ABSTRACT

Evidence of gas hydrate and free gas occurrences in a 3D seismic volume from the West-Central Coastal Province of the Congo Fan, offshore Angola, illustrates all the components of a complete petroleum system. Analysis and interpretation are based on the information in attributes calculated from three 3D time-migrated common-angle seismic volumes; the attributes include seismic amplitude, spectral components, dip magnitude, amplitude variation with angle, and instantaneous frequency. The source is organic-rich muds associated with late Cretaceous to early Tertiary channels, the migration paths are along growth faults, and the traps are partly defined by the gas hydrate stability zone (for the gas hydrate), partly by the bottom-simulating reflector (for the subhydrate free gas), and partly by faults (for both). The spatial distribution of free gas is further supported by the associated seismic bright spots, and also by the attenuation of high frequencies of P-waves that traverse the gas-saturated zone. Locally higher temperatures, associated with upward fluid circulation along fault zones, facilitate gas transmission through the gas hydrate and forms gas chimneys that extend to the sea floor.

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