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.

Abstract Tullich Field, operated by Kerr-McGee Oil (UK) plc with 100% interest, lies in UK block 9/23a, to the southeast of Gryphon, and to the east of the Harding fields. The field is a subsea tie-back from a central manifold to the Gryphon ‘A’ FPSO (floating production, storage and offloading) facility. The southern part of the field was discovered by Hamilton in 1991–1992. Well 9/23a-27 and its sidetrack encountered thin oil and gas bearing sandstones of Lower Eocene age from the Balder Formation, which tested 570 bpd of 25.8 degree API oil. Kerr-McGee recorded a 3D seismic survey over the area in 1990, and in 1999 acquired a 3D OBC seismic survey over the Gryphon Field and the northern part of block 9/23a. The similarity of the inverted shear wave seismic data over the Gryphon Field and the northern part of block 9/23a was the catalyst for the further exploration and appraisal of the area. This led to an exploration drilling programme in 2001, which included the acquisition of full waveform shear sonic data and core. In July 2001, a separate discovery was made by Kerr-McGee with the drilling of 9/23a-29A, to the north of the 9/23a-27 wells. Thick oil bearing sandstones of excellent reservoir quality were encountered in the Balder Formation. The field was delineated between July and December 2001 by sidetracking 9/23a-29A and drilling the 9/23a-31 well. From a windowed extraction of peak amplitudes of the shear seismic data, it was noted that at Gryphon the high amplitudes corresponded to the presence of thick reservoir sandstones, whilst at Tullich it appeared that the high amplitudes related to a non-reservoir argillaceous sandstone sequence rather than the much thinner reservoir sandstones. These reservoir sandstones lie at different stratigraphic levels in the B2 unit of the Balder Formation but appear to be sufficiently in communication to act as a single reservoir unit. They are currently interpreted as sandstones deposited by turbidity currents which pinch out against the plunging nose of the Crawford Ridge. In order to take advantage of synergy with other work in the Gryphon area, a tight timeframe was imposed on the project. A four-well development drilling programme began immediately once the Department of Trade and Industry had given their approval of the Field Development Plan (FDP) in March 2002 and was completed by October 2002, with first oil achieved at the end of August 2002.

Abstract The Gryphon Field was discovered in 1987 in Quadrant 9 in the Beryl Embayment. Oil was encountered in a thick Balder Formation sandstone, and the reservoir was interpreted as lobes of a submarine fan system, such as many of the prolific early Tertiary fields in the North Sea. After an extensive appraisal phase, oil production started in 1993 through the Gryphon floating production, storage and offloading vessel. After a successful initial development phase, the integration of production data, improved and regularly acquired seismic data, and a better geological understanding resulted in the identification of sandstone intrusions. These have since been interpreted to form a volumetrically significant part of the Gryphon reservoir. The drilling of further infill wells, and the development of satellite fields Maclure, Tullich and the future Ballindalloch, ensued from this change to the geological model. To date, the Gryphon, Maclure and Tullich fields have produced more than 200 MMbbl of oil compared to an initial reserve estimate of 151 MMbbl. Although the current and mid-term focus remains on maximizing oil production, the final phase of the wider Gryphon area fields’ development should see the production of the regional gas cap.

Abstract Several productive Paleogene deepwater sandstone reservoirs in the North Sea show evidence of having undergone post-depositional remobilization and clastic injection, which can result in major disruption of the primary reservoir distribution (e.g ., Alba, Forth/Harding, Balder, and Gryphon fields). Case studies of deepwater sandstones from UK Quadrants 9, 15, 16 and 21 are presented to illustrate the wide spectrum of remobilization features, which range from centimeters (e.g ., core-scale) to hundreds of meters (e.g ., seismic-scale). Most common are clastic injection structures such as dikes and sills. Sills of massive sand, over 20 m thick, have been identified. Intrusions associated with the propagation of syn- to early post-depositional, dewatering-related polygonal fault systems in adjacent deepwater mudrocks are also common. The scale of the clastic intrusion and remobilization has significant impact on reservoir architecture and production performance, including changes in (a) original depositional geometries; (b) reservoir properties; (c) connectivity, (d) top reservoir surface structure, (e) reservoir volumetrics, and (f) recovery/performance predictions. There are several prerequisites for sandstone intrusions to form: the source sediment must be uncemented, and the ‘parent’ sand body must be sealed such that an overpressure with a steep hydraulic gradient can be generated. The seal on the overpressured sand body must then be breached for the sand to fluidize and inject. The stress state within the basin, burial depth, fluid pressure and the nature of the sedimentary host rock all contribute to the final style, geometry and scale of intrusion. At shallow depths, within a few meters of the surface, small irregular intrusions are generated, more commonly forming sills, whereas at greater depth larger and more continuous dikes and sills form clastic intrusion networks. Field examples from the Ordovician in Ireland, and Panoche Hills in California are used to illustrate the control of burial depth/stress on intrusion scale. Earthquake induced liquefaction, tectonics stresses and build-up of excess in-situ pore pressure are the most commonly cited explanations for the occurrence of clastic intrusions. However, our work suggests that the large-scale, ‘catastrophic’ sandstone intrusions within the North Sea Paleogene, which remobilized hundreds of cubic meters of sediment, probably require the presence of fluids migrating from deeper within the basin (e.g ., gas charge) to drive the injection. Deepwater sand bodies within the North Sea that appear most susceptible to remobilization occur in mud-dominated successions and include (1) narrow, elongate channel or gully-filled sands (i.e ., non-leveed channel systems), and (2) isolated sand-rich mounds (e.g ., ‘ponded’ sand bodies and terminal fan lobes). Sand bodies located above rift-related basin-forming faults, which periodically appear to have acted as vertical fluid escape pathways, were especially susceptible to remobilization. Sand remobilization may influence reservoir distribution in other mud-dominated, deepwater depositional systems.