Geochemistry of Hydrocarbon Gases and Volatile Organic Acids from Sediments; Results from the India NGHP Expedition 01

Authors: Lorenson, Thomas D. (U.S. Geological Survey, speaker), Hunsinger, Glendon, Rao, B.R., Sinha, A.K., Woodong, J.

Venue: India’s Directorate General of Hydrocarbons’ International Conference on Gas Hydrates in Nodia (New Delhi), India, February 6–8, 2008 (http://www.dghindia.org/site/pdfattachments/upcomingevents/Updated_Programme_gAS[1].pdf [PDF-external site]).

Abstract: Coring to determine the gas hydrate endowment of India was carried out under the 2006 National Gas Hydrate Program (NGHP) Expedition 01 in four Indian offshore basins: Kerala-Konkan, Krishna-Godavari (KG), Mahanadi, and Andaman. A total of 39 holes were drilled or cored at 21 sites. Shipboard organic geochemical studies included analysis of the composition of volatile hydrocarbons, including methane, ethane, and propane and fixed natural gases (i.e., O2, CO2, and N2+Ar) from headspace void gas and gases evolved from sediment collected at pressure. Shore-based isotopic studies are still underway; however, preliminary results of molecular and stable carbon isotopic composition will follow. In addition, volatile organic acids were analyzed from pore water samples to monitor the concentration of microbial fermentation substrates. Gas hydrates, composed mainly of methane, were found in large quantities in Indian offshore areas, particularly the KG basin; therefore, the genesis of methane is of particular interest in resource evaluation. The gas composition of sediments—mainly methane—in each basin strongly suggests that the sources of hydrocarbon gases are microbial. Isotopic studies conducted by the India Oil and Natural Gas Corporation confirm that the methane is of microbial origin and that the ethane is also of microbial or early diagenetic origin. The high concentration of microbial hydrocarbon gases, coupled with young sediments, suggests that methane generation is an ongoing in situ process and that the migration distances of gases is relatively short compared with thermogenic processes and migration. Volatile organic acid analysis of pore waters for formate, acetate, propionate, and lactate demonstrate that these microbial fermentive substrates are available and a potential source of microbial methane. The most ubiquitous and important substrate, acetate, is present in concentrations ranging from 3 to 775 micromolar. Typically, acetate concentrations are lower in shallow sediments and increase to local maxima in areas of high methane concentrations, such as the free gas near bottom-simulating reflectors or gas hydrate localities. This relationship suggests that microbes relying on acetoclastic methanogenesis are inhibited by locally high methane concentrations. Similar relationships of abnormally high acetate concentrations associated with gas hydrates have been documented along the Cascadia margin during ODP Leg 204, and IODP Leg 311, and the Blake Ridge, ODP Leg 164, suggesting a fundamental process.

Related NETL Project
The USGS conducts scientific studies of natural gas hydrates to support DOE efforts to evaluate and understand methane hydrates, their potential as an energy resource, and the hazard they may pose to ongoing drilling efforts. This project, DE-AI26-05NT42496, extends USGS support to the DOE Methane Hydrate Research Program previously supported under DE-AT26-97FT34342 and DE-AT26-97FT34343.

NETL Project Contacts
NETL – Robert Vagnetti (robert.vagnetti@netl.doe.gov or 304-285-1334)
USGS – Deborah R. Hutchinson (dhutchinson@usgs.gov or 508-457-2263)