Appearance and preservation of natural gas hydrate from Hydrate Ridge sampled during ODP Leg 204 drilling Gerhard Bohrmann a, ⁎ , Werner F. Kuhs b , Stephan A. Klapp a,b , Kirsten S. Techmer b , Helmut Klein b , M. Mangir Murshed b , Fritz Abegg a a Fachbereich Geowissenschaften, Universität Bremen, Postfach 330440, 28334 Bremen, Germany b GZG, Abt. Kristallographie, Universität Göttingen, Goldschmidtstr. 1, 37077 Göttingen, Germany Received 27 May 2006; received in revised form 12 March 2007; accepted 12 May 2007 Abstract The state of preservation of natural gas hydrate samples, recovered from 6 sites drilled during ODP Leg 204 at southern summit of Hydrate Ridge, Oregon Margin, has been investigated by X-ray diffraction (XRD) and cryo-scanning-electron-microscopy (cryo-SEM) techniques. A detailed characterization of the state of decomposition of gas hydrates is necessary since no pressurized autoclave tools were used for sampling and partial dissociation must have occurred during recovery prior to the quench and storage in liquid nitrogen. Samples from 16 distinct horizons have been investigated by synchrotron X-ray diffraction measurements at HASYLAB/ Hamburg. A full profile fitting analysis (“Rietveld method”) of synchrotron XRD data provides quantitative phase determinations of the major sample constituents such as gas hydrate structure I (sI), hexagonal ice (Ih) and quartz. The ice content (Ih) in each sample is related to frozen water composed of both original existing pore water and the water from decomposed hydrates. Hydrate contents as measured by diffraction vary between 0 and 68 wt.% in the samples we measured. Samples with low hydrate content usually show micro-structural features in cryo-SEM ascribed to extensive decomposition. Comparing the appearance of hydrates at different scales, the grade of preservation seems to be primarily correlated with the contiguous volume of the original existing hydrate; the dissociation front appears to be indicated by micrometer-sized pores in a dense ice matrix. © 2007 Elsevier B.V. All rights reserved. Keywords: gas hydrate; X-ray diffraction; Ocean Drilling Program; FE-SEM 1. Introduction Natural gas hydrate occurs worldwide in oceanic sediments and polar areas, where temperature and pressure conditions of the gas hydrate stability field are reached and sufficient gas is available (Matsumoto et al., 2000; Kvenvolden and Lorenson, 2001; Bohrmann and Torres, 2006). Based on global estimates, although still uncertain, gas hydrates in ocean and permafrost areas are considered as significant reservoirs for natural gases (in particular methane) along with societal and economic points of view. Natural gas hydrates might therefore become a major hydrocarbon source which may help to meet the increasing demand due to global consumption of hydrocarbons (Max et al., 2006). Natural gas hydrates have also captured the attention of geologists as slope stability around continents seems to be influenced by the presence of hydrates in the pore space of margin deposits (Paull et al., 2003). Marine Geology 244 (2007) 1 – 14 www.elsevier.com/locate/margeo ⁎ Corresponding author. Tel.: +49 421 218 8639; fax: +49 421 218 8664. E-mail address: gbohrmann@uni-bremen.de (G. Bohrmann). 0025-3227/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2007.05.003