Pore Pressure Monitoring of Active Fault-Fluid-Hydrate System in Deep Water, Nigeria

Abstract

To characterize the link between faults and fluid release and to identify the role of fluid flow in gas hydrate occurrences, four piezometers were deployed at the border of a shale-cored anticline in the eastern Niger delta. Two of those piezometers were deployed along a major fault linked to a shallow hydrocarbon reservoir while the two others were deployed within two pockmark fields. Pore pressures were measured for periods ranging from 370 and 435 days. The two piezometers deployed along the fault simultaneously registered a prolonged fluid flow event lasting 90 days. By combining and analyzing acquired data, we show that the fluid-fault system operates according to the following three stages: i) upward pore fluid migration through existing conduits and free gas circulation within several shallow sandy layers intersecting the major fault, ii) gas accumulation and pore pressure increase within sandy-silty layers and iii) hydro-fracturing and fluid pressure dissipation through sporadic degassing events, causing pore fluid circulation through shallow sandy layers and attracting seawater into the sediment.The present work clearly demonstrates how an integrated approach based on seafloor observations, in situ measurements, and monitoring is essential for understanding fault-fluid-hydrate systems.