Significant hydrocarbon concentrations were detected at three sites during Leg 184 (Fig. 27). Abundant methane of biogenic origin was inferred only from Site 1144. High TOC abundance and complete sulphate reduction in the upper few meters of this site (and the absence of significantly heavier hydrocarbons downhole) are characteristic of methanogenesis in immature sediments.
In contrast, the downhole increase in hydrocarbon abundance at Sites 1146 and 1148 is characteristic of thermogenic generation with age and increasing temperatures with depth. These two deeper penetration sites on the continental margin have predicted bottom-hole temperatures of only 35°C at Site 1146 (607 mbsf) but 71°C at Site 1148 (853 mbsf). The temperatures in the lower interval of Site 1148 are thought to be high enough to begin the process of thermal generation of hydrocarbons from the low amounts of organic carbon (0.3% to 0.5%) and may well explain the observation of sparse heavy hydrocarbons (C2+) at the site. None of the Leg 184 sites revealed conditions for source rock production, and no gas hydrates were observed.
Synthesis-Interstitial Waters: Silica and Sulfate
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