Jean K. Whelan

Amino acids in a surface sediment core of the Atlantic abyssal plain

Abstract The amino acids in a 56 cm sphincter core (K-19-4-9, Woods Hole Oceanographie Institution) from 30°N latitude, 60°W longitude—approximately 300 miles southeast of Bermuda—at a water depth of 5454m were analyzed by gas and liquid chromatography. Amino acids decreased rapidly with depth with β-alanine and γ-aminobutyric acid predominating in deeper sections. Most of the amino acids are tightly bound in the sediment and are released only upon hydrolysis with 6N HC1. Amino acids accounted for only 0.40% of total nitrogen in the non-carbonate minerals in the top 0–8 cm and 7.9 × 10 −4 percent in the bottom 48–56 cm. The remaining nitrogen is removed from the sediment by amino acid extraction.

Laboratory and theoretical constraints on the generation and composition of natural gas

Hydrous pyrolysis experiments were conducted at 125 to 375°C and 350 bars to constrain factors that regulate the generation and relative abundance of hydrocarbon and nonhydrocarbon gases during thermal maturation of Monterey, Eutaw, and Smackover shale. Thermogenic gas was generated at temperatures as low as 125°C and increased in abundance with increasing temperature. The relative abundance of individual hydrocarbons varied substantially in response to increasing time and temperature reflecting the chemical processes responsible for their formation. The hydrocarbon fraction of low maturity gas produced via primary cracking of kerogen was composed predominantly of methane. With increasing thermal maturity, the onset of bitumen generation produced longer-chain hydrocarbons causing a decrease in the relative abundance of methane. At high levels of thermal maturity, the absolute and relative abundance of methane increased due to decomposition of bitumen. In all experiments at all temperatures, carbon dioxide was the most abundant volatile organic alteration product. Carbon dioxide was produced directly from kerogen at low thermal maturity and via the decomposition of bitumen and/or kerogen at high thermal maturity. During early stage alteration, kerogen likely represents the dominant source of oxygen in carbon dioxide while at high thermal maturities water may represent an abundant and reactive oxygen source. Hydrogen released during the disproportionation of water is likely consumed during hydrocarbon generation. Theoretical reaction path modeling suggests that the precipitation of calcite may effectively remove carbon dioxide from natural gas if a source of Ca is available within the rock. Thus, carbon dioxide-rich natural gas may be relatively pristine while methane-rich natural gas may reflect the occurrence of secondary reactions involving inorganic sedimentary components. Kinetic analysis of the experimental data indicates a narrow range of activation energies for the generation of C1-C4 hydrocarbons from the Monterey, Smackover, and Eutaw shales. Carbon dioxide generation from the Monterey and Eutaw shales is accounted for by a substantially broader range of activation energies. Application of these data to predict gas formation at temperatures and time scales typical of subsiding sedimentary basins suggests that C1-C4 generation is restricted to relatively high temperatures while carbon dioxide generation occurs at both low and high thermal maturities. Thus, in contrast to the bulk of C1-C4 generation which is predicted to occur after peak bitumen generation, production of carbon dioxide will occur before, during, and after the generation of liquid hydrocarbons.

Chemical Methods for Assessing Kerogen and Protokerogen Types and Maturity

Kerogen is the complex, high-molecular-weight, disseminated organic matter (OM) in sediments. It is operationally defined as OM that is insoluble in nonpolar organic solvents and in nonoxidizing mineral acids (HCl and HF). It is generally believed to be the major starting material for most oil and gas generation as sediments are subjected to geothermal heating in the subsurface. It is the most abundant form of organic carbon on Earth—about 1000 times more abundant than coal, which forms primarily from terrigenous higher plant remains. Kerogen consists of the altered remains of marine and lacustrine microorganisms, plants, and animals, with variable amounts of terrigenous debris in sediments. It represents about 1% of the organic matter which originates from biological sources and undergoes extensive degradation and alteration within the water column and at the sediment-water interface prior to burial in sediments (Hedges and Prahl, this volume, Chapter 11). The structured terrestrial (e.g., woody) portions of kerogen have elemental compositions similar to that of coal (Hunt, 1979, pp. 279–280; Hunt, 1991).

