Paul A. Comet

13C12c ratios in individual fatty acids of marine mytilids with and without bacterial symbionts

Abstract No evidence was found for isotopic fractionation at methyl carbon during methylation of fatty acids with BF 3 -methanol. It is therefore possible to evaluate intermolecular carbon isotopic variations in natural fatty acids by determining the δ 13 C composition of the corresponding methyl esters after derivatization. We illustrate the usefulness of the technique to the evaluation of dietary strategies of marine mytilids in two contrasting marine environments, a normal coastal Newfoundland estuarine system and a cold hydrocarbon seep benthic community from the Gulf of Mexico. Mussels from coastal Newfoundland have fatty acid δ 13 C compositions of −34.4 to −24.9 0 00 , whereas those from the Gulf of Mexico showed a range of −56.9 to −49.0 case:0 00 . This difference is primarily ascribed to the difference in the ultimate source of Cl carbon in the two environments (CO 2 in coastal Newfoundland and CH 4 in the Gulf of Mexico). Differences observed in the nature of intermolecular carbon isotopic patterns of saturated and unsaturated fatty acids of mytilids from the two marine localities reflect the differences in the carbon pathways utilized by the organisms in their respective growth environments.

The origins of petroleum in the northern Gulf of Mexico

Abstract The distribution and chemistry of oils in the northern Gulf of Mexico and the adjacent onshore can be explained by multiple sources, fades variations, maturation, and post-generation alteration. Genetic families include Jurassic Smackover, Flexure Trend, Upper Cretaceous, and Paleogene oils. Smackover oils have high sulfur contents, pristane/phytane ratios C 35 C 34 hopane ratios ≥1.0, and C 30 C 29 hopane ratios usuall ≤ 1.0. Flexure Trend oils are similar and contain abundant extended hopanes, high sulfur contents, and V (Ni + V) > 0.5 ; these oils are inferred to be sourced in Upper Jurassic/Lower Cretaceous strata. Upper Cretaceous oils contain a distinctive suite of tricyclic and nonhopanoid triterpanes; oleanane is low or absent. Paleogene oils can be recognized by the presence of 18α(H) oleanane and a lack of extended hopanes. These oils occur from south Texas to southern and offshore Louisiana. Two subfamilies can be recognized based on the relative abundance of triterpanes and steranes. These oils have a presumed Paleogene source. Mixing of oil types is quite prevalent at the geographic boundaries of oil types. The initial control on the distribution of oil and gas in the northern Gulf of Mexico is provided by the areal extent of source rocks. Secondary control is due to Cenozoic deposition, which provides the thermal stress to generate, as well as destroy, oil. Salt tectonics provide conduits for migrating fluids to escape the zone of thermal destruction. Mesozoic source strata again become important in the deep Gulf of Mexico, where Cenozoic sediments thin.

Gulf of Mexico hydrocarbon seep communities: XI. Carbon isotopic fractionation during fatty acid biosynthesis of seep organisms and its implication for chemosynthetic processes

Abstract The individual fatty acids of mytilids and vestimentiferan ( Escarpis sp.) from hydrocarbon seeps exhibit light δ 13 Cvalues: from −56.9 to −49.0‰ for the mytilids and from −38.6 to −31.6‰ for the vestimentiferan. Unsaturated fatty acids have lighter δ 13 C than saturated ones. The variations in δ 13 C are up to 5.1–6.7‰ (mytilids) and 7.0‰ (vestimentiferan) within a single specimen. It is suggested that a kinetic isotopic effect in the biosynthesis of fatty acids and intermolecular isotope fractionation during fatty acid desaturation and elongation are responsible for the observed distribution pattern. Fatty acids are depleted in 13 C relative to the gills of the mytilids, whereas fatty acids of the vestimentiferan are enriched relative to trophosomes. The difference in δ 13 C of fatty acids between mytilids and vestimentiferan reflects the differences in substrates (methane vs. CO 2 ), and the different chemosynthetic processes of the invertebrates.

