The most common method of hydrocarbon monitoring used in ODP's operation is the analysis of gas samples obtained from either core samples (headspace analysis) or from gas expansion pockets visible through clear plastic core liners (vacutainer analysis).
In headspace analysis, the composition and concentrations of hydrocarbons are analyzed by GC with the following technique.
A 5-cm3 sample (one per core), taken from the core immediately after retrieval on deck, is placed in a 20-cm3 glass vial that is sealed with a septum and a crimped metal cap. When consolidated or lithified samples are encountered, chips of material are placed in the vial and sealed. From cores where an interstitial water (IW) sample is obtained, the headspace sample is taken from the top of the section immediately next to the IW sample, whenever possible. The vial is labeled with the core, section, and interval from which the sample was taken. The vial is then placed in an oven at 70°C for 30 min. A 5-cm3 (5 mL) volume of gas extracted through the septum is then injected with a glass syringe into a gas chromatograph (GC3 and NGA when the sediment contains high levels of hydrocarbons or nonhydrocarbons such as CO2 and H2S).
When a core comes on deck, it is checked for gas pockets, bubbles, or frothing within the liner or bulging end caps of sealed core liners. These gas pockets are sampled using a gas sampling device that is a liner penetrator tool equipped with a valve and needle. The gas is transferred into a pre-evacuated, septum-sealed glass tube (20 cm3) or a 50-cm3 syringe equipped with a three-way stopcock valve. To sample a void, the device is inserted into the core liner where the void is located. The syringe/vacutainer is placed on the valve, and the valve is switched open. In case of syringe sampling, note that the stopcock handle is always over the closed port. With the glass vacutainer, one first has to place a needle on the valve and then open the valve by turning the switch so that it is parallel with the needle. After 10 s, the valve is turned back off and the vacutainer is removed. Portions of gas in the syringe/vacutainer are then injected for analysis in either or both the GC3 and NGA.
The GC3, a Hewlett Packard (HP) GC model 6890, is used to accurately and rapidly measure the concentrations of methane (C1), ethane (C2), ethylene (C2=), propane (C3), and propylene (C3=). The GC3 is configured to use an 8 ft x 1/8-in stainless steel packed column filled with HayeSep R porous polymer packing (80/100 mesh). The injector consists of a 1/16-in Valco union with a 7-µm screen connected to a Valco-to-Luer lock syringe adaptor. This injector connects to a 10-port Valco valve that is switched pneumatically by a digital valve interface (DVI). The injector temperature is set at 110°C. Samples are introduced into the GC via a 0.25-cm3 sample loop connected to the Valco valve. The valve can be switched automatically to backflush the column. The oven temperature is programmed to be initially at 100°C for 5.50 min then increased to 140°C for 4 min at a rate of 50°C/min. Helium is the carrier gas. Initial helium flow on the column is 30 mL/min. Flow is then ramped to 80 mL/min after 3 min to accelerate elution of C3 and C3=. The run time is 10.30 min. The GC is equipped with a flame ionization detector (FID) set at 250°C. The GC is also equipped with an electronic pressure control (EPC) module to control the overall flow into the GC.
