanding sequence, C3 = seaward-expanding sequence.

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Figure 73. Seismic time section in the raft domain of the Kwanza Basin, Angola (Hudec and Jackson, 2004).

Figure 74. The rise and fall of diapiric structures during extension (Jackson, 1995).

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