ORGANIC GEOCHEMISTRY OF THE ORINOCO DELTA, VENEZUELA: A STUDY OF RECENT SEDIMENTS AND THEIR SIZE FRACTIONS.

摘要

The Orinoco River is building a large delta in the North-eastern part of Venezuela, South America. Twenty-five bottom grab samples ("Shipek") from the delta were analysed using standard organic geochemical techniques. Selected samples were separated into 4(phi) (62(mu)) to 10(phi) (0.98(mu)) fractions by sieving and pipette analysis. The organic carbon content of each size fraction was determined and the organic matter characterized by pyrolysis and pyrolysis-gas chromatography.; Organic carbon content of the sediments averaged 0.7 weight percent, and decreased with distance offshore. Extractable bitumens averaged 292 ppm, with aliphatic, aromatic and NSO compounds averaging 34 ppm, 27 ppm, and 230 ppm, respectively. Normal alkanes are dominated by odd-carbon number hydrocarbons (nC(,21), nC(,25), nC(,29)), suggesting terrestrial derivation, and this is supported by the dominance of pristane over phytane, and high nC(,29)/nC(,17) ratio. A low level contamination is indicated by some organic geochemical parameters. Pyrolysis response/gram of organic carbon also indicates a terrestrially-dominated environment. Microscopic examination of kerogens confirms the dominance of woody organic matter in the samples, and elemental analysis of one sample indicated a type III kerogen.; Kerogen densities average 1.48 g/ml for terrestrial organic matter, and 1.04 g/ml for algal organic matter. They decreased with distance and depth offshore, at the rates of 0.009 g/ml/mile and 0.004 g/ml/m., respectively.; For sediment size fractions, most organic geochemical parameters (organic carbon, pyrolysis yield, pyrolysis ratio, isoprenoid ratios, and hydrocarbon contents) were found to increase with decrease in size grade.; Hydrocarbon compositions of pyrolysates for various size fractions, plotted on ternary diagrams, with nC(,5)-nC(,10), nC(,11)-nC(,15) and nC(,15+) hydrocarbons as end members, shifted either towards the nC(,15+) end member (offshore sample), or towards the nC(,5)-nC(,10) end member (nearshore sample), with decrease in size.; These observed trends are attributed to the surface active properties of the finer size grades and/or sorting of organic matter by physical characteristics. Deviations occur in the nearshore areas, where the terrestrial detritus is differentially concentrated on the coarse .4(phi) fraction, relative to the finer size grades.