Elizabeth W. Chinn

Chemosynthetic bacterial mats at cold hydrocarbon seeps, Gulf of Mexico continental slope

Abstract White and pigmented filamentous bacterial mats dominated by several undescribed species of Beggiatoa were sampled during research submersible dives to cold hydrocarbon seep sites on the upper continental slope off Louisiana (130–550 m). Mats occur at the interface between reducing sediments and the oxygenated water column. They are localized at sea floor features related to seepage of biogenic methane and crude oil, but there is little evidence that the organisms utilize the hydrocarbons directly. Granules of elemental sulfur (S0) are visible within cells of Beggiatoa, and mat material is characterized by high contents of S0 (up to 193,940 ppm). The Beggiatoa biomass is isotopically light ( δ 13 C = −27.9‰ PDB ). Our geochemical data suggest that the Beggiatoa species are part of a complex bacterial assemblage in cold seep sediments. They oxidize H2S derived from the bacterial sulfate reduction that accompanies bacterial hydrocarbon oxidation when O2 is depleted in sediments, and fix isotopically light carbon from CO2 that is the result of bacterial hydrocarbon oxidation. Beggiatoa mats appear to retard loss of hydrocarbons to the water column by physically retaining fluids in sediments, a function that could enhance production by other bacteria of the H2S and CO2 needed by Beggiatoa.

Fixed-ammonium in clays associated with crude oils

Abstract The association of ammonium (NH 4 + ) silicates with organic-rich sedimentary environments has stimulated interest in the chemical cycle of N, and its possible application as an indicator of in situ organic maturation reactions or crude oil migration. Fixed-NH 4 in clay minerals was determined from three hydrocarbon occurrences of similar depositional environment but different ages, depth and thermal maturity, to determine whether anomalously high NH 4 -substitution occurs near mature hydrocarbons. Results show higher fixed-NH 4 concentrations in marginally mature mudstones than in immature sediments. The highest fixed-NH 4 concentrations were found in clays from sandstone reservoirs containing migrated crude oil. Fixed-NH 4 in clays from Holocene oil seep sediments in the Gulf of Mexico continental slope, offshore Louisiana, averages 0.08 wt % and increases with depth in shallow cores (420 cm), reflecting an early diagenetic trend that is apparently not influenced by migrating crude oil. Programmed pyrolysis shows that the sediments are thermally immature (av.T max = 419°C). High Hydrogen Index values (av.= 359mg/g) are the result of biodegraded crude oil, and a high Oxygen Index (av.= 182mg/g) reflects the presence of authigenic carbonate. Fixed-NH 4 averages 0.16 wt % in Wilcox Group (Eocene) mudstones enclosing two sandstone reservoirs at Fordoche Field, onshore Louisiana. In comparison to these mudstones, anomalously high NH 4 -fixation appears to occur in reservoir clay minerals. Pyrolysis shows that the sediments are marginally mature for crude oil generation (av.T max = 432°C). Average Hydrogen Index (187 mg/g) and Oxygen Index values (75 mg/g) are consistent with oil-prone Type II and Type III kerogen. Increased pyrolysis Production Index values and solvent extraction shows the presence of migrated crude oil. This suggests that a reaction which releases NH 3 during crude oil generation or migration is recorded byNH 4 + substitution in clays. Fixed-NH 4 and total organic carbon (TOC) at Fordoche Field show no statistically significant correlation, suggesting that NH 4 + substitution in clay minerals is not simply related to the amount of organic matter in the section, but is also influenced by the presence of crude oil. Once NH 4 + has been fixed in clays, it is a more stable hydrocarbon proximity indicator than pore fluid tracers, because it is less influenced by later chemical or geological changes.

Deposition of magnetic pyrrhotite during alteration of crude oil and reduction of sulfate

Abstract Crude oil seepage is widespread in carbonate cap rock and in overlying sediments of the Damon Mound salt dome, Brazoria County, Texas. Scanning electron microscopy of calcite-lined cavities provides insight to the latest stage of mineral deposition in this salt dome environment. Solid crude oil residues and microbes occur in association with surface minerals such as gypsum and barite. Deposition of pyrrhotite, pyrite, marcasite, sphalerite, and elemental sulfur is attributed to microbial sulfate reduction, and deposition of aragonite and calcite is related to microbial oxidation of crude oil hydrocarbons. The C1—C4 hydrocarbons in pyrrhotite-bearing cap rock samples are a biodegraded residue derived from crude oil. Higher molecular weight crude oil components in cap rock are altered by water washing and to a lesser extent by biodegradation. Hydrocarbon compositions could be explained by limited aerobic biodegradation, but a component of anaerobic biodegradation cannot be excluded. Some Damon Mound samples are highly magnetic because of abundant ferrimagnetic pyrrhotite. This study provides new evidence of a link between crude oil migration, microbial activity, and diagenetic magnetic anomalies in rocks.

