ORGANIC GEOCHEMISTRY

The concentration of methane (headspace analysis) in sediments from Hole 1148A remained at background levels (<10 ppmv) to 440 mcd. Below this depth, concentrations increased to 53 ppmv at 475 mcd but decreased again between 475 and 505 mcd. A second increase in methane to the bottom of the hole (711 mcd) was accompanied by the presence of ethane and propane as well as heavier hydrocarbons (HC) downhole. Maximum methane and ethane concentrations were detected at 593 mcd (569 and 25 ppmv, respectively). From the first detection of ethane at 480 mcd, the C₁/C₂ ratio declined rapidly from 99 to a minimum of 15 at the bottom of Hole 1148A. Gases were measured in Hole 1148B below 614 mcd to compare with Hole 1148A and below 715 mcd for safety and pollution prevention monitoring. Between 715 and 851 mcd, methane concentrations remained low (<200 ppmv) and generally decreased with depth downhole. The C₁/C₂ ratio decreased to as low as 4; however, this is expected for the small amounts of organic matter in these poor source rocks as they enter the zone of petroleum maturation. As much as 50 ppmv of C₅ and lesser amounts of other light hydrocarbons were detected. The concentration of carbonate ranged from 1 to 76 wt%. Total organic carbon (TOC) obtained by difference (total carbon [TC] - inorganic carbon [IC]) decreases systematically from a maximum of 0.8 wt% at the top of the hole to <0.2 wt% by 130 mcd. It remains at this level to 485 mcd (Fig. F19B). A sudden increase in TOC is noted below 485 mcd (>0.4 wt%). The concentration of TOC remains near this value downhole (0.2-0.5 wt%), with just one exception coincident with a calcite-rich layer. Based on C/N values, a purely marine organic source for organic matter (OM) is suggested for the upper 130 m of Hole 1148A; higher C/N values at the bottom of the hole may indicate increasing terrestrial input. However, the interval of low abundance of organic carbon (130-485 mcd) results in a similarly low C/N ratio, and the values cannot be used to characterize OM. The variation in sulfur abundance follows that of TOC in the top 130 m of the hole, decreasing slowly with depth but exhibiting a normal marine S/C ratio (0.4). Below this, S values are zero, coincident with very low to zero OM. From 485 mcd to the bottom of the hole, total sulfur (TS) concentration increases, following TOC. However, the S/C ratio is anomalously high for normal marine sediments (>1), suggesting the addition of S from another source.

Headspace gas analysis was performed on every core taken from Hole 1148A. Sampling and analysis were conducted as described in "Organic Geochemistry" in the "Explanatory Notes" chapter. The concentration of methane (headspace) is low (<10 ppmv) to a depth of 440 mcd, increasing gradually downhole (Table T12; Fig. F20A). A significant increase is then observed to the top of a carbonate-rich layer at 475 mcd (53 ppmv). The concentration of methane is once again low (<30 ppmv) within this lithologic unit, coincident with low core recovery. Below 510 mcd, methane increases rapidly to a maximum concentration of 569 ppmv at 593 mcd. A brief decrease occurs at 574 mcd (64 ppmv). Upon further heating (20 min) of the headspace sample, we concluded that such a reduction may be produced by heating time insufficient to completely release all trapped hydrocarbon gases. No significant changes in sediment composition were reported within this nannofossil clay interval.

No higher hydrocarbons were noted above 480 mcd. Below this depth, the abundance of both ethane and propane follow a similar trend to that of methane, peaking at 583-593 mcd (25 and 10 ppmv, respectively). Methane concentrations increase in the final Core 184-1148A-77X to 327 ppmv, and ethane and propane increase as well, achieving a second maximum of 21 and 11 ppmv, respectively. Ethylene (C₂H₄) was observed between 480 and 567 mcd, coincident with the onset of low core recovery (Fig. F20A). Ethylene was observed previously during ODP Leg 112 (Shipboard Scientific Party, 1988a, 1988b) and is the product of high drill-bit temperatures (caused by loss of drill-water circulation). Pyrolysis of OM in the surrounding sediment under dry conditions (suggested by the dry condition of the core in the core catcher) also produces additional methane and ethane that cannot be distinguished from biogenic or catagenic methane by gas chromatographic analysis. This may account for the increased methane, ethane, and propane in this interval. From the initial appearance of ethane at 480 mcd, the methane/ethane ratio declines rapidly from 99 to a minimum of 14 at 505 mcd (Table T12; Fig. F20B). The ratio then increases to 26 at 567 mcd but steadily declines to the base of the hole (ratio of 15.7).

