Extensive authigenic quartz overgrowths in the gas-bearing Haynesville-Bossier Shale, USA

In sandstone reservoirs, despite grain rearrangement during compaction, significant pore volumes may be retained prior to the onset of late diagenetic quartz cementation. In mudstone reservoirs, grain rearrangement during compaction results in significant pore volume reduction prior to late diagenesis. Where quartz overgrowths have been previously reported in post-compaction mudstones they have been in volumetrically low concentrations and interpreted as anomalous occurrences. Quartz cementation alters rock brittleness resulting in changes to mechanical fracture properties. Quartz overgrowths reduce reservoir porosity and permeability. We present petrographic evidence of two phases of quartz cement in the Haynesville-Bossier Shale: (i) grain replacive and (ii) quartz overgrowths. Carbonate grain replacement is volumetrically low (< 1%). Quartz overgrowths identified from SEM-CL imaging are volumetrically more significant (8–13%). Quartz overgrowths were most commonly observed in the coarse mudstone microfacies, but are present in both medium and fine mudstone microfacies. Petrographic evidence indicates three processes in the development of quartz overgrowths. Mica and pyrite are (i) engulfed and (ii) displaced by quartz overgrowth cement. The absence of a supportive, primary granular framework surrounding engulfed detrital and early authigenic minerals would indicate that quartz overgrowths are also replacive. Pressure dissolution of quartz silt grains and smectite-to-illite transformation are likely sources of silica for quartz cement. This study is the first to document large-scale, replacive, authigenic quartz overgrowth development within a producing mudstone.