Petrography and diagenesis of the Bunter Sandstone Formation in the UK Southern North Sea

Rushton, J.C. ORCID:; Hannis, S.; Milodowski, A.E.. 2023 Petrography and diagenesis of the Bunter Sandstone Formation in the UK Southern North Sea. Nottingham, UK, British Geological Survey, 59pp. (OR/23/054) (Unpublished)

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This report describes a regional petrographic study of the Triassic aged Bunter Sandstone Formation (BSF) in the northern part of the UK Southern North Sea. To date, the spatial relationships leading to the prediction of regional reservoir quality in the BSF in terms of its potential for storage of CO2 or hydrogen have not been fully understood. Previous studies have been largely focused on the hydrocarbon fields and these have shown that the presence or absence of diagenetic cements can both create barriers and enhance pathways for fluid flow. More recent studies investigating CO2 storage potential through regional dynamic modelling have demonstrated the importance of physical and chemical property distributions. This applies not only to the immediate injection and containment of the CO2 plume but also to the impacts of the pressure footprints created from injecting a large volume of fluid and the potential interaction between neighbouring sites. However, a consensus allowing an extrapolation of reservoir quality prediction to the more data-poor saline aquifer parts of the BSF has not proved straightforward as evidenced by results of a CO2 storage exploration well drilled in 2010. This has highlighted the importance of improving regional understanding of reservoir storage potential for future CO2 or hydrogen appraisal activities. This report adds to the body of research attempting to discern reservoir rock property distribution in the BSF through detailed petrographic observations to understand their diagenetic histories through the analysis of 83 samples across 12 wells. Diagenetic observations are described and presented as a proposed diagenetic history in the figure below. Detrital and near-surface diagenesis (eodiagenetic) characteristics of the BSF are consistent with terrestrial deposition under arid conditions, principally with fluvial origins and a minor aeolian input. The eodiagenetic phases that are particularly characteristic of arid conditions include widespread and locally abundant nodular grains (carbonate and sulphate) and cements (also carbonate and sulphate) that preserve un-compacted and expanded (displaced) grain frameworks. Additionally, some cements have evaporitic textures (e.g. pseudomorphed ‘desert rose’ forms, enterolithic anhydrite). The carbonate nodules (calcite and dolomite) are an abundant framework grain constituent throughout the BSF. These are characterised by dominantly rounded sand sized forms with concentric structures defined by sequential zones of micritic and/or radial fibrous carbonate, and hematitic clays. Nodule cores comprise a mix of silt-grade silicate grains, mixed micrite and clays, and nodule fragments with evidence of varied degrees of reworking. They are most abundant in wells from the central to eastern parts of the study area (Quadrants 43 and 44). These nodules are not ooids sensu stricto, because evidence that they formed through both surface and shallow sub-surface processes is abundant and widespread. Carbonate nodules are locally concentrated in laminations and, together with associated eodiagenetic cements, form dolocrete and calcrete layers, mostly hosted in finer grained laminations with sub-millimetre to centimetre (plus) scale thicknesses. Variable lateral continuity typically reflects the structure of the hosting sedimentary laminations. These features present partial barriers to larger scale porosity interconnectivity and are of sub-seismic-resolution. Subsequent burial diagenesis (mesodiagenesis) is dominated by the formation of further widespread pore-filling cements, mostly of anhydrite and halite. These cements are typically also partially replacive of eodiagenetic nodules. Several episodes of cementation and some of dissolution, have been identified. These are generally poikilotopic cements, which in the case of anhydrite, differentiates it from eodiagenetic sulphate nodules. Anhydrite largely pre-dates halite. Enclosure of anhydrite by halite is largely passive (i.e. anhydrite crystals have euhedral margins), but there is local evidence for dissolution of anhydrite prior to, or during, halite emplacement. Both of these cement phases are preferentially developed in coarser sandstones. Major halite cement is only observed at and below a current burial depth of ~1400 m. This suggests that the halite distribution must be, in part, controlled