Applications of thermal distillation—pyrolysis to petroleum source rock studies and marine pollution

Abstract The technique of thermal distillation—pyrolysis involves heating a 0.5–50 mg sample of wet sediment from 200 to 800°C at 20° min and measuring evolved hydrocarbons as a function of temperature. Unaltered absorbed hydrocarbons evolve at 100–150°C, and cracked or pyrolyzed hydrocarbons at 650–800°C in two well-separated peaks, P 1 and P 2 . The compounds in P 1 and P 2 are analyzed by capillary gas chromatography (GC) and GC—mass spectrometry. An increasing ratio of P 1 (P 1 + P 2 ) indicates increasing petroleum source rock maturity. Data are presented for known source rocks and a test well (COST 1, Gulf of Mexico, U.S.A.). Applications of the method to examination of oil and chemical pollutants in organisms and surface sediments are given. Results to date have shown that an increasing degree of pollution causes an increasing P 2 P 2 ratio and increasing complexity of the P 1 peak. The hydrocarbon composition of P 2 has been used to fingerprint and trace high molecular weight organic-rich particles in the marine environment.

Phase fractionation at South Eugene Island Block 330

Persistent gas flux can dissolve, remobilize and alter reservoired or migrating oil through a process of phase fractionation. Moving gas, when flowing through an oil, can dissolve large fractions of that oil. The composition of the oil dissolved in the gas is dependent on the pressure-temperature conditions of the oil and the fluid flow history of the basin. The composition of the residual oil can be interpreted to yield both the depth at which the oil fractionated and the volume of gas required to fractionate the oil. South Eugene Island Block 330 in the U.S. Gulf Coast is a hydrocarbon province that has recently experienced large gas fluxes. Some of the oils in the region show signs of progressive fractionation and remobilization by gas transport. For example, the oils are more aromatic and less paraffinitic than unfractionated oils of similar maturity from the same area. The altered oils are also depleted of light n-alkanes. We have developed a computer-based model of oil alteration based on a fluid phase equilibria algorithm to simulate progressive fractionation of oil by gas. Application of the model to the South Eugene Island Block 330 area shows that several of the oils in the area have compositions that are compatible with alteration caused by equilibrating with approximately 12 to 14 mol of gas per mol of oil (2 to 2.7 g of gas per g of EI oil). The oils appear to have fractionated at approximately the depths of their present reservoirs. The model has great potential to examine hydrocarbon fluids for evidence of past migration and mixing.

Organic geochemical indicators of dynamic fluid flow processes in petroleum basins

A variety of preliminary geophysical, geochemical, and geological observations (Anderson et al., Geophysics, 12-17, April 1991a, b; Anderson, Oil & Gas J. 85-91, 26 April 1993), along with well production data (Schumacher, AAPG Ann. Conv. Abstr., April 1993), have led to the hypothesis that fluid injection into Eugene Island Block 330 (EI-330) in the U.S. Louisiana Gulf Coast may be a continuing episodic process, with the most recent injections having occurred on short geologic time scales (10,000 yr or less). Organic geochemical evidence presented here suggests these processes may occur on very short time scales (years). A process, called «dynamic fluid injection», was proposed to explain these observations and is envisioned as episodic pressure build-up followed by rapid escape of gas and oil through geopressure and injection into overlying reservoirs

Contribution of “Old” carbon from natural marine hydrocarbon seeps to sedimentary and dissolved organic carbon pools in the Gulf of Mexico

Natural radiocarbon (14C) abundances and stable carbon isotope (δ13C) compositions were measured for sediment total organic carbon (TOC), and total lipid fractions of sediments, bottom water, and hydrate-water collected from two hydrocarbon seepage sites in Green Canyon, Northern Gulf of Mexico to determine the contribution of “old” carbon from seeps to sediment TOC and dissolved organic carbon (DOC) pools. Our results indicate that 40–60% of the organic carbon preserved in the sediments and 30% of the DOC in the deep water above the seeps were seep-derived 14C-depleted organic carbon. This new evidence along with our previous studies suggest that natural marine hydrocarbon seepage could be a significant source contributing “old” carbon to the marine environment. Our findings suggest that the global importance and the long-term impact of this contribution to biogeochemical carbon cycling in the ocean need to be more thoroughly investigated.

