Previous   |    Next

doi:10.2204/iodp.proc.331.201.2016

Methods and materials

Exteriors were removed from samples (mostly wedges taken from the working half of cores) prior to being dried and solvent extracted with 97:3 dichloromethane (DCM)/methanol MeOH (v/v) in a Soxhlet apparatus for 48 h. Solvent extracts were separated by column chromatography with activated (120°C overnight) silica. Aliquots of the extracts (~0.01–0.05 mg) were absorbed onto activated silica (~500 mg). After conditioning the column with 5 mL of hexane, saturated fractions were eluted with 3 mL of hexane, aromatic fractions with 3 mL of a mixture (3:1) of hexane and DCM, and the polar fraction with 3 mL of a mixture (2:1) of DCM and MeOH. The polar fraction was reduced to dryness under N₂, redissolved in a mixture of (5:1) hexane/DCM, transferred to a clean vial (this leaves asphaltic material in the original vial) and derivatized with BSTFA (N,O-bis [trimethylsilyl] trifluoroacetamide) to convert alkenoic acids to their silylesters.

Gas chromatography–mass spectrometry (GC-MS) was carried out on an Agilent Technologies (AT) 6890N network GC (pulsed splitless) system fitted with a 30.0 × 250.0 µm internal diameter (ID) film thickness 0.25 µm fused capillary column coated with DB-5 coupled to an AT 5975 quadrupole mass- selector detector (electron input energy 70 eV, source temperature 250°C) with helium as carrier gas. Fractions were analyzed using an oven programmed temperature of 60°C (2.0 min) to 120°C at 20°C/min to 290°C at 4°C/min and held at 290°C for 23 min. The mass-spectrometer was operated in selective ionization mode (SIM) with compounds identified by relative retention times and comparisons to well-characterized samples. Concentrations of fatty acids are reported relative to an internal standard of nonadecanoic acid.

A level of thermal maturity or thermal alteration was determined using biomarker proxies and expressed as a vitrinite reflectance equivalence (%VRE). Two tools were used. For Site C0017 and surface sediments at Site C0014, the hopane %22(S) parameter was converted to %VRE using comparative diagrams published in Peters et al. (2004) and Killops and Killops (2005). At Site C0013 and for the deeper horizons at Site C0014, the conversion equations used to obtain an approximate %VRE from the methylphenanthrene index were Rm (%) = 0.38 + 0.61 MPI 1 as presented in Radke (1988) (see also Radke et al., 1980, for additional context). Given the limited mixing thermally mature and immature bitumen inferred for these settings and the general level of error involved in using the approaches described above, the level of error for measurements is approximately ±0.15% on the VRE scale. Thermal maturation was modeled using the EASYvit model described in Sweeney and Burnham (1990). Within the model, durations of heating were varied (1y to 1 My), whereas present-day downhole temperature profiles were kept constant to produce predictions of levels of thermal alteration for different depths.

Top of page   |    Previous   |    Next