Geothermal Gradient Estimate in Pilot Study Areas of Gas Hydrates in the Colombia Caribbean Sea

Calle, Andres E.; Vesga, Alejandra

Gas hydrates are crystalline solids that consist of gas molecules, usually methane, trapped in a cage of water molecules, they occurred when pressure, temperature, gas saturation, water salinity with other specific conditions, made them stable. In the Colombian Caribbean sea, some work have been done to estimate the potential of gas hydrates as a energy resource, specifically in Lopez and Ojeda (2006) they computed the geothermal gradient following the methodologies from Shipley, et al., (1979), Yamano et al. (1982) Kaul et al., 2000 and Vohat et al., (2003) for the entire Caribbean sea (170 km offshore). Results show that there are three different areas following the BSR distribution, to north, central, and south part called Guajira, Magdalena's delta and Sinú respectively, and with the remarks that the geothermal gradient ranged between 20 to 40° C/km, despite the fact that the southern part is dominated by an accretionary prism, which turn to be anomalously cold.

However, the beginning of a research exploration project of gas hydrates in the Caribbean Sea using 3-D seismic, in which the BSR was interpreted, was necessary to review the geothermal gradient variations to define more precisely the gas hydrate stability zone, for better understanding of the nature of this type of unconventional resource. The methodology estimates the thickness of the stability zone using high resolution seismic processing velocities, from the 3-D volume and 2-D lines, following Shankar 2010, and travel times from seafloor reflector and BSR. The pressure is estimated from sea level to BSR, according Shankar 2010, considering all as hydrostatic pressure contribution and moreover this Pressure corresponds to equilibrium phase of the gas hydrate formation. Assuming pure methane and salinity of 35%, which allowed us to use an equation for estimating the phase change temperature, Quinby-Hount Dickens 1994. Seafloor temperature is computed by introducing Melo 2008, and thus a detail geothermal gradient grid is computed in the areas.

The methodology used is more accurate, with probably lesser uncertainty than any previous work done in the areas, showing details in the geothermal gradient variations, far from being homogeneous, ranging from 30 to 50°C/km and certainly with anomalous values. Thus requiring different scenarios: structural analysis, migration pathways like permeable faults, or tectonic activities, note that permeable structure like mud volcanoes was isolated. Finally this geothermal gradient map is the starting point for the design of heat flow, CTD, and piston core acquisitions in the areas of interest.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013