Nonmethylene-interrupted fatty acids of hydrocarbon seep mussels: Occurrence and significance

Abstract 1. 1. Characterization of fatty acid composition of hydrocarbon seep mussels reveals that the mussels contained a striking diversity of nonmethylene-interrupted dienoic (NMID) and trienoic (NMIT) fatty acids. 2. 2. The concentrations of the NMI acids are unusually high, accounting for 12.2–17.7% of the total fatty acids. 3. 3. The NMI acids are primarily ω7 and ω9 series, and the position of double bonds of the NMI acids are primarily at or near the center of the fatty acyl chains. 4. 4. The biosynthesis of NMI acids and their physiological significance are discussed.

Geochemistry of oils from the northern Timan-Pechora Basin, Russia

Abstract A geochemical study of 36 oils from the northern Timan-Pechora Basin indicates four oil types. Two types of sulfur-rich oils (range 1.4–2.5% sulfur) can be identified on the basis of both gross and molecular properties. These oils are found primarily in the Toravey, South Toravey and Roman Trebs fields. They are also present in the offshore Prirazlom field, thereby extending the geographic range of sulfur-rich oils into the Pechora Sea. Another oil type exhibits a distinctive odd predominance of n -alkanes in the range n -C 11 through n -C 19 and a diminished abundance of n -alkanes greater than C 19 . These oils, which occur primarily in the A. Titov and Labogan fields, show striking similarities to Ordovician-sourced oils from other Paleozoic basins throughout the world. Their biomarker and isotopic compositions, for example, bear a strong resemblance to oils generated from the Ordovician Viola Limestone of the Anadarko Basin. The fourth oil type occurs in the northwestern portion of the basin in the Khylchuyu field. Some mixing between the various types is evident. Light oils and condensates of the Peschano-ozyor, Shtokman, Rusanov and Vasilkovo fields cannot be readily assigned to any of these four types. The Upper Devonian Domanik Formation is generally thought to be the major oil source in the Timan-Pechora Basin. However, geochemical characteristics of Timan-Pechora oils analyzed do not closely resemble those of other Upper Devonian-source oils, such as those generated from the Duvernay Formation of Western Canada. These differences might reflect variations in the organic matter preserved in the two basins during the late Devonian or may indicate that the Upper Devonian is not an important hydrocarbon source in the northern Timan-Pechora region. The results suggest that the Timan-Pechora oils analyzed are generated from Paleozoic carbonate or calcareous shale source rocks, at least one of which appears to be of Ordovician age.

Triterpenes in a Nigerian oil

Abstract A series of monounsaturated triterpenes have been identified in an oil from Utue Field of the Niger delta. The triterpenes are the angiosperm-derived olean-13(18)-ene, olean-12-ene, olean-18-ene, baur-7-ene, and urs-12-ene. The presence of these compounds in oils is attributed to the extraction of thermally immature Agbada shales by the migrating fluid and in the reservoir. The terpane biomarker signature of the oil does not appear to have been altered by this phenomenon.

Sterane and triterpane patterns as diagnostic tools in the mapping of oils, condensates and source rocks of the Gulf of Mexico region

