Regional characterisation of the Triassic Bunter Sandstone: implications for reservoir connectivity and fluid flow

Introduction

The Bunter Sandstone Formation is a proven gas reservoir in the Southern North Sea (e.g. Hewett, Esmond and Forbes fields) as well as a key reservoir interval that may facilitate carbon storage in the UK Southern North Sea and beyond. Despite increased interest in the formation’s CO₂ storage potential and numerous wellbore penetrations, there remains notably little published information on reservoir composition and lithofacies heterogeneity at a regional scale. To fully utilise this key interval, characterisation of the lithofacies will enable interpretation of the regional depositional environment and hence prediction of the presence and scale of any potential baffles and barriers to flow, is needed. The Bunter Sandstone Formation and its equivalents in the study region comprise laterally variable continental red bed sequences deposited under fluctuating arid to semi-arid conditions in endorheic ephemeral fluvial/lacustrine-palustrine and aeolian environments. The formation is capped by mudstones and evaporites of the overlying Haisborough Group and the equivalent onshore Mercia Mudstone Group.

Method

This work uses data from a series of wells in the Southern North Sea, as well as a previously unstudied onshore borehole, to build a broad understanding of spatial and temporal variability in depositional environments. The wells cover a wide study area across the UK onshore, UK offshore and Dutch offshore sectors. A depositional model has been constructed through detailed sedimentary logging of 1.2 km of core that is linked to an extensive high-resolution porosity and permeability database. Over 200 samples have been collected and analysed via a combination of SEM-EDS and petrographic techniques to gain detailed compositional and diagenetic information. This is supplemented by over 1800 XRF measurements, which provide a comprehensive dataset that is used to understand how sandstone composition varies at a high resolution.

Examples

Results show the occurrence of low porosity, low permeability facies, primarily within playa, playa margin and overbank depositional settings, throughout the region, albeit with some spatial variability. Such, lithological baffles within the studied core can be metres thick although the lateral extent is strongly governed by depositional setting. The presence of these baffles will have a major impact on fluid flow within the sandstone dominated reservoir. Various cement mineral phases are observed within the samples including dolomite, calcite, anhydrite and halite. These cements can obliterate porosity. The occurrence of these cement minerals varies between wells but also within wells on a centimetre scale. Understanding which cement mineral phases are more likely to be present within the reservoir will help predictions of sandstone reactivity upon CO₂ injection. Comparison of sandstone composition between wells reveals spatial trends in the dominant feldspar and cement phases that may be linked to provenance and/or diagenesis.

Conclusions

This work documents and characterises lithological baffles within the Bunter Sandstone that are the main cause of the lowest reservoir quality intervals. Diagenetic cement phases vary in proportion and type, often having a major impact on porosity and permeability. The effects of these diagenetic and sedimentological complexities on hydrocarbon reservoir quality vary spatially and can serve to compartmentalise reservoir facies. Moreover, the numerous baffles and barriers within the target reservoir intervals, will likely add complexity to CO₂ plume pathways within the reservoir and enhance residual trapping.