Several samples analyzed on the GC3 (flame ionization detector [FID] 1) indicated a significant presence of higher HC. Six samples were therefore re-analyzed on the natural gas analyzer (FID 2) to determine concentrations of C₄-C₅ (Table T12; Fig. F20A; see "Organic Geochemistry" in the "Explanatory Notes" chapter). C₅ was first detected at 583 mcd, but the lack of a C₄ peak may suggest contamination of this sample (see "Organic Geochemistry" in the "Site 1143" chapter). The first conclusive occurrence of C₁-C₅ is in Core 184-1148A-75X. Total concentrations of C₄ and C₅ appear to increase downhole, with a maximum abundance at 711 mcd (28 and 32 ppmv, respectively). The methane/ethane ratio calculated from these few samples resembles that calculated from the GC3.

Hydrocarbon gas abundance was measured in Hole 1148B below 614 mcd (Table T13). Concentrations similar to those in Hole 1148A were detected above 715 mcd. Below 715 mcd, methane concentrations remained low (38-190 ppmv) and decreased with depth. The C₁/C₂ ratio reached a minimum of 4. Although these conditions might normally be cause for concern and even hole abandonment (see "Volatile Hydrocarbons" in "Organic Geochemistry" in the "Explanatory Notes" chapter), the small amounts of organic matter in these poor source rocks and low absolute concentrations of gas did not suggest a potential petroleum reservoir. As much as 50 ppmv of C₅ and lesser amounts of other light hydrocarbons were measured, suggesting limited maturation. Ethylene was again detected in zones of poor recovery as a result of drill-bit heating.

In summary, the most significant change in HC gas abundance occurs immediately beneath the lithologic change evident between 475 and 480 mcd, just below the slumped sediments of Unit VI in an interval of poor core recovery (see "Lithostratigraphy"). The maintenance of higher HC abundance below this depth suggests in situ generation of HC (see "Organic Matter Characterization"), without the addition of HC by migration. It is possible that the increase in methane between 445 and 485 mcd results from gas-phase migration through an underlying low permeability layer. No evidence exists for aqueous-phase migration of heavier HC, nor was any significant pore-water diffusion observed (see "Inorganic Geochemistry"). The lack of significant variation in the C₁/C₂ ratio below 490 mcd suggests minimal lateral migration to the site and is consistent with very high sedimentation rates (see "Biostratigraphy"). Bottom-hole thermal gradient estimates suggest a sediment temperature of ~60°C (see "Physical Properties"), which is generally considered high enough for the onset of petroleum generation. This confirms the observation of increasing sediment maturation and heavier HC generation with depth (see "Organic Matter Characterization") instead of the alternative explanation of migrated HC. Gas values for the onset of petroleum generation are 10^-2 to 10^-3 g, C₁ to C₅ per gram of carbon (Barker, 1979). We have detected ~0.5 to ~10^-3g, C₁ to C₅ per gram of carbon.

Sampling for carbonate was conducted in three sections per core for Hole 1148A (Table T14; Fig. F19A). The distribution of carbonate downhole can be subdivided into five intervals, approximately corresponding to lithologic Units I, II, III-V, VI, and VII (see "Lithostratigraphy"). The interval 0-180 mcd includes samples with the lowest carbonate abundance, ranging from 1.2 to 26.7 wt% (average [AV] = 12.4 wt%; standard deviation [SD] = 6.1). A significant increase occurs at 183 mcd. The second interval (183-320 mcd) contains samples of higher, but significantly more variable, carbonate content (AV = 33.0 wt%; SD = 8.7), coinciding with a shift from clay with nannofossils to nannofossil clay that contains alternations of yellowish and reddish brown clayey ooze (see "Lithostratigraphy"). A strong change in appearance of sediments below 320 mcd marks a transition to an interval of higher and less variable carbonate content. This interval (323-453 mcd) exhibits gradually increasing carbonate abundance (AV = 39.6 wt%; SD = 7.2) but cannot be conclusively subdivided according to lithologic Units III, IV, and V. A brief and dramatic increase in carbonate abundance (29-76 wt%) occurs between 457 and 476 mcd (lithologic Unit VI). The final sample of this interval (184-1148A-50X-2, 107-108 cm) is nannofossil chalk. Very poor recovery was noted in Cores 184-1148A-51X through 56X, the upper cores of lithologic Unit VII and the lowermost carbonate interval (480-715 mcd). This lithologic unit is better defined by the higher abundance of TOC (see "Organic Carbon and Nitrogen") but is also characterized by lower carbonate abundance (AV = 32.5 wt%; SD = 7.1). It contains one sample of unusually high carbonate content (66 wt%; Sample 184-1148A-65X-1, 103-105 cm), taken from a discrete interval of coarse-grained calcite-rich sediment underlying a calcite/quartz sand layer.

Coulometer reaction times of complete dissolution of carbonate were observed to increase toward the bottom of the hole, which coincides with the appearance of authigenic rhombohedral dolomite crystals in smear slides (see "Lithostratigraphy"). The longer reaction times may also indicate carbonate bound with increased amounts of OM.

