Abstract: Geologic Controls on the Gas Hydrate Distribution, Offshore Congo

Lindholm, Rosanne M. and Robert Cunningham - Exxon Exploration Co.

The distribution of gas hydrates observed on seismic data has been mapped offshore Congo (Fig. 1). The area mapped represents the northern part of the Congo (Zaire) fan, a present-day active fan and one of the largest deep-water depocenters in the world. Gas hydrates occur 200 to 600 meters below mudline in water depths greater than 1,000 meters. A typical expression of the hydrates (Fig. 2) is a moderate to strong bottom stimulating reflector (BSR) that is opposite polarity to the sea floor. Below the BSR are 200 to 300 milliseconds of high amplitude reflections interpreted to represent free gas that has accumulated below the BSR.

Detailed mapping of gas hydrate in the area north of the Congo Canyon indicates that the hydrate has variable distribution. Correlation of the hydrates with underlying geology indicates that hydrate occurrence is controlled by a combination of stratigraphic and structural factors. In the southern part of the study area, the primary control on hydrate distribution is the presence of the Congo Canyon. A semi-continuous sheet of hydrate and underlying free gas extends approximately 20 km laterally away from both canyon walls. Hydrates can also occur inside the canyon within canyon fill. The association of hydrates with the canyon is inter- preted to be due to rapidly deposited organic-rich sediments flanking the canyon walls that yield biogenic gas. The gas reaches the hydrate stability zone via slump and dewatering faults.

Between 20 and 40 km north of the canyon, hydrates become discontinuous. Here, hydrate occurrence can be correlated with the presence of underlying upper Miocene deep-water confined channel complexes. As shown in Figure 2, high amplitude reflections can be seen in the interval between the channels and the BSR suggesting that biogenic gas is percolating through the permeable upper Miocene channel complexes and accumulating beneath the BSR. Biogenic gas migration is also occurring via extensional salt tectonic faults and small dewatering faults.

Greater than 40 km north of the canyon, the hydrate distribution becomes more discontinuous. Upper Miocene deposition characteristically consists of small deep-water channels and distributary lobe complexes. Here, the dominant control on hydrate distribution appears to be the presence of faults and structural elements. Hydrates commonly occur as "flags" on deep-seated faults associated with salt-cored anticlines and extensional grabens. This distribution suggests that the source of the biogenic gas is from upper Miocene or older sediments. The association of the hydrates with deep-seated faults and structures also suggests the possibility of thermogenic gas input to the hydrate stability zone.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil