Hydrocarbon generation is the natural result of the maturation of buried organic matter. The sources of organic matter in ocean sediments include plankton, bacteria, and land-derived material. The organic matter can be transformed into hydrocarbons, depending on burial depth and temperature. Hydrocarbons such as methane, ethane, propane, and butane are usually found in the gaseous phase in sediments close to the surface. Microbial activity is most often the source of these gases at shallow depths, but one of the exciting discoveries during ODP was evidence of microbial activity deeper in the crust.
Hydrocarbon monitoring was one of the primary reasons for gas sampling and analysis. During ODP, the JOIDES Resolution was not designed to drill in areas where oil or gas could be encountered. Proposed sites were intensively studied and reviewed in order to ensure those sites did not have factors conducive to hydrocarbon accumulation. Headspace gas samples and gas from expansion voids were analyzed as part of an active monitoring program. In addition to hydrocarbons, gases such as hydrogen sulfide represented a potential safety problem.
Gas analyses were conducted on headspace samples (gas obtained from sediment samples) and Vacutainer samples (void pockets within the core before the liner had been breached). Headspace samples were taken from the core immediately after the core was brought on deck. A 5-cm3 sample was placed in a glass vial, sealed, and heated for 30 min. A 5-mL aliquot of gas was extracted and analyzed by GC.
Vacutainer samples were also taken immediately after the core came on deck if there were gas pockets, bubbling, or frothing within the liner, or bulging end caps. A liner penetrator tool equipped with a valve and needle was used to collect gas samples. A preevacuated sealed glass tube or syringe was placed on the valve and the valve was opened for a few seconds. In the laboratory, some of the gas was extracted from the tube or syringe and analyzed by GC.
Over the span of ODP, several instruments were used to analyze HS and VAC gases. Several HP gas GCs were used, from the HP 5890A GC used during the early part of ODP to the HP 6890 GC instruments in the laboratory at the end of ODP. Instrumentation in the chemistry laboratory usually included a GC with both a FID and TCD and a GC with a NGA, FID, and TCD. A Carle Series 100 analytical GC was used for rapid determinations of methane, ethane, and propane.
Preanalyzed standards were run to ensure the chromatographic responses were calibrated. The calibration measurements were not archived or entered into the Janus database. Additional information about ODP gas analyses can be found in Technical Note 30 (Pimmel and Claypool, 2001).
Early in ODP, gas data were collected on log sheets which were sent to ODP/TAMU at the end of each cruise. The data were entered into the S1032 database, and the log sheets were microfilmed for archival storage. Data entry routines were implemented so that data entry could be done on the ship and the practice of collecting data on log sheets ended. Gas analyses were stored in the S1032 database until the Janus database became operational during Leg 171.
The data model for gas elements data can be found in "Janus Gas Chromatography Data Model" in "Appendix B." Included are the relational diagram and list of the tables that contain data pertinent to gas analyses, column names, and definition of each column attribute. ODP Information Services Database Group was responsible for the migration of pre-Leg 171 data to Janus.
Gas analyses can be retrieved from Janus Web using a predefined query. The gas elements query Web page allows the user to extract data using the following variables to restrict the amount of data retrieved: leg, site, hole, core, section, depth range, and latitude and longitude ranges. The gas query also gives the user the option of extracting data by sampling method, instrument used to analyze the gas, or detector type.
Table T4 contains the data fields retrieved from the Janus database using the Janus Web predefined query. The first column contains the data item; the second column indicates the Janus table or tables in which the data were stored; the third column is the Janus column name or the calculations used to produce the value. "Description of Items from Gas Elements Query" in "Appendix B" contains additional information about the fields retrieved using the Janus Web gas elements query and the data format for the archived ASCII files.
The collection of gas data was vitally important to the safety of drilling operations on the drillship. Much care was taken in order to obtain rapid, accurate results. There are few known instances where there was a major problem with data collection. Anything written or typed was a potential source of error. Analytical results were written on log sheets. These data were then typed into the S1032 database. Data entry programs were implemented to add the data to the S1032 database, but these programs still required manual entry. Typographical errors from writing or typing incorrect information occasionally happened, and some mistakes were not identified. Often, the scientific party found errors and corrected them for the data included in the Initial Reports volume, but data sent to ODP/TAMU were not corrected.
Another error found during the migration of gas data was that samples were missing from the database. In those instances, a sample was entered into the database so that the data could be migrated. The verification of those samples and the verification of the entire gas data set were not completed because of time constraints. Most data collected after the Janus database was operational during Leg 171 were verified as part of the Janus data management and verification procedures (see "Janus Data Management and Verification"). Some verification was done on the pre-Leg 171 data; however, if there is a discrepancy between the database and data in the Initial Reports volumes, the published data should be considered more reliable.
Janus does not contain any calibration information for GCs. Procedures for collecting and storing calibration information in the database were not implemented during ODP. Procedures for collecting data on lipids were also not implemented during ODP.