Abstract Four hundred and eighty-one Gulf of Mexico oils and condensates can be subdivided into nine major families based on their sterane ( m / z 217, 218) and triterpane ( m / z 191, 177) biomarker mass chromatogram patterns. These families are (1) carbonate-sourced oils reservoired in Jurassic-Cretaceous reservoirs onshore and Plio-Pleistocene reservoirs offshore, (2) Austin Chalk reservoired oils, (3) Tuscaloosa oils, (4) Wilcox reservoired oils (La), (5) South Texas Palaeogene reservoired oils, (6) Mississippi delta oils, (7) post-mature oils of uncertain affinities, (8) migration contaminated oils (showing attributes of a mixing of immature and post-mature sources), and (9) oils and condensates lacking biomarkers. Each oil family is concentrated in specific age-defined producing trends, ranging in age from Jurassic to Pleistocene. For Tertiary reservoirs, these producing trends shift geographically with time, in a manner analogous to the clockwise migrating Cenozoic depocenter model for the northern Gulf of Mexico. At least six ages of marine source rock deposition may be implicated in the genesis of these oils (1) The Kimmeridgian-Oxfordian (including Smackover and Norphlet formations), (2) The Aptian (including Pine Island, Bexar and Sunniland formations), (3) The Albian (Paluxy formation), (4) The Cenomanian-Turonian (including Eagle Ford and Tuscaloosa formations), (5) The Coniacian-Santonian (Eagle Ford and Austin Chalk), and (6) The Palaeocene (Wilcox, Midway, Sparta formations). The Jurassic and Cretaceous source rocks correspond to described oceanic anoxic events of probable global significance. This paper will concentrate on describing the biomarker patterns of the oils and postulated source rocks, but due to reasons of space, isotopic, elemental and other data will only be briefly discussed.

Maturity Mapping of Northern Gulf of Mexico Oils Using Biomarkers

ABSTRACT The observed maturity trends determined from 481 northern Gulf of Mexico oils and offshore seeps follow the major geological trends of the basin in a manner consistent with published heatflow measurements. Onshore the Luling-Mexia-Gilbertown-Pickens-Pollard fault zones appear to act as areas of enhanced heatflow, as indicated by the high maturity of nearby oils (near base of oil window). To the north and south of these fault lines, oils of reduced maturity are encountered. Offshore Louisiana, oils of lower maturity, comparable to the middle to early oil window, are found; the low maturity of these oils appears to be linked to salt diapirism. By contrast, the mature condensates offshore Texas appear to link, in part, with shale diapirism and growth faulting. Analyses of oils and seepage from the Flexure Trend area indicates that hydrocarbon maturity is reduced towards the south where the Tertiary overburden thins.

Gulf of Mexico hydrocarbon seep communities—IX. Sterol biosynthesis of seep mussels and its implications for host-symbiont association

Abstract The sterol analysis of six hydrocarbon seep mussels (mytilid II and mytilid Ib) from the Alaminos Canyon in the Gulf of Mexico is reported. The sterol composition of the mussel-symbiotic bacteria complex reflects a preponderant synthesis of 4α-methyl sterols (seep mytilid II), and a predominant biosynthesis of 4-desmethyl sterols with some amounts of 4α-methyl sterols (seep mytilid Ib). This suggests a methane-based symbiotic relationship between the mussels and methanotrophic bacteria. It also suggests that the biosynthesis of sterols in the mussel-bacteria complex is completed to the level of cholest-5-en-3s-ol (mytilid II) or 5α-cholestan-3s-ol (mytilid Ib).

Significance of Lignites in Paleocene Middle Wilcox Stratigraphy in East Central Louisiana

ABSTRACT Lignites are a ubiquitous component of the stratigraphic sequence in the deltaic depositional environment of the Wilcox Group. In the Middle Wilcox, between the Big Shale and the upper Minter oil sands, 11 principal lignites, ranging in thickness from a few inches to 5-10 feet, have been observed on E-logs and corroborated by conventional cores and side wall samples. These extensively correlative lignites commonly overlie distributary channels and overbank bay fills. The lignites are usually overlain by shales indicating that compactional subsidense or sea level rise returned the area to shallow marine conditions. Assuming a lower delta-plain setting, the upper lignite surfaces commonly represent marine-flooding surfaces or parasequence boundaries. Peat accumulation resulting in lignites appears to represent only minor hiatuses between aggradational sands and interdistributary bay marine deposits. Productive Minter oil sands (2 ft - 12 ft) are closely associated with lignites and may be bounded above and below by them (Bee Brake and North Bee Brake Fields). These lignites possibly act as oil migration conduits. Organic petrography and geochemistry indicate that they are good potential source rocks for hydrocarbons. The association of these lignites with possible methane gas production is as yet an uninvestigated possibility in east central Louisiana.