Chapter 3: Carbonate Precipitation at Gulf of Mexico Hydrocarbon Seeps: An Overview
Harry H. Roberts, Dong Feng, 2013. "Carbonate Precipitation at Gulf of Mexico Hydrocarbon Seeps: An Overview", Hydrocarbon Seepage: From Source to Surface, Fred Aminzadeh, Timothy B. Berge, David L. Connolly
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Analyses of large tracts of seismic data reveal thousands of sites on the northern Gulf of Mexico (GOM) continental slope where vertical migration of subsurface fluids and gases reaches the modern seabed. A common characteristic of these sites is the occurrence of locally precipitated 13C-depleted authigenic carbonates. Recent studies of the poorly understood middle and lower slope indicate that the geochemical conditions required for carbonate precipitation exist across the full depth range of the slope. However, a wide range of mineralogical, petrographical, and isotopic variations occurs. Analysis of 450 samples from about 100 sites spread over the entire slope indicates that seep carbonates are composed mostly of high-magnesium calcite (HMC) and aragonite. Minor amounts of dolomite occur in samples from most sites. Samples from a few sites contain more than 50 wt.% of dolomite. These mineralogical compositions are typical of authigenic carbonates from the entire slope. The HMC and dolomite are mineralogies associated mostly with the matrix that formed in the shallow subsurface, whereas aragonite is found mostly as void-filling cement precipitated close to the seabed. Stable isotopic compositions of carbon and oxygen vary within sites (-32.2‰–14.0‰ for δ13C and 2.6‰–6.8‰ for δ18O), but much greater variation (-64.4‰– 14.0‰ for δ13C and 1.5‰–6.8‰ for δ18O) occurs between sites. Between-site variation in δ13C values is attributed primarily to different parent hydrocarbon sources. Within-site variation in δ13C values has many potential origins, including the rate at which hydrocarbons are delivered to the zone of precipitation, differences in biologically forced processes, spatial changes in hydrocarbon source, and differential sequestration of hydrocarbons by gas-hydrate formation. In addition, chemosynthetic communities, such as bathymodiolid mussels and vestimentiferan tubeworms, may have distinct geochemical impacts on associated seep carbonates. Initial studies indicate that carbonates formed in mussel beds and tubeworm communities have different δ13C values even at the same seep site. Carbon isotopic vital effects of seep mussels and tubeworms, fluid physical pumping by mussels, and release of sulfate by tubeworm roots may be responsible for the relatively more negative δ13C values of tubeworm-associated carbonates. Careful petrographic and geochemical analyses of samples from the entire slope provide a general understanding of slopewide variability in seep carbonate properties. In addition to carbonates, barite is common at many seeps on the northern GOM slope. Barite is found disseminated in surface sediments; it also occurs as small mounds, chimneys, cones, and crusts. Mineralogically, samples of barite may contain carbonate minerals, HMC, and dolomite, but aragonite is absent in all samples analyzed so far.
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With the increased resolution power of many geophysical methods, we are seeing direct evidence of seeps on a wide variety of data, including conventional seismic. New methods and technology have also evolved to better measure and detect seeps and their artifacts and reservoir charge and to map migration and remigration routes. In addition, detection of seepage is important for minimizing the risks associated with shallow gas drilling hazards, ensuring platform stability, and preventing well blow-outs.