An Integrated Approach Towards a Holistic Sedimentological and Pore-Scale Characterisation of Fine-Grained (Shale) Unconventional Reservoirs

Abstract

For a holistic characterisation of fine-grained unconventional reservoirs, our integrated approach utilises coherent, reproducible datasets and refined work practices carried out within a comprehensive quality management system. A typical reservoir characterisation includes integration of sedimentological, structural and pore-scale datasets, however, the specific work flow design depends entirely on the nature of the problem and the availability of appropriate data. This paper illustrates an example of unconventional reservoir characterisation of a fine-grained formation from the North Sea where the specific aims were to establish the depositional framework for recognising sedimentary environments, recommend sample locations to target specific queries within the sedimentological context for petrographical and geochemical analyses, to investigate what porosity types are present and to assess 'brittleness' of the rocks.First and foremost, high-resolution interpretative graphic core descriptions were carried out at 1:24 scale, utilising Badley Ashton's mudrock-specific lithotypes and depositional packages schemes. Lithotype characterisation uniquely captures very fine-scale attributes (bed to subbed scale), whilst upscaled depositional packages (bed-stack scale) provide a more holistic characterisation from core, as well as from wireline and image logs, where available. Plugs coded by the above descriptors were selected post-logging, for detailed petrographical and geochemical analysis including Rock-Eval pyrolysis within the sedimentological/structural context. Mineralogical data was acquired by whole-rock and clay-fraction XRD analysis, whilst pore-scale fabric/textural investigation were undertaken by conventional light microscopy and BS-SEM. A subset of the plugs was subjected to FIB SEM analysis to characterise any potentially organic matter associated pore system. All these different strands of data were then integrated to evaluate and link the depositional system/sedimentary environment, storage capacity and brittleness of the reservoir in order to assess the overall reservoir potential of the fine-grained formation.