Animations of Salt Diapirs - Part I

Guglielmo, G., Jr., Vendeville, B. C., Schultz-Ela, D. D., and Jackson, M. P. A., 1995, Animations of salt diapirs, Part I: AGL95-MM-001. A BEG hypertext multimedia publication in the Internet at: https://www.beg.utexas.edu/agl/animations/AGL95-MM-001.

ANIMATIONS OF SALT DIAPIRS - Part I

GUGLIELMO, G., Jr., VENDEVILLE, B. C., SCHULTZ-ELA, D. D., and JACKSON, M. P. A.

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

ABSTRACT

This publication includes three animations: restored photographs of a physical model, restored drawings of a physical model, and a finite-element numerical model. These animations illustrate the following aspects of salt tectonics: (1) evolution of a symmetric diapir during regional extension, showing graben formation, reactive diapirism, and active diapirism (2) decoupled extension above and below an initially tabular salt layer, showing how complete separate grabens can open in the basement and in the cover triggering diapirs, eventually increasing coupling (3) asymmetric active diapirism. Sections on "Potential structural traps for hydrocarbons" suggest how structural traps could be created or destroyed at each stage of these animations.

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INTRODUCTION

In contrast to most brittle rocks in sedimentary basins, salt behaves as a viscous fluid and so can (1) deform into discordant bodies, (2) lubricate and fill in gaps between adjacent fault blocks, (3) flow at low pressure and temperature conditions, and (4) drag adjacent strata. Besides being triggered and controlled by tectonic deformation, these salt-related processes may become self-driven owing to buoyancy forces. Consequently, initially horizontal salt layers in a sedimentary sequence can produce structures that follow complex deformation paths through time. For example, a salt layer can continuously or episodically evolve into irregular allochthonous salt sheets, salt pillows, diapirs, walls with overhangs, etc. Accurate visualization and interpretation of how these salt-related structures develop through time are complex but are motivated by economic implications: salt structures are highly impermeable, so their irregular shapes can (1) control fluid migration paths in permeable adjacent rocks, and (2) combine with sealing faults and shales to form traps for hydrocarbons. Animation is a particularly effective way to study these time-dependent processes. Animations of computer models, physical models, and restorations provide a seamless continuity of events, allowing us to study how movements in one part of a structure are linked to those in another. This publication includes three animations that illustrate the following aspects of salt tectonics: (1) evolution of a symmetric diapir during regional extension; (2) decoupled extension above and below an initially tabular salt layer; and (3) asymmetric active diapirism.

SYMMETRIC DIAPIR

This animation illustrates the initiation and growth of a symmetric salt diapir during thin-skinned extension. This animation was constructed from photographs of four vertical sections cut at the end of four different experiments. Each experiment had a similar initial set-up but was stopped after a different duration. Structural evolution was reproduced consistently in all four experiments. Brittle overburden (red and white layers of sand) overlies viscous salt (black silicone). Initially, all layers were tabular and horizontal. The uppermost massive white layer was added after deformation ended in each model, so it is equivalent to air. Extension rate is one centimeter per hour. Animation time is scaled so that one hour of actual deformation (orange time bar) corresponds to two seconds of animation. Six and a half hours of the experiment are shown in 13 seconds of animation. The resulting deformation can be described in four sequential stages:

Stage 1 (0 to 2 hours): Graben formation: Thin-skinned extension creates a graben in the overburden. The footwall flexes slightly upward to reequilibrate isostatically with its thinned hangingwall. Fault blocks stop sinking as they reach isostatic equilibrium with the salt below.

Stage 2 (2 to 5 hours): Reactive diapirism: A diapir rises at the center of the graben in reaction to extensional thinning and weakening of the overburden. New faults progressively propagate toward the center of the graben above the diapir crest. Room for the reactive diapir was created by extension of the overburden.

Stage 3 (5 to 6.0 hours): Active diapirism: As the overburden becomes thin enough, the diapir rises actively through its roof by buoyancy forces. Shear along the diapiric contact brings faults and beds into parallelism with the boundary of the diapir. Flexure of strata makes most of the room for the diapir and creates an anticline above the crest of the diapir.

Stage 4 (6 to 6.5 hours): Onset of passive diapirism: The diapir emerges at the surface. During passive diapirism there is no room problem because flanking strata are not displaced. Diapir rising causes only minor dragging of strata but no further faulting or folding.

