Fracture-controlled gas hydrate systems in the northern Gulf of Mexico

Abstract

a b s t r a c tHigh-angle, open mode fractures control the presence of natural gas hydrate in water-saturated clays at the Keathley Canyon 151 site in the northern Gulf of Mexico, which was investigated for gas hydrates as part of the Chevron Joint Industry Project drilling in 2005. We analyze logging-while-drilling resistivity images and infer that gas hydrate accumulated in situ in two modes: filling fractures and saturating permeable beds. High-angle hydrate-filled fractures are the most common mode for gas hydrate occurrence at this site, with most of these fractures dipping at angles of more than 40 and occurring between 220 and 300 m below seafloor. These fractures all strike approximately N-S, which agrees with the 165 SE-345 NW maximum horizontal stress direction determined from borehole breakouts and which aligns with local bathymetric contours. In one interval of hydrate-filled fractures, porosity increases with increasing hydrate saturation. We suggest that high pore pressure may have dilated sediments during fracture formation, causing this increase in porosity. Furthermore, the formation of gas hydrate may have heaved fractures apart, also increasing the formation porosity in this interval.

First-Order Estimation of In-Place Gas Resources at the Nyegga Gas Hydrate Prospect, Norwegian Sea

“…As an example, Ruppel et al [32] outline Gulf of Mexico gas hydrate saturations of 1.5-6%, 1-12% and >20% depending on which method for calculation is used [68][69][70].…”

Geologic framework of the 2005 Keathley Canyon gas hydrate research well, northern Gulf of Mexico

“…Common to all of these explanations is the presence of pathways along which fluids can migrate and cause gas hydrate to be concentrated as these fluids move into the HSZ. The gas hydrate inferred at KC 151-2 is most compatible with this third, pathways-based model because of their presumed association with fracture porosity in the 220-300 mbsf interval (e.g., Cook et al, 2008). The seismic data are insufficient to resolve steeply dipping structures such as fractures and are therefore not appropriate for mapping the distribution of the fracture networks.…”

“…The three units identified beneath horizon 5 correlate with fine-grained muds in the drilling. Gas hydrates are associated with two of these deeper, older units and occupy near-vertical fractures or possibly pore space in cleaner lithologies (e.g., Cook et al, 2008). An older family of faults also occurs within these three older units.…”

“…9) further indicate that this interval from 220 to 300 mbsf contains numerous, near-vertical structures interpreted as fractures (Cook et al, 2008). The fracture voids are the most likely explanation for the localized elevated concentrations of gas hydrate (Cook et al, 2008). Fractures within clay-dominated hydrate-bearing sediments have been observed in the Blake Ridge (Dillon et al, 1997;Rowe and Gettrust, 1993).…”

“…Sinusoidal patterns in the resistivity-at-thebit (RAB) imaging for the KC 152-2 hole (Fig. 9) further indicate that this interval from 220 to 300 mbsf contains numerous, near-vertical structures interpreted as fractures (Cook et al, 2008). The fracture voids are the most likely explanation for the localized elevated concentrations of gas hydrate (Cook et al, 2008).…”