Natural gas geochemistry of sediments drilled on the 2005 Gulf of Mexico JIP cruise Thomas D. Lorenson a, * , George E. Claypool b , Jennifer A. Dougherty a a U.S. Geological Survey, Coastal and Marine Geology Team, 345 Middlefield Road, MS-999, Menlo Park, CA 94025, USA b 8910 West Second Avenue, Lakewood, CO 80226, USA article info Article history: Received 1 February 2007 Received in revised form 17 July 2007 Accepted 4 January 2008 Keywords: Gas hydrate Gas geochemistry Microbial methane Gulf of Mexico abstract In April and May 2005, cores were acquired and sub-sampled for gases in lease blocks Atwater Valley 13 and 14 and Keathley Canyon 151 during deep subseafloor drilling conducted as part of the JIP study of gas hydrates in the northern Gulf of Mexico. Sample types included sediment headspace gas, free gas derived from sediment gas exsolution, and gas exsolution from controlled degassing of pressurized cores. The gases measured both onboard and in shore-based labs were nitrogen, oxygen, hydrogen sulfide, carbon dioxide, and the hydrocarbons methane through hexane. The presence of seafloor mounds, seismic anomalies, a shallow sulfate–methane interface, and similar gas compositions and isotopic compositions near the seafloor and at depth suggest an upward flux of methane at both sites. Sediment gases at the Atwater Valley sites, where seafloor mounds and adjacent sediments were cored, strongly suggest a microbial source of methane, with very little thermogenic gas input. Sediment gas from all cores contained from about 96 to 99.9% methane, with the balance composed primarily of carbon dioxide. Methane to ethane ratios were greater than 1000, and often over 10,000. Gases from cores at Keathley Canyon were similar to those at Atwater Valley, however, deeper cores from Keathley Canyon contained more ethane, propane, and butane suggesting mixing with minor concentrations thermogenic gas. The isotopic composition of methane, ethane, and carbon dioxide were measured, and d 13 C values range from 84.3 to 71.5&, 65.2 to 46.8&, and 23.5 to 3.0&, respectively, all consistent with microbial gas sources, early diagenesis of organic matter and perhaps biodegradation of petroleum. The presence of deep microbial gas at these sites here and elsewhere highlights a potentially significant, predominantly microbial gas source in the northern Gulf of Mexico. Published by Elsevier Ltd. 1. Introduction 1.1. Rationale In support of the Chevron Joint Industry Project (JIP) on Meth- ane Hydrates, natural gas components in recovered sediment cores were monitored for methane to butane hydrocarbon concentration during a 2005 drilling cruise aboard the Uncle John. When gas volume was present in sufficient amounts, samples were collected for more detailed chemical and isotopic analyses post-cruise in shore-based laboratories. The gas was analyzed to determine if sediments contained high gas concentrations and a gas composi- tion likely to support the presence of gas hydrate, and to determine the origin of the gas. Possible gas sources evaluated are those produced locally by sedimentary microbial processes, microbial gases that have migrated from other areas (Sassen et al., 2003), or thermogenic gas from underlying petroleum accumulations. Gas hydrates were previously recovered from the seafloor and shallow sediments on many parts of the continental slope of the northern Gulf of Mexico (e.g., Milkov and Sassen, 2001; Fig. 1) and had been studied in detail on a regional basis (e.g., Paull et al., 2005; Winters et al., 2005) and in local areas near enhanced flux features like mud mounds (Ruppel et al., 2005). The areas selected by the JIP project for 2005 coring and logging included both a site with mud mounds (Atwater Valley, shown in Fig. 2) and a site at the edge of a salt withdrawal minibasin and away from enhanced flux features on the seafloor (Keathley Canyon, shown in Fig. 3). During site survey cruises prior to the drilling, these focus areas were evaluated through the acquisition of a variety of datasets, including geochemical data collected on near-surface sediments recovered by shallow coring operations (e.g., Coffin et al., 2008; Pohlman et al., 2008). In this paper, we extend some of these studies to the deeper part of the sedimentary section through analyses on sediment cores recovered from the JIP boreholes. * Corresponding author. Tel.: þ1 650 329 4186; fax: þ1 650 329 5441. E-mail address: tlorenson@usgs.gov (T.D. Lorenson). Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo 0264-8172/$ – see front matter Published by Elsevier Ltd. doi:10.1016/j.marpetgeo.2008.01.017 Marine and Petroleum Geology 25 (2008) 873–883