Several organic geochemical measurements were used during Leg 149 to monitor volatile hydrocarbons and other gases as part of the shipboard safety requirements and to provide an initial characterization of the organic matter in the sediments.
Compositions of low-molecular-weight hydrocarbons and other gases were monitored in each core by the standard ODP headspace procedure (Kvenvolden and McDonald, 1986). About 5 cm3 of sediment was obtained from the end of a freshly cut core section and placed into a 21.5-cm3 glass serum vial. The vial was sealed with a septum and a metal crimp cap and heated at 60°C for 30 min. For each analysis by gas chromatography, a 5-cm3 volume of gas from the headspace in the vial was extracted with a standard glass syringe.
The standard ODP vacutainer method of gas sampling (Kvenvolden and McDonald, 1986) was used whenever gas pockets or expansion voids were observed in cores as they arrived on deck. Vacutainers were pre-evacuated, septum-sealed, 20-cm3 glass tubes. For obtaining a gas sample, a special tool was employed to penetrate the core liner.
Headspace and vacutainer gas samples were analyzed routinely using the Hach-Carle (HC) gas chromatograph. This instrument has been designed to measure, accurately and rapidly, the concentrations of methane, ethane, and propane. Ethene is resolved from ethane and can also be quantified. Samples are introduced into the HC gas chromatograph through a 1.0 cm3 sample loop having manual column backflush. The chromatographic column was a 0.32 cm x 1.8 m stainless steel tubing packed with 80% Porapak N and Porapak Q (80/100 mesh). A flame ionization detector was used, and the chromatographic conditions were isothermal at 90°C, with helium used as carrier gas. A Hewlett-Packard 3365 ChemStation computer data collection and analysis system was used to integrate and store the results of the gas measurements.
Two pyrolysis systems were used during Leg 149 to evaluate the type of organic matter present in sediments and sedimentary rocks: the Delsi-Nermag Rock-Eval II and the Geofina hydrocarbon meter. Both systems use whole-rock pyrolysis techniques to identify the type and maturity of organic matter and to detect petroleum potential and oil shows in sediments, as described by Espitali et al. (1986). Sample size for both instruments was between 50 and 100 mg.
The Rock-Eval system involves a graduated temperature program that first volatilizes existing hydrocarbons at 300°C for 3 min, and then releases hydrocarbons from thermal cracking of kerogen as the temperature increases at 25°C/min from 300° to 550°C. Four parameters characterizing the organic matter are determined:
The Rock-Eval II instrument is equipped with a module that uses an algorithm and the S1, S2, and S3 peak values to estimate the total amount of organic carbon (TOC) in samples.
Organic matter can be characterized from Rock-Eval data using the hydrogen index [(100 S2)OC], the oxygen index [(100 S3)OC], and the S2/S3 ratio. The first two parameters normally are referred to as HI and OI, respectively. Interpretations of Rock-Eval pyrolysis are considered to be unreliable for samples having less than 0.5% TOC (Katz, 1983; Peters, 1986), although a correction procedure has been described for estimating matrix effects and for obtaining reliable values from samples having lower amounts of TOC (Espitali, 1980).
The Geofina hydrocarbon meter (GHM) employs a Varian 3400 series gas chromatograph that has been modified to include a programmable pyrolysis injector. The system has three flame ionization detectors and two capillary columns (25 m, GC2 fused silica). Like the Rock-Eval II, this tool determines S1, the free hydrocarbons that are released up to 300°C, and S2, the pyrolysis products that are generated from the sample kerogen. Tmax of the S2 peak also is determined. The effluent from the furnace is split 20:1 so that the hydrocarbon distributions making up S1 and S2 can be examined in detail by capillary gas chromatography. Rock-Eval and GHM parameters can be used to calculate the production index (PI) = S2/(S1 + S2), and petroleum potential or pyrolyzed carbon index (PC) = 0.083(S1 + S2).
The carbonate carbon content of samples was determined using a Coulometrics 5011 inorganic carbon analyzer (Engleman et al., 1985). A sample of about 20 mg of freeze-dried and ground material was reacted with 2N HCl. The liberated CO2 was titrated in an ethanolamine solution containing a color indicator, and the change in color was measured by a photodetector cell. Carbonate carbon contents are expressed as weight percent CaCO3, assuming all the carbonate was present as pure calcite.
The total organic carbon content (TOC) of sediment samples was determined either by the Rock-Eval TOC module or by the difference between carbonate carbon and the total carbon value determined by the Carlo Erba Model NA 1500 NCS analyzer. In the NCS analyzer, freeze-dried and ground samples were combusted at 1000°C in an oxygen stream. Nitrogen oxides were reduced to N2, and the mixture of CO2, SO2, and N2 was separated by gas chromatography and quantified with a thermal conductivity detector.