Previous   |    Next

doi:10.2204/iodp.proc.337.103.2013

Organic geochemistry

During Expedition 337, organic geochemists investigated gas and solid-phase samples on board the ship and took gas, solid-phase, and interstitial water samples for postcruise research. The shipboard gas program included continuous on-line monitoring of mud gas that was extracted from the drilling mud in a separator unit and transferred to a mud-gas monitoring laboratory unit next to the rig floor, as well as the analysis of gases in samples taken from cuttings, sediment cores, and formation fluid. The formation fluid samples were retrieved under in situ pressure by Downhole Fluid Analysis (DFA). Together with hydrocarbon gases and their stable carbon isotopic composition, hydrogen, carbon monoxide, and noble gases were the most important target compounds, and O₂ and Ar were monitored to account for the introduction of air and for corrosive processes during drilling operations. In addition, radon was recorded by a third-party tool provided by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Institute for Research on Earth Evolution (IFREE).

For shipboard solid-phase analysis, samples were collected from cuttings and sediment cores and cuttings for elemental analysis (C in organic and inorganic form, N, and S) and characterization of the kerogen type via Rock-Eval pyrolysis. In addition, lipids (including fossil hydrocarbons, phospholipid fatty acids, and intact polar lipids) were extracted from cuttings and sediment cores by accelerated solvent extraction for both shipboard and shore-based analysis. The different types of biomarkers will help to characterize the deep coalbed biosphere and constrain the thermal history of the hole.

Fresh sediment samples were taken for shipboard and shore-based incubation experiments to study microbial life. On board the ship, a total of 43 experiments were started and will be continued postcruise: 34 WRCs from selected typical lithologies were processed in ¹⁴C-radiotracer experiments to determine rates of microbial metabolic activities such as methanogenesis and acetogenesis. In addition, nine WRCs were used for a series of stable isotope probing (SIP) experiments in which compounds labeled with either ¹³C or deuterium (D) will serve to track substrate utilization and uptake into cellular biomass. The analysis of gas, solid-phase, and fluid samples together with radiotracer and SIP experiments will allow us to track the carbon flow in the deep subseafloor biosphere within, above, and below the Shimokita coalbeds and will help us test the hypothesis that biogenic methane is formed in situ within coal seams.

Gases

On-line monitoring and sampling of mud gas

The goal of mud-gas monitoring was the real-time characterization of formation gases as they were recovered from the borehole and brought to the surface with the circulating drilling mud. On-line analyses were conducted using a methane carbon isotope analyzer (MCIA), a gas chromatograph (GC)–natural gas analyzer (NGA), and a process gas mass spectrometer (PGMS) while drilling operations were monitored and recorded via the SSX database. The on-line mud-gas analysis of hydrocarbon gases was complemented by dissolved hydrocarbon gas analysis in 48 samples of unwashed cuttings and 65 samples of sediment cores using the headspace technique.

During operations in Hole C0020A, gas monitoring data were recorded nonstop, including periods during which drilling was intermittent, such as when periodic “tripping” or emplacement of new pipe stopped progress, as well as periods of flushing to clean the hole. Because a round trip of drilling mud through the borehole took ~1–2 h (depending on hole depth and mud flow rate), interruptions in drilling operations affected the recovered mud gas after a lag time of 1–2 h. Figure F34 shows the depth from which mud gas was recovered at a certain day and time (note that all activities were recorded in ship time [UTC + 8 h]). We observed that on-line data started to scatter widely when drilling paused and mud gas was recovered from the same depth for >5 min. Therefore, we strictly limited our data interpretation and sampling activities to mud gas that resulted from periods in which drilling advanced into the geological formation and mud flow allowed for sufficient mud-gas recovery. Furthermore, the drilling rate in combination with the mud circulation rate, mud weight, and mud to headspace ratio in the separator impacts the recovery of gas from the formation (Erzinger et al., 2006). Because the conversion of mud-gas contents into absolute gas concentrations in the drilled rock is not straightforward, we limit our presentation of results here to the discussion of the relative ratios of gas species in the mud gas.

For the deepening of Hole C0020A from 647 to 2466 mbsf, we identified 96 discrete time intervals in which drilling actively advanced into the formation (Table T16), totaling 68 h of drilling time during which more than a million discrete data points were recorded for the characterization of mud gas. Mud-gas monitoring yielded an almost continuous depth profile of the various target compounds from 647 to 2466 m MSF. A few data gaps exist, however. They correspond to periods in which no or not enough mud gas could be recovered, either because of low mud flow rates, or because of exceptional events like the complete loss of drilling mud at 1110 m MSF and the clogging of the mud-gas line when drilling mud was arriving from sediment horizons at ~750 and 960 m MSF.

Hydrocarbon gases

The primary and novel continuous monitoring instrument used to measure mud gas was the MCIA. It provides information on the content and carbon isotopic composition of methane in mud gas that is continuously flowing through the instrument without separation of compounds or further addition of carrier gas. In scientific ocean drilling, Expedition 337 provided the first opportunity to test the MCIA in the field, and the accuracy of the recorded data will be carefully evaluated on shore using selected mud-gas samples and isotope-ratio-monitoring gas chromatography/mass spectrometry as a well-established standard method for carbon isotopic analysis of hydrocarbon gases (e.g., Ertefai et al., 2010). By simultaneous monitoring of drilling parameters and MCIA data, we observed that operational changes strongly affected the methane content of the recovered mud gas but did not have an obvious effect on the δ¹³C values of methane. Nevertheless, the potential effect of drilling on the carbon isotopic composition of methane recovered with mud gas deserves careful postcruise evaluation.

