Abstract

We offer a new geochemical method that employs pore-water sulfate gradients as a potential indicator of gas-hydrate presence in deep-water sediments. Gas hydrates are ice-like solids, generally composed of water and methane, which occur naturally in sediments under conditions of high pressure, low temperature, and sufficiently large methane concentrations. As petroleum exploration and development efforts move into deeper water, the possibility of encountering gas hydrate within sediments becomes increasingly likely, and their occurrence may be hazardous to production infrastructure. Operations that significantly alter ambient conditions may cause decomposition of gas hydrate, decrease sediment strength, and potentially induce sediment failure. Thus, recognition of gas hydrate occurrence is necessary for the safe design and emplacement of offshore drilling/production platforms, subsea production equipment, and pipelines. Unfortunately, gas hydrates are not generally sensed by seismic techniques used to identify shallow gas hazards, inviting a geochemical approach to detect gas hydrate in deepwater environments.

Methane concentrations will tend to be elevated in areas prone to gas hydrate formation. However, conventional methane concentration measurements in deep-water sediments grossly underestimate the amount of methane due to outgassing. We have determined that under appropriate conditions, pore-water sulfate gradients of marine sediments can be used as a proxy measurement of methane. When methane is present within sediments in sufficient amounts, most of the interstitial sulfate will be consumed by reaction with methane, affecting the shape of sulfate profiles. Fresh sediment samples containing pore waters are collected from a surface ship via piston coring as an independent operation or during geotechnical drilling. Prior identification of the potential for gas hydrate deposits within shallow-subbottom sediments using this geochemical technique can reduce the threat to seafloor structures by safe location of seafloor structures or accommodation of their design.

Introduction

Successful production of deep-water hydrocarbons requires the safe emplacement of platform pilings, subsea production equipment, and pipelines. Localities with significant occurrences of methane (gas) at the seafloor or in the shallow subsurface have been traditionally avoided to minimize engineering problems and risk to production infrastructure. High-frequency seismic surveys are effective tools for seafloor hazard detection when methane is in the gaseous phase, because seismics are very sensitive to gas bubbles. For example, amplitude anomalies (bright spots)' occur when free gas reaches a few percent of the pore space.2 Thus, the petroleum industry has been extremely successful in avoiding gas hazards in continental shelf and slope settings.

Methane will not typically exist as free gas in the shallow subsurface of deep water environments but rather as dissolved methane and as a solid phase called gas hydrate. Gas hydrates require sufficient methane concentrations to form, so their presence suggests elevated dissolved methane concentrations in pore water. However, because a free gas phase is unlikely in these settings, high-frequency seismic reflection techniques may not sense their presence unless gas hydrates form extensive pavements at the seafloor or form pervasive pore filling cements. Thus, gas hydrates at or near the seafloor may not be detected by conventional seismic surveys.

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