by current and / or recent conditions. Locally, halite dissolution has occurred preferentially along coarser grained sandstone laminations in otherwise fully cemented intervals. Diagenetic silicate cements are rare over most of the study area. The exceptions to this are samples from the western edge of the study area (Quadrant 41 wells) where compaction textures are well developed and quartz cement is widespread. Heterogeneity of compactional textures is a key characteristic of the BSF observed across the study area. On a sub-millimetre scale, areas of well compacted framework grains exist next to areas with open and expanded fabrics. The looser textures can only partially be explained by grain replacement and dissolution, and the current distribution of diagenetic cements. We conclude that the samples were partially cemented prior to maximum burial, but the cement distribution has changed subsequently. It is clear that the halite and anhydrite cements in their current distribution, cannot have been the primary control on the degree of compaction currently observed in the BSF. This observed textural heterogeneity is consistent with our proposed diagenetic model, which infers that the sandstones had abundant, but not complete, early cements that preserved shallow framework fabrics. As these cements were partial, compactional fabrics were created in the surrounding less- or un-cemented zones. Subsequent dissolution, replacement and / or mobilisation of some or all of the cement phases, post maximum burial, has resulted in the widely recognised heterogeneous compaction fabric which does not correspond to current cement distributions. Since both anhydrite and halite show evidence for both multiple phases of formation and partial dissolution, these are the primary candidate minerals for dissolution / mobilisation. As these phases have also partially replaced some of the framework carbonate nodules, then their subsequent dissolution / mobilisation could also create an apparently uncompacted fabric. One expected outcome of abundant, pre-maximum-burial cementation, is that BSF porosities should be detached from a simple linear variation with maximum burial depth. This is what is observed for the BSF. It is recognised that the conclusions of this study are constrained by the limited numbers of samples (for the extent of the study area) and the fact that they are all sourced from hydrocarbon-interest boreholes which have targeted potential reservoir structures. Many of these structures are a consequence of site-specific halokinesis, therefore with potentially atypical thermal, fluid and structural conditions. Whilst we have gained significant insight into the diagenetic paragenesis, we are unable to adequately predict porosity, a major interest for CO2 and energy storage interests. This is a consequence of the heterogeneities of the BSF in texture, cement distribution and paragenesis. To improve the remaining knowledge gaps and predictabilities of major reservoir properties, further studies are needed: 1. To obtain a better understanding of the distributions of grain fabrics and diagenetic cements, in order to improve predictability of pore size and connectivity, and porosity distribution at a regional scale: • Extend the study to include more samples for detailed modal analysis and minus cement porosity calculation. • Apply petrographic image analysis to more samples and a wider range of properties to characterise actual porosity, grain size and compactional fabric distributions. 2. Develop a high-resolution diagenetic sequence through isotopic studies of the main cements, tied to their paragenetic sequencing using: • Strontium isotope analysis (87Sr/86Sr) to inform the origins of solutes in the diagenetic fluids, and extent of rock-water interaction (target phases - calcite, dolomite, anhydrite and halite). • Stable (oxygen, carbon, sulphur) isotope analysis (δ13C, δ18O and δ34S). These techniques would further inform the mineralisation temperature, and carbon and sulphur sources (target phases - anhydrite, dolomite and calcite cements). • U-Pb dating to obtain absolute dates for carbonate mineral formation. Using petrographically-guided targeting, this will place the paragenetic sequence in absolute time. A major issue for this will be potential contamination of the carbonate phases by finely-disseminated hematite, which is known to preferentially concentrate U and Pb.

Item Type: Publication - Report
Funders/Sponsors: British Geological Survey
Additional Information. Not used in RCUK Gateway to Research.: This item has been internally reviewed, but not externally peer-reviewed.
Date made live: 25 Jan 2024 10:50 +0 (UTC)

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