Characterisation of the Early-Late Triassic Stratigraphy of the Solway Firth, UK: Potential for CO2 storage

Abstract

The Solway Basin and onshore Carlisle Basin (Solway Firth) contains the Early-Middle Triassic (Olenekian-Anisian) Sherwood Sandstone Group (SSG) reservoir and the Middle-Late Triassic (Anisian-Rhaetian) evaporitic mudstone seal of the Mercia Mudstone Group (MMG). This succession forms a ‘saline aquifer’ that is locally under-characterised. Imminent CO2 storage in analogous stratigraphy in the depleted hydrocarbon fields of the adjacent East Irish Sea Basin (EISB) raises the question whether the Solway Basin could have similar utility. Onshore outcrop analogues, legacy offshore hydrocarbon data and well cuttings are used to characterise this reservoir-seal system and determine its suitability for CO2 storage. An extensive petrographic, diagenetic, depositional and reservoir quality investigation of the SSG reservoir has been conducted, alongside burial history modelling and stable isotope analysis. The Solway Basin was likely connected to the EISB, forming a distal section of the vast (>400 km) overarching ‘Budleighensis’ river system. A detailed facies evolution study suggests the very fine-grained fluvial facies of the Annan Sandstone Formation, with stacked channels, flood plain fines and playa facies, represents the terminal site of this significant fluvial system. The aeolian dune facies and subordinate playa facies of the Kirklinton Sandstone Formation marks its abandonment and retraction from the Solway Basin. Facies have played a key role in reservoir quality distribution. The heterogenous nature of the Annan Sandstone Formation is reflected in its porosity (2-25%), where reservoir quality is moderate within the ribbon channel facies but has been severely impeded in finer grained facies by two phases of calcite cementation, quartz cementation and pore-filling matrix illite. The Kirklinton Sandstone Formation has preserved excellent reservoir quality (porosity 14-28%), similar to or better than benchmark EISB sandstones despite a finer grain-size, mainly due to clay grain-coats. This petrography is thought geochemically optimum for CO2 sequestration, with a significant storage potential (~500 Mt). The MMG was characterised mainly using XRD, SEM and wire-line data, revealing an excellent, thick (700-800 m) seal of pure halite beds, halite-mudstones and silty to sandy claystones. The illite-rich caprock will likely be stable during CO2 storage geochemically. The greatest risks to caprock integrity are chlorite and dolomite dissolution and heterogenous silt and sand presence. The Solway Basin is seen as an excellent CO2 storage target that is not hindered by the issues of previous hydrocarbon extraction but where characterisation has been aided by regional hydrocarbon reservoir analogues. The facies development and reservoir qualities identified suggest that Triassic targets that were distal or marginal to fluvial systems can form prime CO2 storage sites.