MacDONALD, IAN R.
Texas A&M University - GERG, College Station, TX 77843
Abstract: Focused Flow on the Gulf of Mexico Continental Slope: Open and Closed Cases
In northern Gulf of Mexico, hydrocarbons migrate from deep reservoirs to the sea bottom via fault systems generated by tectonic deformation of a Jurassic Age salt unit. Fluid venting profoundly alters biogeochemical characteristics of the upper sediment column and seafloor at points of discharge. Published analysis of satellite remote sensing data documents 60 active oil seeps in the northern Gulf, but recent, more comprehensive data indicate that this estimate is too low by an order of magnitude. Although geophysical and geochemical surveys have been quite comprehensive, fewer than 5% of probable total number of seeps have been visited by submarines or ROVs and this activity has been heavily biased toward sites at depths <700m. Two robust modes of focused flow are demonstrated by these examples. Open systems vent continuously through craters or pockmarks filled with fine sediment suspended in hypersaline fluid. Venting pockmarks as small as 25 cm and as large as 25 m in diameter have been observed. They shared a scale-independent morphology comprising a low mound with braided overflow channels emanating from on side of the pool. The transition in salinity and temperature between the ponded fluid and overlying seawater is abrupt. Salinity reaches 130 psu or greater. Fluid temperature ranges from 1-35<deg>C above ambient and may vary episodically, which indicates concomitant variability in hydrocarbon fluxes. Closed systems tend to retain and disperse hydrocarbons within larger cross-sections of surficial sediment via several disparate mechanisms that reduce porosity and fluid flux. Most dynamic is formation and dissociation of layers of thermogenic gas hydrate. These layers occur as vein-filling conglomerates of sediment, oil and hydrate and as lenses of relatively pure hydrate that breach the sediment interface and interact with the water column. Recent findings demonstrate a plethora of bacterial and metazoan interaction with shallow hydrate.
AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California