The internal structure of evaporites: a case study of the Zechstein salt using 3D seismic data, field observation, and numerical modeling.
Final Report Abstract
For the first time, the internal structure of thick, deformed rock salt and imbedded evaporite layers was mapped and imaged on the scale of an entire basinal area. The anhydrite‐ and carbonate‐rich Z3AC intra‐salt layer is imaged in 3D seismic data from the subsurface of the Dutch onshore and displays the deformed boundary between the thick Z2 and Z3 salt layers. It was shown that Z3AC experienced several complex phases of dragging, rupture and folding during extensional and compressional deformation stages of the embedding rock salt layers. The regional variability of very different deformation stages and types of the Z3AC and the salt layers was characterized and used to reconstruct the timing and evolution of salt withdrawal, salt flows and salt diapirism in Zechstein Basin of the Northern Netherlands. Kinematic reconstructions have shown that towards the edge of the salt basin in the central Netherlands, the principally de‐coupled deformation of sub‐ and supra‐salt sediments in the deeper basin trends towards coupled faulting and folding of the sub‐ and supra‐salt where the rock salt is wedging out towards structural highs. Furthermore, the early diapirism of the Zechstein salt, which was triggered by thin‐skinned rafting of Triassic sediments during regional extension, caused the development of Jurassic and Cretaceous basins, which today hold hydrocarbon reservoirs in the Netherlands. From seismic and well data investigations it was shown that during the Cretaceous huge amounts of Carboniferous coal gas migrated through areas of massive salt withdrawal into the shallow reservoirs, where it in some places escaped at seafloor, as for example expressed in a field of giant pockmarks in the Lower Saxony Basin. The internal deformation of the salt layers is mainly controlled by salt reacting on regional tectonic impacts and postsalt strata growth as well as the heterogeneous layering of the evaporites, which locally show rigid (carbonate, anhydrite) and ductile inclusions (e.g., K‐Mg salts). Salt flows and the movements of rigid inclusions within the salt are thereby controlled by the current state of stress and the associated rheology of the rock salt. While both pressure‐solution creep (PSC) and dislocation creep (DC) allow for large and fast strains in the salt during high stresses, PSC is almost lacking during low‐ stress scenarios, where the salt becomes immobile due to crystal growth and grain boundary fluids shrinking to bubbles. From reconstruction models and evaporite facies analysis across parts of the Dutch Zechstein Basin it was shown that during the deposition of the Zechstein evaporites syndepositional fault activity in the subsalt caused a sequential renovation of morphology at the top of the precipitating evaporites. This caused the spatially limited occurrence of some very distinct evaporite sequences, for example thick layers of bischofite and carnallite at the southern edge of the Groningen High, or the transition from anhydrite‐ to polyhalite‐rich layers from morphological highs to lows, respectively. Based on these observations the very early stages of Zechstein evaporite deposition and their subsequent deformation could be reconstructed as well as it helped establishing models of the distribution of economically relevant evaporite minerals (i.e., K‐Mg salt). This variability in layer distribution, thickness, rheology and mechanics of the evaporites as well as their partially complex structure make the Zechstein one of the most critical sections in terms of drilling or mining. Some of the major risks are associated with salt creeps, pressure kicks or losses in fractured carbonates and flows from intra‐salt brine pockets. While creeps of squeezing salts and kicks in basal carbonates can be controlled by extra casing and adjustment of the drilling mud weight, kicks from intra‐salt carbonate blocks and brine pockets are still hard to predict and manage. 3D seismic imaging of the regional distribution of different styles of intra‐salt structures compared to the distribution of the different types of drilling issues helped to better predict drilling risks and might reduce well investment in the Dutch subsurface.
Publications
- (2014). Regional variations in the structure of the Permian Zechstein 3 intra‐salt stringer in the Northern Netherlands: 3D seismic interpretation and implications for salt tectonic evolution. Interpretation 2, 4, 1–17
Strozyk, F., Janos L. Urai, Heijn van Gent, Martin de Keijzer, Peter Kukla
(See online at https://doi.org/10.1190/INT-2014-0037.1) - 2014. The internal structure of salt: insights from a regional 3D seismic study of the Permian Zechstein 3 intra‐salt stringer in the Northern Netherlands, and its implications for salt tectonics. EGU General Assembly 2014, Vienna, Austria
Frank Strozyk, Janos L. Urai, Heijn van Gent, Martin de Keijzer, and Peter A. Kukla
- 2015. Seismic interpretation of the internal geometry of the Zechstein evaporites. Gulf Coast Section SEPM, Houston, 2015
Strozyk, F.
- 2016. The structure of the Zechstein 3 stringer in the northern Netherlands, and its implications for salt kinematics and rheology. EGU General Assembly 2016, Vienna, Austria
Strozyk, F., Janos Urai, Shiyuan Li, Joyce Schmatz, Bianca Biehl, Lars Reuning, Alexander Raith, Steffen Abe, Heijn van Gent, Martin de Keijzer, and Peter Kukla
- (2017). The internal structure of the Zechstein salt and related drilling risks in the northern Netherlands. In: J.I. Soto, Flinch, J.F., Tari, G. (Eds.), Permo‐Triassic salt provinces in Europe, North Africa and the Atlantic margins: Tectonics and hydroc
Strozyk, F.
(See online at https://doi.org/10.1016/B978-0-12-809417-4.00006-9) - (2017). The tectonic history of the Zechstein Basin in the Netherlands and Germany. In: J.I. Soto, J. Flinch, G. Tari (Eds.), Permo‐Triassic salt provinces in Europe, North Africa and the Atlantic margins: Tectonics and hydrocarbon potential. Elsevier, Pa
Strozyk, F., Reuning, L., Scheck‐Wenderoth, M., Tanner, D.
(See online at https://doi.org/10.1016/B978-0-12-809417-4.00011-2)
