Sequence Stratigraphic Update of the 3-Series of the Troll Field Reservoir

Thomas-Ideh, Osivwi ¹
(1)Geological modelling, Statoil, Bergen, Norway.

The Troll Field in the Norwegian North Sea is composed of five sequences of stacked coastal to shallow marine parasequences belonging to the Upper Jurassic Sognefjord Formation. This project focuses on the 3-series sequence especially the 3Cc2 system tract. This 3-series was initially interpreted to have been formed during forced regression with the 3Cc2 unit representing an ebb-tidal delta. Wells penetrating the 3Cc2 unit however primarily encountered fine sands where coarse sands were predicted. Results from outcrop studies, 3-D seismic interpretation, core description and wireline-log correlation were used to redefine the stratigraphic evolution of the 3-series and facies distribution within the 3Cc2 system tracts.

Results indicate that the 3-series consist of four system tracts that developed a N/S elongated prograding spit system. However the 3Cc2 lowstand wedge shows a more complex facies distribution. On 3D seismic data the 3Cc2 unit shows an incisional surface to the east with clinoforms prograding towards the west. These corresponds to an upward coarsening succession passing from lower shoreface fine sands upwards into trough cross bedded upper shoreface and planar bedded foreshore coarse sands. Well correlation shows an asymmetry in which silty sand units preferentially occur in the proximal reach of the southern part of the 3Cc2 lobe.

The 3Cc2 system tract is interpreted as a westward prograding asymmetric lowstand delta with facies differing in the updrift and downdrift sector of the incised channel system. The updrift section consists of a coarse grained sand body formed by southwards longshore currents reworking older spit shoreface sands. Fluvial effluent trapped the longshore derived sediments updrift, and a sand body will be constructed downdrift by the fluvio-tidal system. This system supplied fine grain sands associated with the back-barrier tidal system and coarse sand reworked from spit platform forming a sand rich barrier bar system distally with a lagoon behind.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.