The TOC concentration by difference (TC - IC) was determined for one sample per core (Table T14; Fig. F19B). As observed at Site 1147 ("Organic Geochemistry" in the "Site 1147" chapter), TOC declines gradually from a maximum close to the top of Hole 1148A (0.79 wt%; 30 mcd) (high values >1 wt% seen at Site 1147 are missing from the very top; see "Organic Geochemistry" in the "Site 1147" chapter) to 0.2 wt% below 110 mcd. This low TOC abundance is maintained to 485 mcd with concentrations of <0.1 wt% between 280 and 380 mcd. The abundance of TOC increases sharply at 485 mcd to >0.4 wt%, coincident with the top of lithologic Unit VII (see "Inorganic Carbon"). This amount of organic matter is above average for carbonates (0.2 wt%) and below average for shales (0.95 wt%) (Tissot and Welte, 1984) but appears to approximate the minimum of 0.4 wt% (or somewhat less in carbonates) required to be a petroleum source rock (Barker, 1979). With the exception of the calcite-rich sediment found at 592 mcd (Sample 184-1148A-65X-1, 103-105 cm; see "Inorganic Carbon"), TOC remains relatively high downhole (AV = 0.35 wt%; SD = 0.12).

Variations in total nitrogen follow those of TOC, but the calculated C/N ratio appears to be dominated by the concentration of TOC (>0.4 wt%) (Table T14; Fig. F19C). Thus, a degraded marine origin may be suggested for sediments of the upper 130 m in Hole 1148A. Very low to zero OM abundance in the sediment between 130 and 480 mcd cannot be characterized. Below this depth, some elevated TOC concentrations produce C/N ratio values >6 and as high as 13 (Sample 184-1148A-64X-2, 107-108 cm; 584 mcd). This may indicate a possible terrestrial source, but the absence of a marked reduction in nitrogen content makes this unlikely. No conclusive evidence exists for a terrestrial source of OM at this site.

Total sulfur and TOC values covary as expected for a normal marine depositional environment (Berner, 1984) in the upper part of the hole, where the TS values range from 0.1 to 0.4 wt% (Table T14). Below 480 mcd, TS values are much higher (0.3-1.2 wt%). Below 475 mcd, the TS/TOC ratios are two to 10 times greater than those expected for normal marine sediments (Berner, 1984). This indicates either (1) that unusually high amounts of metabolizable organic matter were available from a much shallower (<1000 m) or nearshore environment or (2) that an external diagenetic or epigenetic source for TS (like sulfate or sulfide) existed some time in the past. Such external (nonindigenous) sulfide or sulfate could come from basin brines or migrated H₂S gas, but no clear evidence for these was observed.

Pyrite is described in two ways (see "Lithostratigraphy"): as millimeter and larger crystalline deposits (pyrite) and as pyrite in grains by microscopic smear-slide observation. Black "iron sulfides" are also described in the sediments. The observed pyrite by all these methods agrees with our TS measurements of 0.1-1.0 wt% between 0 and 90 mcd and between 485 and 715 mcd.

Rock-Eval pyrolysis was conducted on samples from three intervals: (1) the top of Hole 1148A (Cores 184-1148A-2H through 10H, 10-90 mcd), (2) the zone of poor core recovery (Cores 184-1148A-51X through 56X, 480-505 mcd), and (3) the bottom of the hole (Cores 184-1148A-64X through 77X, 584-712 mcd). The results are shown in Table T15. Sediments from the upper interval (and Core 184-1148A-51X) exhibit low maturation, having low T_max and hydrogen index (HI) values. Samples from the other two intervals appear to be within a zone of potential petroleum generation. T_max increases steadily from ~400° to 424°C; the latter temperature indicates thermal maturity sufficient for the onset of petroleum generation. In contrast to a noted decline in TOC abundance to <0.1% above the lithology change at 478 mcd, TOC by Rock-Eval pyrolysis in samples from the interval of low core recovery below 493 mcd is ~0.5%; it is 0.4% at the bottom of the hole. These represent a minimum for petroleum source rocks (see "Organic Carbon and Nitrogen"). A low production index for these samples indicates a low evolution level of the organic matter relative to its hydrocarbon generation potential. This is confirmed by low S₁ values (0.01-0.06), indicating a concentration of volatile C₃₊HC of 10-60 ppmv. This correspondence with headspace HC concentration suggests predominantly in situ generation of higher HC. A van Krevelen-type plot of HI vs. oxygen index (OI) for samples from interval 3 shows the organic matter from Hole 1148A to be of marine origin (kerogen type II) and of "intermediate" maturity (Fig. F21).

In summary, sediments from the bottom of Hole 1148A are in an early stage of petroleum generation. The TOC is still relatively abundant (0.4%) but is too low to have the potential of a source rock. Low concentrations of detectable HC suggest an early stage of OM evolution relative to HC generation potential.