Shale Intrusions and Associated Surface Expressions - Examples from Nigerian and Norwegian Deepwater areas
Authors: Heggland R. , Nygaard E. 
Published: October 2008

This paper describes features seen in the upper sediments, and their associated seabed expressions from two different deepwater areas offshore Nigeria and offshore Norway. Data examples from the two areas are presented, showing the different techniques used to map the seabed and the shallow sediments. By using different mapping techniques detailed interpretation of the seabed and upper sediments can be performed on data acquired with less than ideal data acquisition parameters in deep water areas, thus saving acquisition time and costs. The implementation of these mapping techniques may help in planning hazard site surveys as well as drilling and engineering activities in deep water areas.

Introduction

Statoil has performed drilling hazard site surveys in deepwater areas offshore Nigeria and Norway. The areas have different geological settings, but common to both areas is an upper sedimentary section disturbed by shale intrusions, which also creates a complex seabed topography. Although ata acquisition parameters may not have been ideal in these deep water areas, seabed features and shallow sediments have been accurately mapped using various techniques. Seabed features have been mapped using swathe bathymetry, side scan sonar mosaic maps and time maps from 3D exploration seismic. High resolution 2D seismic have been used together with 3D exploration seismic to obtain a detailed interpretation of the shallow sediments. Accurate bathymetry measurements in areas with complex seabed topography can only be obtained with multi beam echosounder. In the exploration phase detailed depth information may not be required. A relatively accurate depth map can be created if the sea bed reflector is picked from a 3D dataset and the resulting time map is depth converted using velocity of sound in water measurements and single beam echosounder data for calibration. The topography can also be imaged using 3D seismic data by the use of dip-maps. Side scan sonar data is required for the identification of potential hazards on the sea floor, but the acquisition of side scan sonar data in deep water creates several problems. A conventional towing method causes extreme laybacks (distance between vessel and sonar fish), making accurate positioning of the fish difficult. If the sonar data is processed and compiled into a mosaic map, this can be compared with the seabed topography map from the 3D seismic and the position of main features on the sea floor can be calibrated. The shallowest sediments in deep water areas often comprise relatively soft clays. A very high frequency source like a hull mounted pinger will normally achieve good penetration. However, due to the large footprint of the signal at seabed in deep water an uneven seabed will cause interference from "off line" features, hence the pinger data may only be interpretable in areas with relatively flat seabed.

Deepwater Norway

The Vema diapir field is located on the inner part of the Vøring Plateau on the Norwegian continental margin (Fig. 1), in 1200 - 1300 m water depth. The available seismic data in this area consists of 3D seismic covering 900 km2 and 2D high resolution seismic covering approximately 210 km2. The latter consists of multichannel data, single and multi beam echosounder, 16 element hull mounted pinger and side scan sonar and was acquired as part of the planning for the first exploration well in the area.