13C values lighter than -55
are thought to have a bacterial origin, with more positive values assumed to have a thermogenic origin (Feux, 1977).
Organic carbon (Corg) contents on samples from Site 1108 ranged from 0.11 to 1.23 wt%, with a mean value of 0.58 wt%. These data are detailed in Table T1. Only two of the samples contained >1.0 wt% Corg.
In contrast, organic carbon contents of the nine samples from Site 1109 ranged from 1.46 to 32.86 wt% (Table T2). Although all of the samples studied from Site 1109 are organic rich and the interval from which they were obtained has been described as coal bearing (Shipboard Scientific Party, 1999c), only samples from Section 180-1109D-38R-1 approach the necessary levels of organic enrichment for classification as a coal. True coals contain a minimum of 50% organic matter by weight (~34 wt% Corg).
Pyrolytic assay provides information about hydrocarbon-generation potential, organic character, and the extent of thermal diagenesis (maturation).
The results of the pyrolytic assay on the Hole 1108B samples are included in Table T1. Total hydrocarbon yields (S1 + S2 = free distillable hydrocarbons [HC] + generatable HC) are all <1.0 mg HC/g rock. Such low hydrocarbon yields indicate no significant hydrocarbon-generation potential is present and that any heavy hydrocarbons generated would be retained within the rock and released as gas following thermal cracking. The free hydrocarbons measured by pyrolysis equate to the more "oily" fraction and do not represent the light (gaseous) hydrocarbons.
The results of the pyrolytic assay for Hole 1109D are included in Table T2. Total hydrocarbon yields range from 0.34 to 98.37 mg HC/g rock. Good and excellent potential and/or effective hydrocarbon source rocks yield >6 mg HC/g rock (Tissot and Welte, 1984). Using this criterion, five of the samples represent good to excellent hydrocarbon source rocks. However, caution should be exercised when evaluating these data because of the suspected low levels of thermal maturity of the sediments. In samples having very low levels of thermal maturity, both the S1 and S2 peaks may not represent exclusively hydrocarbons but may instead contain significant amounts of heterocompounds (i.e., molecules containing O, N, and S). The presence of these labile compounds may result in a slight overestimation of the hydrocarbon yields. The remaining samples, although organic rich, have yields that indicate a significant amount of the organic matter is inert and is not capable of generating hydrocarbons.
Although limited by organic enrichment and mineral matrix effects (Katz, 1983), Rock-Eval pyrolysis can be used to provide insight into the nature of the sedimentary organic matter. The hydrogen index (HI) (in milligrams "S2" hydrocarbons per gram organic carbon) and oxygen index (OI) (in milligrams CO2 per gram organic carbon) and a modified van Krevelen-type diagram, as described by Espitalié et al. (1977), are used to accomplish this interpretation. Most of the data from both of the sites plot below the type III reference curve (Fig. F2), indicating that the organic matter is largely inert or type IV (Tissot et al., 1979). Type IV or inert organic matter forms through oxidation at the sediment-water interface when sedimentation rates are low and exposure time is great, or it may represent sedimentary recycling or the introduction of charcoal. A few of the samples are slightly more hydrogen enriched. Even these samples are, however, principally gas prone.
As noted above, there are problems associated with the pyrolysis assay of immature sediments. Pyrolysis does not accurately characterize organic matter in immature sediments as a result of the abundance of oxygenated functional groups in protokerogen. These functional groups are lost during the formation of kerogen. As a result of the presence of these compounds in very immature sediments, the HI values are often depressed and the OI values are elevated, making the organic matter appear more gas prone. Even when these effects are taken into consideration, the interpretation of the organic-matter character of this data set is not significantly changed.
Neither of the two pyrolysis thermal-maturity indicators, Tmax (temperature at which maximum pyrolytic hydrocarbon-generation occurs) nor the transformation ratio (TR = S1/[S1+ S2]), are reliable for Site 1108 because of the low pyrolysis (S2) hydrocarbon yields. An S2 yield of at least 1.0 mg HC/g rock is considered necessary for a geochemically meaningful Tmax to be determined, and an S1 + S2 yield of at least 1.0 mg HC/g rock is considered necessary for the TR to have significance.
The yields for samples from Site 1109 are sufficient for both pyrolysis thermal-maturation indices to be considered meaningful. These indices indicate that the sampled sequence is thermally immature (i.e., has not yet achieved levels of organic diagenesis consistent with the main stage of hydrocarbon generation and expulsion) (Fig. F3). The Tmax values for all samples with S2 yields >1.0 are <425°C, and the transformation ratios are <0.2 for samples with a total hydrocarbon-generation potential of at least 1.0 mg HC/g rock. The main stage of hydrocarbon generation and release is marked by Tmax values of ~440°C and a TR value of ~0.2 (Espitalié et al., 1977).
