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Abstract

The numerous sandstone injections found associated with the Gryphon field in the United Kingdom North Sea are mostly small-scale intrusions less than 30 cm (12 in.) thick. The largest intrusions identified in core and wire-line-log data from the Gryphon field are approximately 8 m (26 ft) thick, but these large features are uncommon. The intrusions form two main populations of interconnected steeply dipping dikes (≥60°) and sills (≤20°), with a lesser number of intrusions with moderate dips. Although small, centimeter-scale injections dominate the intrusion population, these small intrusions cluster around thicker dikes and sills (>20-30 cm [>8–11.8 in.] thick) that are localized at the margins and above the field. Sandstone injections are found as much as 170 m (557 ft) vertically above the main Gryphon reservoir sandstone and several hundred meters laterally from the parent sandstone body. Greater numbers of dikes exist in the first approximately 80 m (262 ft) above a top reservoir datum, and at higher levels, sills are more numerous. A well-by-well analysis of the intrusion distributions shows that they cluster at different heights above the top reservoir; injections are not equally spaced. Examination of the total cumulative thickness of intrusions measured in the recovered core and intrusion thickness interpreted from wire-line logs beyond the extent of the core suggests that there may be twice the volume of injected sand on the field flanks, margins, and off-field positions than over the center of the main reservoir sandstone. Integration of the observations from core and wire-line logs allows a new model for the sandstone injection complexes on Gryphon to be developed. This model suggests that a complex hierarchy of intrusion scale exists, with thin intrusions branching off the main intrusive network. The dip distribution of the injection population is influenced by the depth relative to the main reservoir sandstone, and the spatial distribution of the intrusion shows that this network is best developed around the margins of the field. Correlation of core and wire-line-log interpretations with seismic data indicates that a seismically identifiable discordant facies is most likely composed of localized interconnected networks of sandstone dikes and sills (an injection complex) in connection with the main reservoir and is not necessarily a single, simple intrusion made up of 100% intruded sand.

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