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

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

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

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)

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.

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of northern Bolivia Eva-Eva thrust Fatima – Caquiahuaca Thrust Section drawn and balanced using LOCACE by F. Peel/BHP unpublished

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

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

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

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

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

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

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

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

location of Angola examples

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

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

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

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

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

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

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

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

roller coaster movement over a deep ramp

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

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

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

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

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

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

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

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

present day geometry

Lower Kwanza Foldbelt example dip line

Progressive growth of folds Display fixed relative to cover section

Progressive growth of folds Display fixed relative to cover section

Progressive growth of folds Display fixed relative to cover section

Progressive growth of folds Display fixed relative to cover section

Progressive growth of folds Display fixed relative to cover section

Progressive growth of folds Display fixed relative to cover section

Lower Kwanza Foldbelt example dip line

sequential flattening

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.

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

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

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

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

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

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

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

Structural evolution of the Western Atwater Fold Belt Deposition on top of Sigsbee salt; downslope glaciation Present day geometry Depth section: v=h ?? ?? K2 (along strike) Mad Dog Frampton

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.

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

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

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

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

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

Structural evolution of the Western Atwater Fold Belt Deposition on top of Sigsbee salt; downslope glaciation Present day geometry Depth section: v=h ?? ?? K2 (along strike) Mad Dog Frampton

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)

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

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

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

Mad Dog present day MCU structure

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

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