Seismic Reflection Characterisation of Shallow Gas Hazards in the Western Niger Delta Offshore Area

Shallow biogenic gas accumulations in the uppermost sedimentary section of the western Niger Delta offshore area represent a significant geohazard for drilling operations, platform installation, and submarine pipeline routing, yet their spatial extent and properties are inadequately characterised by standard exploration-scale seismic data. This study characterises shallow gas occurrences using high-resolution 3D seismic reflection data in a 400-square-kilometre area of the western offshore Niger Delta in Ondo State territorial waters. The seismic volume was subjected to amplitude extraction, instantaneous frequency computation, and acoustic impedance inversion to identify and characterise shallow gas anomalies within 1 second two-way travel time (approximately 750 metres depth below seafloor). Bright spot and flat spot anomalies identified in the seismic data were classified by their frequency content, acoustic impedance contrast, and structural context. A total of 47 shallow gas bodies are mapped in the study area, ranging in lateral extent from 0.1 to 8 square kilometres and estimated thicknesses of 5 to 60 metres. Acoustic impedance inversion resolves gas saturation gradients within several of the larger anomalies, indicating partial gas saturation of 15 to 40 percent. Two of the largest gas accumulations are located within 200 metres of proposed pipeline routes, requiring route deviation. A geohazard probability index map combining gas presence, depth, and overpressure indicators is produced to support well path planning and marine operations management. Keywords: shallow gas, seismic reflection, Niger Delta offshore, geohazard, bright spot