Geohazard Analysis in Deepwater Nigeria

Abstract The understanding and management of geohazards play a prominent role in a successful Exploration and Production venture in the deep offshore environment. Inadequate containment of these hazards could result in significant loss or damage to personnel and equipment. SNEPCO, on behalf of a consortium comprising of AGIP, ESSO, ELF acquired two offshore blocks covering an area of 3500sq.km in deepwater Nigeria in water depths ranging from 200m to 1500m. Within the period 1994-99 SNEPCO has carried out extensive exploration and pre-development work during which, the assessment and management of geohazards have been given a prominent role. The data requirements and methodologies to ensure an effective management of geohazards in offshore operations and SNEPCO's experience are discussed in this paper. Introduction Geohazards in the context of deep offshore operations refer to naturally occurring or man made features on the seabed and subsurface that could pose a danger to exploration, production or development activities. From the onset of its exploration activities SNEPCO had identified the effective management of geohazards in all phases of operations is one of the key success factors. Thus resources were devoted to acquisition of requisite data that will enable the understanding of these geohazards and their management. Deepwater Hazards The geohazards prevalent in deepwater environment are significantly different from those found in shallow marine, shelf and slope environment. Some of the hazards encountered in the deepwater environment include but not limited to:Surface debris and obstructionsIncompetent sedimentsSlump and scour featuresSteep faulting and glide planesShallow gas pocketsMud volcanoesGas Hydrates and MouldsOverpressures In the SNEPCO area of operations, overpressures were only encountered in the deeper sections. Geology Seismic interpretation of deep offshore Nigeria data indicates that the seismic package can be sub-divided into two units. An upper and a lower unit. The upper unit is comprised of chaotic, discontinuous, variable amplitude seismic reflections. These are interpreted as channel levee complexes and slumps of Plio-Pleistocene age. This characteristic chaotic nature, channeling, faulting and slumps constitute majority of the geohazards encountered. The lower unit in this sub-division is made up of mid-slope to slope deposits, consisting of sand rich mounded sheet sands and mid-fan lobes. A number of the prolific reservoirs are found in this unit. The prominent geohazard that is found in this unit is overpressures. Data used for geohazard studies The underlisted data were used for the analysis of geohazards:3D seismic data conventional and re-processedVelocity dataSite Survey data Analogue: -Echo SounderSide scan sonarSub-bottom profiler Digital2D High Resolution Data. The contribution of each of these data sets to the geohazard analysis process are discussed hereunder. 3D conventional and re-processed data. The study of the 3D data enables regional understanding of the environment of deposition and interpretation of sedimentological units. The seabottom may be mapped and using 3D visualization techniques vivid images of the seabottom obtained (Figs 1&2).