Chapter 2 - Sedimentary features of tight gas reservoirs

Abstract

Tight gas sandstones before classified as unconventional reservoirs are primarily grouped in medium-sized clastic rocks, and thus they have the same depositional characteristics as conventional sandstone reservoirs. This means, sediment provenance, and the action of sedimentary characteristics on their composition, texture, and structure for both conventional and unconventional sandstone reservoirs could be similar. Therefore, it is essential to first characterize these reservoirs based on the primary depositional characteristics to reach a comprehensive understanding of the factors controlling their reservoir properties. Texturally, tight sandstones, under the influence of sedimentary environment conditions, show a wide range of grain sizes from very fine to very coarse and granular, and in most cases are characterized by heterolithic lithology in their interval. Mineralogically, tight sandstones are classified into different groups of arenites (quartz arenites, feldspathic arenites, and lithic arenites) and graywackes. However, sandstones with low mineralogical maturity such as feldspathic-lithic-rich ones are more prone to tightness under the effect of diagenetic processes such as compaction and cementation. The main diagenetic cement of tight sandstones is silica, carbonate, and clay minerals. In addition to compaction, this cement by occluding pore spaces and pore throats has a significant effect on reducing the volume and connection of the pore system resulting in the development of low permeability and tight sandstone reservoirs.