Structural controls on shallow fluid flow and associated pockmark fields in the East Breaks area, northern Gulf of Mexico

Abstract

Three-dimensional (3D) seismic data, combined with semi-automated mapping methods in ArcGIS, were used to analyse the morphology and distribution of 720 pockmarks and 62 mud volcanoes in the northern Gulf of Mexico. The relationship amongst salt bodies, faults and the distribution of pockmarks and mud volcanoes stresses the significance of these structures in focusing fluid flow on continental margins. The pockmarks were classified according to their structural setting and depth of source, and later correlated with seep data from the Bureau of Ocean Energy Management (BOEM). Key findings include: a) half of the pockmarks are located within faults rooted on the top of salt diapirs, whilst 96% of the pockmarks are associated with salt diapirs – emphasising the importance of salt and crestal faults in focusing fluid flow to the sea floor; b) diffusion flow through the salt minibasins is evident due to the presence of soft amplitude anomalies (indicating fluids) and pockmarks located far from salt or faults; c) oil and gas are actively leaking to the sea floor; d) a higher density of fluid flow features are found in areas with steeper minibasin dips and greater catchment areas. While no clear correlation is evident between the morphological attributes and depth of source, the shallow plumbing system is dominated by pockmarks whereas mud volcanoes are sourced from the deeper parts of the salt minibasins. In summary, this study uses a novel approach to analyse the plumbing system in a salt-rich basin based on the surface fluid flow features. The importance of characterising the fluid flow features and associated structures to reduce risk and uncertainty is stressed in terms of both shallow gas hazards and hydrocarbon leakage from deeper reservoirs.