The MCIA was operated at a frequency of one measurement per second. During the 68 h active drilling period (Table T16), >186,000 data points were recorded for both the content and isotopic composition of methane in the mud gas continuously flowing through the MCIA. In general, 100–200 data points were obtained per drilled meter of sediment, depending on drilling parameters (i.e., ROP). Depth-based averages were compiled for these data to generate discrete data for corresponding depths (Table T17).

In the drilled depth interval between 647 and 2466 m MSF, methane is relatively depleted in ¹³C, with δ¹³C values ranging from –76.5‰ to –58.8‰ (versus Vienna Peedee belemnite [VPDB]) and averaging around –65.6‰ vs. VPDB. According to the well-established carbon isotopic systematics of methane (e.g., Whiticar, 1999), the carbon isotopic composition of mud-gas methane suggests biogenic methane sources to a depth of 2466 m MSF. Figure F35 illustrates that the carbon isotopic composition of methane varies distinctly with sediment depth. At the shallowest drilled depth of ~640 m MSF, δ¹³C values are around –65‰ vs. VPDB and closely match δ¹³C values of methane that were observed at this site in sediment cores recovered from ~358 mbsf during the Chikyu shakedown cruise (Expedition CK06-06) (F. Inagaki et al., unpubl. data). Below ~1100 m MSF, δ¹³C values decrease with respect to depth and reach a local minimum of around –75‰ vs. VPDB at ~1400 m MSF. With further increasing depth, δ¹³C values first increase to about –62‰ vs. VPDB at 1800 m MSF, remain more or less constant between 1800 and 2050 m MSF, and finally show a slight trend toward more negative values between 2050 m MSF and the bottom of the hole at 2466 m MSF. The correspondence of these isotopic variations with depth to microbial communities and availability of substrates will be further investigated in postcruise studies.

On-line monitoring of methane by the MCIA was supplemented by on-line GC-NGA analysis of methane and higher hydrocarbon gases with a frequency of roughly three analytical runs per hour, which yielded a total of 189 analyses (Table T18) during active drilling times (Table T16). Although the interpretation of methane content alone is not straightforward, as it cannot be directly related to methane concentration in the formation (see above), simultaneous analysis of methane and higher hydrocarbon gases allows calculation of C₁/C₂₊ ratios. The C₁/C₂₊ ratio is a valuable parameter to distinguish between hydrocarbon gases from biogenic and thermogenic sources (e.g., Whiticar, 1999). C₁/C₂ ratios were obtained not only during on-line mud-gas monitoring (Table T18) but also for dissolved gases in cuttings (Table T19) and sediment cores (Table T20) using headspace analysis. The results of the three different methods are depicted together in Figure F36.

Overall, on-line mud-gas monitoring resulted in systematically higher C₁/C₂ ratios than headspace analysis of cuttings, and C₁/C₂ ratios in core samples plot intermediate between the two (Fig. F36A). Methane degasses more easily than ethane, and the distinctly lower C₁/C₂ ratios in cuttings compared to mud gas might result from preferential degassing of methane during sample retrieval. Nevertheless, gas analyses from cuttings, cores, and mud gas altogether point to a biogenic source of the methane observed in Hole C0020A.

Because the occurrence of major amounts of C₂ (to C₅) is associated with thermogenic hydrocarbon generation, high C₁/C₂ ratios indicate biogenic methane formation when methane concentration exceeds 10,000 parts per million (ppm). However, for the interpretation of C₁/C₂ ratios one has to consider that minor amounts of C₂ (and C₃, C₄, and C₅) can also be generated in situ during early 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 (Pimmel and Claypool, 2001; Ocean Drilling Program, 1992). The plot of C₁/C₂ ratios obtained from on-line mud-gas monitoring versus in situ temperature (assuming a geothermal gradient of 24°C/km and a bottom water temperature of 6°C) shows a normal increase with depth and suggests that the gases formed in situ, rather than having been transported from greater depth (Fig. F36B).

Although C₁/C₂ ratios of mud gas generally decrease with increasing depth as expected, they show a distinct excursion toward higher values between 1840 and 2054 m MSF (i.e., the depth interval in which 12 coal layers were observed in sediment cores and during downhole logging). At greater depth, C₁/C₂ ratios decrease again. The excursion of the C₁/C₂ ratio points to the enhanced activity of biogenic methanogenesis in or between the coal layers relative to under- and overlying sediments.

