Frank Strozyk

3D seismic study of complex intra-salt deformation: An example from the Upper Permian Zechstein 3 stringer, western Dutch offshore

Abstract Most of the information on subsurface evaporitic structures comes from 3D seismic data. However, this data only provide limited information about the internal structure of the evaporites, which is known from salt mines and salt diapir outcrops. Brittle intra-salt layers (carbonate, anhydrite, clay) of at least 10 m thickness form good reflectors in evaporites, but the structure and dynamics of such ‘stringers’ in the salt movement are poorly understood. In this study, we investigate the intra-salt Zechstein 3 (Z3) stringer from 3D seismic data in an area offshore the Netherlands. Observations show complex deformation including boudinage, folding and stacking. Reflections from thin and steep stringer parts are strongly reduced, and we present different structural models and tests of these. We compare our observations to structural models from salt mines and analogue/numerical models of intra-salt deformation. A smoothed representation of the upper surface of the stringer fragments follows the shape of Top Salt, but smaller-scale stringer geometries strongly differ from this and imply boudinage. The imaged disharmonic patterns of constrictional folds provide evidence for the complexity of the intra-salt, in agreement with observations in salt mines. This may be explained by interaction of the layered salt rheology, complex three-dimensional salt flow, different phases and styles of basement tectonics and movement of the overburden.

Regional variations in the structure of the Permian Zechstein 3 intrasalt stringer in the northern Netherlands: 3D seismic interpretation and implications for salt tectonic evolution

The late Permian Zechstein evaporites in the northern Netherlands were exceptionally well imaged in ![Formula][1] of prestack depth migration 3D seismic data. Seismic reflections of a 30–150-m-thick Zechstein 3 anhydrite-carbonate stringer, which was encased in thick layers of rock salt, provided an unparalleled, basin-scale view of the 3D internal structure of a giant salt basin. Seismic data were used to map the regional variation of the intrasalt stringer to analyze its role in deformation styles and salt flow as well as its interaction with the sub- and suprasalt sediments. From our interpretation of the stringer, the salt layers, and the encasing sediments, three regional structural stringer styles can be defined and were analyzed in the context of regional salt kinematics. Our results revealed that the current stringer initially formed a continuous sheet of anhydrite and carbonate, embedded in salt of varying thickness. After the onset of syndepositional gravitational gliding of some of the salt masses and passive salt diapirism triggered by differential loading in the Triassic in other areas, salt flow caused rupture and folding of the stringer on a wide range of scales. The thickness and deformation degree of the individual salt layers controlled the development of regionally distinctive styles of intrasalt structures. Although deformation of the salt and the embedded stringer stopped early on morphologic highs, the basinal areas experienced phases of later activation or reactivation of salt structures and sedimentary basins. This was especially the case during the Late Cretaceous to Early Tertiary plate tectonic reorganization in the Central European plate, causing three-dimensionally complex intrasalt structures observable today. [1]: /embed/mml-math-1.gif

Origin and deformation of intra-salt sulphate layers: an example from the Dutch Zechstein (Late Permian)

From salt mine galleries and well data it is known that thick rock salt layers can contain anhydrite and carbonate layers with thicknesses on the millimetre to tens of metre scale. The relatively thick Zechstein 3 anhydrite–carbonate layer in the northern Netherlands has been studied previously using 3-D seismic data. Observations from geophysical well logs in this study reveal the presence of thin sulphate layers on the sub-seismic scale imbedded in the Zechstein 2 (Z2) salt. Core samples, thin sections, seismic data and geochemical measurements were used to determine the mineralogy and origin of one of these Z2 sulphate layers. Bromine analyses show that they mark a freshening event in the Z2 salt, which can be correlated over large distances in the northern Netherlands. Their core-calibrated log signature indicates that the Z2 sulphate layers consist either of pure anhydrite or of anhydrite and polyhalite. The mineralogy and thickness of the sulphate layers are interpreted to vary between synsedimentary morphologic lows (thin anhydrite–polyhalite couplets) and highs (thicker anhydrite layers). Such a combination of core observations and well log analysis is a powerful tool to detect lateral trends in evaporite mineralogy and to reconstruct the environmental setting of their formation. Salt internal geometries can further be used to distinguish between different deformation mechanisms. In our study area, the distribution of sulphate layers within the Z2 salt indicates that subjacent salt dissolution was not the dominant process leading to salt-related deformation.

The Tectonic History of the Zechstein Basin in the Netherlands and Germany

Abstract In northwestern Europe, the upper Permian Zechstein evaporites are a highly efficient seal for Carboniferous gas trapped in clastic Rotliegend reservoirs. The Zechstein evaporite succession and postsalt sediments experienced complex deformation during several tectonic phases in the German and Dutch part of the Southern Permian Basin, including extension and compression or transpression. However, controlled by late Permian topography, the number and thickness of the individual evaporite cycles vary, mainly correlating with the basin, slope, and platform settings of the Zechstein Sea. The main trigger mechanisms for early salt movement comprise extension and rafting of Lower Triassic sediments, syn-depositional fault activity within the subsalt, subsequent differential loading of postsalt sediments, and intrasalt heterogeneities. This latter caused thin-skinned salt tectonics with passive diapirism, which seeded the number of the salt highs observed today. Later reactivation, cessation, or formation of new salt highs was triggered by the massive change from extensional to compressional tectonics that began in Cretaceous times. Regional fault patterns were reactivated or newly formed during these tectonic phases, and they are often marked by elongated salt walls. A large drop in tectonic stresses since the onset of the Cenozoic caused a rather tabular draping of thick clastics and marks the cessation of major salt movements across the greater part of the Dutch and German Zechstein Basin. Since then, most of the Zechstein salt has been considered to be at rest.

