Rising and subsiding diapirs

Guglielmo, G., Jr., Vendeville, B. C., and Jackson, M. P. A., 1998, Animation of rising and falling salt diapirs: A BEG hypertext multimedia publication on the Internet at: https://www.beg.utexas.edu/agl/animations/AGL98-MM-005.

ANIMATION OF RISING AND FALLING SALT DIAPIRS

GUGLIELMO, G., Jr., VENDEVILLE, B. C., and JACKSON, M. P. A.

Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas 78713-8924 USA

ABSTRACT

An animation showing a palinspastic reconstruction of a physical model illustrates the evolution of rising and sagging diapirs. The deformation includes domino-style faulting; reactive-, active-, passive-, and subsiding diapirs; and salt welding. Analogous structures are found in hydrocarbon-bearing basins in both sides of the South Atlantic, Gulf of Mexico, and Red Sea, among other areas.

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ANIMATION

INTRODUCTION

Allochthonous salt sheets can extend for hundreds of square miles down the continental slope. These sheets spread under their own weight and under the weight of overlying sedimentary layers, which glide down slope carried by the underlying mobile salt. Regional extension creates grabens in the overburden, which trigger diapiric walls that rise and eventually sag. The animation illustrates the sequence in which these structures form, which controls the timing of migration and entrapment of potential hydrocarbons in the overburden. See Guglielmo et al (1999) for 3-D visualization and interpretation, scroll down on this page.

DESCRIPTION AND INTERPRETATION

The animation above shows a computer-restored section from a scaled physical model that illustrates the rising and subsequent subsidence of salt diapirs. The model (Vendeville and Jackson, 1992), which introduced the process of diapiric fall induced by extension , was restored using Restore® by Schultz-Ela, (1992). The same restoration, combined with photographs of a section of the model, forms the basis for this animation.

Initially, a prekinematic layer (sand, shown in blue, white, and black layers at A) overlies a flat subhorizontal salt basin or sheet (viscous silicone shown in black at B). The model is then tilted 2 degrees to the right to simulate regional extension by gravity spreading and gravity gliding. Flat synkinematic layers of sediments (gray, red, and white sand at C) are then added throughout extension. The section "Potential structural traps for hydrocarbons" suggests how structural traps could be created or destroyed. The animation lasts 16 s. Percentages of elapsed time listed below show the approximate age and duration of each stage with respect to the entire history. Deformation can be described in 4 sequential stages:

Stage 1 (0–25% time elapsed) - Onset of gravity gliding and spreading; Reactive diapirs: At the onset of deformation, extension produces brittle failure of the overburden. Fault blocks rotate counterclockwise in "domino style" causing layers within each fault block dip to the left, that is, to the opposite direction of regional extension. Reactive diapirs begin to rise at block sutures where the overburden is the thinnest and weakest. Dragging locally changes the dip direction of strata that is next to, and stratigraphically below the crest of, each diapir

Stage 2 (25–60% time elapsed) - Active diapir rise: At more advanced stages of extension, the roof of each diapir becomes thin enough to cause active rise of the underlying diapir. Each diapir eventually pierces its roof, asymmetrically splitting each original fault block into two: a small block (triangular in cross section at D) to the left and a larger block (E) to the right of the diapir.

Stage 3 (60–95% time elapsed) - Diapir sagging: When the diapirs nearly emerge, synkinematic sedimentary layers are progressively deposited (dark gray, red, and white sand in that order). Diapirs start to subside because regional extension continuously widens the diapirs. Salt could not be imported rapidly enough from the depleted source layer to supply salt to the widening diapirs. Thus, these diapirs began to sag. Diapir subsidence creates horn-like prominences in diapir crest that, in seismic images, could be misinterpreted as injections of salt (F). Depocenters (G, H) form at the roof of each diapir: A left-dipping growth fault form above the diapir on the left and a graben from above the diapir on the right. Layers within these depocenters dip regionally to the right, opposite to the dip direction of the prekinematic layers. These depocenters flank a central horst (I).

Stage 4 (95–100% time elapsed) - Salt welding: Salt supply diminishes to the point that prekinematic sediments weld against the basement (J). Contact with the basement rotates both prekinematic and synkinematic layers within the central horst clockwise.

