Mixing of methane and sulphate due to fluid flow in the Barbados accretionary prism

Conference · Tue Dec 31 04:00:00 EST 1996

OSTI ID:425673

Laier, T ^[1]

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1. Geological Survey of Denmark and Greenland, Copenhagen (Denmark)

Methane concentrations above background level in sulphate-containing (15 mmol/l) pore waters have been observed in the d6collement zone of the Barbados accretionary prism. The peak in methane concentration in the decollement was found at a number of sites by headspace analysis of cores retrieved during ODP Legs 110 & 156 at the toe of the accretionary prism. {delta}{sup 13}C{sub 1} values between -22{per_thousand} and -36{per_thousand} indicate that methane oxidation occurs possibly due to sulphate reduction. Thus, the presence of both methane and sulphate at the same depths suggests mixing of fluids due to fluid flow. Fluid flow is also indicated by the distinct minima in chloride concentrations at the same depths. In the case of on-going methane oxidation, mixing of sulphate and methane fluids is anticipated to have occurred fairly recently. Sulphate concentration decreases only little with depth in the Pleistocene to lower Miocene sediments where TOC is very low, <0.2 %. Sulphate decreases more rapidly with depth in the Oligocene to Eocene sediments where numerous relatively thin turbidites occur. The turbidites have significantly higher TOC, 0.5-1.5 %, than the interbedded hemipelagic sediments, TOC <0.2 %. High methane concentrations were not found in any of the boreholes, but the trends in sulphate and methane in boreholes indicate that high methane concentrations exist in older sediments not reached by drilling. The decollement zone is composed of lower Miocene to upper Oligocene sediments near the toe of the prism, but deepens into stratigraphically lower sediments prism ward. Thus, methane originating from these older sediments may have been brought to shallower depths by active fluid flow in the decollement.

• OSTI ID: 425673
• Report Number(s): CONF-960527--
• Country of Publication: United States
• Language: English

Related Subjects

03 NATURAL GAS
BARBADOS
FLUID FLOW
GEOCHEMISTRY
METHANE
NATURAL GAS DEPOSITS
RESERVOIR FLUIDS
RESERVOIR ROCK