Volatile C1-C7 organic compounds in surface sediments from Walvis Bay

Abstract C1-C7 volatile organic compounds were analyzed in three gravity cores taken from Walvis Bay shelf. The compounds detected included alkanes (methane, ethane, propane, i- and n-butane, and i- and n-pentane, and heptane), alkenes (2-methyl-2-butene, dimethylcyclopentenes, cyclohexene), oxygen containing compounds (2- and 3-methylfuran, 2,5-dimethylfuran, 2- and 3-methylbutanal and 3-pentanone), sulfur compounds (dimethylsulfide, thiophene, 2- and 3-methylthiophene) and aromatic compounds (benzene and toluene). In situ biological and low temperature chemical (less than 15°C) formation processes are proposed, possibly from marine terpene precursors. Subsequent to this work, these compounds were found to be widely distributed in surface gravity cores from other areas. Many of these compounds do not survive deeper burial. Furans, ketocompounds, and alkenes are generally not found in more than trace quantities in deeper (⪢10m subbottom) DSDP cores we have examined from other areas.

C1C8 hydrocarbons in sediments from Guaymas Basin, Gulf of California—Comparison to Peru Margin, Japan Trench and California Borderlands

Surface seafloor sediments, hydrothermal vent samples, and Deep Sea Drilling Project sediments (Hole 481A) from the Guaymas Basin were examined for C1C8 hydrocarbons. The proportions of various classes of compounds were examined and compared to those from other geographic areas (Peru upwelling region and Japan Trench) to gain insight into the relative importance of thermal generation, migration and biodegradation. Concentrations of C2C7 hydrocarbons were about 10–10,000 times higher in geothermally warm (estimated to have been exposed to maximum temperatures in the range of 30–150°C) Guaymas Basin sediments in comparison to the low concentrations (0.1–10 ppb per compound) typical of geothermally cold (maximum thermal exposure less than 20°C) seafloor and DSDP diatomaceous sediments. However, one sediment sample from DSDP Site 477, estimated to have been exposed to temperatures of 300°C or higher in the past, showed only a limited hydrocarbon composition, consisting of C1C3 alkanes and aromatic hydrocarbons only. Alkene/alkane ratios of 0.1 or greater were typical of both geothermally cold sediments and also of very hydrocarbon-richAlvin samples recovered from the seafloor. Because little or no alkene was generally detected in buried sediments exposed to geothermal temperatures greater than 30°C, it is suggested that the alkenes are produced by biogenic processes. Normal alkenes predominated over cyclic and branched structures in geothermally cooler (<20°C) sediments, with the proportion of cyclic and branched compounds increasing in hotter sediments. Concentrations ofgem-dimethyl and aromatic compounds generally remained approximately constant or increased slightly with temperature in comparison with geothermally cold shallow sediments. Similarities in compositions of branched and cyclic compounds were observed in some pairs of bitumen-rich Guaymas seafloor samples recovered from different areas, suggesting common mechanisms of light hydrocarbon generation and/or migration. Localized increases in ratios of specific cycloalkane ratios were observed adjacent to sill intrusions.

Sedimentary and geochemical expressions of oxic and anoxic conditions on the Peru Shelf

Abstract Sediments from drill hole transects across the Peru shelf and upper slope show that the extent and intensity of the oxygen minimum zone (OMZ) has varied considerably from the early Pleistocene to the present. Primary sedimentary features, such as laminations and bioturbation, and the hydrogen index of sedimentary organic matter, appear to covary with the presence and intensity of the OMZ. Sediments deposited under dysoxic to anoxic bottom-water conditions are significantly more hydrogen-rich relative to bulk TOC. The molecular composition of pyrolytic hydrocarbons and heterocompounds, however, is surprisingly uniform, and is indicative of neither oxic nor anoxic degradation at either the sediment-water interface or in the water column. The concept of better preservation of organic matter under anoxic conditions in the bottom water is reflected in the bulk parameter of pyrolyzable organic matter content relative to TOC, but not in individual compound classes incorporated into kerogen. The most important control on the hydrogen-richness of organic matter appears to be sediment reworking and redeposition.