Seismic Expressions of Thermogenic Seeps

Abstract

Natural leakage of thermogenic fluids along continental margins is of major interest in the context of global climatic change because the release of methane and heavier hydrocarbons significantly increase the green house effect. Quantifying the total amount of expelled hydrocarbons is compulsory to understand the implication of natural leakage on global warming. Sea surface indicators of oil leakage are also widely used by petroleum companies. The identification of recurrent oil slicks at the sea surface provides possible evidence of active petroleum systems. Synthetic aperture radar (SAR) systems generate repeated images all over the world in continental margins, allowing multi-temporal observations of thermogenic fluid escape from buried reservoirs. The Lower Congo basin is considered a key area for the identification of thermogenic seeps on SAR data. Compilation map established from a stack of SAR images reveal “flower patterns” formed by individual oil slicks diverging from a mean impact point at the sea surface. Each flower pattern is interpreted as an active oil seep that can be reliably associated to a seabed vent thanks to low oceanic drift. From 3D exploration seismic and high-resolution 2D AUV (Autonomous Underwater Vehicle) seismic data, we distinguished different classes of fluid escape features from their morphological signature on the seabed. The combination of SAR observations over 18 years and the analysis of geophysical data allowed sorting seabed features depending on oil release activity. Six hundred pockmarks aligned along syncline axes and 900 funnel-shaped scattered pockmarks were mapped, but none of these are currently expelling oil; these are likely associated to dewatering. Seven seabed mounds, 26 irregular pockmarks and 55 clustered pockmarks are responsible for oil leakages which represent respectively 30%, 15% and 50% of the total identified. High resolution surveys also highlight several meter-high asphalt mounds above underlying dim zones surrounding active hydrocarbon vents. These mounds represent serious geological risks for petroleum installations and shall be evaluated from geophysical dataset analysis simultaneously with SAR observations. Integrated studies combining SAR observations and geophysical data hereby show their potential in determining locations and morphological characteristics of oil-releasing features on seabed.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015