Sand injectites are the products of post-depositional remobilisation and injection of sand during shallow burial; they comprise sand dykes, sills and injection breccias.

Outcrop examples of these features have been known for well over a century (e.g. Murchison 1827; Newsom 1903), but they were only recently recognised in the subsurface of the northern North Sea where they constitute a prolific play in the deep-water Paleogene (Jenssen et al. 1993; Newman et al. 1993; Newton & Flanagan 1993; Timbrell 1993; Dixon et al. 1995; Lonergan et al. 2000; Duranti et al. 2002). Sand injectites occur in most siliciclastic environments but they are particularly common in association with the deep-water reservoirs of the North Sea Paleogene (Figs 1.1, 1.2). An increasing number of observations indicate that sand injectites occur in several other hydrocarbon plays, such as the Upper Jurassic and Lower Cretaceous of the North Sea, the Jurassic, the Cretaceous and Paleogene of the NW European Atlantic Margin, and in the deepwater offshore West Africa.

Sand injectites in the subsurface are most readily identified in cores where they display crosscutting relations with the encasing shales (Figs 1.3, 1.4; Dixon et al. 1995; Duranti et al. 2002). They have often been encountered when exploring for deeper targets or during the appraisal and production phase of depositional Paleogene reservoirs in the northern North Sea. Fields associated with sand injectites in core often also display anomalous crosscutting events on 3D seismic data (Fig. 1.4), and it has been shown that the crosscutting wing-like anomalies along the edges of the Alba reservoir represent large-scale injected sand bodies (Lonergan & Cartwright 1999; MacLeod et al. 1999).

The processes leading to the intrusion of large volumes of sand in reservoir caprocks are still relatively poorly understood, but it is assumed that large-scale intrusion of sand takes place as fluidised flow driven by the pressure differential between an overpressured reservoir and a less overpressured, shallower aquifer or the seabed. Triggering mechanisms leading to caprock failure may include tectonic shaking, polygonal faulting, fluid injection and hydrofracturing (Lonergan & Cartwright 1999; Hillier & Cosgrove 2002; Jolly & Lonergan 2002).
The sand injectites drilled in the North Sea are often highly porous with Darcy-range permeabilities, making them effective both as reservoirs and as thief sands (MacLeod et al. 1999; Lonergan et al. 2000; Duranti et al. 2002a). Recognition and correct evaluation of sand injectites can thus be vital for the appraisal and production of reservoirs affected by remobilisation and injection (Fig. 1.4).

Fig. 1.3

Fig. 1.4

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References

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Duranti, D., Hurst, A., Bell, C. Goves, S. & Hanson, R. 2002. Injected and remobilised sands from the Alba Field (Eocene, UKCS): core and wireline log characteristics. Petroleum Geoscience 8, 99-107.

Hillier, R. D. & Cosgrove, J. W. 2002. Core and seismic observations of overpressure-related deformation within Eocene sediments of the Outer Moray Firth, UKCS. Petroleum Geoscience 8, 141- 149.

Jenssen, A. I., Bergslien, D., Rye-Larsen, M. & Lindholm, R. M. 1993. Origin of complex mound geometry of Paleocene submarine-fan reservoirs, Balder Field, Norway. In: Parker, J. R. (ed.) Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. Geological Society, London, 135-143.

Jolly J. H. R. & Lonergan L., 2002. Mechanisms and control on the formation of sand intrusions. Journal of the Geological Society, London 159, 605-617.

Lonergan, L. & Cartwright, J.A. 1999. Polygonal faults and their influence on reservoir geometries, Alba Field, United Kingdom Central North Sea. AAPG Bulletin 83, 410-432.

Lonergan, L., Lee, N., Johnson, H. D., Cartwright, J. A. & Jolly, R. J. H. 2000. Remobilization and injection in deepwater depositional systems: implications for reservoir architecture and prediction. In: Weimer, P., Slatt, R.M., Coleman, J., Rosen, N.C., Nelson, H., Bouma, A.H., Styzen, M.J. & Lawrence, D.T. (eds) Deep-water reservoirs of the World. GCSSEPM Foundation, 20th annual Conference, Houston, 515-532

MacLeod, M. K, Hanson, R. A., Bell, C. R. & McHugo, S. 1999. The Alba Field ocean bottom cable seismic survey: Impact on development. The Leading Edge, November 1999, 1306-1312.

Molyneux, S., Cartwright, J. & Lonergan, L. 2002. Conical sandstone injection structures imaged by 3D seismic in the central North Sea, UK. First Break 20, 383-393.

Murchison, R.I. 1827. Supplementary remarks on the Oolitic Series in the Counties of Sutherland and Ross, and in the Hebrides. Transactions of the Geological Society 2, 353.

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Newton, S. K. & Flanagan, K. P. 1993. The Alba field: Evolution and depositional model. In: Parker, J. R. (ed.) Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. Geological Society, London, 161-171.

Newsom, J. F. 1903. Clastic dikes. Bulletin of the Geological Society of America 14, 227-268. PESGB 2000. Structural framework of the North Sea and Atlantic Margin (2000 edition). Map, Petroleum Exploration Society of Great Britain.

Timbrell, G. 1993. Sandstone architecture of the Balder Formation depositional system, UK Quadrant 9 and adjacent areas. In: Parker, J. R. (ed.) Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. Geological Society, London, 107-121.

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