A.G. Requejo

Bacterial methane oxidation in sea-floor gas hydrate: Significance to life in extreme environments

Samples of thermogenic hydrocarbon gases, from vents and gas hydrate mounds within a sea-floor chemosynthetic community on the Gulf of Mexico continental slope at about 540 m depth, were collected by research submersible. The study area is characterized by low water temperature (mean = 7 C), high pressure (about 5,400 kPa), and abundant structure II gas hydrate. Bacterial oxidation of hydrate-bound methane (CH{sub 4}) is indicated by three isotopic properties of gas hydrate samples. Relative to the vent gas from which the gas hydrate formed, (1) methane-bound methane is enriched in {sup 13}C by as much as 3.8% PDB (Peedee belemnite), (2) hydrate-bound methane is enriched in deuterium (D) by as much as 37% SMOW (standard mean ocean water), and (3) hydrate-bound carbon dioxide (CO{sub 2}) is depleted in {sup 13}C by as much as 22.4% PDB. Hydrate-associated authigenic carbonate rock is also depleted in {sup 13}C. Bacterial oxidation of methane is a driving force in chemosynthetic communities, and in the concomitant precipitation of authigenic carbonate rock that modifies sea-floor geology. Bacterial oxidation of hydrate-bound methane expands the potential boundaries of life in extreme environments.

Organic geochemistry of sediments from chemosynthetic communities, Gulf of Mexico slope

We used a research submersible to obtain 33 sediment samples from chemosynthetic communities at 541–650 m water depths in the Green Canyon (GC) area of the Gulf of Mexico slope. Sediment samples from beneath an isolated mat of H2S-oxidizing bacteria at GC 234 contain oil (mean = 5650 ppm) and C1–C5 hydrocarbons (mean = 12,979 ppm) that are altered by bacterial oxidation. Control cores away from the mat contain lower concentrations of oil (mean = 2966 ppm) and C1–C5 hydrocarbons (mean = 83.6 ppm). Bacterial oxidation of hydrocarbons depletes O2 in sediments and triggers bacterial sulfate reduction to produce the H2S required by the mats. Sediment samples from GC 185 (Bush Hill) contain high concentrations of oil (mean = 24,775 ppm) and C1–C5 hydrocarbons (mean = 11,037 ppm) that are altered by bacterial oxidation. Tube worm communities requiring H2S occur at GC 185 where the sea floor has been greatly modified since the Pleistocene by accumulation of oil, thermogenic gas hydrates, and authigenic carbonate rock. Venting to the water column is suppressed by this sea-floor modification, enhancing bacterial activity in sediments. Sediments from an area with vesicomyid clams (GC 272) contain lower concentrations of oil altered by bacterial oxidation (mean = 1716 ppm) but C1–C5 concentrations are high (mean = 28,766 ppm). In contrast to other sampling areas, a sediment associated with the methanotrophic Seep Mytilid I (GC 233) is characterized by low concentration of oil (82 ppm) but biogenic methane (C1) is present (8829 ppm).

Polynuclear aromatic hydrocarbons (PAH) as indicators of the source and maturity of marine crude oils

Whole oil GC-MS was used to characterize selected polynuclear aromatic hydrocarbons (PAH) in a suite of marine crude oils derived from source rocks deposited under different conditions. The selected PAH consist primarily of 2- and 3-ring aromatic hydrocarbons, including naphthalenes (to C 4 ), fluorenes (to C 3 ), phenanthrenes (to C 4 ) and dibenzothiophenes (to C 3 ), as well as several 4- and 5-ring compounds. Concentrations of the selected PAH range from 2294 to 129,170 ppm and typically comprise between 1% and 20% of the total C 12+ aromatic fraction. PAH compositions in all source types are dominated by naphthalenes. These compounds comprise between 41.9% and 88.9% of the total PAH measured. The greatest difference between oils lies in the relative abundance of dibenzothiophenes. Their abundance rivals that of naphthalenes in the carbonate oils (up to 41.7% of total PAH) but is exceedingly low in the paralic oils ( < 2.1% of total PAH). These differences, which reflect differences in clastic content, Eh-pH, and availability of reduced sulfur in the source rock depositional environment, are captured in the ratio DBT/P, calculated as the sum of all dibenzothiophenes relative to phenanthrenes. DBT/P is greater than 1.0 in most carbonate-sourced oils but ranges between 0.08 and 0.18 in the paralic-sourced oils. Siliciclastic-sourced oils are intermediate, with values ranging from 0.14 to 0.87 (average 0.40). All oils show a predominance of alkylated PAH homologues over the unsubstituted parent, however, the maximum degree of alkylation (the most abundant alkylated homologues within any given PAH series) varies. Carbonate-sourced oils show a high degree of alkylation, often maximizing at C 3 or C 4 , while paralic-sourced oils generally exhibit a lower degree of alkylation in which the C 1 or C 2 isomers predominate. Siliciclastic-sourced oils are intermediate. with alkylation maximizing between the C 2 and C3 isomers. We propose that these alkylation trends reflect differences in the PAH precursor moieties in source kerogens and/or variations in the thermal history of the source types, i.e., milder thermal history for the carbonate oils relative to paralic and siliciclastic oils.