The NGA is an HP 6890 multivalve, multicolumn GC. It is equipped with both thermal conductivity (TCD) and FID detectors. The multiple-valve switching system is used to direct flows through various sample loops and columns. Helium is used as the carrier gas. Four columns are used sequentially to provide rapid partitioning and measurement of N2, O2, CO2, H2S, CS2, and C1-C10 hydrocarbons (12 min run). The multi-column system is composed of a 6-in stainless steel column packed with Poropak T (50/80 mesh) in line with a 3-ft column packed with molecular sieve 13x (60/80 mesh), a 6-ft stainless steel column packed with 80/100 mesh Haysep R (acid washed), and a 60 m x 0.32 mm capillary column coated with a 1-µm film thickness of DB-1 (J&W, Inc.). The injection is performed via a 1/16-in Valco union connected to a Valco-to-Luer lock syringe adaptor. This injector connects to a six-port Valco valve (V1) switched pneumatically by a DVI. When V1 is in the LOAD position, samples will be introduced onto the entire set of columns via three sample loops (0.25 cm3, 1 cm3, and 0.5 cm3). When V1 is in the INJECT position, the sample will be loaded onto the capillary column for separation of C1 to C7 hydrocarbons. Valve 2 (V2) is a four-port Valco valve that can be switched (using a DVI) to backflush the capillary column. Valve 3 (V3) is an eight-port Valco valve connected to a 1-cm3 sample loop and the HayeSep R column, which is used to separate air and methane from CO2, ethylene, ethane, H2S, propylene, and propane. After propane has eluted, V3 is switched OFF to backflush the Haysep column. Valve 4 (V4) is a 10-port Valco valve connected to a 0.5-cm3 sample loop and the Poropak T and molecular sieve columns, which are used to separate N2, O2, methane, and CO. Valve 4 is switched OFF immediately after V3 is opened to backflush the columns. The chromatographic separation on the TCD portion of the GC is carried out isothermally at 80°C. This separates O2, N2, C1, C2, C2=, H2S, C3 and C3=. The hydrocarbon separation on the FID portion of the GC system is carried out isothermally at 50°C. The FID routinely detects C1 to C7 but can also separate "out" through C10. The TCD injector and detector temperatures are 80° and 201°C, respectively, and the corresponding temperatures for the FID are 80° and 250°C, respectively. The NGA is equipped with two EPC modules.
An HP ChemStation computer data acquisition and analysis system is used to integrate and store the results of the gas measurements. Chromatographic responses are calibrated against preanalyzed standards, and the gas contents are reported in parts per million.
For safety consideration, gas composition is commonly expressed as C1/C2 ratio (obtained from the GC data) and plotted vs. depth. This ratio is generally used to get rapid information about the origin of the hydrocarbons (i.e., to distinguish between biogenic gas and gas migrated from a deeper source of thermogenic hydrocarbons). When high amounts of C1 are present (>10,000 ppm), very high C1/C2 ratios indicate a gas (C1) formation by biological processes. On the other hand, the occurrence of major amounts of C2 (to C5) in shallow depths is associated with thermogenic hydrocarbon generation. When interpreting the C1/C2 ratios, it has to be considered, however, that minor amounts of C2 (and C3, C4, and C5) can also be generated in situ during early (low temperature) diagenesis of organic matter. The importance of this process increases with increasing burial depth, resulting in a consistent ("normal") decrease in C1/C2 with increasing temperature. The relationship of C1/C2 and sediment temperature (Fig. F2) can be used as one criterion to evaluate the "normal" vs. "anomalous" nature of the hydrocarbon occurrence. Anomalously low C1/C2 ratios suggest the presence of migrated thermogenic hydrocarbons (Ocean Drilling Program, 1992).
The C1/C2 vs. temperature guidelines developed by the JOIDES Safety Panel (Ocean Drilling Program, 1992) were based on analysis of vacutainer or gas expansion pocket samples. Rudy Stein and others adapted and calibrated the C1/C2 vs. temperature diagram (Fig. F2) for headspace analysis (Shipboard Scientific Party, 1995).
Figure F2 includes a pattern that approximately separates anomalous vs. normal C1/C2 ratios. Also shown by the two lines is the approximate influence of different levels of organic carbon content. Headspace and vacutainer techniques generally give slightly different results, especially at shallow depths. Vacutainer C1/C2 ratios are higher than headspace ratios because the vacutainer technique retains more of the methane, but the overall trends should be similar.
Figure F3 shows the separation pattern of normal vs. anomalous zones for both headspace and vacutainer techniques for Leg 151 Hole 909C. At Site 909, the headspace C1/C2 ratios show the normal general decrease with depth and temperature, from values of >10,000 in shallow depths to a value of ~50 at 1061 mbsf (or 93°C in temperature). Vacutainer C1/C2 ratios are slightly lower (19) but show the same trend. Despite heavy hydrocarbon (C2-C7) occurrence after 700 msbf (or 60°C), their smooth increase with depth was normal and drilling was continued without incident until 1061.8 msbf. A sharp change in the hydrocarbons (abrupt increase of C3-C7) and other geological observations resulted in the termination of drilling of Hole 909C.
"Appendix A" describes the procedure used to inject a gas sample into the GC3 and NGA. "Appendix B" describes the procedure used to send gas data to the Janus database. "Appendix C" describes the features of the GAR program used to monitor gas data in real time.