Analysis of sedimentary hydrocarbon gases in Ocean Drilling Program (ODP) sediments is carried out routinely on board ship for safety and pollution prevention. Low molecular weight hydrocarbon (LMWH) gases (methane, ethane, and propane) are monitored in each core following a standardized headspace sampling method (Kvenvolden and McDonald, 1986). In this method, a sediment sample is placed in a glass serum vial and sealed and heated to 60°C for 30 min. A 5-cm3 subsample of the headspace gas is then analyzed by gas chromatography (GC). When heavier molecular weight hydrocarbons (C3 and above) are detected, the sample is analyzed by a natural gas analyzer (NGA) used to quantify hydrocarbons up to C6 and also nitrogen, oxygen, and carbon dioxide. This technique provides consistent order-of-magnitude data on both methane and ethane and a meaningful C1/C2+ gas ratio utilized for safety monitoring. However, it suffers from the disadvantage that in routine analytical procedures, compounds >C3 will be missed if propane is not initially detected in the gas sample.
This study examined the LMWH distributions in sediments from two sites (Sites 1109 and 1115) in the western Woodlark Basin using purge-trap thermal adsorption/desorption gas analysis. The advantage of this method over normal shipboard headspace analysis is that all the hydrocarbons from the sediment sample are flushed from the vial headspace and concentrated on an absorbent trap, reducing the detection limit of volatile sedimentary hydrocarbons and also giving a complete analysis of C2-C6 hydrocarbon components.