Guglielmo, Giovanni, Jr., J. Hossack, M. P. A. Jackson, and B. C. Vendeville, 1995, Animation of allochthonous salt sheets: AGL96-MM-002. BEG hypertext multimedia publication on the Internet at http://www.beg.utexas.edu/indassoc/agl/animations/AGL96-MM-002/index.html.

 

ANIMATION OF ALLOCHTHONOUS SALT SHEETS

G. GUGLIELMO Jr., J. R. HOSSACK*, M. P. A JACKSON, and B. C. VENDEVILLE

Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas 78713

* BP Exploration, Uxbridge, Middlesex UB8 1PD, U.K.

ABSTRACT

An animation of a palinspastic reconstruction of a physical model shows the evolution of complex allochthonous salt sheets during progradation. This 14-stage deformation involves box folding of the overburden; multiple episodes of reactive, active, and passive diapirism; creation and filling of minibasins; multiple salt extrusive breakouts and flows; thrusting; extension; rafting; and salt welding. The original animation lasts 14 seconds and represent 54 hours of the original experiment. Eighteen potential structural traps for hydrocarbons are formed, and 8 are destroyed during deformation.

ANIMATION

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INTRODUCTION

A single allochthonous salt sheet in the Gulf of Mexico or elsewhere may extend up to hundreds of square kilometers sealing and obscuring enormous volumes of subsalt reservoirs. However, whether hydrocarbons are present in these reservoirs depends, in part, on whether or not the evolution of structures before and after emplacement of the sheet concentrates hydrocarbons. For example, a successful trap could form where normal faults or folds create structural highs in permeable sands subsequently sealed by the overlying salt. Conversely, potential traps could loose their value if evacuation of salt, which is required to form the allochthonous sheet, displaces hydrocarbons by (1) juxtaposing initially isolated reservoirs, which creates conduits or (2) changing dips of overlying reservoirs, which changes migration paths of hydrocarbons. A series of static images may, in some cases, be adequate to illustrate individual processes. However, on a regional scale, several such processes occur simultaneously at separate locations and static images are insufficient to show how these processes interact. By providing a seamless continuity of events, the following animation illustrates where, when, and how complex structures related with allochthonous salt sheets could form and interact on a regional scale to create or destroy hydrocarbon traps.

 

DESCRIPTION AND INTERPRETATION

This publication includes an animation of a computer-restored cross section from a scaled physical model illustrating the complex evolution of an allochthonous salt sheet driven by sedimentary progradation. Only the right part of the experiment (yellow rectangle) shown in the opening photograph was restored and animated. Throughout the experiment, synkinematic sediments (sand) prograde from left to right across a flat-topped, horizontal salt basin or allochthonous sheet (pink viscous silicone, animated in black) that pinches out seaward below a thin prekinematic layer (sand, animated in light and dark blue). The prekinematic layer at the beginning of the animation comprises irregular fragments (deformed fault blocks) reassembled into the nearest semblance of its undeformed state as a horizontal, tabular layer.

The resulting deformation can be described in 9 sequential stages. The section "Potential structural traps for hydrocarbons" suggests how structural traps could be created or destroyed at each stage. Percentages of elapsed time listed below show the approximate age and duration of each stage with respect to the entire history. The animation, which was constructed from 14 key restoration frames, lasts 14 seconds and represent 54 hours of the original experiment. 18 potential structural traps for hydrocarbons are formed, and 8 are subsequently breached or drained during deformation.

 Stage 1 (0–12% time elapsed)—Box folding: At the onset of deformation, the weight of overlying prograding sediments (lowermost 4 wedges visible in the opening photo, pale green wedge tip in animation) pressurizes the salt and squeezes it seaward (to the right). The pressurized salt lifts and deforms the prekinematic layer into a seaward-verging bending (not buckling) box fold.

Stage 2 (12–16% time elapsed)—Grabens and reactive diapirs: A foregraben and a backgraben form at the crests of the box fold. These grabens were very clear in top view of the actual experiment but are not readily distinguishable in the restoration because later faulting cut up the roof of the box fold. The grabens are best recognized in the animation by their later effects: the foregraben becomes the site of the first breakout of salt in Stage 4; the backgraben becomes overlain by locally thick, dark-green strata in Stage 3. These grabens locally cause differential loading and structural weakening of the overburden, triggering reactive diapirs below each graben.

Stage 3 (16–22% time elapsed)—Slumped sediments fill local basins: The oversteepened forelimb of the box fold collapses and sheds pale-blue sediments into a local basin at the base of the forelimb.

Stage 4 (22–40% time elapsed)—First breakout of salt: The weight of prograding sediments continuously pressurizes salt in the box fold. The reactive diapir in the foregraben becomes active and emergent. Salt expelled from the foregraben (breakout 1), flows glacially seaward down the resedimented forelimb. Traction exerted by the base of the glacial extrusion folds the partially collapsed forelimb into a small isoclinal, recumbent syncline. The rear diapir (below scale mark 2-6) continues to rise reactively. Extrusive salt spreads radially as lobes in the abyssal plain that resemble piedmont glaciers.

 Stage 5 (40–44% time elapsed)—Canopies: These salt flows coalesce laterally to form a canopy (allochthonous sheet 1) with a scalloped leading margin (not visible in vertical section). The diapir at the backgraben evolves to a passive stage after emerging through the stretched dark-green layer.

Rafts: The roof and backlimb of the box fold are stretched by flow of the underlying salt and segmented into two rafts that migrate seaward as overthrusts carried by the salt sheet. The overthrusting rafts above the isoclinal syncline create a triple stratigraphic repetition of the prekinematic layer.