In contrast to on-line mud-gas monitoring, analysis of dissolved gases in cuttings and cores by headspace sampling provides some clues on the concentration of hydrocarbon gases, though most of the gas dissolved in pore fluid may have escaped prior to sampling because of depressurization of the core during recovery. We collected 48 cuttings samples as well as 65 headspace samples from sediment cores, including four that we took directly from the coal layers. For better comparability, all concentrations are reported in moles hydrocarbon per liter interstitial water, based on concentration measurements in vials and the mass and porosity of sediment sampled. In several samples, the hydrocarbon concentration was greater than the calculated values for in situ saturation of interstitial water despite some degassing during core recovery. This suggests that substantial free or sorbed gas existed in the sediments at depth. In cuttings, methane was still present in concentrations ranging from 3 µM to ~1.4 mM. Methane concentrations reached particularly high values at ~1820 and 1920 m MSF (Table T19), suggesting a methane source in the coal layers found at this depth. This trend is even more obvious in headspace samples taken from cores. For samples taken from the sediment cores without coal layers, methane concentrations were usually <5 mM (Table T20), but in samples taken from coal layers or adjacent sediments, methane concentrations were distinctly elevated, exceeded 50 mM in several cases, and reached a maximum of ~270 mM at 1920 m CSF-B. The latter concentration exceeds the in situ solubility of methane and points to the presence of coalbed methane, which is typically sorbed in micropores of organic matter. We noted bubbling of free gas from adjacent sediments in the coal-bearing cores and took four void gas samples from Core 337-C0020A-22R, in which the methane content exceeded 80%.

In the process of riser drilling, drilling mud is recycled (the mud is pumped through the hole, returned to the ship, and used again). In order to ensure that the gases measured during mud-gas logging were derived from the formation, it was necessary to determine if carryover of gases existed in the mud gas that was to be reused. Over the course of 10 days, 14 samples were taken from the drilling mud tank. Over this period, the drilling mud had an average concentration of 25 ppm methane and traces of higher hydrocarbons (<12 ppm; Table T21) in the gas phase of headspace vials. These levels are insufficient to strongly impact concentration measurements in formation-derived samples, which sometimes released several thousand parts per million methane in the headspace of vials using the same sample/headspace ratio.

Oxygen, argon, and nitrogen

A major impetus for the use of a PGMS for on-line mud-gas monitoring was the determination of ratios of oxygen/argon and nitrogen/argon to detect the presence of corrosive processes occurring at the drill bit. Atmospheric concentrations of nitrogen, oxygen, argon, and hydrogen are 78%, 20%, 0.93%, and 550 ppb (v/v), respectively. These concentrations yield an atmospheric signature of nitrogen/argon and oxygen/argon ratios of 83 and 21, respectively. The nitrogen/argon and oxygen/argon ratios throughout the drilling procedure averaged 84 and 22, respectively, and point to the absence of oxidative processes (Table T22). Nitrogen, oxygen, and argon concentrations were 78.2% ± 3.7%, 20.7% ± 0.6%, and 0.9% ± 0.03%, respectively, and showed little variation with depth throughout Hole C0020A (Fig. F37). These concentrations are similar to those in air and indicate that a major portion of the mud gas was air originally contained in drilling mud or intruding from the degasser system. Concentrations of xenon and helium were below the detection limit of the PGMS. Note that on-line mud-gas monitoring by the PGMS only started at 1005 m MSF because of insufficient gas flow to the instrument during the first day of mud-gas monitoring (15 August 2012).

Continuous on-line monitoring of oxygen, argon, and nitrogen by the PGMS was complemented by sporadic mud-gas analysis using the GC-NGA (with a thermal conductivity detector [TCD]) (Table T23). In general, oxygen, argon, and nitrogen concentrations were typical of atmospheric concentrations.

Hydrogen and carbon monoxide

During mud-gas monitoring, volumetric hydrogen concentrations as determined by the PGMS were typically 2 orders of magnitude higher in the mud gas than in ambient air (Fig. F37). The average hydrogen concentration in the mud gas was 2.4% (Table T22). Although oxygen concentrations were not suggestive of corrosion, elevated hydrogen concentrations in the mud gas point to the formation or release of hydrogen during the drilling process. Sporadic analysis of the mud gas by the GC-NGA (using a TCD) showed carbon monoxide concentrations exceeding atmospheric levels (Table T23).

For determining the concentrations of dissolved hydrogen and carbon monoxide, the investigation of dedicated samples taken from sediment cores is more informative than continuous on-line mud-gas analysis by the PGMS and GC-NGA. A combination of two methods was used for this purpose (i.e., the extraction and incubation methods). The extraction method is based on the direct extraction of dissolved hydrogen and carbon monoxide into the helium headspace of a sample vial and is conducted immediately after sample recovery. This method was used not only to analyze samples taken from sediment cores, but also to check the hydrogen and carbon monoxide background in drilling mud that was caught between the sediment and the core liner. We collected a total of 63 samples for hydrogen and carbon monoxide analysis from the sediment cores (~2 samples per core) and 25 corresponding samples of drilling mud from the core liners (Table T24).

The depth profile of hydrogen concentration displays no systematic trends, and within one single core, hydrogen concentration often varies substantially (Fig. F38). The dissolved hydrogen concentrations span a range of 0.398–562 µM and average ~48 µM. Carbon monoxide concentrations vary throughout the drilled sediment interval. Carbon monoxide concentrations in sediment samples and drilling fluid recovered from above 1821 m CSF-B were <400 nM, but below 1920 m CSF-B, concentrations were higher than in the shallower sediment and drilling fluid and reached a maximum of ~100 µM within the coal layer at 1920 m CSF-B. The depths of the carbon monoxide pulses seem to correspond to coalbeds.