SHORT-CHAIN (C21 AND C22) DIASTERANES IN PETROLEUM AND SOURCE ROCKS AS INDICATORS OF MATURITY AND DEPOSITIONAL ENVIRONMENT

Geochemical analysis of steranes in source rocks from the Western Canada Basin has allowed the identification of several C2, and C2* short-chain sterane isomers. The principal compounds in carbonates from the U. Jurassic Nordegg Formation have been identified as diginane and homodiginane, more thermodynamically-stable isomers of the compounds pregnane and homopregnane that have 5a, 14p, 17p( H) stereochemistry. The Triassic Doig formation also contains two compounds tentatively identified on the basis of geologic and geochemical data as diapregnane and diahomopregnane, rearranged isomers of pregnane and homopregnane with methyl substitution at the 5- and 14-carbon positions. The distribution of these compounds parallels that of the higher molecular weight diasteranes. A well-defined relationship is evident between the ratio Fe/S and short-chain diasterane content: samples with Fe/S approximately 0.90 exhibit high diasterane content. This threshold Fe/S value is nearly equal to the value associated with stoichiometric pyrite (0.87) and suggests that the abundance of short-chain diasteranes is related to excess iron associated with detrital clays. This is consistent with the accepted mechanism of sterane rearrangement, which is thought to proceed via catalysis of clay minerals. In contrast to the higher molecular-weight diasteranes, which increase in abundance relative to other sterane isomers with increas- ing thermal maturation, the abundance of the rearranged short-chain compounds varies little relative to diginane or homodiginane in a maturity suite from the U. Devonian Duvemay Formation. Predicted stabilities based on computational chemistry agree with the observed distributions; diginaneldiapregnane and homodiginane/homodiapregnane represent compound pairs with similar thermal stabilities. The relative invariance of short-chain sterane distributions with maturity suggest that these compounds may be better suited for estimating the elastic content (i.e., Fe/S ratio) of source rocks than the higher molecular-weight steranes, which are both maturity and facies dependent. Copyright 0 1997 Elsevier

Geochemical characterization of lithofacies and organic facies in Cretaceous organic-rich rocks from Trinidad, East Venezuela Basin☆

Abstract Organic-rich Cretaceous sediments from southern Trinidad show differences in hydrocarbon source potential which can be geochemically related to lithofacies and organic facies. Sediments which exhibit good to excellent potential for oil were deposited during the Campanian through Cenomanian in clastic-starved environments, as evidenced by the inverse relationship between Hydrogen Index and the parameters Al2O3/TOC and Th/U. Principal Components Analysis (PCA) of combined TOC, Rock-Eval, major oxide and trace element data shows three lithofacies—carbonate, siliceous and clastic—within samples exhibiting hydrocarbon source potential. The highest petroleum potential is associated with the siliceous and carbonate lithofacies, which consist of hemi-pelagic sediments deposited under low oxygen conditions. The clastic lithofacies,have lower petroleum potential and represent nearshore sediments which have been transported to the continental slope via submarine canyons. Extractable saturated and aromatic hydrocarbon compositions vary with lithology. PCA of conventional biomarker and 2- and 3-ring aromatic hydrocarbon distributions shows that the carbonate lithofacies are characterized by an enrichment in αββ steranes and dibenzothiophenes. Two organic facies can be recognized within the carbonate lithofacies that differ in their relative bundance of tricyclic terpanes gammacerane and bisnorhopane. The siliceous lithofacies are characterized by an abundance of bisnorhopane, ααα (20R) steranes and a slight enrichment in napthalenes relative to phenanthrenes. The clastic lithofacies exhibit high hopane contents relative to steranes, an enrichment in moretanes and 18α (H)-oleanane, and an enrichment in phenanthrenes relative to napthalenes.

Association of Oil Seeps and Chemosynthetic Communities with Oil Discoveries, Upper Continental Slope, Gulf of Mexico

ABSTRACT Geochemical fingerprinting of many upper Gulf slope oils indicates an origin from deeply buried Mesozoic carbonate source rocks. Substantial vertical migration (>6 km; 19,680 ft) along salt and fault migration conduits must be invoked to explain the emplacement of Mesozoic-sourced oils in shallow Miocene to Pleistocene reservoirs. Gulf slope seeps provide evidence of oil and gas charge in a rapidly subsiding salt basin, where oil and gas migration is a geologically recent or ongoing event. Sea-floor oil and gas seeps, gas hydrates, chemosynthetic communities, authigenic carbonate derived from hydrocarbon oxidation, and natural oil slicks are diagnostic indicators of charge. There is a regional association between seeps and the upper Gulf slope oil province offshore Louisiana and Texas. There is also a subregional relationship between seeps and the localized charge systems that give rise to individual oil accumulations. Key case histories include Marquette Field, Jolliet Field, and the GC 228 discovery in northwest Green Canyon, as well as the Cooper and Auger discoveries in northeast Garden Banks.

Origin of High-Sulfur Oils, Gulf of Mexico Continental Slope: ABSTRACT

Oils of the Gulf of Mexico slope typically show higher sulfur contents, higher metal contents, and lower API gravities than do oils of the Gulf shelf. These properties result in increased costs of production, transportation, and refining, thereby diminishing their value. In contrast to shelf oils, many of which are derived from lower Tertiary shale source rocks, the sources of slope oils are thought to be of Mesozoic age. Since deeply buried source rocks have not been encountered during drilling on the slope, we focus on detailed geochemical characterization of the slope oils to infer their origin. Biomarkers suggest variable clastic-starved source facies containing marine kerogen, such as marls and carbonates. Higher sulfur contents and lower API gravities are thought to correlate with increasing carbonate content of the source facies and milder thermal maturity histories. Carbonate-sourced oils from the Upper Jurassic Smackover and Lower Cretaceous Surmiland trends show some similarities but are not strong analogs.