Mud volcanoes in deepwater Nigeria

Detailed study of 3D seismic data from deepwater Nigeria has revealed the presence of features interpreted to be mud volcanoes. They occur in an upper slope environment seen as 1-2 km circular features at the seabed. Seabed cores from the mud volcanoes contain oil, gas and sand/shale-clast content richer than the seabed background. Pliocene fossils have been identified in the cores, demonstrating transport of material from depth. The features show a high seabed seismic amplitude above a chimney of chaotic seismic reflections and data wipe-out. The mud volcanoes in the study area shows two distinct clusters located over deeper structural culminations. Four active mud volcanoes (Area 1) are located above a rollover anticline in the central part of the area. Cuspate listric faults reach the seabed on the up-slope side of these mud volcanoes. Four abandoned mud volcanoes have also been identified, with progressively older ages towards the crest of the underlying structure. These abandoned features are associated with an extremely chaotic seismic signature. A combination of over-pressured carrier beds and low integrity top seals are believed to be responsible for the formation of these mud volcanoes. It is further believed that gas expansion, subsequent to seal failure, was the main driving force for what must have been violent eruptions. The long lived mud volcano activity over the deep structural closure suggests a plumbing system that focuses on escaping compaction water and hydrocarbons through time. In the south west of the study area, another cluster of five mud volcanoes is located above a shale diapir. The seabed expression of the members of this cluster is more varied. Some show a positive relief at the seabed while others show circular depressions. The largest feature represents an assemblage of many smaller mud volcanoes with a common root system. Their seismic expression is also different from that of Area 1, with a well-defined sediment wedge at surface and a shallower root system. These features are believed to represent a less violent type of mud volcano characterised by ductile flow. Tectonic stress, due to growth of the underlying diapir, is thought to have played an important role during eruption, in addition to focused methane escape and the low mechanical strength of the overburden. (C) 2000 Elsevier Science Ltd. All rights reserved.