The data gathered from Hole 1108D as part of this study indicate that no liquid hydrocarbon source was penetrated and that the stratigraphic sequence has only minimal gas source potential. A review of the shipboard data reveals that even with an expanded sample population the penetrated section has only limited gas source potential (Shipboard Scientific Party, 1999b). The organic matter present is largely inert (type IV, according to the scheme of Tissot et al., 1979).
Although the penetrated section at Site 1108 exhibits insufficient hydrocarbon-generation potential for commercially significant gas, sufficient organic matter is present to explain the limited gas present within the recovered cores. Shipboard measurements report a maximum headspace methane concentration of 59,668 ppmv for the sample obtained from 370.71 mbsf. The increase in gas wetness with depth (i.e., lower methane/ethane ratios) (Fig. F4) observed by the Shipboard Scientific Party (1999b) is not associated with an increase in gas abundance. These observations are consistent with an in situ origin for the gas at Site 1108. The increases in gas wetness can be attributed to a continuum of microbial and low-temperature nonbiological processes. The increase in gas wetness with depth does appear to be more rapid than at other ocean drilling locations (Claypool and Kvenvolden, 1983). This more rapid increase in gas wetness may be a reflection of the site's elevated geothermal gradient, which approaches ~100°C/km (Shipboard Scientific Party, 1999b). The elevated geothermal gradient acts to compress the different hydrocarbon-generation zones. The measured temperatures are, however, still consistent with microbial and low-temperature (<75°C) nonbiological gas generation rather than high-temperature thermogenic gas generation (>75°C).
The examined interval at Site 1109 included a number of highly organic enriched zones. A significant portion of the organic matter is inert and has relatively limited hydrocarbon-generation potential. Although most of the samples appear to be dominated by inert material, both the extractable fraction and the pyrolysis products suggest that the organic facies vary. The pyrolysis gas chromatography data indicate that some of the samples may contain slightly higher percentages of exinitic material capable of generating higher molecular weight products. These variations in organic facies may reflect changes in both the character of the organic input as well as organic preservation potential. These variations could be reflecting the suspected changes in the salinity of the lagoon in which they were deposited (Shipboard Scientific Party, 1999c).
The apparent in situ origin of the gas and the limited gas generation potential of the section suggest that Site 1108 could be revisited and the primary objective reached safely. However, because of the elevated geothermal gradient, careful hydrocarbon monitoring would be required. The greatest gas risk is associated with a stratigraphically equivalent organic-rich interval similar to that observed at Site 1109. The high geothermal gradient and the elevated generation potentials of the coaly zone could result in elevated gas concentrations.