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ABSTRACT

The Permian Upper Rotliegend sandstones from the Upper and Lower Slochteren formations form the main gas reservoir of the Netherlands and are the host for the giant Groningen gas field. Fractures in the Rotliegend are quite rare in the Dutch subsurface, based on the very large core and log database that has been acquired over half a century. The fractures therefore are not considered to be a major control on the reservoir response and gas recovery in most of these areas. The Rotliegend can have tight reservoir properties due to the combined effects of more pronounced mechanical compaction and diagenesis. In these situations the presence of natural fractures and creation of hydraulic fractures can assist in improving the vertical and lateral connectivity. The impact of fracture presence depends on the types of natural fractures and their orientation, which define preferred well paths and steer the selection of the optimal hydraulic fraccing methods to deploy in the tight gas reservoirs.

Many Rotliegend fields are compartmentalised by faults, creating baffles and barriers to fluid flow on a production timescale and more rarely on a geological timescale. Hydrocarbon columns in certain Rotliegend fields are longer than can be expected based on mapped structural closure, because of fault seal. Understanding fractures—their origin, orientation, and properties—assisted in improving the understanding of the properties of faults and the different potential fault-seal mechanisms that are observed in the Rotliegend. Fractures are in a way smaller representations of the larger faults in the subsurface.

The various Upper Rotliegend fracture types include cataclastic, cemented, shale smear, phyllosilicate framework, and open fractures. The most common types in the Upper Rotliegend are cataclastic and cemented fractures. Detailed laboratory analysis of fractures in cores has shown that both cemented and cataclastic fractures can hold significant pressure differences. However, cemented fractures and faults are not continuous and therefore likely leak through weaker, poorly cemented windows along the fault and fracture surfaces. Cataclasis may be continuous along the entire fault and fracture surfaces and may form a sealing mechanism in the high-net-to-gross Upper Rotliegend sandstones.

The Rotliegend fracture types and their properties are defined by the conditions of temperature, pressure, and stress during the deformation phase during which these fractures developed and by possible additional effects of conditions at subsequent tectonic phases. Knowledge of the geotectonic history of the Rotliegend is therefore of relevance to improve the understanding of the presence and properties of fractures.

This paper provides an overview of the tectonic history of the Rotliegend, describes fracture origin and propagation, addresses the various Rotliegend fractures in detail, and discusses implications for Rotliegend field development and exploration prospectivity. The purpose of this overview paper is to contribute to Rotliegend structural characterisation, primarily by providing a catalogue of Rotliegend fractures. Reference is made in some places to fields and pressure and fluid data, but the intent is not to provide detailed field cases or fault-seal examples.

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