Drilling mud contained high hydrogen concentrations (333 nM to 74 µM) but carbon monoxide contents were low (<0.5 µM), except for a few cases in the coalbeds, in which shreds of coaly sediment had been blended into the drilling mud (1–1.4 mM) (Table T25). Overall, there was no obvious relationship between hydrogen concentrations in sediment samples and associated drilling mud. Although it seems likely that high hydrogen concentrations in the drilling mud would lead to the contamination of the sediment cores, hydrogen concentrations in sediment samples were, in some cases, orders of magnitude higher than in the corresponding drilling mud (e.g., Cores 337-C0020A-6R and 11R). In particular within the coal-containing intervals, hydrogen concentration in sediment was distinctly higher than in drilling mud. This suggests the release of hydrogen from the coaly sediment.

The observation of high hydrogen concentrations in the drilling mud is in agreement with the observation of hydrogen contents above atmospheric levels during PGMS analysis of mud gas and underlines that the analysis of hydrogen in interstitial water with the extraction method might be impacted by hydrogen formation during the drilling process.

In contrast to the extraction method, the determination of hydrogen concentrations with the incubation method is likely to be less impacted by the presence of hydrogen in the drilling fluid. This is because hydrogen that might initially be present in the sample is flushed out before experiments are initiated to monitor the evolution of hydrogen in the incubated samples over time. The observation of hydrogen concentrations over time aims to identify the establishment point at which a steady state is reached between biogenic processes that produce and consume hydrogen. We collected 64 sediment samples, approximately two per core, for determining the hydrogen concentration by the hydrogen incubation method (Table T26). We analyzed a total of three time points from all vials between sampling and the end of the expedition, including the initial time zero measurement. Time Point 1 was sampled after ~1 week of incubation, followed by a second time point 1 week later. The blank or initial hydrogen values were typically 1.5 ppm or less. Within 1 week of incubation, hydrogen contents increased on average by 54 ppm (n = 64) but varied largely from 1.5 to >2500 ppm in the individual samples. To test the exceedingly high increase in hydrogen concentrations observed in some samples, we performed a test of hydrogen increases in paired samples with one control sample killed by three cycles of deep freezing at –80°C (samples from Sections 337-C0020A-32R-1 and 32R-6). Control samples had similar or higher hydrogen concentrations after a 4 day incubation with 2199 and 70 ppm in live samples versus 1517 and 259 ppm in killed samples, respectively. This result is consistent with hydrogen being released abiotically from the sediments rather than being produced by biological processes. Hydrogen concentrations did not systematically vary over depth.

Carbon monoxide analysis was completed concurrently with hydrogen analysis on the reduced gas analyzer provided as a third-party tool by JAMSTEC Institute of Kochi Core Sample Research. Though vials were purged, a substantial accumulation of carbon monoxide (95 ppm) was observed in coal-associated lithologies at ~2001 m CSF-B. These data suggest that carbon monoxide might play an important role in carbon cycling in coalbeds.

On-line analysis of radon

Radon (Rn) is an inert radioactive gas of the daughter nuclei of uranium and thorium with rather short half-lives. Among Rn isotopes, ²²²Rn has the longest half-life of 3.82 days, followed by ²²⁰Rn with a half-life of only 55.3 s. Rn has widely been monitored to detect microcracking prior to seismic activities (e.g., Igarashi et al., 1995). In scientific drilling, concentrations of Rn dissolved into the circulation mud are anticipated to reflect the lithologic units of the formation; its parental elements, uranium and thorium, are generally rich in the terrigenous sediments and felsic rocks. In addition, some coalbeds contain abundant uranium up to 200 ppm (Takeda, 1981). The typical environmental background of Rn on the Chikyu was on an order of several Bq/m³. During on-line mud-gas monitoring, Rn concentrations were distinctly higher than this background and reached as high as 120 Bq/m³. Data are reported in Table T27.

Gases in fluids retrieved by DFA

A total of six formation water samples were sampled from 1279 to 1978 m WMSF by DFA and analyzed for methane, ethane, propane, and n-butane as well as molecular hydrogen and carbon monoxide (Table T28). Because samples were recovered under in situ pressure and did not suffer loss of gases during retrieval, the resulting concentration data are likely to be the most representative data for in situ concentrations obtained during Expedition 337. Methane concentrations ranged from 17.3 to 39.6 mM. The concentrations were significantly higher than those in headspace samples from sediment cores (<5 mM) but lower than the in situ solubility of methane and indicate the absence of free gas in the sampling intervals. Methane was the dominant hydrocarbon gas. Ethane concentrations were 1 order of magnitude lower and ranged from 1.3 to 4.1 µM, whereas concentrations of ethane and n-butane were in the nanomolar range. The corresponding C₁/C₂₊ ratios ranged from 1629 to 2995 and point clearly to biogenic methane sources (Whiticar, 1999). Although the ratios in DFA samples from 1808 to 1978 m WMSF were higher than those from on-line mud-gas monitoring, the depth profile of DFA samples is consistent with those from mud-gas monitoring, showing a distinct excursion toward higher values between 1840 and 2045 m WMSF.

Hydrogen concentrations ranged from 10.7 to 492 µM (Table T28). Hydrogen concentrations from 1279.5 to 1844 m WMSF were higher than those from the sediment core samples retrieved from roughly corresponding m WMSF. Hydrogen concentrations from 1901.5 to 1978 mbsf were similar to those from the sediment core samples. The high hydrogen concentrations compared to the sediment samples suggest that concentrations in the DFA sample are in situ concentrations. We cannot, however, eliminate the possibility of contamination from drilling mud because the drilling mud inside the borehole was not sampled by DFA. Therefore, the in situ hydrogen background in drilling mud remains unknown.

