Physical and Geochemical Evidence of Methane Hydrate in Marine Sediments

Abstract

Natural gas hydrates exist in continental margin sediments, where high pressure and low temperatures allow for hydrate stability. In this project, I analyzed a core from the Gulf of Mexico Gas Hydrate Join Industry Project Leg 1, collected in in Keathley Canyon Block 151 for artifacts of hydrate occurrence. Currently, no artifacts of natural gas hydrate have been identified in standard sediment cores, aside from pore water salinity measurements collected shortly after core recovery. In my research, I show evidence of former natural gas hydrate occurrence using ion chromatography and δ13C analysis. On geophysical image logs, gas hydrates were found to occupy near-vertical fractures in Keathley Canyon Block 151. Using x-ray computed tomography I examined the core 17H-4, from 259 mbsf, to identify near-vertical fractures. These fractures were sampled for inorganic and organic carbon isotope analysis using a Picarro cavity ring-down spectrometer. Organic δ13C values ranged from -26‰ to -31‰ while inorganic δ13C was -6‰ to -12‰. Samples were taken from fractured and un-fractured areas in three measurements, Shallow, Mid, and Deep. The Shallow background dissolved organic and inorganic measurements are heavier than the Mid measurement in four of six measurements. This observation is reversed in all the samples from the fractured zones where the Mid measurements are isotopically heavier than the Shallow measurements. The pattern reverses in both DOC and DIC measurements within the fracture plane demonstrating a difference in the fractionation of organic and inorganic carbon when compared to the background samples. Either this variation is natural and coincidental, or a microbial process is altering the δ13C of both organic and inorganic carbon differently with respect to fractures and background sediment.

Samples were also collected for wet chemistry analysis to compare salinity of the fracture surface compared to the background salinity for Core 17H-4. These results showed that evaporation has altered pore-water chemistry, and a correlation between fracture plane and porewater freshening was observed in the first round of testing. The geochemical evidence presented in this thesis demonstrates the relevance of using wet chemistry and carbon isotope analysis on traditional core segments to identify previous occurrence of natural gas hydrate in marine sediments.