PIERCEMENT SHALE DIAPIRISM IN THE DEEP-WATER VEMA DOME AREA, VORING BASIN, OFFSHORE NORWAY

Within the Norwegian 15th Round deep-water production Licence No. 217 on the Voring Plateau on the Norwegian continental slope, there is an unusually rugged seafloor topography. Our study addresses new 2D-, 3D- , and bathymetric data acquired in the Vema Dome area, where evidence for active piercement shale diapirism is found. The cores of the shale diapirs consist of coccolith oozes deposited during the Paleocene and Miocene. Individual shale diapirs and diapiric escarpments rise up to 70 m above the general sea floor (at 1200 m water depth). One of the key questions in our study has been to find out why diapirism is trigg ered over the Vema Dome and not over the nearby Nyk High, which has si milar buried deposits of coccolith oozes. On the basis of interpretati ons of the new data, we conclude that the triggering is caused by a co mbination of at least three factors : (a) a buried low-density, high-p orosity layer of deformable material (the coccolith ooze); (b) a domin g substratum which causes extension and faulting; (c) migration of lig ht hydrocarbons through some of the faults and fault zones. A fourth f actor which probably contributes to triggering is tectonic instability . The one factor that discriminates the Vema Dome area from the neighb ouring Nyk High area is the dome itself. We therefore suspect that thi s in some way is the main factor responsible for the triggering. Furth ermore, we conclude that the process of piercement shale diapirism in the Vema Dome area is probably still active: but, because the movement of the sediments is suspected to be very slow, it should be of only m inor concern to future oil field development. (C) 1998 Elsevier Scienc e Ltd. All rights reserved.