1. Styles, Mechanisms and Hydrocarbon Implications of Syndepositional Folds in Deepwater Fold Belts
Examples from Angola and the Gulf of Mexico
Frank Peel, BHP Billiton Petroleum, Houston
with the invaluable assistance of Gulf of Mexico and Angola Joint Venture team members

2. The real subject of the talk
The importance of precursor fold structures:
Why they exist
How they evolve
Their impact on structure, crestal faulting, and reservoir systems

3. What are the significant differences between deepwater toethrust foldbelts and onshore orogenic foldbelts?
Orogenic thrust belts are linked to plate movement. Shortening will occur whether or not there is a good decollement; the nature of the decollement controls the style, NOT the existence, of the fold belt rg
Passive margin fold belts are driven by gravity. Movement occurs only if the geometry is suitable AND there is an excellent decollement – strongly overpressured mud or salt

4. gravity driven fold belts: gravity sliding vs
gravity driven fold belts: gravity sliding vs. gravity spreading above salt
gravity sliding margin:
The sediment sequence will slide downhill as long as the basal slope is maintained. The rate limiting factor is the effective viscosity of the basal layer. Movement should therefore be continuous and fairly steady
e.g. Angola (Kwanza)
early GoM
gravity spreading margin:
The sediment sequence will spread as long as the surface slope is maintained. The rate limiting factor is sedimentation in the shelf and upper slope. Movement should therefore be episodic and tied to sedimentation.
e.g. Miocene GoM (Louisiana)

5. Implications of driving mechanisms
contraction in orogenic foldbelts is fast (~1cm/yr), steady and continuous.
passive margin foldbelts are governed by sedimentation rates and the viscosity of the basal layer. Movement rates are slow (~0.1-1 mm/yr), and may be variable and episodic in gravity spreading margins.

6. Characteristics of orogenic fold belts Frontal structure of a typical orogenic fold/thrust belt: the Subandean belt of northern Bolivia
Eva-Eva thrust
Fatima – Caquiahuaca Thrust
Section drawn and balanced using LOCACE
by F. Peel/BHP unpublished

7. Deformed state and fully restored section
eroded
stratigraphy
Restored section
at same scale
preserved
stratigraphy
High amount of shortening (ca. 70%)
Large volume of sediments eroded
For most of the section, maximum depth of burial occurred before thrusting
The actively shortening area is subject to erosion
50km v=h

8. Frontal structure of an orogenic fold/thrust belt: An example from Pakistan
Molasse
synorogenic sediments R
Even in orogenic fold belts where there is significant synorogenic sedimentation over the foldbelt, there is usually net erosion over the crests of active structures
5km
v=h

9. effect of syntectonic erosion on fold consistency
eroded fold crest remains the weakest link

10. effect of syntectonic erosion on thrust consistency
eroded thrust crest remains the weakest link

11. effect of syntectonic erosion on the consistency of structural style
In a region which is experiencing no deposition, and especially where there is erosion of the crests of rising structures:
thrust and fold anticlines remain thin and weak;
therefore the same folds and thrusts tend to remain active throughout the folding/thrusting episode
This leads to consistency of structural style throughout the folding/thrusting: folds don’t change wavelength, the same thrusts tend to remain active

12. What makes passive-margin toethrust fold belts different from onshore, orogenic fold belts?
abundant sediment supply, usually turbiditic
weak erosion environment
several km of potential accommodation space
slow rate of structure growth

13. example of sedimentation outpacing fold growth (salt-cored fold anticline, offshore Angola)

14. syntectonic deposition over folds
typical passive-margin toethrust foldbelt
typical onshore orogenic foldbelt
This has great significance for deepwater fold belts, which will be developed within this presentation

15. location of Angola examples

16. the Lower Kwanza : a classic gravity sliding margin
Low-dip region
Slope toe
Kwanza slope
Fold belt at slope toe
10km
80km
note: line is vertically exaggerated

