Hill I A Taira A Firth J V et al 1993 Proceedings of the Ocean Drilling Program Scientific Results Vol 131 15 LIGHT HYDROCARBONS IN SEDIMENTS OF THE NANKAI ACCRETIONARY PRISM LEG 131 SITE 808 1 Ulrich Berner2 and Eckhard Faber2 ABSTRACT A suite of gas samples obtained from gas pockets and sediments of the Nankai accretionary prism Site 808 has been analyzed for their gas composition and carbon and hydrogen isotope ratios Gases collected from gas pockets between 10 and 555 mbsf consist of CH4 and CO2 Stable carbon isotope ratios of these two components point to a bacterial formation of methane via CO2 reduction that is also supported by D H ratios of methane Methane desorbed from sediments by a vacuum acid treatment is of bacterial and thermal origin Mixing between these gas types is indicated by molecular composition and carbon isotope ratios Diagenetic processes at low temperatures can explain ethane to pentane concentrations from 0 to 850 mbsf Between 850 mbsf and the basaltic basement hydrocarbon occurrences are related to catagenetic processes at elevated temperatures Thermal alteration of organic matter is reflected through different gas parameters Propane carbon isotope values of a sample from the zone of the frontal thrust indicate that the gas likely migrated from sediments of a higher maturity into the immature sediments at 366 mbsf INTRODUCTION ODP Site 808 Fig 1 drilled the complete sedimentary section of the Nankai accretionary prism Japan penetrating the frontal thrust d collement and basaltic basement Taira Hill Firth et al 1991 The primary objectives of Leg 131 were to obtain information on the temperature history the pore fluids the mechanical state and the physical properties of the deformed sediments As the formation reactions of light hydrocarbons are strongly temperature dependent their analysis at Site 808 can represent a sensitive indicator for the thermal hydrocarbon generation process They also may provide information on the thermal evolution of the accretionary prism Molecular gas composition and isotope ratios of individual components have been determined on gases from Site 808 to determine 1 the origin and 2 migration of hydrocarbons and 3 the thermal structure of the accreted sediments METHODS At Site 808 gases from 22 gas pockets in the depth range 10 555 mbsf were expanded into glass Vacutainers on board JOIDES Resolution before cutting the liners for detail cf Taira Hill Firth et al 1991 In addition 44 sediment samples were frozen at liquid nitrogen temperatures directly after cutting the cores Gases of the frozen sediments were released by a vacuum acid treatment of Horvitz 1972 modified by Faber and Stahl 1983 The light hydrocarbons collected by this method are gases which are dissolved in the interstitial fluids and gases which are adsorbed on the sedimentary particles The combination of these two fractions is referred to as sediment gases in the text All gas samples sediment gases and Vacutainer gases were analyzed for their molecular composition their carbon and depending on the amount for their hydrogen isotope ratios of methane The molecular composition of gases was determined by standard flame ionization detector FID and thermal conductivity detector TCD analyses utilizing a Siemens Sichromat 2 gas chromatograph with a capillary pre column 50 m and a Al2O3 filled capillary main mil I A Taira A Firth J V et al 1993 Proc ODP Sci Results 131 College Station TX Ocean Drilling Program 2 Federal Institute for Geosciences and Natural Resources Stilleweg 2 D 3000 Hannover 51 Federal Republic of Germany column 50 m Nitrogen was used as a carrier gas at a flow rate of 2 93 mL min The chromatographic separation was carried out isothermally at 90 C Gas concentrations of gas pockets are reported as vol Table 1 whereas gas concentrations of sediment samples are given as nanograms per gram ng g wet sediment Table 2 For the isotope analyses the individual gas components were separated chromatographically and subsequently combusted to CO2 and H2O using a preparation line described by Dumke et al 1989 The combustion product H2O is reduced to H2 by reaction with zinc in sealed glass tubes at 450 C Coleman et al 1982 Dumke et al 1989 For the subsequent carbon and hydrogen isotope analyses Finnigan MAT 251 and Finnigan Delta mass spectrometers were used Carbon isotope values are reported in reference to the PDB PeeDee Belemnite standard whereas hydrogen isotopes refer to the SMOW Standard Mean Ocean Water standard The reproducibility of isotope measurements is 0 for carbon and 2 for D H ratios respectively The isotope data of gases from gas pockets is given in Table 1 and isotope ratios of sediment gases are compiled in Table 2 RESULTS AND DISCUSSION Gas Pockets Between the sediment surface and 3 mbsf sulfate content of pore waters decreases from seawater concentrations to 0 mM due to bacterial sulphate reduction Taira Hill Firth et al 1991 Gamo et al this volume Gas samples have been collected from gas pockets below this zone of sulfate reduction Between 10 and 555 mbsf gases consist predominantly of CH4 and CO2 Table 1 Methane dominates over carbon dioxide Ethane is only present in traces at a depth of more than 170mbsf Table 1 Stable carbon isotope ratios of methane and CO2 point to a bacterial formation of methane via CO2 reduction Fig 2 as described by Whiticar et al 1986 Carbon isotope values of carbon dioxide of Vacutainer gases are depleted in heavy carbon isotopes by approximately 14 o compared to the ZCO 2 of the interstitial waters Gamo et al this volume This difference can be attributed to a degassing fractionation during sample retrieval when dissolved CO2 is released from pore waters due to the decreasing pressure The carbon isotope ratios of CO2 of the gas pockets can therefore only be used as a rough estimate in gas characterization However the bacterial origin of methane in the gas pockets is supported by D H and 13C 12C ratios of methane Fig 3 gas pockets Carbon isotope values of methane from gas pockets seem to be depleted in heavy carbon isotopes by only 2 e compared to gases 185 U BERNER E FABER Shikoku Basin 30 Figure 1 Location map of Site 808 released by the vacuum acid treatment of the sediment Tables 1 and 2 The differences are also related to a slight degassing fractionation during sample retrieval when methane is released from sediments due to decreasing pressure The small deviation of carbon isotope ratios in methane of gas pockets