ROCK EVALUATION

RE analyses were used to evaluate type and maturity of organic matter, calculate petroleum potential, and detect oil shows. These measurements are usually done by exploration companies looking for oil and gas. The RE data were generated using the Delsi Nermag Rock-Eval II Plus TOC instrument for whole-rock or sediment pyrolysis.

Oil exploration was not the reason that rock evaluation analyses were done by ODP. There was a significant amount of planning and research done to avoid drilling in areas that had potential for oil or gas. These data were used on the ship as an interpretive tool for monitoring hydrocarbon safety levels, in addition to providing information about the organic matter in the sediments. Because of the length of time necessary to complete the analysis of a sample, RE results were not normally used as a part of the real-time hydrocarbon monitoring program.

Data Acquisition

RE data have been collected since ODP Leg 101. Samples analyzed for rock evaluation were usually subsamples of the freeze-dried and crushed material collected for carbon analyses. The RE method consisted of programmed-temperature heating to quantitatively determine any free hydrocarbons contained in the sample and the hydrocarbon- and oxygen-containing compounds that are volatilized during heating.

Five basic parameters, S1, S2, S3, temperature maximum (Tmax), and TOC, were measured:

  1. S1: amount of free hydrocarbon (gas and oil). If S1 > 1 mg/g, it may be indicative of an oil show. S1 can be contaminated by the drilling fluids and mud.
  2. S2: amount of hydrocarbon generated through thermal cracking of nonvolatile organic matter. S2 is an indication of the quantity of hydrocarbon that the sediments could potentially produce should burial and maturation continue.
  3. S3: amount of CO2 produced during pyrolysis of kerogen. S3 is an indication of the amount of oxygen in the kerogen.
  4. Tmax: temperature at which the maximum release of hydrocarbons from cracking of kerogen occurs during pyrolysis. Tmax is an indication of the stage of maturation of the organic matter.
  5. TOC: total organic carbon, in weight percent, can be determined by oxidizing the organic matter remaining in the sample after pyrolysis. TOC is determined by adding this residual organic carbon to the pyrolized organic carbon.

From these measurements, four additional parameters can be calculated that describe the type and maturity of the organic matter:

  1. Productivity index [S1/(S1 + S2)]: characterizes the evolution level of the organic matter. In an ideal situation with increasing burial depth, S1 should increase and S2 should decrease resulting in PI increasing with depth and maturation.
  2. Petroleum potential or pyrolized carbon [0.083 x (S1 + S2)]: corresponds to carbon content, the maximum quantity of hydrocarbons capable of being produced from the source rock given sufficient depth and time.
  3. Hydrogen index [(100 x S2)/TOC]: parameter used to characterize the origin of the organic material. Marine organisms and algae have higher H/C ratios than land plants. HI typically ranges from ~100 to 600 mg HC/g sediment in geological samples.
  4. Oxygen index [(100 x S3)/TOC]: parameter that indirectly correlates the ratio of oxygen to carbon. OI values range from ~0 to 150 mg CO2/g sediment.

Additional information about rock evaluation measurements can be found in Technical Note 30 (Pimmel and Claypool, 2001).

Archive

Pre-Janus Archive

Early in ODP, RE data were collected on log sheets 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. RE data were stored in the S1032 database until the Janus database became operation during Leg 171.

Migration of RE Data to Janus

The data model for RE data can be found in "Janus Rock-Eval Data Model Stored in Carbonate Tables" in "Appendix L." Included are the relational diagram and the list of the tables that contain data pertinent to rock evaluation, column names, and the definition of each column attribute. ODP Information Services Database Group was responsible for the migration of pre-Leg 171 data to Janus. When the Janus data model was implemented, tables to store RE data were created. After Janus became operational, it was determined that it was more practical to store the data in the carbonate tables, especially since many of the RE analyses were done on splits of the carbonate samples.

Organic carbon (ORG_C) determined by coulometer as part of the carbonate analyses was often used instead of the TOC measurement from the RE instrument to calculate the HI and OI parameters. One problem that recurred periodically throughout the migration was determining whether the organic carbon value was ORG_C (coulometer) or TOC (RE). The tables in the Initial Reports volumes often did not discriminate between ORG_C and TOC.

Janus RE Data Format

RE analyses can be retrieved from Janus Web using a predefined query. The rock-eval 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, or latitude and longitude ranges. In addition, this query includes the organic carbon values obtained from the carbonate analysis. Often, this value was used in place of TOC to calculate the HI and OI parameters.

Table T30 lists the data fields retrieved from the Janus database for the Rock-Eval predefined query. The first column contains the data item, the second column indicates the Janus table or tables in which the data were stored, and the third column is the Janus column name or the calculations used to produce the value. "Description of Data Items from Rock-Eval Query" in "Appendix L" contains additional information about the fields retrieved using the Janus Web Rock-Eval query, and the data format for the archived ASCII files.

Data Quality

The RE data in Janus represent an extensive collection of organic carbon analyses in sediments from ocean basins throughout Earth. More than 9300 samples were analyzed to characterize organic carbon. A common error found during the migration 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. Another common error falls in the general category of operator error. Analytical results were written on log sheets. These data were then typed into th S1032 database. Data entry programs were implemented to add the data to the S1032 database, but the programs still required manual entry. Data acquisition programs were later implemented to collect RE data, but the operator manually entered the sample information. Mistakes in logging samples, logging data, an typing data into the database occasionally happened and were not always identified. Often, the Scientific Party found errors and corrected them for the data included in the Initial Reports volume, but data sent back to ODP/TAMU were not corrected.

Verification of the entire RE data set was not completed because of time constraints. One result of this is that the ORG_C data is not always retrieved, even though it exists in Janus. If ORG_C data are missing from the Rock-Eval query, those data should be available through the Carbonate query. 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 rock evaluation. Procedures for storing calibration information were not implemented during ODP.

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