Geochemical and Physical Evidence of Methane Hydrate in Marine Sediments
Buchwalter, Edwin R.; Cook, Ann; Welch, Susan; Sheets, Julie; Rose, Kelly; Disenhof, Corinne
Natural gas hydrate exists in continental margin sediments, where high pressure and low temperatures allow for gas hydrate stability. In this project we are looking for artifacts of hydrate occurrence in marine sediment cores to better understand their formation, source, and distribution in sediment. Currently, no artifacts of natural gas hydrate have been identified for standard sediment cores, aside from pore water salinity measurements collected shortly after core recovery. In marine muds natural gas hydrate occur as fractures and veins which dissociate during core removal. We hypothesize that the dissociating gas hydrate may leave behind physical and chemical artifacts in sediment cores such as fractures, tunnels, carbon isotope anomalies, salinity variability in areas surrounding fractures, as well as mineralization artifacts such as pyrite and calcite precipitation resulting from the biogeochemical transformation of methane.
Cores used in this project are from the Gulf of Mexico Gas Hydrate Join Industry Project Leg 1, collected in Hole KC151-2 in Keathley Canyon Block 151. We examined the interior of the core for fractures using x-ray computed tomography. In situ gas hydrate filled fractures have been observed on logging-while-drilling resistivity images in Keathley Canyon; these fractures are near vertical with apertures on the millimeter-scale. Similar near-vertical fractures have been attributed to gas hydrate rather than fractures formed during the removal process. These fractures were sampled for carbon isotope analysis using a Picarro CO2 elemental analyzer to measure d13C of organic and total carbon present and compare these data with carbon isotope data from un-fractured core sections. Preliminary results indicate an isotopically light ratio of -21 to -31 consistent with marine alga. Other samples will be collected from the potential hydrate fractures for wet chemistry analysis to compare salinity levels of the fracture surface versus the background salinity level for Hole KC151-2. We hypothesize this will be markedly different as the dissociating hydrate will flush the fracture with fresh water. Further samples will be taken from the core surrounding the fractures and from cores that do not contain hydrate fractures to analyze the mineralization artifacts present using the scanning electron microscope. We hope to develop a set of criteria that can be used to identify former occurrence of natural gas hydrate in sediment core.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013