Active Seafloor Seepage Along Hydraulic Fractures Connected to Lateral Stress From Salt‐Related Rafting: Regab Pockmark, Congo Fan

• Published in: Journal of geophysical research. Solid earth Vol. 123; no. 5; pp. 3301 - 3319
• Main Authors: Wenau, S., Spiess, V.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-05-2018
• Subjects: Basins; Biology; Continental margins; Deformation; Flow system; Fluid dynamics; Fluid flow; Fractures; Geology; Geophysics; Hydraulic fracturing; Hydraulics; Hydrocarbons; Lateral stress; Lower Congo Basin; Migration; Ocean basins; Ocean floor; Orientation; Overburden; Overpressure; Rafting; Regab; Reservoirs; Salts; salt‐induced stress; seafloor seepage; Seepage; Seismic data; Seismological data; Stresses; Uplift
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1002/2017JB015006

Seafloor seepage is a widespread phenomenon within salt‐influenced basins as the deformation provides pathways for hydrocarbons to reach the seafloor. However, only minor attention has been given to the distal parts of such systems where the impact of salt tectonic deformation is relatively unpronounced. The stress put on the sedimentary column by moving salt on a continental margin may influence fluid flow systems even outside of the salt province. This stress may lead to overpressure formation within reservoirs and determine the orientation of overpressure‐induced fractures. Seepage in the Congo Fan has been discovered in such a distal position at the Regab pockmark, about 35 km west of the salt front, and its geology and biology have been studied extensively in recent years. We present high‐resolution multichannel seismic data from the Regab pockmark that reveal the underlying migration pathways from a buried channel flank 300 m below seafloor to the seafloor via hydraulic fractures in the sealing overburden. Local doming of the reservoir and the remobilization and uplift of sedimentary strata along the migration pathways are interpreted as the result of overpressure within the reservoir. The orientation of the hydraulic fractures is WSW‐ENE, and the fracture outline corresponds to the area of most intense seepage activity within the seafloor pockmark. Along with a similar orientation of other fractures in the vicinity, we propose that this alignment is due to the stress imposed on the sedimentary column in the fan by the seaward moving salt and rafting sedimentary packages of the salt province further east.
Key Points
The Regab pockmark is fed through hydraulic fractures from a buried channel flank
The orientation of mapped fractures suggests lateral stress in a WSW‐ENE orientation
Stress induced by westward moving salt is proposed as responsible for overpressure generation and fracture orientation