Spatial variation in quartz cement type and concentration: An example from the Heidelberg formation (Teufelsmauer outcrops), Upper Cretaceous Subhercynian Basin, Germany

Abstract

Spatial heterogeneity of cement abundance in sandstones is an important factor for reservoir compartmentalization. Here we present results on the variety of diagenetic products and spatial distribution of intense and focused silica cementation within otherwise friable sandstone without silica cements. The Cretaceous quartz arenite of the Teufelsmauer outcrops near Quedlinburg (Upper Cretaceous Heidelberg formation, Subhercynian Basin, Germany) is cemented by various types of diagenetic silica. The highly cemented sandstone sections contain both syntaxial quartz cement (concentrations: 11–31vol.%) and microcrystalline pore-filling quartz cement (10–34vol.%). Microcrystalline cement generations form the latest SiO2 precipitations and overgrow moderate to intense syntaxial rims.At outcrop and sedimentary basin scales, the occurrence of highly cemented sandstone is limited to an area of several hundreds of meters in lateral extension, but does not exceed a layer thickness of about 5m. The highly cemented sandstone shows internal heterogeneity in cementation with preferred orientation of their length axis, suggesting mainly stationary fluid flow pathways during cementation of several silica generations. Most likely, the source of all diagenetic silica is owing to enhanced dissolution at cataclastic deformation bands, which are found within the same sandstone strata. As a mechanism for locally focused precipitation, we suggest a combination of (1) local enhancement of silica dissolution due to deformation and cataclasis and (2) enhanced fluid flow and expulsion during deformation related to the synchronous thrusting of the Harz block onto the Cretaceous Subhercynian Basin. We interpret that the expelled silica-enriched formation water experienced fluid mixing with ascending water from an overpressured, deeper regime, implying that the observed silica generations trace this unique zone of fluid expulsion, ascension, and mixing.Our investigations show that silica precipitation may result in a pattern of spatially-oriented highly-cemented sections within poorly-cemented sandstone. The type of silica cement does, however, not influence size, frequency, and shape of highly cemented bodies. Implications range from specific conclusions about silica export from cataclastic deformation bands and adjacent precipitation to general conclusions about partitioning in cemented sandstone reservoirs, including information about the spatial scales of fluid movement, silica redistribution (dissolution–precipitation), and heterogeneity and sequence of cement types (syntaxial quartz and microquartz).