Carbon monoxide concentrations ranged from 0.857 to 6.70 µM (Table T28). Carbon monoxide concentrations from 1279.5 to 1844 m WMSF were 1 order of magnitude higher than those from sediment samples. Carbon monoxide concentrations from 1901.5 to 1978 m WMSF were similar to those from the sediment samples.

Solid phase

Total carbon, nitrogen, and sulfur contents, inorganic carbon, organic carbon, and carbonate content of the solid phase

Carbonate and elemental analyses have been carried out on 51 core samples from Units II to IV (Table T29), and 34 selected cuttings samples from the four stratigraphic units (Table T30) and are summarized in Figure F39. Inorganic carbon contents were relatively uniform and generally <0.5 wt% for both core and cuttings samples throughout the hole. Several relatively high inorganic carbon values (1.6–6.3 wt%) were observed for the core samples, two of which were cemented sandstones from Unit III. Inorganic carbon values from the cuttings samples were higher than those from the corresponding core samples by factors of 1.1–74. Total organic carbon (TOC) contents of the core samples showed strong lithologic control: coal had the highest average TOC contents (40.9 ± 9.9 wt%), followed by mudrocks (clay-rich lithologies and shales) (1.4 ± 1.0 wt%), silty (0.43 ± 0.29 wt%), and sandy (0.26 ± 0.18 wt%) lithologies. Therefore, Unit III, with its heterogeneous lithologic composition, showed a large downcore variation in TOC contents. Most of the cuttings samples from Units II to IV, except for those taken near the coal layers, had TOC contents higher than the corresponding core samples. These high values are the combined effects of depth averaging of low-TOC strata with organic-rich strata (such as clasts of coal) and input from the drilling mud, which contains organic additives. Accordingly, TOC values in Unit I, represented only by cuttings samples, might be overestimates of the actual values in individual strata. Total nitrogen (TN) contents of core samples of different lithologies followed a similar pattern to that of TOC: coal = 0.84 ± 0.26 wt%, shale = 0.07 ± 0.02 wt%, siltstone = 0.04 ± 0.02 wt%, and sandy lithologies = 0.03 ± 0.02 wt%. However, unlike the TOC record, 70% of the cuttings samples from Units II to IV had TN values lower than the corresponding core samples. TN contents of cuttings from Unit I had an average value (0.07 ± 0.02 wt%) close to that of shale. Both depth averaging and the drilling mud may have affected the TN record of cuttings. The molar TOC/TN ratios of the core samples ranged from 3 to 65 (Fig. F39). The samples with low TOC/TN values (<5) usually had a TOC content <0.1 wt%; accordingly, the values are more strongly impacted by imprecise analysis as well as inorganic N contributions. For samples with TOC content >0.1 wt%, both the sandy and silty lithologies had an average TOC/TN ratio of ~13, whereas for the clayey sediment and coal, the average went up to 22 and 58, respectively. However, the clastic sediment had a large within-group variation in the TOC/TN ratio (1σ = 7–10), making it difficult to link the origin of organic matter (terrestrial or marine) to lithologic compositions. The TOC/TN ratios from cuttings samples in Units II–IV were generally higher than the corresponding core samples because of the overestimation of TOC and underestimation of TN. Total sulfur (TS) values in the hole were generally low (from below detection to 1.4 wt%) and showed no clear association with lithology or specific trends with depth. The cluster of samples with low TS contents in Unit III were from a variety of different lithologies, including coal.

Characterization of the type and maturity of organic matter by Rock-Eval pyrolysis

Rock-Eval pyrolysis was carried out on cuttings samples from Unit I (Table T31) and core samples from Units II–IV (Table T32). The data are displayed in Figure F40. Compared to the core samples, the cuttings samples yielded lower peak temperature maxima (T_max) values (319° ± 47°C), larger amounts of free hydrocarbons (S1: 0.86 ± 0.40 mg hydrocarbon [HC]/g sediment), and higher hydrogen index (HI) values (227 ± 83 mg HC/g TOC). T_max values from the cuttings samples were lower than those reported for shallower, young shelf sediment (e.g., D’Hondt, Jørgensen, Miller, et al., 2003) and were regarded as signals of incompletely removed organic contaminants from the drilling mud. Four of the 51 core samples yielded poorly resolved S2 peaks with T_max values between 496° and 604°C and were excluded from the data set presented in this report. In Units II–IV, most of the core samples yielded T_max values in the range of 400°–440°C, indicating organic matter in a thermally immature to early mature state. Some of the sandy samples had lower T_max values, probably as a consequence of contamination from drilling mud.

Most of the core samples had no detectable S1 peak. The four samples in Units II and III with S1 yields of 0.1–0.6 mg HC/g sediment had low T_max values (<400°C) and were considered contaminated. There is a large variation in the S1 yield among different coal samples: although the shallower coal had barely detectable S1 peaks consistent with a relatively low coal rank (e.g., less than that of bituminous coal), the deepest coal bore a S1 content of 2.2 mg HC/g sediment. The very high production index (PI) values (and relatively high S1 values) observed within drilling cuttings (Table T31) could reasonably represent (1) the presence of low molecular weight humic and fulvic acids, (2) instances of drilling mud infiltration, or (3) nonindigenous bituminous organic matter (Peters, 1986). For data acquired from core samples (Table T32), except for a few samples, the PI values >0.001 all correspond to sandstones—a permeable lithology that when poorly cemented is easily contaminated. Thus, as for drill cuttings, these high values do not represent thermally driven conversion of kerogen to bitumen but either thermally immature organic matter or infiltration of drilling fluid.

