The Formation of Advancing Pockmarks: An Interplay between Hydrocarbon Leakage and Slope Sedimentation

Advancing pockmarks composed of multiphase, migrated infill-sets are observed above gas-bearing channels and deep-seated extensional faults in a high-resolution 3D seismic survey of the Lower Congo Basin, offshore Angola. They occur within a Neogene-Quaternary aged sedimentary sequence deformed by polygonal faults. “Advancing” pockmarks are so termed based on the presence of migrating stratal packages which represent episodes of infill, i.e. where apices are laterally offset by hundred(s) of meters. Migration occurs systematically in the same direction over the evolution of the pockmark array. “Normal” vertical stacked up pockmarks on the contrary show vertically stacked successive episodes of individual pockmark formation.

The objective of this research is to establish what factors control the development of advancing pockmark arrays and what implications they have for leakage of hydrocarbons. Advancing and “normal” pockmarks occur in different settings. “Normal” pockmarks develop on horizontal slopes whilst “advancing” pockmarks are located on inclined slopes or in channels. Migration occurs in the downslope direction implying that bottom currents play a role in the migration process. These pockmarks developed above a turbidite channel which is overlain by a gas hydrate highlighted in seismic data by a bottom-simulating reflector (BSR). The BSR is restricted to channel areas. High seismic amplitudes within the pockmarks below the BSR suggest that they are filled with free gas.

The following emplacement mechanism is interpreted for advancing pockmarks arrays. 1) Initial slope topography of the system is created by expulsions of gas focused through the underlying turbidite channel, 2) Fine-grained sands were draped over the pre-existing pockmark crater, 3) The vortex resulted from a higher influx rate of bottom currents, in combination with seepages, resuspended the unconsolidated sediments on the downstream pockmark sidewall. A local starving area was thus created. This area covered directly by another fine-grained layer which in turned to be deformed in the downstream side. The same process repeated, starving areas stacked up and vertically connected to each other. A permeable pathway for fluid flow was developed along the downstream pockmark sidewall. 4) Fluid leakage through the permeable pathway determines the location of future pockmarks which are preferentially developed on the downslope flank. The result is a downslope migrating pockmark fill. In conclusion the two main factors that control the formation of advancing pockmarks are: 1) fluid flux, and 2) the interaction of local bottom currents with sedimentation and pre-existing pockmark topography.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California