Drilling for Gas Hydrates: Ocean Drilling Program Leg 164

Abstract

Abstract Leg 164 of the Ocean Drilling Program (ODP) was devoted to investigating the amount and in situ characteristics of natural gas hydrates stored in marine sediments. Three sites (penetration depths of 700 to 750 m) were drilled on the Blake Ridge, an area characterized by prominent bottom simulating reflectors and simple geology. Here gas hydrates typically occur in a zone 200 to 450 meters below the sea floor. Based on interstitial geochemistry, well-logging, and vertical seismic profiling data. gas hydrates occupy 1–8% of the sedimentary section in this zone and are mostly finely disseminated. However, gas hydrate veins and nodules also occur. Samples of gas generated from decomposition of the gas hydrate nodules are -99% methane and I% carbon dioxide. Free gas, similar in composition, is also prevalent throughout a few hundred-meter-thick zone below the gas hydrate-bearing lone. Because geophysical data indicate that gas hydrates are distributed throughout a large area beneath the Blake Ridge, enormous amounts of methane (-35 Gt) are stored in this region. Short holes (50–67 m penetration) were also drilled on the crest of two diapirs on the Carolina Rise that intrude sediments associated with bottom simulating reflectors. Salinity anomalies indicate that these are salt-cored structures. Site 996 was located in a sea floor pockmark near the crest of a diapir on the uppermost Blake Ridge. Active venting of methane is occurring along a fault that extends to near the depth of a bottom simulating reflector below. Massive gas hydrates are common in the near-sea-floor sediments (e.g .. 020 m depths) in this area and the associated pore waters are charged with potentially hazardous levels of hydrogen sulfide Introduction Gas hydrates are a solid phase composed of water and low molecular weight gases (predominantly methane), which form under conditions of low temperature, high pressure, and adequate gas concentrations: I conditions that are common in the upper few hundred meters of rapidly-accumulating marine sediments. 2 Although gas hydrates may be a common phase in the shallow geobiosphere. they are unstable under normal surface conditions, and thus surprisingly little is known about them in natural settings. Enormous volumes of natural gas may be associated with sediments containing gas hydrate. 3 Estimates suggest that there are 103 to 104 Gt (Gt = 10 15 gm) of carbon stored in gas hydrates, an amount which rivals the estimated size or all other fossil fuel deposils .3Moreover. there may he considerable volumes of dissolved methane in pore fluids and free gas trapped beneath gas hydrate-bearing zones. However. at present we know too little about the distribution and abundance of natural gas hydrates to he confident about these global estimates. Seismic Detection of Marine Gas Hydrates. Gas hydrates are believed to be common because seismic reflection data have indicated their presence in every ocean basin.4 Prominent reflectors in seismic reflection profiles. called bottom simulating reflectors (BSRs), are frequently associated with gas hydrate-bearing sediment.