Cementation and structural diagenesis of fluvio-aeolian Rotliegend sandstones, northern England

Abstract

Quartz cementation in sandstones is closely linked to grain coating phases and diagenetic alteration. Grain coatings consisting of illite smectite stained with iron oxides and hydroxides are able to preserve large amounts of porosity by preventing the formation of syntaxial quartz overgrowth cement. The Penrith Sandstone Formation was chosen as an analogue for Rotliegend reservoirs to test the impact of grain coatings on quartz cementation. This adds to an existing model of cementation. Differences of grain coating coverage can be linked to grain size. Extensive grain coatings are present in finer grained laminae in some samples. Coarser grained laminae contain less extensive grain coatings. The analysis of grain coatings based on standard petrographic analyses is combined with high resolution QEMSCAN® analyses. Structural features include deformation bands of different ages. Diagenetic alterations around faults, recorded by grain coatings, allow the delineation of relative temporal relations, revealing at least two generations of deformation band formation associated with normal faulting. In the Vale of Eden succession one normal faulting event postdates burial diagenetic quartz cementation as is evident by fault focused fluid flow and associated bleaching of iron and absence of quartz overgrowth. Repository KITopenDies ist ein Postprint/begutachtetes Manuskript.

Three‐Dimensional Phase‐Field Investigation of Pore Space Cementation and Permeability in Quartz Sandstone

“…Simulations of constrained growth in multigrain system signify the following characteristic similarities of the numerically cemented grains (visualized along central plane of 3‐D domain) with the natural thin sections of Busch et al (): Flat facets (euhedral form) are developed when enough space is available for syntaxial overgrowth cementation. Randomly shaped grains are formed due to constrained growth when sufficient space in not present. Grains appear from adjacent planes (which further advocates the advantage of 3‐D numerical studies over 2‐D simulations). …”

“…Moreover, 3‐D phase‐field simulations of cementation in sandstone with the calibrated value of c / a ratio were successful in reproducing natural crystal morphologies and size effects as enumerated below. Simulations of undisturbed cement growth indicate that fine grains develop their euhedral form faster compared to coarser ones, as reported in experiments (Lander et al, ). Simulations of constrained growth in multigrain system signify the following characteristic similarities of the numerically cemented grains (visualized along central plane of 3‐D domain) with the natural thin sections of Busch et al (): Flat facets (euhedral form) are developed when enough space is available for syntaxial overgrowth cementation. Randomly shaped grains are formed due to constrained growth when sufficient space in not present. Grains appear from adjacent planes (which further advocates the advantage of 3‐D numerical studies over 2‐D simulations). Simulations of different‐sized multigrain systems indicate two important aspects about kinetics of pore space sealing: The nonlinear porosity‐time curve for each grain size r is convex with respect to time, as suggested by Wangen (). The rate of porosity loss in finer grain packs is larger than that in coarser ones. This signifies faster cementation of finer sands that is in agreement with the experimental works of Heald and Renton () and Lander et al (). The analysis of pore size statistics at different stages of cementation reveals that the relative frequency distribution evolves from bell‐shaped curve to lognormal patterns. CFD analysis performed on the numerically cemented microstructures for computing the permeability at different stages of cementation reveals two important aspects of the obtained permeability‐porosity relationships: Coarse sands possessing the same porosity as finer ones have a comparatively larger permeability that is in agreement with the published literature (e.g., Füchtbauer, ). Permeability‐porosity curves follow a power law relationship with exponents lying between 3 and 5, as suggested by empirical equations of Timur (). …”

“…Progression is shown at representative time steps. (h) Visualization along the central plane of 3‐D domain for comparison with thin sections of natural samples (Busch et al, ). (i) At t = 245, following similarities in patterns between simulation result and natural sample (in outlet picture) can be observed—(I) constrained growth due to grain boundary interference leading to random shapes, (II) development of flat facets (euhedral form) when enough pore space is available for growth, (III) appearance of grains from adjacent planes, as highlighted with green lines.…”

“…2. Simulations of constrained growth in multigrain system signify the following characteristic similarities of the numerically cemented grains (visualized along central plane of 3-D domain) with the natural thin sections of Busch et al (2017): a. Flat facets (euhedral form) are developed when enough space is available for syntaxial overgrowth cementation.…”

“…Differences in thermal exposure result in different volumes of quartz cements within similarly composed sandstones. For instance, the lower thermal exposure of less compacted Rotliegend sandstones in northern England results in a smaller amount of quartz cements and more retained porosity, shown in blue color in Figure a (Busch et al, ), as opposed to Rotliegend sandstones from northern Germany (shown in Figures c and d; Busch et al, ). The pore space in samples that experienced higher thermal exposure is completely occluded by quartz cements; see Figures c and d.…”