Abstract
Abstract. Fracture attribute scaling and connectivity datasets from analogue systems are widely used to inform sub-surface fractured reservoir models in a range of geological settings. However, significant uncertainties are associated with the determination of reliable scaling parameters in surface outcrops. This has limited our ability to upscale key parameters that control fluid flow at reservoir to basin scales. In this study, we present nine 1D-transect (scanline) fault and fracture attribute datasets from Middle Devonian sandstones in Caithness (Scotland) that are used as an onshore analogue for nearby sub-surface reservoirs such as the Clair field, west of Shetland. By taking account of truncation and censoring effects in individual datasets, our multiscale analysis shows a preference for power-law scaling of fracture length over 8 orders of magnitude (10−4 to 104 m) and kinematic aperture over 4 orders of magnitude (10−6 to 10−2 m). Our assessment of the spatial organization (clustering and topology) provides a new basis for up-scaling fracture attributes collected in outcrop- to regional-scale analogues. We show how these relationships may inform knowledge of geologically equivalent sub-surface fractured reservoirs.
Highlights
Fractures – used in this paper as a general term to include faults, joints and veins – fundamentally control the fluid flow and mechanical properties of many crustal rocks, including many sub-surface reservoirs holding oil, gas or water (e.g. Nelson, 1985; Sibson, 1996; Adler and Thovert, 1999; Odling et al, 1999) or potential sub-surface repositories (De Dreuzy et al, 2012)
The Group 3 structures of the Orcadian Basin clearly formed in the sub-surface, and we argue that they are the best direct analogue for the oil-bearing fracture systems that occur in the Clair Group reservoir
Fracture attribute analyses are often conducted on field outcrop analogues because they can provide useful information to bridge the gap between faults imaged in geophysical datasets and fractures observed in borehole data
Summary
Fractures – used in this paper as a general term to include faults, joints and veins – fundamentally control the fluid flow and mechanical properties of many crustal rocks, including many sub-surface reservoirs holding oil, gas or water (e.g. Nelson, 1985; Sibson, 1996; Adler and Thovert, 1999; Odling et al, 1999) or potential sub-surface repositories (De Dreuzy et al, 2012). Establishing the size, spatial organization, connectivity, scaling and fracture-fill properties of fluid-conductive structures is crucial to understanding the performance of sub-surface reservoirs in a range of lowporosity/permeability rock types (see review by Laubach et al, 2019). In sub-surface reservoirs, fracture description is typically performed on image logs and drill cores that provide high-resolution (10−4 to 100 m) but highly censored (size limited by borehole diameter), spatially limited and biased 1D samples (e.g. Odling et al, 1999; Zeeb et al, 2013). Berkowitz and Adler, 1998); reservoir analogues can give access to fracture datasets across many scales (10−2 to 106 m scales) and in one, two and three dimensions for use in reservoir models (Jones et al, 2008). Statistical analysis of fracture attributes from appropriate outcrop analogues can provide reliable and robust geological (conceptual models) and quantitative (attribute and scaling) information to inform. Dichiarante et al.: Fracture attribute scaling and connectivity in the Devonian Orcadian Basin the planning of exploratory and development drilling, and design and conditioning of reservoir simulation models (Mäkel, 2007)
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