Sedimentology, stratigraphy and reservoir architecture of the Lower Triassic Main Buntsandstein in the Roer Valley Graben, the Netherlands

Abstract

The Main Buntsandstein Subgroup in the Roer Valley Graben in the southern Dutch subsurface is a sand-prone sedimentary interval deposited in a fluvial-aeolian environment, and is currently investigated for its suitability as target for low-entalpy geothermal exploration. The current depositional models in the Roer Valley Graben do not fully address the facies heterogeneities within and between Buntsandstein sedimentary units and their impact on the prediction of reservoir architecture. A detailed analysis of the Main Buntsandstein sedimentary facies heterogeneities to de-risk future sustainable energy operations is therefore crucial. In the present study, the sedimentology and lithostratigraphy of the Buntsandstein are assessed in a multidisciplinary analysis by use of a subsurface dataset composed of well cores, gamma-ray logs, and thin section data. The deposition of the Main Buntsandstein sediments in the Roer Valley Graben is dominated by different fluvial processes, with minor aeolian reworking. River planform style evolved through geological time from highly mobile and ephemeral to more perennial in nature. These changes in river style seem to be dictated by a decrease in climatic aridity along with a decrease in tectonic activity. The depositional processes resulted in the development of six lithofacies associations, developing three different types of reservoir architectures with their own set of heterogeneities at different spatial scales. Amalgamated, stacked sandstones have the highest net-to-gross (N/G) with a high degree of lateral and vertical connectivity, and the highest average porosity and permeability. Compensational-stacked sandstone reservoirs have a lower N/G and are the most heterogeneous due to the frequent occurrence of cemented intervals as well as mud drapes in the sandstone bodies. Marginal isolated sandstones show a well preserved relationship between reservoir properties and depositional facies, while more data are needed to resolve the spatial connectivity and lateral continuity of these sandstone bodies. The results of this study enhance the understanding of Lower Triassic reservoir architecture and sedimentary heterogeneities in the Roer Valley Graben that can be applied well beyond the area and provide a solid basis for future investigation of the relationship between sedimentary facies, diagenesis, and reservoir quality.