Abstract The reservoir rock of the Schoonebeek oil field is formed by the sandstones of the Bentheim Sandstone Member. The sedimentology and depositional environment of this sandstone have been extensively studied, but the relationship between the geometry of the sandstone and tectonic activity in the Schoonebeek area remains poorly understood. 355 boreholes and two three-dimensional (3D) seismic surveys were used to study this relationship. An eroded zone in the west of the field and an area where the original depositional thickness is still intact were identified. Using the ezValidator software package it can be seen that uplift of a local anticline played an important role in the erosion of the sandstone. Deposition of the sands of the Bentheim Sandstone Member and the overlying Vlieland Sandstone and Claystone formations occurred on an unstable changing palaeotopography, whereby the instability was probably driven by halokinetic movement of the underlying Late Permian Zechstein salt. Syndepositional tectonic movements affected local thickness variations in the Bentheim Sandstone Member in the west of the field, leading to westwards thinning.

ABSTRACT The Bentheim Sandstone Formation is of Valanginian age and consists of several sheet-like bodies (each up to tens of meters thick) located in the western part of the Lower Saxony Basin of NW Germany. This study compares the reservoir characteristics of the Bentheim Sandstone Formation at outcrop in the Bad Bentheim area and in the subsurface where it forms an oil reservoir in the Riihlermoor field. The formation is interpreted to have been deposited along a tidal, embayed coastline at the margin of a small, partially enclosed sea. Rapid thickness variations of the total unit suggest it was deposited in a syn-rift setting of horst and graben. Toward the south and east, the sandstone interfingers with basinal shales. Outcrop facies analysis has identified three main facies associations: (1) a tide-influenced fluvial deltaic association; (2) a wave-influenced fluvial deltaic association, and (3) a tide-dominated deltaic association. All three facies associations are sand-dominated. Progradation of the deltaic system in all three cases resulted in the formation of sandier-upward facies successions composed of mudstone or heterolithic facies in the lower part and cleaner, more massive sandstone in the upper part. Comparison with contemporaneously deposited reservoir sandstones has been assisted by the collection of outcrop gamma ray and permeability data. A stratigraphic methodology derived from study of outcrop can be applied to the subsurface. In both outcrop and subsurface, the Bentheim Sandstone Formation can be subdivided into three genetic sequences (Bentheim 1, 2 and 3) which are retrogradationally stacked, back-stepping toward the basin margin in a north-westerly direction. Genetic sequences are used to define progradational-retrogradational phases of deposition within which stacked, sandier-upward successions form a stratigraphic subunit correlatable over several kilometers. These subunits are recognized in the subsurface and are interpreted to possess a sigmoidal geometry. Sedimentation was strongly influenced by synsedimentary tectonism. This tectonic influence focused deltaic depositional systems into a series of W-E and NW-SE oriented grabens. In both the outcrop and subsurface areas this influence is strong and has a marked control on the distribution of facies and, consequently, on reservoir thickness and quality.

Abstract The Netherlands, with only scarce occurrences of outcropping or shallow buried natural stone, has over centuries imported huge quantities of Early Cretaceous Bentheim Sandstone and Obernkirchen Sandstone from Germany. The present paper provides an overview of their distribution and properties relevant to their use as building stone, and their mutual differences and comparative weathering. Evidence in Dutch architecture for the onset of quarrying of the Bentheim Sandstone is presented, and an overview is given of the use of Bentheim Sandstone and Obernkirchen Sandstone in the Netherlands and Belgium.

Abstract In this study we review recent advances in our understanding of anisotropy in rocks, focusing on dilatant and compactant failure in sandstones and in a foliated metamorphic rock. In sandstones, the anisotropy can be associated with bedding, as in the Rothbach sandstone, or it can also be due to shape anisotropy of the grains and/or the pores, as in the Bentheim sandstone. Two scenarios are proposed for the development of anisotropy in these two end members. In a metamorphic rock with strong foliation like the Four-mile gneiss, it has been commonly observed that the brittle strength is minimum at a foliation angle of about 30°–45°. A damage mechanics model is proposed that underscores the dominant role of biotite foliation in the development of microcracking. In contrast it is often observed in sandstones with strong bedding that the strength is minimum in the direction parallel to bedding. New results for the Rothbach sandstone showed that compared to parallel-to-bedding samples: (i) in the brittle faulting regime the perpendicular-to-bedding samples have both a higher strength and dilatancy stress; and (ii) in the cataclastic flow regime the compactive yield envelope for the perpendicular-to-bedding samples expands significantly towards higher stress values.

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 km 2 -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.