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The AAPG/Datapages Combined Publications Database
Light-hydrocarbon (C1 to C4) concentrations and compositions were determined in sediments from 23 piston cores on the Florida-Hatteras slope and innermost Blake Plateau. On the basis of seismic profiles, cores were taken from slump masses, an accretionary wedge, channel cuts, fault zones, and over a diapir.
Maximum total C1 to C4 hydrocarbon concentrations in the sediments analyzed were less than 38 ppm. Light-hydrocarbon concentrations of less than 10 ppm were found in most samples, and methane, ethane, and ethylene were generally the major components. At two sample sites, however, concentrations were greater than 10 ppm. Samples from a channel cut contained hydrocarbons through butane with total C1 to C4 concentrations of nearly 25 ppm. Samples from the diapir site contained mainly methane although ethane and propane were present; the total C1 to C4 concentrations were under 38 ppm.
Geochemical surveys that measure light hydrocarbons in surficial sediments have been used as a prospecting tool in offshore petroleum and natural gas exploration. In the shelf and slope areas being considered, background distributions must be established to distinguish anomalous hydrocarbon concentrations that result from natural seeps. Extremely low concentrations of light hydrocarbons and the presence of biologically formed ethylene in the sediments from this study area imply that background levels are possibly due to microbial production of gas from near-surface organic matter rather than by diffusion from underlying petrolific sources. The ubiquitous occurrence of low concentrations, in the ppb range, of the hydrocarbons through butane in slope sediments may arise from microbial p oduction either directly as metabolic by-products or indirectly as degradation products. The possibility that microbes may be related to the occurrence of low concentrations of light hydrocarbons through butane in surface sediments must therefore be considered when evaluating petroleum potential from near-surface data.
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