ALBERT

Description: Hyaloclastites commonly form high-quality reservoir rocks in volcanic geothermal provinces. Here, we investigated the effects of confinement due to burial following prolonged accumulation of eruptive products on the physical and mechanical evolution of surficial and subsurface (depths of 70 m, 556 m, and 732 m) hyaloclastites from Krafla volcano, Iceland. Upon loading in a hydrostatic cell, the porosity and permeability of the surficial hyaloclastite decreased linearly with mean effective stress, as pores and cracks closed due to elastic (recoverable) compaction up to 22-24 MPa (equivalent to ~1.3 km depth in the reservoir). Beyond this mean effective stress, denoted as P∗, we observed accelerated porosity and permeability reduction with increasing confinement, as the rock underwent permanent inelastic compaction. In comparison, the porosity and permeability of the subsurface core samples were less sensitive to mean effective stress, decreasing linearly with increasing confinement as the samples compacted elastically within the conditions tested (to 40 MPa). Although the surficial material underwent permanent, destructive compaction, it maintained higher porosity and permeability than the subsurface hyaloclastites throughout the experiments. We constrained the evolution of yield curves of the hyaloclastites, subjected to different effective mean stresses in a triaxial press. Surficial hyaloclastites underwent a brittle-ductile transition at an effective mean stress of ~10.5 MPa, and peak strength (differential stress) reached 13 MPa. When loaded to effective mean stresses of 33 and 40 MPa, the rocks compacted, producing new yield curves with a brittle-ductile transition at ~12.5 and ~19 MPa, respectively, but showed limited strength increase. In comparison, the subsurface samples were found to be much stronger, displaying higher strengths and brittle-ductile transitions at higher effective mean stresses (i.e., 37.5 MPa for 70 m sample, 〉75 MPa for 556 m, and 68.5 MPa for 732 m) that correspond to their lower porosities and permeabilities. Thus, we conclude that compaction upon burial alone is insufficient to explain the physical and mechanical properties of the subsurface hyaloclastites present in the reservoir at Krafla volcano. Mineralogical alteration, quantified using SEM-EDS, is invoked to explain the further reduction of porosity and increase in strength of the hyaloclastite in the active geothermal system at Krafla.

Description: Deeply buried sandstones in sedimentary basins typically have low porosity due to cementation and compaction. There are several known causes of anomalously high porosity in sandstones, one of which is microquartz coatings on sand grains that seem to inhibit growth of quartz cement. However, there has been no mechanistic understanding of why or how microquartz grows, or why it maintains high porosity in sandstones. Here we have used high-resolution scanning electron microscopy, electron backscattered diffraction, and transmission electron microscopy to study the microquartz-cemented Late Cretaceous Heidelberg Formation, Germany. We have revealed that a nanofilm of amorphous silica (50–100 nm) and a layer of chalcedony are between the detrital grain and microquartz cement. The amorphous silica insulates the detrital quartz grains and prevents syntaxial growth, while microquartz adopts the orientation of the underlying chalcedony with its fast-growth c axis parallel to the grain surface, thus preventing growth into the pore. Now that we know what controls microquartz growth and why it preserves porosity, it can be used to help identify, rank, and appraise deeply buried petroleum accumulations.