Tore M. Loseth1, Ian Sharp2, Michael Whitaker2
(1) University of Bergen, N-5007 Bergen, Norway
(2) Norsk Hydro ASA, Bergen, Norway
ABSTRACT: Interplay Between Extensional Tectonics and Sedimentation within the Middle Jurassic Tarbert Formation, Oseberg Field, Norwegian North Sea
The Oseberg Field is situated on the Oseberg Fault Block in the Norwegian North Sea. The Late Bajocian Tarbert Formation, deposited during the infant stages of active rifting, is an important shallow-marine reservoir unit in the area, whereas the overlying lower Heather Formation dominantly consists of offshore mudrocks.
Improved depositional models for the formations, emphasising the linkage between sedimentation and tectonics, is provided from integrated structural, sedimentologic and palynologic studies of core data from 24 wells and wireline log data from about 150 wells, covering the stratigraphic interval of the uppermost Ness, Tarbert, and lower Heather formations.
The Tarbert Formation was deposited in a wave-dominated deltaic depositional system that prograded south to north. Fault activity had limited, but important depositional control in this period. Both the presence of a transgressive tidal unit below the ravinement surface at the base of the Tarbert Formation and a general aggrading to backstepping stacking pattern, both confined to hangingwall areas, are indicative of this minor fault activity. Internal truncation surfaces within the Tarbert Formation, as predicted in some recent works, are not present in the study area.
Major active rifting and fault-block rotation commenced in the latest Bajocian. As a result, the Oseberg Fault Block was subaerially exposed in up-dip areas, causing an up-dip unconformity between the Tarbert and Heather formations. Three episodes of rotation, superimposed on an overall subsidence, are recorded in the studied lower Heather Formation. The first two episodes are associated with intra-Heather sandstones redeposited from exposed up-dip areas.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado