Fluid Venting Structures of the Lower Congo Basin on 3-D Seismic: Gas Flux Variations Recorded by the Vertical Evolution of Pockmarks and Associated Amplitudes Anomalies

The Neogene succession of the oil and gas-bearing Lower Congo Basin contains a number of venting structures such as acoustic pipes, pockmarks and amplitude anomalies which are observed in 3-D high resolution seismic data.

Positive high amplitude anomalies (PHAAs) associated with pockmarks or acoustic pipes have recently become the focus of many studies. These PHAAs in the study area presented here, occur either at the top of acoustic 'pull-up' pipes, or at the base of pockmarks .

Their high, positive amplitudes relative to laterally equivalent sediments indicates a localized increase in acoustic impedance. In addition, seismic pullup effects below the PHAAs show that they correspond to a material with higher internal velocity. Combined with the very local character of the anomalies in the fine-grained clastic environment of the Neogene interval in this basin, PHAAs are interpreted as seep carbonates, possibly associated with gas hydrates.

Methanogenic carbonates can form either at the seafloor or in the sulfate methane transition zone (SMTZ), which occur a few tens of meters below at the maximum. Below the seafloor, they mostly result from anaerobic oxidation of methane (AOM), while at the seafloor they may result from AOM or from the development of chemosynthetic communities. Fluid leakage at the seabed can be indicated directly by pipes, pockmarks and also PHAAs. Among these indicators only PHAAs are diagnostic of methane gas.

The fact that PHAAs form close to the seafloor (within the limits of SMTZ) means that a vertical succession of PHAAs could reflect a prolonged phase of venting. The size, number and morphology of the PHAAs are interpreted to reflect relative quantities of methane escape. In combination with the presence of pockmarks and pipes associated with vertical successions of PHAAs on seismic, the dynamics of fluid expulsion and the flux rate variations over time can be estimated.

The workflow used in the analysis presented here were derived from a framework previously established by several authors.

Linear PHAAs without pockmarks or depressions were observed above faults. This type of linear PHAAs characterizes relatively slow fluid seeps. Sub-circular PHAAs are hosted within depressions. This succession records the following variations in flux rate. An initial moderate rate of flux which increased in more recent times until a eruption threshold was reached and a crater was created. Using these principles regional fluid flux rates can be reconstructed.

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