Geophysical evidence of gas hydrates in shallow submarine mud volcanoes on the Moroccan margin

Gas hydrates inside mud volcanoes have been observed in several locations but are generally found at water depths of 1000 m and deeper. We present the first observation of the base of a gas hydrate stability zone within a shallow mud volcano in the El Arraiche mud volcano field on the Moroccan Atlantic margin. The mud volcano base is located at about 475 m and is over 125 m high. On high-resolution seismics we observed an anomalous but coherent reflection under the slopes of the mud volcano. The event was interpreted as the base of a gas hydrate stability zone because of its inverse polarity and its morphology. Far from the crater, the event is nearly parallel to the seafloor. Closer toward the crater, the event shallows. Inside the mud volcano crater, no event is observed. A stability model using thermogenic gas compositions is applied to local P-T conditions, indicating that thermogenic gas hydrates can be stable at this depth. The high modeled heat flow in the crater of the mud volcano indicates a focused flow of warm fluids. Below the slopes of the mud volcano, the inferred heat flow is also elevated but less high. In areas of thermogenic gas production, gas hydrates can occur at shallow water depths, even in areas with high heat flow. This also suggests that dewatering of the accretionary wedge complex is mainly focused along fault surfaces and through seafloor structures, such as mud volcanoes