For the other Rock-Eval parameters presented in Figure F40, a link to lithology was observed. The average values of S2, S3, and HI decreased in the order of coal > mudrocks (shales and other clayey lithologies) > silty or sandy lithologies, suggesting higher contents of hydrogen-rich organic matter in fine-grained sediment and coal. The high yields of S3 components (15.2 ± 2.8 mg CO₂/g sediment) in coal are consistent with the expectation of abundant lignocellulosic materials, which are found to contribute substantially to S3 signals (Carrie et al., 2012). The opposite order applies to the average oxygen index (OI) values, with 85% of the sandy and silty lithologies having OI values >100. Because of the high values contributed by the coal samples, the downcore profiles of S2 and S3 appeared rather uniform except for the few excursions from coal, whereas the distributions of HI and OI values were highly scattered as a reflection of the lithologic changes. The summed values of pyrolyzable carbon and residual organic carbon generally correlate well with TOC values, with the high proportion of residual organic carbon found in many samples indicating the presence of recalcitrant organic materials, not readily amenable to pyrolysis or even combustion, which can be particularly abundant in coals.

Lipid analysis

Shipboard lipid analysis focused on fossil hydrocarbon markers to characterize the sources of sedimentary organic matter and to construct a thermal history of Site C0020. Shipboard work included measurement and limited exploratory investigation of data. From this, it can be seen that in many cases biomarker characteristics vary more by lithology than by the depth interval. Data are presented by compound type for coal, siltstone, and mudstone lithologies; rock types that are typically associated with high concentrations of organic matter and considered the most resistant to contamination by drilling fluid infiltration (Peters et al., 2005). Biomarker data derived from cuttings have been included in a preliminary exploration of data but are presented separately.

n-Alkanes

n-Alkanes were detected in all lithotypes with samples of coal, mudstone, and siltstone always exhibiting an odd-over-even predominance (Fig. F41). In all cases, except where petroleum biomarkers (biomarkers associated with processes or conditions that generate petroleum) are present, carbon number distributions show a strong odd-over-even preference and were skewed toward high carbon numbers (e.g., >C₂₅). This distribution preference is consistent with the sedimentary organic matter being derived from higher plants (Meyers and Ishiwatari, 1993). However, land plant–derived organic matter is not the only source of high carbon number n-alkanes, and the solvent extracts of many phytoplankton species also yield n-alkanes in the higher carbon number range (Volkman et al., 1998). Additionally, the rapid solvent evaporation procedure used to facilitate shipboard work also has the potential to remove low carbon numbered compounds of relatively high volatility and may have enhanced any skew toward higher carbon numbers.

The carbon preference index (CPI) for n-alkanes (Table T33) varies downhole, with slightly lower values measured in samples for the deepest cores. This is consistent with a higher thermal maturity for these deeper samples but still indicates a very low level of thermal maturation. Where pristane/phytane ratios can be reliably measured, pristane typically predominates in siltstone and coal, indicating greater inputs of tocopherol precursors that contain C₁₉ isoprenoids (these compounds are abundant in woody organic matter and lignocelluosic organic matter) and/or higher oxidation rates of potential phytane precursors (Peters et al., 2005). Slightly lower values in mudstone may represent reduced inputs of woody organic matter and less oxidizing conditions in the depositional environment.

Polycyclic aromatic hydrocarbons

Shipboard biomarker work focused on three- and four-ring polycyclic aromatic hydrocarbons (PAHs). These compounds are common in most coals at even low thermal maturities (Radke et al., 1980), although when present at very low thermal maturities (e.g., R_o values of 0.25% or less), their interpretation as indicators of thermal maturity is not possible. Within all samples PAH abundance varies, with phenanthrene being variably prominent in mass chromatograms of diagnostic ions (Fig. F42). Unlike in cuttings, the anthracene isomer is not seen in core samples and neither is the 4-methylphenanthrene (MP) isomer observed. This indicates that these compounds may have been introduced into cuttings samples during drilling operations. The variable prominence of phenanthrene makes it difficult to apply for geochemical purposes, and, as was the case for low carbon number n-alkanes, this may result from the solvent concentrating procedure used to facilitate shipboard work that has the potential to remove relatively volatile compounds.

The tetracyclic PAHs are more abundant, and summed ion chromatograms are provided in Figure F43. Pyrene and fluoranthenes (a nonalternant PAH associated with low-temperature combustion) (Killops and Killops, 2005) are present in all lithologies and noticeably more abundant in coal. Within some sections, a trend to higher proportions of fluoranthene relative to pyrene with higher HI values is observed, likely reflecting increased proportions of oxidized (Type IV) organic matter in these sections.

n-Alkanoic acids and n-alkanols

Given the strong odd-over-even predominance seen for n-alkanes and the presence of wood fragments in many samples, the even-over-odd predominance for high carbon number n-alkanoic acids (>C₂₄) evidenced in mass chromatograms in Figure F44 is to be expected (Meyers and Ishiwatari, 1993; Killops and Killops, 2005) (Table T34). As for n-alkanes, there is a skew toward higher carbon numbered n-alkanoic acids in many but not all coal, mudstone, and siltstone samples. Variation in the even-over-odd predominance of n-alkanoic acids with depth may represent both variation in the source of organic matter (reduced inputs of terrestrial organic matter) and increased levels of thermal maturation. However, the sudden drop toward the bottom of Unit III (~2000 m CSF-B) is unlikely to be solely the result of thermal maturity (Fig. F45).

n-Alkanols are visible but less prominent on ion chromatograms, with the C₁₈ homolog being the most abundant, except in a clast of coal taken from coal-bearing breccias in Unit II (interval 337-C0020A-2R-2, 0–15 cm). Of the higher carbon n-alkanols with the C₂₄, C₂₆, and C₂₈ alkanols are typically the most abundant, making their carbon number distribution similar to that of the n-alkanoic acids.

