Importance of orientational rearrangement during vitrification of hydrocarbons: dependence on molecular shape

On the basis of Monte Carlo calculations of 2,2-dimethylpropane (neopentane), n-pentane, and 2,2-dimethylbutane (neohexane) at several temperatures, thermodynamic properties and radial distribution functions as well as dimerization and bonding energy distribution functions are reported for both liquid and glassy states. Changes in the radial distribution functions on cooling depend on whether the groups are accessible (peripheral) or inaccessible. Peaks in the radial distribution functions corresponding to peripheral groups do not shift to lower distances on cooling and at times display a large increase in the intensity of the first peak. The peaks due to inaccessible groups, on the other hand, shift to lower distances on cooling. The magnitude of the reorientational contribution in determining the resulting structure of the glass is estimated for the different hydrocarbon molecules investigated. The reorientational contribution is highest for neopentane (26%) followed by isopentane (24%), neohexane (22%), and n-pentane (0%). It appears that molecular geometry has an important role in determining the magnitude of the reorientational contribution to the structure of the glass.

Low-molecular-weight hydrocarbons in sediments at DSDP Sites 89-585 and 89-586

C2-C8 hydrocarbon concentrations (about 35 compounds identified, including saturated, aromatic, and olefinic compounds) from 38 shipboard sealed, deep-frozen core samples of Deep Sea Drilling Project Sites 585 (East Mariana Basin) and 586 (Ontong-Java Plateau) were determined by a gas stripping-thermovaporization method. Total concentrations, which represent the hydrocarbons dissolved in the pore water and adsorbed on the mineral surfaces of the sediment, vary from 20 to 630 ng/g of rock at Site 585 (sub-bottom depth range 332-868 m). Likewise, organic-carbon normalized yields range from 3*10**4 to 9*10**5 ng/g Corg, indicating that the organic matter is still in the initial, diagenetic evolutionary stage. The highest value (based on both rock weight and organic carbon) is measured in an extremely organic-carbon-poor sample of Lithologic Subunit VB (Core 585-30). In this unit (504-550 m) several samples with elevated organic-carbon contents and favorable kerogen quality including two thin "black-shale" layers deposited at the Cenomanian/Turonian boundary (not sampled for this study) were encountered. We conclude from a detailed comparison of light hydrocarbon compositions that the Core 585-30 sample is enriched in hydrocarbons of the C2-C8 molecular range, particularly in gas compounds, which probably migrated from nearby black-shale source layers. C2-C8 hydrocarbon yields in Site 586 samples (sub-bottom depth range 27-298 m) did not exceed 118 ng/g of dry sediment weight (average 56 ng/g), indicating the immaturity of these samples.

Low-molecular-weight hydrocarbon concentrations, organic carbon contents, and Rock-Eval pyrolysis hydrogen indices at DSDP Holes 75-530A and 75-532

A series of C2-C8 hydrocarbons (including saturated, aromatic, and olefinic compounds) from deep-frozen core samples taken during DSDP Leg 75 (Holes 530A 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 C2-C8 hydrocarbon concentrations vary from 3,500 to 93,100 ng/g Corg, reflecting drastic differences in the hydrogen contents and hence the hydrocarbon potential of the kerogens. The highest concentrations measured of nearly 10**5 ng/g Corg 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 530A 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 Corg), 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 530A, where several organic-carbon lean samples were enriched by migrated gaseous hydrocarbons. The core samples from Hole 530A 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.

Gas hydrates at DSDP Leg 84

On DSDP Leg 84, gas hydrates were found at three sites (565, 568, and 570) and were inferred, on the basis of inorganic and organic geochemical evidence, to be present at two sites (566 and 569); no evidence for gas hydrates was observed at Site 567. Recovered gas hydrates appeared as solid pieces of white, icelike material occupying fractures in mudstone or as coarse-grained sediment in which the pore space exhibited rapid outgassing. Also a 1.05-m-long core of massive gas hydrate was obtained at Site 570. Downhole logging indicated that this hydrate was actually 3 to 4 m thick. Measurements of the amount of methane released during the decomposition of these recovered samples clearly showed that gas hydrates had been found. The distribution of evolved hydrocarbon gases indicated that Structure I gas hydrates were present because of the apparent inclusion of methane and ethane and exclusion of propane and higher molecular weight gases. The water composing the gas hydrates was fresh, having chlorinities ranging from 0.5 to 3.2 per mil. At Sites 565, 568, and 570, where gas hydrates were observed, the chlorinity of pore water squeezed from the sediment decreased with sediment depth. The chlorinity profiles may indicate that gas hydrates can often occur finely dispersed in sediments but that these gas hydrates are not recovered because they do not survive the drilling and recovery process. Methane in the gas hydrates found on Leg 84 was mainly derived in situ by biogenic processes, whereas the accompanying small amounts of ethane likely resulted from low-temperature diagenetic processes. Finding gas hydrates on Leg 84 expands observations made earlier on Leg 66 and particularly Leg 67. The results of all of these legs show that gas hydrates are common in landward slope sediments of the Middle American Trench from Mexico to Costa Rica.

C1-C8 hydrocarbons in DSDP Leg 64 Holes sediments

Sediments from the Baja California Continental Margin Transect - Sites 474 and 476 - showed small amounts of C2-C8 hydrocarbons and functionalized compounds (alkenes) typical of organic-rich, Recent, cold (<30°C) marine sediments. In contrast, some samples from Sites 477, 478, 479, and Hole 481A in the Guaymas Basin, an active spreading center, showed the characteristics of thermally generated hydrocarbons. These include an increase (sometimes exponential) in amount and diversity of C2-C8 hydrocarbons and a decrease in alkenes in more thermally mature sediments. The results indicate that the injection of basaltic sills has minimal effect on C2-C8 hydrocarbon generation except in the immediate vicinity of the sill. The absence of light hydrocarbons close to the hottest sills suggests that the compounds distill away as they are formed in these areas of very active hydrothermal circulation. A sample of young sediment exposed to very high temperatures (>300°C) from deeper thermal sources at the hottest site, 477, showed a very limited hydrocarbon distribution, including primarily ethane, benzene, and toluene, together with smaller amounts of propane and butane.

C1-C7 volatile organic compounds in sediments at DSDP Legs 56-57 Holes

Volatile C1-C7 components in sediments were examined for Japan Trench DSDP Sites 438, 439, 435, 440, 434 and 436, proceeding from west to east. Levels of all components are lowest in the highly fractured sediments of Sites 440 and 434. A number of alkenes, furans, and sulfur compounds were detected in concentrations higher than noted in any other DSDP sediments examined to date. The types, amounts, and specificity of occurrence are similar to those for 1-meter gravity cores we have examined which bear a significant biological imprint. Site 436 shows high levels of saturated and aromatic hydrocarbons, as well as olefins, including traces of dimethycyclopentanes and the highest level of cyclohexene detected in any DSDP sediment we have examined to date. The results from Site 436 were unexpected, considering the low organic-carbon content, absence of biogenic methane, and evidence of an aerobic depositional environment at this site.