Giant pockmark formation from Cretaceous hydrocarbon expulsion in the western Lower Saxony Basin, The Netherlands

Abstract A field of giant pockmarks was discovered at the base of the Upper Cretaceous Chalk unit in the westernmost Lower Saxony Basin in The Netherlands. 3D seismic and well data show that mostly circular, 300–850 m-wide and 10–50 m-deep, pockmarks formed at the top of the Lower Cretaceous Upper Holland Marl Formation, which overlies oil- and gas-filled Lower Cretaceous sandstone reservoirs in the vicinity of the study area. Based on our interpretations, we present a scenario of early gas generation in Carboniferous coals and a localized migration of the gas from its original subsalt reservoirs through a salt weld in the Zechstein evaporites into the shallow Cretaceous sandstone reservoirs and the fine-grained marl above. Diapiring salt walls thereby limited the gas migration and trapping to a 150 km2-sized basin. A sea-level drawdown during Base Chalk formation possibly led to excess pore pressure in the reservoir and the breaching of the seal close to the seafloor, which caused a short-lived expulsion of the gas and pockmark formation. While hydrocarbon generation, migration and trapping are common processes in this region, gas escaping at the seafloor with pockmark generation appears to be a rather rare and complex phenomenon. In general, the presence of pockmarks associated with salt welds may be used to constrain the timing and migration pathway of hydrocarbons from subsalt into shallower reservoir levels. Both features may imply a general reservoir potential for regions where suitable source rocks are missing in the post-salt succession.

The Internal Structure of the Zechstein Salt and Related Drilling Risks in the Northern Netherlands

Abstract The internal structure of the Late Permian Zechstein evaporites in the Netherlands is dominated by deformed rock salt with imbedded fragments of relatively brittle anhydrite and carbonate layers as well as highly ductile intervals of potassium salts. The largely broken and folded Zechstein 3 anhydrite-carbonate layer is fully encased in the rock salt and can be resolved in the seismic data. It was therefore able to study, using 3D seismic interpretation, the variability of intrasalt structures and regional salt flows in the Dutch onshore, and to indicate the salt rheology and composition. When drilling toward subsalt Rotliegend gas reservoirs, one of the most critical overburden sections, in terms of drilling hazards, is the Zechstein, especially in areas of complicated intrasalt structures, which occasionally coincide with the occurrence of reservoirs in the subsalt. Some of the major risks during drilling of the Zechstein section are associated with salt creeps, pressure kicks and losses within fractured carbonates, and flows from intrasalt brine pockets. While creeps of squeezing salts and kicks in the basal Z1 and Z2 carbonates can be controlled by extra casing and adjustment of the mud weight, respectively, kicks and losses in Z3 intrasalt carbonate blocks as well as brine pockets are still hard to predict and manage. Studies on the regional distribution of different styles of intrasalt structures and the distribution of the different types of drilling issues helped to better predict drilling risks, significantly reducing well investment.

Reconstructing the Upper Permian sedimentary facies distribution of a tight gas field in Central Europe on the basis of a modern analog field study in the Panamint Valley, western U.S.

Comparison of modern deposits in the Panamint Valley, western United States, to core and geophysical data from a Permian (Rotliegend, Germany) tight gas field allows for improved understanding of the interaction of tectonics and sedimentary processes during Rotliegend deposition. The Panamint Valley was selected for a modern analog of the subsurface Rotliegend Basin because both study sites are characterized by (1) elongated grabens with large-scale bounding fault zones resulting from synsedimentary transtensional tectonics; (2) fault-controlled paleotopography as key controlling parameter for the sediment facies distribution, including alluvial fans, dunes, wet and damp interdune sandflats, and ephemeral dry lake deposits; and (3) local sediment provenance from sedimentary and volcanic rocks. The analysis of satellite images and field data from the Panamint Valley enabled the development of a conceptual model involving topography, synsedimentary faulting, and wind activity as controlling factors for the sediment facies distribution. The application of the model to the reconstructed Rotliegend paleotopography of the German subsurface study site allows for prediction of the facies distribution prior to the Triassic–Cretaceous tectonic overprinting. As a consequence, we expect a sediment facies succession from (1) alluvial fan deposits along the hanging walls of the basin-bounding fault zones to (2) distributary fluvial channel deposits toward the basin center and (3) ephemeral lake deposits in the deepest basin area. (4) Eolian dune accumulation and preservation is mainly concentrated on hanging-wall locations. However, additional dune deposits are proposed above overlapping step faults and on footwalls of synsedimentary active faults. (5) Sandflats occur on the upwind and downwind margins of the dune field. These predictions are calibrated to core and geophysical well log data.

Quantifying the Key Role of Slope Material Peak Strength – Using Discrete Element Simulations

This study investigates how progressive oversteepening and fault kinematics impact on slope failure initiation and subsequent landsliding along subsiding basin flanks using 2D DEM simulations. We use large assemblages of granular particles to simulate the deformation behaviour of slope sediments with varying peak strength. Sediments with high peak strength deform preferentially on major faults and produce a stepped topography and a stable slope in the long-term. Mass failures in these sediments occur as large, compact slides of short run out. In contrast, slopes with lower peak strength deform diffusely and present large numbers of faults that fail frequently and maintain the slope at its critical angle of inclination. The resulting slope topography is smoother and laterally more elongated. These differences in mass movements are governed by (i) characteristic fault patterns, and (ii) repeated oversteepening during ongoing basin subsidence, which is an important prerequisite for failure initiation. Our experiments indicate quantitatively that the failure distribution, dimension, and transport mechanism, as well as the recurrence rate of landslides are essentially controlled by the peak strength of the failed material.