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Lidia Lonergan1,
Richard J.H. Jolly2,
Nick Lee1,
Howard D. Johnson3,
Joseph A. Cartwright4,
Steve Molyneux5
(1) Imperial College, London, United Kingdom
(2) Golders Associates, UK
(3) Imperial College, United Kingdom
(4) Cardiff University
(5) Imperial College
Abstract: Remoblisation and injection in deepwater depositional systems
Productive Paleogene deep water reservoirs in the North Sea, (e.g. Alba, Harding, Balder, Gryphon) show evidence of having undergone major post-depositional remobilisation and clastic injection from core to seismic scales. Predicting the nature and the degree of modification of primary sand body geometry, requires an understanding of the mechanisms of the clastic intrusion process. Field studies of remobilised deepwater sandstones undertaken in Ireland, California and France provide insights on intrusion mechanisms and geometries.
Stress state, burial depth, fluid pressure and the nature of the sedimentary host rock control intrusion styles, geometry and scale. For a clastic intrusion to form the source material for the intrusion must be uncemented and sealed such that overpressure is generated within the sand pocket. The cohesivity of the host rock controls whether the intrusion is emplaced by stoping (the incorporation of host rock material as rafts in the intrusion), or dilation (the forceful pushing apart of the host rock to create space), resulting in diverse styles of intrusive geometry.
Seismicity, tectonics and build up of excess in situ pore pressure are most commonly cited explanations for the occurrence of clastic intrusions. However, the scale of North Sea Paleogene deep-water intrusions suggests that the presence of an external fluid (e.g. gas) is required to drive the injection. Deep water sand bodies that appear most susceptible to these processes include (1) narrow, elongate channel sands, (2) isolated sand-rich mounds, and (3) sand bodies/fan systems located above deep basinal faults, which periodically acted as vertical fluid escape pathways.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana