--> Abstract: The 3d Geometry of the Zechstein Z3 Carbonate/Anhydrite Member: Implications for Salt Dynamics and Hydrocarbon Production, by Heijn W. van Gent, Janos L. Urai, and Peter A. Kukla; #90082 (2008)

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The 3d Geometry of the Zechstein Z3 Carbonate/Anhydrite Member: Implications for Salt Dynamics and Hydrocarbon Production

Heijn W. van Gent1, Janos L. Urai1, and Peter A. Kukla2
1Structural Geology, Tectonics and Geomechanics, RWTH-Aachen University, Aachen, Germany
2Geological Institute, RWTH-Aachen University, Aachen, Germany

The study of large rock inclusions in salt structures is of broad economic interest as they constitute hydrocarbon reservoirs and/or represent potential drilling risks due to their high overpressures. The Zechstein Z3 carbonate/anhydrite member (Z3 ‘stringer’) of the Southern Permian Basin is fully encased in Zechstein evaporites. Despite a significant acoustic impedance contrast, the stringer is often misinterpreted as missing where it dips steeply, leading to drilling risks. Interpreting the 3D geometry of the Z3 stringer is also relevant regarding the contrasting rheologies of the Z3 carbonates and anhydrites and the surrounding rock salt during deformation. Complex geometries of the Z3 stringer were interpreted from two (3D) seismic surveys of the Dutch on- and offshore.

In the onshore survey, the thickness of the flat lying stringer is fairly constant (about 35 m). However, discrete zones with thicknesses up to 150 m are observed, resulting from internal stringer thrusting and stacking. A further structure is asymmetrical folding on pillow flanks, and no stringer in the crest. We propose a complex interaction of pillow growth and differential sedimentation, tilting, gravity gliding and thrusting of the stringer.

In the offshore survey, the Z3 stringer geometry is more complex and segmented. Only the base stringer is interpreted, but observations include stringer ‘grave yards’, roofed stringers and open as well as closed, isoclinal folds perpendicular to the main salt structure. Geometries are reminiscent of boudinaged competent layers in soft matrixes, observed in field and experimental studies.
The presence of intrasalt stringers is known from a number of petroliferous evaporite basins (e.g. European Zechstein, Oman, Yemen), where the structural evolution of these stingers remains still unclear. The findings of this study may be applied in a larger setting.

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