17. Low-dip region Slope toe Kwanza slope
note: line is vertically exaggerated

18. Low-dip region Slope toe Kwanza slope
note: line is vertically exaggerated

19. Low-dip region Slope toe Kwanza slope Fold belt at slope toe
note: line is vertically exaggerated

20. Low-dip region Slope toe Kwanza slope Fold belt at slope toe
note: line is vertically exaggerated

21. Low-dip region Slope toe Kwanza slope Fold belt at slope toe
note: line is vertically exaggerated

22. Low-dip region Slope toe Kwanza slope Fold belt at slope toe
note: line is vertically exaggerated

23. 10km 80km Low-dip region Slope toe Kwanza slope Fold belt at slope toe
note: line is vertically exaggerated

24. roller coaster movement over a deep ramp

25. Angola Outer Kwanza Basin gravity sliding and folding
present day depth section

26. reconstruction to v horizon
Low-dip region
Slope toe
Kwanza slope
v=h
no vertical exaggeration

27. reconstruction to s horizon
Low-dip region
Slope toe
Kwanza slope
incipient fold belt
with very short wavelength
v=h
no vertical exaggeration

28. reconstruction to q horizon
Low-dip region
Slope toe
Kwanza slope
Fold belt at slope toe
Formation of early small-wavelength folds
v=h
no vertical exaggeration

29. reconstruction to p horizon
Low-dip region
Slope toe
Kwanza slope
Fold belt at slope toe
Start of formation of longer-wavelength folds
v=h
no vertical exaggeration

30. reconstruction to m horizon
Low-dip region
Slope toe
Kwanza slope
Fold belt at slope toe
amplification of longer-wavelength folds
v=h
no vertical exaggeration

31. reconstruction to f horizon
Low-dip region
Slope toe
Kwanza slope
Fold belt at slope toe
further amplification of longer-wavelength folds
v=h
no vertical exaggeration

32. present day Low-dip region Slope toe Kwanza slope
Fold belt at slope toe
further amplification of longer-wavelength folds –
and possible start of overprinting longest-wavelength folds
v=h
no vertical exaggeration

33. present day geometry

34. Lower Kwanza Foldbelt example dip line

35. Progressive growth of folds Display fixed relative to cover section

36. Progressive growth of folds Display fixed relative to cover section

37. Progressive growth of folds Display fixed relative to cover section

38. Progressive growth of folds Display fixed relative to cover section

39. Progressive growth of folds Display fixed relative to cover section

40. Progressive growth of folds Display fixed relative to cover section

41. Lower Kwanza Foldbelt example dip line

43. sequential flattening

44. When the fold wavelength changes, why does it tend to treble rather than double, as might intuitively seem more likely?
wavelength doubling
initial fold
new fold with double wavelength
Combination fold is mechanically unfeasible
wavelength trebling
initial fold
new fold with three times the wavelength
Combination fold is mechanically easier to form
In general, if folding and deposition are continuous, we expect to see this wavelength multiplying effect.

45. Overlapping fold belts in the US Gulf of Mexico
Mississippi
Canyon
Shelf edge
Green Canyon
Atwater
Valley
Walker Ridge
Lund

46. mapped extent of the Neogene fold belt in offshore Louisiana
“Mississippi Fan Fold Belt”
“Western Atwater Fold Belt”

47. mapped extent of a Paleogene fold belt in offshore Louisiana
“Cascade Fold Belt”

48. Structure map
This map shows the structure of a region SW of the prolific Western Atwater Fold Belt.
The structure of this region appears to be dominated by large salt pillows and withdrawal basins.
Cascade
Chinook

49. Paleostructure map
The paleostructure, revealed by isochron mapping, is radically different. A compressional fold belt of Paleogene age is clearly defined by the isochron thins and thicks.
inflated salt
high
fold belt
anticlines
limit of deep salt

