C2-C8 Hydrocarbons in Sediments from Deep Sea Drilling Project Leg 75, Holes 530A, Angola Basin, and 532, Walvis Ridge

Abstract

A series of C 2-C 8 hydrocarbons (including saturated, aromatic, and olefinic compounds) from deep-frozen core samples taken during DSDP Leg 75 (Holes 53OA and 532) were analyzed by a combined hydrogen-stripping/thermovaporization method. Concentrations representing both hydrocarbons dissolved in the pore water and adsorbed on the mineral surfaces vary in Hole 530A from about 10 to 15,000 ng/g of dry sediment weight depending on the lithology (organic-carbon-lean calcareous oozes versus "black shales")-Likewise, the organic-carbon-normalized Q-Cg hydrocarbon concentrations vary from 3,500 to 93,100 ng/g C 0Ig , reflecting drastic differences in the hydrogen contents and hence the hydrocarbon potential of the kerogens. The highest concentrations measured of nearly I0 5 ng/g C org are about two orders of magnitude below those usually encountered in Type-II kerogen-bearing source beds in the main phase of petroleum generation. Therefore, it was concluded that Hole 53OA sediments, even at 1100 m depth, are in an early stage of evolution. The corresponding data from Hole 532 indicated lower amounts (3,000-9,000 ng/g C org), which is in accordance with the shallow burial depth and immaturity of these Pliocene/late Miocene sediments. Significant changes in the light hydrocarbon composition with depth were attributed either to changes in kerogen type or to maturity related effects. Redistribution pheonomena, possibly the result of diffusion, were recognized only sporadically in Hole 53OA, where several organic-carbon lean samples were enriched by migrated gaseous hydrocarbons. The core samples from Hole 53OA were found to be severely contaminated by large quantities of acetone, which is routinely used as a solvent during sampling procedures on board Glomar Challenger.

Generation and Migration of Low-Molecular-Weight Hydrocarbons in Sediments of Deep Sea Drilling Project Leg 79, Sites 544, 545, and 547, offshore Morocco

“…The modification of the method was necessary because most of the core samples turned out to be extensively contaminated by acetone. Similar contamination effects have been documented in samples from various DSDP holes Leythaeuser, 1984). The severe contamination is caused by the usage of large quantities of acetone during on board core-cutting and sealing procedures, in particular, by rinsing the plastic caps with this solvent before they are pressed on the core-liner segments.…”

“…in a hydrogen flow of 5 ml/min. Prior to the chromatographic separation, the mobilized compounds are passed through a filter of a mixture of dry CaCl 2 and silica gel in order to prevent the pore water and the acetone from entering the gas chromatographic column [for analytical details see and Schaefer and Leythaeuser (1984)]. We have indications that the yields obtained by this procedure represent nearly the absolute quantities of light hydrocarbons present, both dissolved in the pore water and adsorbed on mineral surfaces.…”

“…It is commonly accepted that the quantity and distribution of low-molecular-weight hydrocarbons (Q-Q range) represent a sensitive indicator for the thermally controlled hydrocarbon generation processes in low-maturity sediments (see, e.g., Hunt, 1981, 1982;Schaefer and Leythaeuser, 1984, and references cited therein). The exception is methane which can, in addition, be formed in large quantities by bacterial carbonate reduction or acetic acid fermentation-type reactions.…”

Hydrocarbons as Indicators for Provenance and Thermal History of Organic Matter in Late Cenozoic Sediments from Hole 909C, Fram Strait

“…Nevertheless, although there appears to be an approach to the diagenesis-catagenesis boundary, the onset of thermal hydrocarbon generation by kerogen cracking was not reached in Hole 909C. Thus, the high concentrations of volatile hydrocarbons detected during drilling most likely represent mainly migrated material from greater depth, although part of them may derive from in situ, low-temperature diagenesis as observed in other deep-sea sediments (e.g., Schaefer and Leythaeuser, 1984;Shipboard Scientific Party, 1984) Table 1 for complete sample codes, a = 22, 29,30-rrmor-«é?o-hop-13(18)-ene, b = 22,29,30-trinor• hop-17(21)-ene, c = 22,29,30-fπnor-17α-hopane, d = 25, 28, 30-trinor-17α, 18α-hopane (position indicated), e = 22,29,30-rrmor-17ß-hopane, f = 24,28-Jmor-17α-lupane, g = 24,28-dmor-17ß-lupane, h = 28,30-d/nor-17α, 18α-hopane, i = 30-no/ -hop-17(21)-ene, j = 30-«or-«eo-hop-13(18)-ene, k = 30-nor-la, 21ß-hopane, 1 = hop-17(21)-ene, m = 30-nor-17ß, 21α-hopane, n = 17α, 21ß-hopane, o = «eo-hop-13(18)-ene, p = 30-rcor-17ß, 21ß-hopane + 17ß, 21α-hopane, q = 30-homo-lla, 21ß-hopane (22S), r = 30-homo-lla, 21ß-hopane (22R), s = 17ß, 21ß-hopane + 30-/iomo-17ß, 21α-hopane, and t = 30-λomσ-17ß, 21ß-hopane.…”

Site 995

“…Comparable concentration vs. depth profiles have been observed at other ODP sites (e.g., at Site 688 [Kvenvolden and Kastner, 1990]). The occurrence of C 4 through C 6 hydrocarbons is more likely related to diagenesis than to thermal generation, as suggested by the greater concentration of the isoalkanes relative to the n-alkanes (Schaefer and Leythaeuser, 1987).…”