POTENTIAL STRUCTURAL TRAPS FOR HYDROCARBONS

Stage 1: The upward flexing of the footwall may cause broad structural highs forcing hydrocarbon migration along strata toward the graben. These hydrocarbons could be trapped within reservoirs sealed by faults of the graben.

Stage 2: The number of sealing faults increases and compartmentalizes any reservoirs at the flanks crest of the diapir. Hydrocarbons could be trapped by these new faults within the graben and against the flanks of the diapir.

Stage 3: Hydrocarbons could be trapped against the flanks of the diapir and in the anticline above the active diapir.

Stage 4: Anticlinal hydrocarbon reservoirs at the crest of the diapir could be pierced and breached.

COUPLING ABOVE AND BELOW A SALT LAYER

This animation illustrates decoupled extension above and below a salt layer. The animation is made of schematic tracings representing the reconstruction backward in time of a vertical section of a physical model. Basement (sand, animated in brown), salt (silicone, black), and overburden (sand, yellow) forms a package of tabular and horizontal units that extend at a rate of 1.8 mm per hour. Seven seconds of the animation represent 43.5 hours of the experiment.

Stage 1 (0–30% time elapsed) - decoupling: The thick salt layer initially decouples basement from cover during regional extension. Accordingly, separate grabens open in the basement and in the cover. Grabens in the overburden initiate reactive diapirs, which in turn, evolve through the active and passive stages.

Stage 2 (30–100% time elapsed) - coupling: One synkinematic sand layer (gray) is added. Continuous widening of basement grabens and the overburden diapirs depletes and thins the source layer, which causes sagging diapirs and increases coupling between cover and basement. As a result, synclinal sags form above basement grabens, and a drape anticline with crestal graben forms above the basement horst.

POTENTIAL STRUCTURAL TRAPS FOR HYDROCARBONS

Stage 1: Traps for hydrocarbons could be associated with diapirs evolving through the active, reactive, and passive stages as described in REACTIVE.MOV. Additionally, salt extrusion during the passive stage could produce areally large overhangs that could trap hydrocarbons.

Stage 2: Sagging diapirs could reverse the dip direction of overlying reservoirs within synclines of the gray layer and direct hydrocarbons away from diapirs. Synclinal sags above basement grabens create symmetric dip gradients that could direct hydrocarbons away from the syncline to be trapped (1) in the three anticlines above the basement horsts and (2) by sealing faults in the overburden above the flanks of the basement graben. Additionally, subsalt traps could form where hydrocarbons within the basement are sealed by (1) overlying salt or (2) faults associated with the basement graben.

ASYMMETRIC ACTIVE DIAPIR

This animation illustrates asymmetric active diapirism. It shows the central part of a finite-element (GEOSIM-2D) model, where the diapir (white) and overburden have physical properties and dimensions typical of physical models. Before deformation, the diapir is 2.5 cm wide and the roof above the diapir varies in thickness from 1.125 cm to 0.625 cm. No tectonic forces are applied, and all deformation is due to buoyancy of the salt layer. Plastic strain in the overburden increases from blue to red contours and indicates fault zones. 16 seconds of animation represent 7.2 hours of deformation.

Active diapirism is enhanced by (a) high density contrasts between salt and overburden (b) mechanically weak overburden (c) large height of diapir relative to thickness of overburden, and (d) topographic depression of the overburden. In the initial stages of deformation, reverse faults form at both corners of the roof, and a graben forms at the crest of the diapir. As deformation proceeds, the reverse fault zone to the right of the diapir becomes nearly adjacent to the crestal graben.The rotated roof between the left reverse fault and the graben represents the rotated flap flank of the diapir. As is typical for all the active diapirs we have modeled, initial angularities in the diapir contact become increasingly rounded during piercement. The diapir nearly emerges at the end of deformation.

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PREVIOUS RELEASE

The material in this publication was previously released to AGL Industrial Associates. All animations were released in November 1993. Additionally, static images were released as follows: 4 images released in 1990 (used in REACTIVE.MOV), 4 in October 1993 (COUPLING.MOV), and 1 in October 1993 (ASYMMETR.MOV).

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ACKNOWLEDGMENTS

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.