Alkenoic acids were detected in some samples at very low concentrations, but are likely to have been introduced during drilling operations (e.g., they were not common in laboratory blanks). Thus, as laboratory contamination can be ruled out, the two remaining possible origins are infiltrated drilling fluid or a sedimentary origin (e.g., they could be indigenous to the formation).

Some siltstone samples evidence a series of broad peaks (marked by an asterisk in Fig. F44) that are also seen in control samples that contain drilling mud (see “Cuttings” below). These compounds dominate the chromatograms of loosely consolidated sandstones and the solvent-soluble components of drilling mud washed from the surface of cuttings samples. This indicates that some siltstone samples have been contaminated, albeit to a far lesser degree than sandy lithologies.

High relative abundances of C₁₅ and C₁₆ iso- and anteiso-alkenoic acids and also C₁₆ and C₁₈ n-alkenoic acids are found at ~2000 m CSF-B (Fig. F46). Iso- and anteiso-fatty acids were not found or are not as abundant in coal and samples from the deepest sections of Core 337-C0020A-25R, nor are they as abundant in overlying mudstone units or clasts of coal from coal-bearing breccias in Unit II. Methyl-branched fatty acids are common biosynthetic products produced by bacteria (Kaneda, 1991), and their presence presumably indicates a bacterial contribution to the fatty acid pool in this interval. Because only free fatty acids were considered, it is not possible to directly infer the presence of an active deep biosphere from this observation alone, but this observation indicates a localized input of bacterial organic matter to these intervals that is not as prominent in other samples.

Steroids

Steroidal biomarkers are known to undergo significant changes in the subsurface in the 40°–60°C temperature range (Mackenzie et al., 1982) and have recently been applied to elucidating subsurface thermal regimes in Northern Hokkaido (Amo et al., 2007) (Table T35). The C₂₇ to C₂₉ homologs of the Δ⁴ and Δ⁵ sterenes and Δ^4,22 and Δ^5,22 steradienes are identified in Figure F47 (tabulated parameters and abundances are in Table T36). Although steradienes are encountered throughout Hole C0020A, they are hard to interpret because of coelution of other compounds and were not interpreted further during shipboard work. Neither stenols nor stanols were detected in core samples, a finding that is similar to that from previous studies in Hokkaido (Amo et al., 2007) that investigated organic matter of similar thermal maturity.

The abundance of a given sterene homolog varies considerably with lithology and also with depth at Site C0020. Coals contain the greatest proportion of C₂₉ sterene homologs, representing a higher proportion of terrestrially derived plant material in sedimentary organic matter (C₂₉ sterols such as β-sitosterol and stigmasterol are proportionally more abundant in plants). By comparison, some of the mudstone samples (Fig. F48) contain greater proportions of sterenes that derive from C₂₇ and C₂₈ sterol homologs, probably representing a greater input from aquatic organisms, particularly algae (Volkman, 1986).

Steranes, by comparison, are far less abundant than sterenes but show similar preferences with regard to carbon number at Site C0020. The thermally immature 5β,14α,17α(H) and 5α,14α,17α(H)20(R) isomers generally predominate. In some instances, sterane isomers that are characteristic of higher thermal maturities are present and are found with triaromatic steroids and thermally mature hopanes—biomarkers found in oils (see Fig. F49). These oil-window biomarkers could indicate migrated petroleum or the erosion and redeposition of an older geological formation that contained thermally mature organic matter including hydrocarbon biomarkers (e.g., Parnell et al., 2007). Drilling during Expedition 337 did not encounter source rocks sufficiently thermally mature to have generated oil; thus, a sedimentary origin seems more feasible until evidence of oil-prone source rocks can be proven.

The conversion of sterenes to steranes is thermally driven and occurs at geologically mild temperatures (40°–60°C) and completes prior to the onset of catagenesis and petroleum formation (Mackenzie et al., 1982). Pathways for the thermal evolution of the sterane/sterene parameter shown in Figure F50 were calculated for Hole C0020A using geothermal gradients of 22°C/km, 24°C/km, and 26°C/km and validate other studies on temperature controls in Amo et al. (2007). It is important to note the considerable scatter evident in sterene data is because of the comparison of data from different lithologies and because this situation did not apply for the data acquired at Hokkaido, shown in the inset of Figure F50. Different mineral surfaces are known to variably catalyze and promote the conversion of sterenes to steranes, and the coal-rich lithologies seem to differ considerably from other lithotypes in both the rate of sterane formation and the relative proportion of the Δ⁴ and Δ⁵ isomers. Despite this, the point at which pre-oil-window biomarkers begin to form is denoted by a pronounced increase in the parameter between 1800 and 2000 m MSF.

