Gas Hydrate Distribution from Mud Volcanoes from Offshore Trinidad

Sean M. Sullivan¹ and Lesli J. Wood^2
1Jackson School of Geosciences, The University of Texas at Austin, Austin, TX
²Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

Scientists have long recognized that many offshore mud volcanoes are intimately linked to gas hydrate accumulations, where the expulsion of warm, methane-rich fluids from the deep subsurface via mud volcano conduits can influence both the distribution and stability of gas hydrate. Marine mud volcanoes and other types of seafloor hydrocarbon seeps show many characteristics that typify gas hydrate deposits, those being: 1) the presence of a bottom-simulating reflector in seismic reflection data, 2) vertical variations in pore-water chlorinity profiles, and 3) the presence of chemosynthetic communities.

The relationship between gas hydrate and mud volcanoes and the fact that mud volcanoes typically emit gases that are > 95% methane has led scientists to surmise that mud volcanoes may play an important role in the global carbon cycle. Three-dimensional seismic reflection data, covering over 8000 km², provides a detailed look into the vertical and horizontal distribution of gas hydrate (assuming that the bottom-simulating reflector marks the base of gas hydrate stability) at over 100 mud volcanoes from offshore Trinidad. The main observations of this study are that the bottom-simulating reflector can be either uplifted or depressed at mud volcano peaks and can vary considerably over the width of the mud volcano. Since gas hydrate is present at predictable pressures and temperatures, the variation of the bottom-simulating reflector at mud volcanoes is believed to signify deviations in heat flow and fluid flow. Such indicators can be used to estimate the activity of mud volcanoes, how variations in fluid flow control their morphology and in addition, this seismic reflector can help constrain how mud volcanoes contribute to global geochemical budgets, most importantly the carbon cycle.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas