Clay mineral transformation as a major source for authigenic quartz in thermo-mature gas shale

Studies of laboratory-treated Na+-smectite and Pierre Shale and natural thermo-mature gas shale indicate that, large quantities of silica are released during clay mineral transformation to form quartz cement (within a closed shale system). Two types of quartz cement are recorded in lab treated samples; granular silica, formed with all lab treated samples (Na+-smectite and Pierre Shale) at treatment temperatures (100–150 °C) and durations (up to 4 months), was seen as rims over the minerals remnance; sheet-like silica, formed on Pierre Shale samples only that was treated at 150 °C for more than a month, was seen as very thin sheets covering the mineral remnance or cementing the granular silica. The granular shape silica was seen on natural gas-shale samples as silica rims or silica-rich micro-grains contain significant amounts of inter-grain micropores while the sheet type silica was seen as micro sheets (< 100 nm) along the bedding cementing the micrograins. Lab diagenesis of Pierre Shale at 150 °C for 4 months produced minerals and textures similar to those of studied natural gas shale which could be formed through similar conditions, within a closed system. The quartz cement acts as shale stiffening agent while the sheet-like quartz enhances shale anisotropy and fracability. Detailed study of sheet-like quartz will have important implications in modeling the elastic behavior, fluid flow and the gas storage mechanism.

► Laboratory-treated Na+-smectite and Pierre Shale and natural gas shale were studied. ► Free silica is released during clay mineral transformation to form quartz cement. ► Quartz cement is recorded in lab treated samples as granular and sheet-like shape. ► Lab diagenesis of clay rich shale produced minerals and textures similar to gas shale. ► Silica cement acts as stiffening agent while the sheet quartz enhances the anisotropy.