--> --> The Evolution of the Rotliegend Play in the Permian Basin (NW Europe) Through Geological Time and Industry Wisdom

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The Evolution of the Rotliegend Play in the Permian Basin (NW Europe) Through Geological Time and Industry Wisdom

Abstract

The Permian palaeogeography of the Southern North Sea, or Southern Permian Basin (SPB), and the distribution and variability of the depositional environments of the Rotliegend Group have been of continuing interest for the last 55 years, since the discovery of the Groningen Gas Field. In the early ages, hydrocarbon exploration in the Rotliegend sandstone was based on the pioneering sedimentological work of Glennie (1972), followed by Ziegler's tectonic and paleogeographic reconstructions (1975, 1978). These maps initially identified broad depositional realms of aeolian, fluvial, and playa environments, distributed in a concentric spatial pattern to form a broadly saucer-shaped basin. These early maps, associated with the good reservoir properties of thick aeolian sandstones, led industry to develop the 40-years-long wisdom of considering the Rotliegend reservoir as a laterally homogenous sandbox, easily represented by a layer-cake architecture. This conceptual geological model persisted over the years, although Ziegler's maps published between the mid 80s to early 90's provided an updated, more detailed understanding of tectonics and palaeogeography. Over the last 10 years, exploration has moved away from the aeolian sandstone fairway, either towards the center or the margin of the SPB. New discoveries have been enabled by implementing better techniques for seismic acquisition and processing and logging tools (e.g. high-resolution borehole images). This led to an improved understanding of reservoir geology and especially of the geometry of m-scale features, such as small dunes and/or sheetflood sandstones (good reservoir) interbedded within interdune or playa deposits (poor and non reservoir), and their lateral correlatability. Recent evidence also highlights the stratigraphic variability in certain basin areas, thus challenging the old wisdom of layer-cake stratigraphy and proving that tectonics played an important role in defining the resulting reservoir architecture. Similarly, Ziegler's suggestion of the occurrence of large fluvial systems, entering the SPB from both southern and northern margins, provided crucial information to locate the best dune reservoirs. This has been recently demonstrated by extensive field observations on modern depositional environments, which identify a strong dependency of dune-field development upon the location of fluvial systems which supply sand to the wind, and upon topographic slope. These examples demonstrate the importance of Ziegler's maps which, beside guiding a multitude of geologists working in the SPB region, at an early stage offered a series of original intuitions at both basin and local scale which are now being recognised, years after their first publication. Ziegler's insightful research concepts foreshadowed the modern developments in our understanding of the SPB petroleum geology.