Assaba

Collaborating Authors

Overcoming Deepwater Geohazards In West Africa

OnePetroMay, 2005

Abstract Woodside and its joint venture partners have encountered a full range of deep water geohazards offshore Mauritania. The excellent quality exploration 3d seismic data allowed identification of potential geohazards; gas hydrates, deep water coral, mass transport complexes, seafloor pockmarks, buried pockmarks, shallow gas, fluid migration and shallow faults. Surface geochemical cores provided regional shallow geotechnical information. Geohazard analysis based on this information allowed nine exploration wells to be successfully drilled during the period 2001 to 2003. This programme resulted in discoveries at Chinguetti, Banda, Tiof and Tevet and to further geohazard analysis. The Chinguetti Field was surveyed using multi beam echosounder, ultra high resolution multi-channel seismic and sub bottom profiling. This was supported by a geotechnical investigation using a DP geotechnical rig. Spectacular mass transport complexes were observed adjacent to Tiof, and this area was surveyed using an AUV. This paper describes the geohazards encountered and the staged survey approach to closing these out at Chinguetti and Tiof. Side scan sonar and multibeam data has mapped the seafloor. Seafloor video has provided close up views of the seafloor. Seismic data ranging from exploration 3d seismic, 10 cu inch mini air gun 2d seismic to chirp profiler has been used to map the sub bottom. Coring and CPT data has provided ground truth for the interpretations and allowed engineering calculations. Examples of each of the data sets are presented. Introduction Woodside Mauritania Pty Ltd and it Joint Venture Participants, have been exploring for hydrocarbon deposits on the Northwest African margin offshore from the Islamic Republic of Mauritania since 1998. The area lies in a finegrained clastic slope apron on which the pelagic and hemipelagic sedimentation is overprinted by downslope gravity flows and along slope bottom currents. The sediments are derived from 'background' sedimentation processes, e.g. settling of planktonic material and wind blown terrigenous dust. The area is considered prone to shallow gas accumulations and detailed analysis of tophole conditions is routinely completed for all wells. The majority of the area is considered deep water, with water depths ranging from 200m to 1500m. Figure 1 Location Map (Available in full paper) This presentation is an overview of the identification and analysis of geohazards. The information derived from the exploration 3d seismic surveys was considered adequate for geohazard management for the short term exploration drilling. Fortunately, the early exploration drilling was successful with the discovery of Chinguetti (2001), Tiof (2003), Teve't (2004) and Banda (2002). In the last quarter of 2004 development drilling commenced over the Chinguetti Field and appraisal drilling at Tiof. Geohazards addressed include:deep water coral gas hydrates seafloor instability sediment waves pockmarks fluid vent shallow gas shallow water flow shallow faults Further work is progressing to close out the potential geohazards, in particular the question of slope instability near Tiof. In parallel to the field development, further exploration and appraisal drilling and exploration 3d seismic acquisition (Tanit, Kiffa and Atar Surveys) continues.

OnePetro

doi: 10.4043/17496-MS

OTC

OTC-17496-MS

Offshore Technology Conference