Vertical evolution of fluid venting structures in relation to gas flux, in the Neogene-Quaternary of the Lower Congo Basin, Offshore Angola

摘要

The processes preceding the genesis of pockmarks and their subsequent evolution has been rarely studied since pockmarks were discovered in 1970. A high-resolution 3-D seismic survey from the continental slope of the Lower Congo basin allows us to study in detail the structures that developed before the formation of pockmarks, as well as their subsequent geometric evolution. This paper aims at reporting new type of fluid-related structures that relate to pockmarks. The objective of this research is to understand: 1) the evolution of these structures and their genetic relationship with pock-marks, and 2) how their morphologies may be influenced by changes in venting intensity over time. Pockmark and chimney structures in this survey are hosted within Late Miocene to Quaternary hemipelagites, above clastic reservoirs in a petroliferous basin. They are all associated with positive high amplitude anomalies (PHAAs) on seismic data. PHAAs are interpreted from their dramatically higher acoustic impedance relative to the background sedimentary values of the host units, and by their association with seismic pull-up effects. From their planform and cross-sectional geometry, PHAAs are primarily interpreted as being due to the development of methane-related carbonates but they may be associated with gas hydrates in one or two examples.