The internal structure and composition of salt diapirs: What do we know, what might we want to know and why might it be important?

Presented as part of the Geological Society of America (GSA) Distinguished Lecturer Tour 2017

The internal structure and composition of salt diapirs: What do we know, what might we want to know and why might it be important?

Understanding intrasalt structure and composition helps to: (i) determine the kinematics of the growth of salt diapirs; (ii) predict drilling hazards; and (iii) build better subsurface velocity models. Insights into intrasalt structure are provided salt diapirs exposed in the field and in diapirs exposed in mines. These data indicate that despite complex intrasalt deformation, stratigraphic layering is generally preserved though highly strained. However, exposures of natural examples of salt diapirs are largely restricted to broadly two-dimensional horizontal slices, and 1D sampling in mines provides, at best, only a quasi-3D understanding of intrasalt structure. Physical models also provide insights into the style and origin of intrasalt structure, typically documenting simple inward flow and thickening of salt during diapir growth. Seismic reflection data offer the as-yet unrealised potential for full 3D imaging of intrasalt structure, although salt is typically poorly reflective owing to very high strains, steep dips and homogenous lithology. However, recent studies show that in layered salt, intrasalt reflections can act as strain markers to image complex intrasalt shear zones and folds.

 

In this talk I will use 3D seismic and borehole data from the Santos Basin, offshore Brazil and the Egersund Basin, offshore Norway to highlight variations in the internal structure and composition of salt diapirs, and the kinematics of salt diapirism. I will demonstrate that in the Santos Basin, a range of complex structures may be developed in salt diapirs, including simple, upright, internal anticlines, recumbent, isoclinal synclines and intrasalt shear zones. We infer that these structure record: (i) initial diapir inflation; (ii) rise of lower mobile halite through an arched and thinned roof of denser, layered evaporites, and emplacement of an intrasalt sheet or canopy; (iii) formation of synclinal flaps kinematically linked to salt breakout and emplacement of the intrasalt allochthonous bodies; and (iv) late-stage diapir squeezing. In the Egersund Basin, diapiric Upper Permian Zechstein salt is seismically transparent due to homogeneous salt and/or high strains. Seismic data suggest that a weld is developed along the diapir stem, although borehole data indicate that the stem consists of an inner, c. 1500 m thick, halite-dominated zone, and an outer, c. 250 m thick, anhydrite-dominated ‘sheath’. We interpret that the anhydrite represents ‘lateral caprock’, which formed late in the basin history in response to the migration of NaCl-poor fluid up the margins of the diapir, and dissolution of halite.

 

The results I present in this talk represent only the preliminary steps in improving our understanding of the internal structure and composition of salt bodies, providing some initial insights into the type of structures that may be encountered during drilling through thick salt. Furthermore, the complex intrasalt structures and lithological variations described here may cause relatively large, abrupt variations in subsurface velocity, thus posing a challenge to velocity model building and imaging of sub-salt and salt-flank exploration targets.