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: As fault blocks rotate counterclockwise, their elevated edges could be brought above the "Carbon Compensation Depth", where carbonate could precipitate and form reservoirs. This rotation could also cause most of the hydrocarbons within the fault blocks to migrate to the right towards the elevated edge of each block. These hydrocarbons could be trapped (1) at the carbonate reservoirs or (2) in the corner formed by the left side of the right-dipping fault and left-dipping impermeable strata. At stratigraphic levels lower than the crest of each diapir and downslope from the diapir, minor amounts of hydrocarbons could migrate to the left to be trapped between the diapir and locally "bent-upwards" impermeable strata.

Stage 2: The roof of each diapir would be hydrocarbon-free during piercement because most hydrocarbons would have migrated to the right during stage 1. Thus, the triangular region between each diapir and the next major faults to the right would more likely be hydrocarbon-free.

Stage 3: Within the graben above the diapir in the right, hydrocarbons could migrate from the roof of the diapir to be trapped against sealing faults bounding the graben. Within the depocenter formed by the growth fault above the diapir in the left, hydrocarbons could migrate to the left to be trapped by the next normal fault (beyond the frame of the animation). Traps under the overhangs formed by the horns of the diapir in the right would not contain hydrocarbons because reservoirs on both sides of this diapir were drained during stage 1.

Stage 4: Salt welds could juxtapose permeable layers above and below the salt layer creating conduits that could drain subsalt hydrocarbons to replenish the reservoirs in the prekinematic layers above. Clockwise rotation of the central horst tilts its horizontal synkinematic layers to the right and would significantly displace hydrocarbons. This Rotation also affects the prekinematic layers within the horst but, because these layers already dip heavily to the right, the additional rotation is less likely to have a significant impact on oil migration.

Samples of visualization and interpretation of rising and falling salt diapirs.

Hot colors represent structural highs.

Figure 1 — Structure contours on top of the salt. Regional extension produced listric faults that triggered the rise of continuous reactive salt walls (A and B) and intervening salt-wall relay systems. Active diapirs emerged and evolved into flat-topped passive diapirs that were later buried. Extensional rejuvenation then produced a central spinelike reactive crest (B and Figure 4). Diapir sagging and indentation of crestal grabens into the salt produced a double-crested salt wall (B), where diapirs widened faster than they could import salt from their depleted source layer.

Figure 2 — Hydrocarbon migration patterns. Early regional extension produces rotational faulting in prekinematic blocks so that the landward edges of each block (purple) subside the most into the salt. At this stage, hydrocarbon migration (white arrows) tends to be seaward (to the right).

Figure 3 — Late diapir sagging and grounding of the underlying fault block reverses the rotation and causes the seaward edge (A) of synkinematic fault blocks to subside. Regional hydrocarbon migration (black arrows) tends to be landward, opposite to that in the prekinematic layers (Figure 2). Farther landward, extension was much less and created several en-echelon grabens. There, within the soft-linked zone of relay ramps (B), subsidence was minimal because total regional extension was distributed among more grabens than elsewhere (C and D). Within each graben, hydrocarbons would migrate up the relay ramps (blue arrows). This local migration would be orthogonal to regional migration elsewhere in prekinematic or synkinematic strata.

Figure 4 — Early extension creates faults (A) flanking the base of a reactive salt wall. The crest of this wall was flat (yellow) because the diapir emerged and became passive before being buried by the blue-gray layer. Later extension created a graben (B) in the new sedimentary roof, which triggered rise of a narrow second-generation reactive diapir (red) above the broad crest of the passive diapiric wall.

Figures 5 and 6 — In a soft-linked zone, gray salt walls enhance deformation of older prekinematic layers (brown, green, and blue). The map shows the structure of the brown layer, which pinched out (B) as an apparent downlap against the underlying green layer. Horsts (bounded by faults C) warped along strike to form an anticline (most apparent in the oblique view). Arrows show potential hydrocarbon migration paths toward the anticlinal culmination of this fault-bounded turtle structure

REFERENCES

Guglielmo, G., Jr., Vendeville, B. C., and Jackson, M. P. A., 1999, Isochores and 3-D visualizations of rising and falling slat [sic] diapirs: Marine and Petroleum Geology, v. 16, p. 849–861.

Schultz-Ela, D., 1992, Restoration of cross-sections to constrain deformation processes of extensional terranes: Marine and Petroleum Geology, v. 9, p. 372–388.

Vendeville, B. C., and Jackson, M. P. A., 1992, The fall of diapirs during thin-skinned extension: Marine and Petroleum Geology, v. 9, p. 354–371.

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ACKNOWLEDGMENTS

This animation was released to the AGL's Consortium in 1996. Static images were released in 1990. This article is published by permission of the Director, Bureau of Economic Geology, The University of Texas at Austin.