Diagenetic controls on the reservoir quality of the tight gas Collyhurst Sandstone Formation, Lower Permian, East Irish Sea Basin, United Kingdom

Abstract

The Lower Permian Collyhurst Sandstone Formation is an important gas reservoir both offshore and onshore in the East Irish Sea Basin of north-western UK, and is considered to be a tight gas reservoir. Diagenetic processes have significantly influenced the reservoir quality of the sandstone. This study employs petrographic, mineralogical, and stable isotopic analyses on two different onshore core successions to investigate the composition and texture, the diagenetic history, and the impact of lithology and diagenesis on reservoir quality. The Collyhurst Sandstone Formation consists of fine- to very coarse-grained, poorly to well-sorted, sub-arkosic to lithic arenite sands. The reservoir properties of the sandstone are relatively poor, with average porosity and permeability of 6.6% and 0.387 mD respectively. On the basis of petrographic analyses, five sandstone lithofacies were identified: fine-grained, conglomeratic, carbonate-cemented, quartz-cemented and matrix-free. Contrasting characteristics between the reservoir units suggest the development of divergent diagenetic pathways. Features observed during early diagenesis include compaction, growth of pore-filling early carbonate cements (calcite I, dolomite and siderite I) and vermicular kaolinite. Late diagenetic features include: further carbonate cementations (ferroan dolomite/ankerite, calcite II and siderite II-III), clay mineral cementation (illite and chlorite) with both grain-coating and pore-filling textures. Also common in late diagenesis are syntaxial quartz overgrowths. Albite, barite and anhydrite are minor late diagenetic cement phases. Compaction processes rather than cementation accounts for the major destruction of the initial sandstone porosity. But cementation still accounts for up to 32% of porosity loss. Carbonate cements mostly fill intergranular pores, thus block pore throat and prevent migration of fluid. Clay minerals bridge and occlude pores, resulting in further destruction of the reservoir quality. The study provides insights into how lithofacies (primary composition, texture and sorting) and diagenesis can influence reservoir qualities of tight gas sandstones.