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Mini thief zones : sub-centimetre sedimentary features enhance fracture connectivity in shales

McCay, Alistair T. and Shipton, Zoe K. and Lunn, Rebecca J. and Gale, Julia F. (2019) Mini thief zones : sub-centimetre sedimentary features enhance fracture connectivity in shales. American Association of Petroleum Geologists Bulletin, 103 (4). pp. 951-971. ISSN 0149-1423

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    This study investigates the influences on fluid flow within a shale outcrop where the networks of two distinct palaeo-flow episodes have been recorded by carbonate-filled veins and green alteration halos. Such direct visualisation of flow networks is relatively rare and provides valuable information of fluid flow behaviour between core and seismic scale. Detailed field mapping, fracture data, and sedimentary logging were used over a 270m2 area to characterise the palaeo-fluid flow networks in the shale. Distal remnants of turbidite flow deposits are present within the shale as very thin (1-10mm) fine grained sandstone bands. The shale is cut by a series of conjugate faults and an associated fracture network; all at a scale smaller than seismic detection thresholds. The flow episodes utilised fluid flow networks consisting of subgroups of both the fractures and the thin turbidites. The first fluid flow episode network was mainly comprised of thin turbidites and shear fractures, whereas the network of the second fluid flow episode was primarily small joints (opening mode fractures) connecting the turbidites. The distribution of turbidite thicknesses follows a negative exponential trend; which reflects the distribution of thicker turbidites recorded in previous studies. Fracture density varies on either side of faults, and is highest in an area between closely spaced faults. Better predictions of hydraulic properties of sedimentary-structural networks for resource evaluation can be informed from such outcrop sub-seismic scale characterisation. These relationships between the sub-seismic features could be applied when populating discrete fracture networks models, for example, to investigate such sedimentary-structural flow networks in exploration settings.