The proportion of the C₂₉ sterene homolog is plotted in Figure F51 and compared to the proportion of sediment (sand, mud, and silt) lithologies logged from cuttings samples, which yielded a continuous record over the drilled interval. The highest proportion of mudstone corresponds to a minimum in the proportion of C₂₉ sterenes. Key decreases in the parameter occur at ~1200, 1600, and 2000 m CSF-B, where the proportion of mud also drops. Presumably this is consistent with decreased proportions of terrestrially derived sedimentary organic matter that contains C₂₉ sterols, which are the chemical precursors of the C₂₉ sterenes (Volkman, 1986).

Hopanoids

Time did not permit the satisfactory elucidation of all major peaks in the time range over which hopanes and other pentacyclic terpanoids and terpenoids elute on a m/z 191 ion chromatogram (Fig. F49). Diploptene was only detected in cuttings samples from the shallowest interval. The C₃₀ and C₃₁ homologs of the Δ¹³ and Δ¹⁷ hopenes are generally the most abundant hydrocarbon terpenoids in deeper intervals. The 14β,17β(H) isomer configuration dominates in most intervals, except where there is evidence of a fossil fuel contribution to samples. This is the case for Sample 337-C0020A-1R-2, 65–81 cm, in which the 14α,17β(H) hopane isomers are dominant. Sample 337-C0020A-1R-2, 65–81 cm, also has a high proportion of lower carbon n-alkanes, as does Sample 2R-3, 60–81 cm, for example, which also possesses an unresolved complex mixture and is dominated by 14α,17β(H) hopane isomers. Note that the distribution of hopane carbon numbers in these samples is different to that seen in drilling mud components; ASTEX-S SAS had not been added to drilling mud at this stage and in any case has higher proportions of the C₃₅ 14α,17β(H) 22 S and R homologs (see Fig. F52). Visual core description did not note the presence of oil in this interval, but headspace gas monitoring detected notably high proportions of butane in Core 2R. These latter observations, in combination with the high proportions of lower carbon n-alkanes, may suggest a lighter oil. A light oil would be difficult to identify during visual core description without recourse to an ultraviolet (UV) light source to induce fluorescence.

Cuttings

Chromatograms for extracts obtained from cuttings are presented together with core samples in Figures F51 and F53. The distribution of C₂₇ to C₂₉ Δ⁴ and Δ⁵ sterenes for Samples 337-C0020A-25-SMW (646.5–656.5 m MSF) and 81-SMW (1096.5–1106.5 m MSF) and a total ion chromatogram for Sample 25-SMW (646.5–656.5 m MSF) are given in Figure F53. Data for parameters derived from cuttings are presented in Table T36. Extract yields for n-alkanes and sterenes are equitable to those reported for mudstone and siltstone sampled from core, indicating that data for these compounds are reliable. An additional level of quality control is outlined in the following paragraph.

Assessment of the effect of drilling operations on hydrocarbon biomarkers

To gauge the impact of the immersion of cuttings within drilling mud, cuttings samples were subjected to sequential extraction. The solvent-soluble products obtained by sonicating samples in solvents were stored and subjected to analysis. The surface of the samples was then rinsed by sonicating samples prior to collecting the solvent to obtain a second extract. Cuttings samples thus cleaned were then crushed and solvent extracted, yielding a third extract. Chromatograms of the products from each stage are displayed in Figure F53. The first extraction stage yielded broad, late-eluting peaks that interfere with both chromatography and mass spectrometry. The second stage products are far cleaner, indicating that the cuttings surfaces have been cleaned. The final extraction stage yields thermally immature biomarkers similar to those found in coal and mudstone samples, suggesting that relatively uncontaminated signals can be produced from cuttings that have been aggressively cleaned. Such an approach, where samples are aggressively preextracted, although possibly losing some analyte, has undoubtedly been used before but differs from that presented in literature for petroleum exploration and production, which places an emphasis on either not losing analyte or disrupting samples by cleaning with solvent (Peters et al., 2005). This latter approach is no doubt intended to preserve analyte in porous samples (e.g., reservoir rock), but for scientific, riser-equipped drilling, the loss of analyte for an increase in sample fidelity by the removal of contamination is perhaps preferable.

SAS is a commercially traded drilling additive used to reduce instances of downhole sticking. It comprises a mixture of asphalt (Gilsonite) and mineral phases. GC-mass spectrometer analysis of its content revealed a series of thermally mature components such as hopanes with an 14α,17β(H) configuration. Distinctive features of this hopane fingerprint include high proportions of the C₃₅ homologs (Fig. F52) and an absence of bisnorhopane (a norhopane that is enriched in many biodegraded petroleum residues). This hopane fingerprint is distinct from both the oil shown in interval 337-C0020A-2R-3, 60–81 cm, and typical samples encountered during Expedition 337 (Fig. F49) and made co-mingling of SAS with hopanes indigenous to the hole relatively simple to spot during Expedition 337. An example of a sandstone sample infiltrated by SAS is shown in Figure F52.

Procedural blanks

Analysis of procedural blanks indicated that little hydrocarbon biomarker contamination was introduced to samples during laboratory handling. Some peaks occur prior to 25 min, but they were present at too low a concentration to identify by analysis in scan mode (Fig. F54).

Top of page   |    Previous   |    Next