Diagenetic effects on porosity–permeability relationships in red beds of the Lower Triassic Bunter Sandstone Formation in the North German Basin

Abstract

Carbonate and anhydrite cement, clay clasts and inter-granular clay are the main components that reduce reservoir quality in the studied Bunter Sandstone Formation. The impacts of these parameters on porosity and permeability are determined by combining petrographic mineral quantification with conventional core analysis of samples from the Danish part of the North German Basin. The depositional environments are considered because they largely control the distribution of cements, clays and grain sizes. The lateral variability of depositional environments is defined by the position in the basin and the proximity to the source areas. The stratigraphic distribution of depositional environments is related both to local topography and to climate because high aridity promoted aeolian deposition.The Bunter Sandstone Formation has high porosity and permeability in most of the sandstone intervals in the northern North German Basin. The reservoir quality is good as long as the cements and clays are present as confined bodies that leave the remaining pore spaces available for flow. In contrast, inter-granular clay and pervasive cementation hinder virtually all flow through the sandstone. The ephemeral fluvial deposits have an average porosity and permeability of 20.3% and 810mD, respectively, and the values are 24.6% and 807mD for the aeolian sandstones, excluding the unconsolidated aeolian sands which presumably have higher porosity and permeability.The aeolian sandstones of the Volpriehausen Member have very good reservoir quality since they have a thickness of about 25m, are laterally continuous, are largely clay-free and the cement occurs in small amounts. The sandstones of the Solling Member consist mainly of ephemeral fluvial deposits, which generally have good reservoir quality. However, some intervals have high contents of inter-granular clays or pervasive carbonate, anhydrite or halite cement and these components reduce the permeability significantly. The lateral distribution of the ephemeral fluvial sandstones is variable and therefore difficult to predict when planning a geothermal exploration well. Thus, the Volpriehausen Member is the preferred target.