The carbon isotope composition of methane and other light hydrocarbons is dependent on bacterial contribution, level of source thermal maturity, and alteration history. The methane stable-carbon isotope composition increases (i.e., becomes more positive) with increasing thermal maturity. Methane 13C values lighter than -55 are thought to have a bacterial origin, with more positive values assumed to have a thermogenic origin (Feux, 1977).
The methane 13C values range from -60.36 to -32.70. These data are summarized in Table T3 and Figure F4. Many of the samples are significantly heavier than would routinely be associated with a biogenic origin (-55) and may therefore be interpreted to indicate a thermogenic origin. There is, however, no clear trend in isotopic composition with depth as would be expected if these data reflected thermogenic hydrocarbon generation. Partial microbial oxidization can result in the isotopic enrichment of the residual gas through the preferential consumption of the isotopically lighter carbon. Such enrichment could result in the masking of the true mode of gas formation and/or thermal maturity (Coleman et al., 1981) and would result in a more mature appearing gas than would be observed in an unaltered gas.
More likely, the reported values reflect fractionation of methane as a result of the degassing of the samples during handling and storage. Faber and Stahl (1983) have shown that degassing of sediment results in the enrichment of 13C in the residual gas. Berner and Bertrand (1991) reported similar fractionation while examining sediments from Site 768. They reported isotopically lighter values for gases obtained from gas pockets compared to "total" gases, which included desorbed gases. Therefore, although these values do not represent in situ gas composition, they clearly do not appear to support a thermogenic origin. The isotopically lighter values observed below 400 mbsf might be a reflection of increases in the degree of lithification and the associated decrease in degassing, which in turn would result in reduced isotopic fractionation (G. Claypool, pers. comm., 2000).
The total bitumen (or total organic extract [TOE]) yields for the samples from Site 1109 range from 899 to 56,929 ppm (Table T4). The extractable fraction is dominated by nonhydrocarbons (resins and asphaltenes; >73%), and the saturate/aromatic hydrocarbon ratios are typically <1.0. Such characteristics are consistent with the previously suggested low level of thermal maturity of the sample suite (Le Tran et al., 1974). With increasing thermal maturity there is an increase in both the hydrocarbon (saturate + aromatic) content and the saturate/aromatic hydrocarbon ratio.
Results of the saturated hydrocarbon-fraction gas chromatography are presented in Figure F5. These chromatographic signatures are also consistent with low levels of thermal maturity. The chromatograms either display a bimodal character with a full suite of n-alkanes or a unimodal pattern dominated by steranes and terpanes (biomarkers) with the n-alkanes being nearly absent. The bimodal samples also typically display a more pronounced naphthenic envelope or "hump." This hump represents a complex mixture of unresolved hydrocarbon compounds. With increasing thermal maturity, the bimodality decreases and there is an increase in the relative abundance of n-alkanes and corresponding decrease in the relative abundance of biomarker compounds.
A comparison of these data with the other available geochemical data reveals no clear relationship between the chromatographic signature and level of organic enrichment, free hydrocarbon content, or kerogen type. This suggests that there is no clear relationship between molecular and bulk chemistry. These observed differences might be the result of variations in both organic input and preservation caused by the suggested changes in salinity within the lagoon (Shipboard Scientific Party, 1999c).
Pyrolysis gas chromatography provides additional information about the nature of kerogen (Larter and Douglas, 1980) as well as a qualitative assessment of principal hydrocarbon products that may be generated upon thermal maturation (Dembicki et al., 1983).
The pyrolysis gas chromatograms are presented in Figure F6. As with the C15+ saturate hydrocarbon-fraction gas chromatograms, the pyrolysis gas chromatograms can be divided into two groups. However, unlike the saturate fraction gas chromatograms, there does appear to be a relationship between the chromatographic pattern and the pyrolysis results. Samples with low Rock-Eval S2 yields (e.g., Sample 180-1109D-38R-5, 21-22 cm) had chromatograms dominated by lower molecular weight compounds and generally lacking higher molecular weight components. In contrast, samples with S2 yields >10 mg HC/g rock (e.g., Sample 180-1109D-38R-2, 29-31 cm), although dominated by lower molecular weight compounds, did contain a full suite of alkane-alkene doublets, extending beyond n-C30. The presence of these higher molecular weight compounds suggests that the samples with the higher hydrocarbon yields contain a greater percentage of exinitic or waxy material. Differences in maceral composition could result from variations in the nature of organic input and/or selective preservation, both of which would change as depositional conditions evolve.
Both chromatographic signatures confirm that the gas would be the primary product generated at the appropriate levels of thermal maturity. Relatively small amounts of a waxy crude oil could be generated by those samples with higher hydrocarbon yields. Although this coaly material could generate longer-chain hydrocarbons, it is not considered likely that they would be expelled as liquids. As a result of their limited volumes, high molecular weights, and the physical structure of the coal, it is more likely that these hydrocarbons would be retained within the coal until they are cracked and released as gas at higher burial temperatures (Katz et al., 1991).
