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

Abstract

Detrital grain assemblages in mudrocks establish a key starting condition for the diagenetic pathways that drive the evolution of rock properties in unconventional petroleum systems. This case study is based on petrographic examination (light microscopy, elemental mapping, and cathodoluminescence imaging) of 41 subsurface samples of Silurian mudrocks from the western margin of the East European Craton (EEC) in Poland. About 2/3 of the samples have grain assemblages composed by >90% of grains of quartz, feldspar, mica, and clay of extrabasinal derivation. Mechanical compaction is the principal cause of porosity decline in these terrigenous-argillaceous mudrocks (tarls). Where thermal maturation has reached a level sufficient to induce smectite illitization, minute amounts of authigenic silica (≪1 vol%) are observed in the small (~100 nm) primary intergranular pores that survived compaction. Organic-rich mudrocks in this succession also display prominent compaction and contain little organic-matter-hosted porosity. Although these organic-rich tarls may have served as effective source rocks as unimpeded compaction drove expulsion of generated hydrocarbons, they do not in general have sufficient porosity and stiffness to serve as effective reservoirs.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.