Stage 6 (47–51% time elapsed)—Salt sheet inflation: Further sedimentation (orange and yellow layers) buries these allochthonous rafts, the diapir at the backgraben, and the salt extrusion. This confinement of allochthonous sheet 1 causes it to inflate actively by continuous salt expulsion from the deflating salt basin and sagging diapir on the left.

 Stage 7 (51–62% time elapsed—Second breakout of salt: Newly deposited yellow sediments define a scarp analogous to the Sigsbee Escarpment. Pressurized salt eventually breaks through the yellow layer (Breakout 2, below scale mark 20), creating a second allochthonous salt layer above this yellow layer. This second breakout causes local folding and slumping downslope from, and next to, the breakout point (not visible at the scale of the animation).

 Stage 8 (62–72% time elapsed)—Segmentation of second flow, listric faults, reactive diapirs, and subsidence of diapir: The second salt flow is completely covered by further sedimentation (yellow). More extension above the flowing sheet causes differential loading and thinning of the yellow overburden. This triggers a large seaward-dipping listric fault (below scale mark 12), and two small reactive diapirs at the seaward edge of the basin, which continue to evolve in Stage 9.

Sedimentary loading and expulsion of salt enhance further subsidence of the rear diapir (below scale mark 7) as salt escapes seaward through breakouts 1 and 2. Both flanks of the rear diapir subside and tilt toward it as the yellow layers locally thicken above its sagging crest. Continuous rotation of the left flank ultimately produces a structure that resembles a landward-dipping (counter-regional) growth fault at the left end of the animation window. However, no slip occurred along this pseudo-fault, which formed entirely by expulsion of salt.

Stage 9 (72–85% time elapsed)—Welding of breakouts: The weight of overlying sediments, combined with overthrusting, join strata at the breakout conduits created in Stages 4 and 7, restricting flow of salt, and eventually producing salt welds. Diapirs rise reactively above the second buried salt sheet. Remaining time (85–100% time elapsed): The animation dissolves back into the photograph of the final cross section.

 

POTENTIAL STRUCTURAL TRAPS FOR HYDROCARBONS

Each stage described above could create or destroy structural traps for hydrocarbons. The following assessment is based on the two-dimensional section and would depend on structural or stratigraphic closure in the third dimension and on the existence of reservoirs.

 Stage 1: Shaly facies of prograding sediments may seal the faulted culmination of the box fold. Local traps for hydrocarbons could form in crestal culminations of this fold.

 Stage 2: The reactive salt wall beneath the backgraben could produce diapir-related structural traps (a) where upraised sediments are truncated against the deepest flanks of the diapir; (b) in graben footwalls around and above the diapir.

 Stage 3 and 4: Hydrocarbons could be trapped at breakout 1 of the pressurized salt. These traps could be (a) within the overturning forelimb of the box fold, and (b) within sediments slumped from the box-fold crest. Both these traps are local structural highs sealed by the overlying salt flow. These salt-breakout traps also form at breakout 2 at the edge of the basin during Stage 7.

 Stage 5 and 6: The salt flow (allochthonous sheet 1) could produce traps by sealing bathimetric highs of underlying prekinematic sands downslope. These broad traps become even larger as flows coalesce into canopies. After the ends of the rafts subside to form extensional turtle structures (when the seaward raft reaches scale mark 20), hydrocarbons could be trapped in their anticlinal culminations. Additionally, prograding muds draping rafts (e.g., below scale mark 13) could provide the necessary seal for a trap. These rafts could form isolated reservoirs.

 Stage 7: As in stages 5 and 6, this second salt sheet could also seal underlying structures creating additional subsalt traps. Salt-breakout traps could form as in Stages 3 and 4: folding and slumping associated with the salt breakout creates a local structural high downdip, which is immediately sealed by the overlying salt flow to form a subsalt trap.

 Stage 8: The two youngest diapirs at the seaward edge of the basin (below scale marks 26 and 31) could produce reactive diapir-related structural traps as described in Stage 2. Also, gradual inward tilting of flanking strata toward the backgraben reactive diapir (below scale mark 7) could redirect hydrocarbons away from the original traps at the overburden/diapir interface. Hydrocarbons would migrate (a) landward, into the broad crest of the pseudo-rollover anticline shown in the opening photograph, and (b) seaward, to be possibly trapped in the footwall of the next seaward-dipping growth fault (below scale mark 12).

Stage 9: Complete evacuation of impermeable salt during welding could juxtapose permeable strata from both sides of the salt layer. This would create windows through which hydrocarbons could drain upward away from the subsalt reservoirs formed at Stages 3, 4, and 7. The latest draping of sediments (light yellow) over the structural high between the larger sagging salt bodies (the backgraben diapir and the first salt flow formed at Stage 5) might create an anticline structural trap and associated onlaps stratigraphic traps (below scale mark 11).

 

ACKNOWLEDGMENTS

Tucker Hentz read the manuscript. The physical model on which this animation was based was designed and run by Bruno Vendeville, with assistance by Martin Jackson, and was originally discussed in the following abstract: Jackson M. P. A. and B. C. Vendeville, 1993, Extreme overthrusting and extension above allochtonous salt sheets emplaced during experimental progradation, (abs): AAPG Annual Convention Program, New Orleans, p. 122–123. This animation has been previously released to the AGL's Industrial Associates and is published by permission of the Director, Bureau of Economic Geology, The University of Texas at Austin.


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