51. Miocene anticlines of the WAFB
Atwater
Valley
Neptune
Green Canyon
Mad Dog
Neptune
Atlantis
Frampton
Cascade pillow
Lund
Miocene anticlines of the WAFB
Walker Ridge

53. Precursor structures of the Cascade Fold Belt
Western Atwater Fold Belt
“Cascade Fold Belt”
Frampton
Precursor structures of the Cascade Fold Belt

55. Structural evolution of the Western Atwater Fold Belt Deposition on top of Sigsbee salt; downslope glaciation
Present day geometry
Depth section: v=h ?? ??
K (along strike)
Mad Dog
Frampton

56. Structural evolution of the Western Atwater Fold Belt Early salt pillowing and precursor structures
short-wavelength fold growth ?? ??
The following slides show the sequential evolution of the WAFB based on the palinspastic restoration of a regional section. This was made on 2D data prior to the drilling of Atlantis and Mad Dog, and the detailed picks and structural history have subsequently been refined. However, it does show the general principles which still seem to be valid.

57. Structural evolution of the Western Atwater Fold Belt Early Miocene: start of main folding phase
short-wavelength fold growth ?? ??
Fold belt at regional
decollement edge
limit of deep salt

58. Structural evolution of the Western Atwater Fold Belt Continued folding
overprinting by longer-wavelength folds ?? ??
Fold belt at regional
decollement edge
limit of deep salt

59. Structural evolution of the Western Atwater Fold Belt Emplacement of Sigsbee salt sheet Final folding
growth of longer-wavelength folds ?? ??
Fold belt at regional
decollement edge
limit of deep salt

60. Structural evolution of the Western Atwater Fold Belt Inflation of Sigsbee salt sheet Folds are no longer growing ?? ??
Fold belt at regional
decollement edge
limit of deep salt

61. Structural evolution of the Western Atwater Fold Belt Inflation of Sigsbee salt sheet Folds are no longer growing ?? ??
Fold belt at regional
decollement edge
limit of deep salt

62. Structural evolution of the Western Atwater Fold Belt Deposition on top of Sigsbee salt; downslope glaciation
Present day geometry
Depth section: v=h ?? ??
K (along strike)
Mad Dog
Frampton

63. Significance of precursor structures 1. Fold shape
anomalously broad crest (e.g. Mad Dog)
Structure shape and evolution
The final shape of a structure is strongly influenced by its evolution. The final fold structure can not be understood without knowing the nature of the precursor structures
anomalously narrow crest (e.g. Frampton)

64. Significance of precursor structures 2. Crestal faulting
Precursor structures are the dominant control on the crestal faulting pattern.
TWT structure
Crestal faulting image
Precursor anticlines

65. Significance of precursor structures 3. Local reservoir distribution
In this example from Angola, it is obvious that a precursor syncline now occupies the crest of the later fold, and that if reservoir were present it would be controlled by the precursor structure. The older and deeper the target, the more this needs to be considered
Mad Dog
In this example from the Gulf of Mexico, we can not image the crest of the field; but we know that there were precursor structures and we should not assume that reservoir distribution is shaped by the later structure
Frampton

66. Conclusions
Deepwater passive margin fold belts are different from onshore orogenic fold belts; we expect there to be deposition over the fold belt as it shortens
Folding of thin sediment sequences creates small-wavelength folds; later folding, involving a thicker sediment pile, favors longer-wavelength folds.
If sedimentation and shortening are continuous, a 3x wavelength switch is favored, but other changes are possible.
The extent of small-wavelength precursor folding is under-recognized, especially in poorly imaged structures (e.g. sub-salt).
The precursor folds control the final structure shape and orientation, which cannot be understood in isolation
Precursor structures have strong influence on reservoir distribution in the older, deeper parts of the structures

67. Mad Dog present day MCU structure

68. Mad Dog Paleogene paleostructure (“MCU” to top Eocene isopach)

69. Mad Dog Cretaceous paleostructure (top deep salt to “MCU” isopach)