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Implications of Mesozoic and Cenozoic Tectonism for Inner Moray Firth Prospectivity

UNDERHILL, JOHN R., University of Edinburgh, Edinburgh, U.K.

The Inner Moray Firth (IMF) lies immediately adjacent to the Sutherland coast in northeast Scotland and contains the Beatrice oil field. Mesozoic basin development of the IMF has classically been regarded as being the result of accommodation to strike-slip motion on the Great Glen fault zone (GGF). However, a seismic interpretation, consisting of over 5000 km of Geco/Merlin seismic data, which integrates all available well data and onshore exposures, suggests that previous models of basin development are incorrect. Instead, it is proposed that the GGF did not play a significant role in the Mesozoic but that extensional, dip-slip motions on other faults controlled its basin geometry and sediment dispersal patterns. The present expression of the GGF only attests to its Cenozoic reactiva ion as an oblique-slip fault.

The new model for basin development has considerable implications for the understanding of IMF prospectivity, particularly timing of structuration, source rock maturation, and hydrocarbon migration. It is believed that the Beatrice structure and its counterparts formed as extensional traps (e.g., closures due to footwall uplift) during Late Jurassic rifting. They were probably charged by Middle Jurassic and older source intervals from a kitchen area that lays beneath a Late Jurassic-Cretaceous half-graben depocenter to the northwest. Subsequent Cenozoic regional uplift and oblique-slip motion on the GGF have served to raise and dissect the kitchen area, tilt preexisting structures, and reactivate several faults. Future exploration success may lie in the effective recognition of suitab e unbreached paleostructures or stratigraphic traps.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)