Grain assemblages and diagenesis in the tarl-dominated Lower Silurian mudrock succession of the western margin of the east European craton in Poland and Lithuania

Around 27% of the sample set has grain assemblages that include biogenic grains in amounts over 10%. Most of these are calcareous tarls (calcareous-terrigenous argillaceous mudrocks), but do not display discernible carbonate cements. Only three of the studied samples contain grain assemblages that originally included a significant component of biogenic silica. These three samples contain microquartz cement (up to 30% of rock volume) and migrated pore-filling organic matter that has developed secondary porosity. Where thermally mature and sufficiently thick, these local intervals of silica-cemented mudrocks may offer interesting targets for development of unconventional gas.