TIME-LAPSE SEISMIC MONITORING OF A TRANSIENT, FAULT-CONTROLLED THERMOGENIC HYDRATE SYSTEM AT WOOLSEY MOUND, GULF OF MEXICO

Woolsey Mound, located at MC118, is the site of the Gulf of Mexico hydrate research consortium’s (GOMHRC) seafloor observatory, where hydrates outcrop at the seafloor. It is a 1 km diameter hydrate complex where hydrates have been investigated using a wide range of geophysical, geological, and geochemical studies. The presence of hydrates in shallow sediments was confirmed by coring. Craters, pockmarks, chemosynthetic communities, and authigenic carbonates populate the seafloor in Woolsey Mound. Morphologically, MC 118 is divided into three craters (NW, SW, and SE craters). Each crater is characterized by a network of shallow crestal faults that connects the mound to the underlying salt body.

Gas hydrate evolution at Woolsey Mound is investigated by conducting a time-lapse seismic analysis. Four sets of collocated 3D seismic reflection data that span over 14 years were analyzed in this study. The oldest data collected in 2000 was used as the ‘base’ in this study. The rest of the three datasets collected in 2003, 2010, and 2014 were analyzed to study the real-time changes in the subsurface pore-fluid content. Acquisitional differences in geometry, sample rate, phase, etc., were minimized in 4-D processing. Our results indicate a significant volume of hydrate had dissociated in three years. Major dissociation occurred along the southern portion of the crestal fault in the SE crater. The dissociation is less prominent in the SW crater. A larger volume of methane seems to have formed between 2000 and 2010. These methane formations are mostly concentrated in the SE crater. The location of seafloor methane seeps observed in 2011 and the residual amplitude anomalies observed between 2000 and 2010 are spatially correlated. The anomaly observed between 2000 and 2014 seems weaker, implying the methane venting had been reduced significantly. Most of the hydrate dissociations and formations observed at various time intervals are spatially correlated to the crestal faults.