Origin of Crude Oil in the Wilcox Trend of Louisiana and Mississippi: Evidence of Long-Range Migration

ABSTRACT Geochemical characterization of crude oils from Wilcox reservoirs in central Louisiana and southwest Mississippi suggests that they represent a single crude oil family that is distinct when compared to crude oils in deeper Tuscaloosa and Smackover reservoirs. This observation is consistent with geologic constraints that suggest an origin of crude oil from within the Wilcox Group itself. Although shales of the shallow Wilcox Group in central Louisiana and southwest Mississippi contain gas-prone kerogen and are thermally immature, a more oil-prone source facies is present in marine shales of the deep Wilcox Group in southcentral Louisiana. Thermal maturity measurements based on pyrolysis suggest a broad area of effective Wilcox source rock in southcentral Louisiana. Migration distances from source to reservoir rocks of the downdip Wilcox Trend of southcentral Louisiana appear to be relatively short. However, long-range updip migration (sometimes greater than 100 km) from deeply buried Wilcox source facies provides the best explanation for emplacement of crude oil in the shallow Wilcox Trend of central Louisiana and southwest Mississippi.

Recent hydrocarbon alteration, sulfate reduction and formation of elemental sulfur and metal sulfides in salt dome cap rock

Abstract Geochemical analysis of hydrocarbons and scanning electron microscopy (SEM) provide insight to recent alteration of crude oil, microbial sulfate reduction and mineral deposition in carbonate cap rock at Damon Mound, Brazoria County, Texas, U.S.A. Previous work showed that intense aerobic biodegradation of hydrocarbons occurs in shallow cap rock where S 0 and metal sulfides are not abundant. The present study focuses on some deeper carbonate cap rock characterized by more limited oxygen availability. The C 1 -C 4 hydrocarbons in deeper cap rock are a biodegraded residue of hydrocarbons derived from crude oil. Alteration of higher-molecular-weight crude oil components is best explained by the effects of water washing accompanied by some biodegradation. SEM of cavities in carbonate cap rock shows microbes in association with globular masses of solid crude oil and sulfate minerals. Late-forming minerals in cavities include S 0 , pyrrhotite, pyrite, sphalerite and aragonite. The presence of S 0 and sulfide minerals indicates that microbial sulfate reduction is associated with limited hydrocarbon biodegradation in this deeper cap rock environment. Similar processes could explain the origin of some S 0 and metal sulfides at Damon Mound in the geologic past.

Fluorescent microscopy of particulate organic matter: Sparta Formation and Wilcox Group, South Central Louisiana

Abstract Particulate organic matter (POM) isolated from core samples of mudstones in the Mid-Eocene Sparta Formation and the Late Paleocene-Early Eocene Wilcox Group in Louisiana was characterized using transmitted and fluorescent light microscopy and Rock-Eval pyrolysis. Observations of maceral fluorescence combined with programmed pyrolysis results better define source potential than visual examination using transmitted (white) light alone. Transmitted light microscopic characterization of POM indicates that amorphous macerals of mixed terrestrial and marine origin dominate samples from both the lagoonal and shelf depositional environments. Microscopic examination under fluorescent light indicates that some of the amorphous matter, including terrestrially derived material, fluoresces to some degree. Hydrogen Index values show a positive correlation with the total number of fluorescing particles in the isolate, as well as with the relative fluorescence levels. Comparison of Sparta and Wilcox samples indicates that significant changes occur in the assemblages at levels of thermal maturation associated with the onset of petroleum generation. Thermally mature samples contain a higher percentage of moderately fluorescent particles and lesser percentages of highly and weakly fluorescent particles. This phenomenon is readily apparent in specific particle types, namely amorphous nonstructured protistoclasts, miospores and dinoflagellate cysts.

Effects of elemental sulfur during programmed pyrolysis of kerogen

Abstract The effects of elemental sulfur during programmed pyrolysis of Eocene Green River Shale and common rock matrix minerals were assessed experimentally. Mixtures of elemental sulfur and Green River Shale with calcium carbonate, silicon dioxide, calcium sulfate, and zinc sulfide were analyzed. With the weight fraction Green River Shale held at near constant values, the addition of elemental sulfur resulted in substantial decreases in Hydrogen Index and substantial increases in Oxygen Index. The effect is so significant that thermally immature Type I kerogen can yield a pyrolysis response typical of Type III kerogen. Moreover, elemental sulfur appears to influence estimations of thermal maturity during programmed pyrolysis. Only minor differences in pyrolysis response were noted for the different matrix compounds without elemental sulfur. This study suggests that programmed pyrolysis results on rocks containing unusually high contents of elemental sulfur should be interpreted with caution.

Origin of the Tertiary reservoired hydrocarbons along the central Texas and Louisiana Gulf Coast rim

Tertiary reservoired hydrocarbons along the central Texas and Louisiana Gulf Coast rim were most likely derived from Paleocene/Eocene Wilcox Group and Sparta Formation marine shales. Sixteen total soluble extracts and >200 oil samples were analyzed using carbon isotopic techniques ({delta}{sup 13}C) and gas chromatography-mass spectometry (GC-MS). Results demonstrated that interpretations must use all types of data because Cretaceous derived and Tertiary derived oils overlap in southern Louisiana. When isotopic, sterane, hopane, and light hydrocarbon data are combined separation of classes become possible. Cretaceous oils and extracts have a full range of extended hopanes, a characteristic peak eluting immediately after C{sub 30} hopane and no oleanane. Paleogene oils and extracts have oleanane and a restricted range of extended hopanes. Regional trends indicate that eastern Louisiana oils were derived from the Sparta or a Sparta/Wilcox mix, the Mississippi delta oils from a Cretaceous clastic source, and western Louisiana and Texas oils from the Wilcox source. Regional variations in GOR/CGR are a function of timing and mechanism of migration.