Carbonate diagenesis and feldspar alteration in fracture-related bleaching zones (Buntsandstein, central Germany): possible link to CO2-influenced fluid–mineral reactions

Abstract

Fracture-related bleaching of Lower Triassic Buntsandstein red beds of central Germany was related to significant carbonate diagenesis and feldspar alteration caused by CO 2 -rich fluids. Using cathodoluminescence microscopy and spectroscopy combined with electron microprobe analysis and stable carbon isotope study, two major fluid-mineral interactions were detected: (1) zoned, joint-filling calcites and zoned pore-filling calcite cements, the latter replacing an earlier dolomite, were formed during bleaching. During the calcite formation and dolomite-calcite transformation, iron was incorporated into the calcite cement crystal cores due to Fe availability from the coeval bleaching. The dedolomitisation was ultimately associated with a volume increase. The related permeability decrease implies a certain degree of sealing and increasing retention of CO 2 , and the volume increase offers a minor CO 2 sink. Carbonate-rich sandstone, therefore, can provide advantages for underground CO 2 storage especially when situated in the fringes of the reservoir. (2) Alkali-feldspar alteration due to the bleaching fluids is reflected in cathodoluminescence spectra predominantly by the modulation of a brown luminescence emission peak (*620 nm). This peak represents a newly discovered effect related to alkali-feldspar alteration not solely associated with bleaching. Its modulation by the bleaching is interpreted to be due to Na depletion or a lattice defect in the Si-O bonds of the SiO 4 -tetrahedron. Alteration reflected by this luminescence feature has a destructive effect on the feldspars implying the possibility of diminished rock integrity due to bleaching and, hence, CO 2 -rich fluids. Two further CL spectral changes related to bleaching occur, (a) decreased intensity between around 570 nm assigned to Mn-depletion, and (b) increased amplitude and wavelength shift of the red (*680 nm) band. Converging evidence from carbonate and feldspar diagenesis, stable carbon isotope data and analysis of fracture directions suggests that CO 2 fluids contributed to a significant extent to the bleaching phenomena and alteration in the studied Buntsandstein strata.

Cathodoluminescence of moldavites

“…Another explanation for the 580 nm emission is a possible structural defect (strong local disorder) as it was reported from altered feldspar crystals (Gonz alez-Acebr on et al 2012; Wendler et al 2012). In these altered feldspars a structural disorder led to a local amorphization and produces an emission band at around 590 nm.…”

“…; Wendler et al. ). In these altered feldspars a structural disorder led to a local amorphization and produces an emission band at around 590 nm.…”

The Role of Mudstone Baffles In Controlling Fluid Pathways In A Fluvial Sandstone: A Study In the Pennsylvanian-Permian Fountain Formation, Northern Colorado, U.S.A

“…Other studies have described ''bleaching'' of red beds likely caused by migration of oil, methane, reducing brines, or CO 2 -charged brine (e.g., Beitler et al 2005;Loope et al 2010;Wendler et al 2012;Wigley et al 2012). Whitening associated with hydrocarbon migration and seeps is well documented in numerous previous studies (see compilation in Petrovic et al 2008).…”

Basin‐scale fluid movement patterns revealed by veins: Wessex Basin, UK

“…Conversely, closed system diagenesis in sandstones is assumed by many of those who study silicate diagenesis and sandstone reservoir quality of (Lander & Walderhaug ) although recent studies have suggested that material flux via fault systems may be a significant aspect of sandstone diagenesis in some circumstances (Wendler et al . ).…”

“…Now of interest as natural analogues to CO 2 geological sites, it is evident that movement of large quantities of CO 2 is a normal aspect of basin evolution (Baines & Worden 2004). Conversely, closed system diagenesis in sandstones is assumed by many of those who study silicate diagenesis and sandstone reservoir quality of (Lander & Walderhaug 1999) although recent studies have suggested that material flux via fault systems may be a significant aspect of sandstone diagenesis in some circumstances (Wendler et al 2012).…”