Thermogenic and Bacterial Hydrocarbon Gases (Free and Sorbed) in Middle Valley, Juan de Fuca Ridge, Leg 139

Abstract

The sorbed gases at all four Ocean Drilling Program Leg 139 sites (855, 856, 857, and 858) showed the unmistakable presence of thermogenic hydrocarbons. No indication of abiogenic gas was found. The evidence for the thermogenic hydrocarbons includes elevated contents of higher hydrocarbons (i.e., C λ l{C 2 + C 3 ] ca. 2-20), δ 13 C CH4 between -30‰ and -45‰ (PDB), 8 13 C C2 H 6 between -18.7‰ and -26. l‰, and δ 13 C c H from -20.5‰ to -25. l‰. The carbon isotope ratios of C 2 and C 3 indicate that the organic matter that generated these hydrocarbons is mature to overmature (1.8% to 4% Ro equivalent) reflecting the range in higher heat flow in the region. Considering the geologic setting, it is highly probable that the thermogenic gas was formed by hydrothermal processes. The presence of considerable amounts of ethene and propene (up to 7.1 hydrocarbon percent [h.c.%]) along with the accelerated maturation of organic matter support this conclusion. Some of the sorbed gases, especially at Sites 857 and 858, have a possible bacterial gas component admixed with the thermogenic, as is also seen in the free gases.The free gases are generally distinct from the sorbed gases and have C]/(C 2 + C 3 ) ratios up to 200 and δ 13 C CH as light as -56. l‰. The bacterial hydrocarbons in the free gas are most prominent at depths where bacterial sulfate reduction has removed most or all of the dissolved sulfate.It is uncertain from the geochemical information whether or not the hydrothermal gases are autothonous or have migrated into the sediments vertically or laterally. However, the general increase in hydrocarbon concentration with depth could indicate a limited upward diffusion or advection which is consistent with the heat and fluid flow information for these holes.

Geochemistry of low-molecular weight hydrocarbons in hydrothermal fluids from Middle Valley, northern Juan de Fuca Ridge

“…The isotopic composition of methane in vent fluids from Middle Valley varies between -50.8 and -55.5 ‰, ( Table 3 and Site 856 (Dead Dog) during ODP Leg 139 are characterized by isotopic compositions that are virtually identical to those reported here (WHITICAR et al, 1994). WHITICAR et al (1994) estimated that between 60 and 85% of the methane in pore fluid samples is derived from bacterial sources, although this result is highly dependent on assumed values of -55 ‰ and -38‰…”

“…WHITICAR et al (1994) estimated that between 60 and 85% of the methane in pore fluid samples is derived from bacterial sources, although this result is highly dependent on assumed values of -55 ‰ and -38‰…”

Organic Geochemistry of Gases, Fluids, and Hydrates at the Cascadia Accretionary Margin

“…The sampling and analytical methods employed and the data generated have been described and reported in detail by the Explanatory Notes and respective Site Summaries of Whiticar et al, 1994;Whiticar, in press;Whiticar and Cederberg, in press;MJ. Whiticar and M. Hovland in Shipboard Scientific Party, 1994;M.…”

Hydrocarbon‐Rich Groundwater above Shale‐Gas Formations: A Karoo Basin Case Study

“…A comparison of δ 13 C‐CH 4 and δ 13 C‐C 2 H 6 reinforces the interpretation that a low abundance of thermogenic natural gas is present in these samples (Figure ). When thermogenic natural gas is generated by increasing heat and pressure, the isotopic signatures for both methane and ethane evolve simultaneously with increasing temperature along a generally linear trend and record important clues about the genetic source, thermal maturity, and subsequent postgenetic alteration of natural gases (Whiticar et al ). In general, the δ 13 C‐C 2 H 6 values from this study area are consistent with the expected composition of thermogenic gases derived from either marine (e.g., shale) or terrestrial (e.g., coal) organic matter (Whiticar et al ).…”

“…When thermogenic natural gas is generated by increasing heat and pressure, the isotopic signatures for both methane and ethane evolve simultaneously with increasing temperature along a generally linear trend and record important clues about the genetic source, thermal maturity, and subsequent postgenetic alteration of natural gases (Whiticar et al ). In general, the δ 13 C‐C 2 H 6 values from this study area are consistent with the expected composition of thermogenic gases derived from either marine (e.g., shale) or terrestrial (e.g., coal) organic matter (Whiticar et al ). The broad range of δ 13 C‐C 2 H 6 (−42.65‰ to −35.29‰) equates to vitrinite reflectance values of approximately 0.33%R o to 0.72%R o (solid black line in Figure ) (Whiticar et al ).…”