Martha E. Tarafa

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.

Effect of hydrocarbon volatility and adsorption on source-rock pyrolysis

Abstract Petroleum source-rock evaluations by pyrolysis are based on the concept that free hydrocarbons in rock samples are volatilized below 300°C while hydrocarbons cracked from kerogen come off at higher temperatures. The pyrolysis of pure hydrocarbons with different mineral matrices shows that free hydrocarbons containing 16 or more carbon atoms may not be evolved eblow 300°C, but at varying higher temperatures. The extent to which this occurs depends on the hydrocarbon volatility, the mineral matrix and the pyrolysis instrument design. Source-rock parameters which use the P1 (S1) peak may be not be reproducible between instruments if the rock contains appreciable amounts of high molecular weight hydrocarbons.

Investigation on the effects of organic solvent extraction on whole-rock pyrolysis: Multiple-lobed and symmetrical P2 peaks

Pyrolysis techniques provide an estimate of oil-generating potential of a sediment based on the amount of P{sub 2} (or S{sub 2}) hydrocarbons. Multiple-lobed and symmetrical P{sub 2} peaks were studied to determine if extractable bitumen affected the P{sub 2} results. Samples from the Tiglukpuk and Ledbetter wells were analyzed as whole, unextracted, ground sediment and after organic solvent extraction using pyrolysis techniques. Results indicate that P{sub 2} for the Tiglukpuk well appears unaffected by solvent extraction. P{sub 2} peaks for the Ledbetter well samples appear to include extractable bitumen or asphaltenes. Pyrolysis-gas chromatography (pyrolysis-GC) and pyrolysis-gas chromatography-mass spectrometry (pyrolysis-GC/MS) determined the composition of the portion of the P{sub 2} peak for the Ledbetter well that was removed by organic solvent extraction. Pyrolysis-GC analysis also demonstrated that for the Ledbetter well gas-generating organic material was not removed by organic solvents.

Bitumen molecular maturity parameters in the Ikpikpuk well, Alaskan North Slope

Abstract Several bitumen molecular maturity parameters: alkylnaphthalene ratios—MNR, ENR, DNR; methylphenanthrene indices—MPI 1 , MPI 2 , DPI; n -C 17 /pristane; pristane/phytane; C 15+ alkane distributions and GC-MS analyses of triterpanes, steranes, diasteranes, triaromatic steroids, and monoaromatic steroids for samples of the Ikpikpuk well, Alaskan North Slope, are reported. The molecular maturity parameters are compared and contrasted with previous data for lower molecular weight C 1 -C 7 hydrocarbons, production index, vitrinite reflectance, and thermal alteration index. Bitumen maturation indices, C 15+ alkane distributions, and distributions of steranes, diasteranes, triterpanes, triaromatic and monoaromatic steroids agree with kerogen maturation indices in the Kingak section of Ikpikpuk well. Bitumen maturation indices and distributions of steranes, diasteranes, triterpanes, triaromatic and monoaromatic steroids diverge from vitrinite maturation profiles for the Shublik section of Ikpikpuk indicating either a major influence of changes in kerogen type or the presence of residues of expulsion/migration in the Shublik. The data are consistent with the Shublik being a source and/or conduit of migration of oil.

Maturity of organic matter and migration of hydrocarbons in two Alaskan North Slope wells

Abstract Plots of T max and production index (P.I.) for seven Alaskan North Slope wells suggest that samples from the Ikpikpuk well are optimal for examination of thermal maturity and the Seabee well for migrational geochemical trends at all maturity levels. Vitrinite reflectance profiles for the two wells are identical and reach maximum values of 2.5% in the deepest rock units. Pyrolysis P1/org C, T max , and P.I. values all behave “normally” in Ikpikpuk but for Seabee show generally lower (by about 60°C) T max values and higher and more scattered P1/org C and P.I. values. Sorbed light hydrocarbon ratios including C2/C3; ic 4/ n C4; i C5/ n C5; 22DMB/ n C6; MCP/benz; benz/tol; paraffin indexes 1 and 2 and C1/(C1 + C2 + C3) are compared for both wells. Light hydrocarbon data for Ikpikpuk generally shows smooth changes expected for maturation across the Kingak shale and Shublik formation with increasing paraffin indexes 1 and 2; decreasing MCP/enz; increasing benz/tol; increasing C1/(C1 + C2 + C3); and a spike in 22DMP/ n C6 at the bottom of the over-mature Sadlerochit sandstone. These ratios are consistent with some localized lateral migration at the bottom of the Sadlerochit Group sandstone and at the top of the pebble shale unit. In contrast, Seabee ratios were very different and showed no particular trends with depth, although some increase in proportions of methane and benzene are observable in the deepest sections of the well. The ratios of pyrolyzable C1/(C1 + C2 + C3) for the Ikpikpuk well were similar to those for the generated sorbed gases within for the catagentic and post mature sections of the well. Seabee showed pyrolyzable C1/(C1 + C2 + C3) ratios similar to those from Ikpikpuk except for a less pronounced bottom hole increase in the ratio. Migration phenomena with preferential retention of polar asphalts or pyrobitumens in strata through which or from which migration has occurred are postulated to be the cause of a low temperature P2 shoulder often observed in and below the catagenic zones of Ikpikpuk. GC/MS data of pyrolyzable material shows that maturation causes a decrease in alkane and alkene pyrolysis products as compared to aromatic and heterocyclic compounds. This low temperature shoulder appears to become the predominant P2 peak in sections of other wells, such as Seabee, which are extensively influenced by migration.