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USGS2021

The Sedimentary Geochemistry and Paleoenvironments Project

Geobiology explores how Earth's system has changed over the course of geologic history and how living organisms on this planet are impacted by or are indeed causing these changes. For decades, geologists, paleontologists, and geochemists have generated data to investigate these topics. Foundational efforts in sedimentary geochemistry utilized spreadsheets for data storage and analysis, suitable for several thousand samples, but not practical or scalable for larger, more complex datasets. As results have accumulated, researchers have increasingly gravitated toward larger compilations and statistical tools. New data frameworks have become necessary to handle larger sample sets and encourage more sophisticated or even standardized statistical analyses.

Geobiology↗

USGS1998

Black shale source rocks and oil generation in the Cambrian and Ordovician of the central Appalachian Basin, USA

Nearly 600 million bbl of oil (MMBO) and 1 to 1.5 trillion ft 3 (tcf) of gas have been produced from Cambrian and Ordovician reservoirs (carbonate and sandstone) in the Ohio part of the Appalachian basin and on adjoining arches in Ohio, Indiana, and Ontario, Canada. Most of the oil and gas is concentrated in the giant Lima-Indiana field on the Findlay and Kankakee arches and in small fields distributed along the Knox unconformity. Based on new geochemical analyses of oils, potential source rocks, bitumen extracts, and previously published geochemical data, we conclude that the oils in both groups of fields originated from Middle and Upper Ordovician black shale (Utica and Antes shales) in the Appalachian basin. Moreover, we suggest that approximately 300 MMBO and many trillions of cubic feet of gas in the Lower Silurian Clinton sands of eastern Ohio originated in these same source rocks. Oils from the Cambrian and Ordovician reservoirs have similar saturated hydrocarbon compositions, biomarker distributions, and carbon isotope signatures. Regional variations in the oils are attributed to differences in thermal maturation rather than to differences in source. Total organic carbon content, genetic potential, regional extent, and bitumen extract geochemistry identify the black shale of the Utica and Antes shales as the most plausible source of the oils. Other Cambrian and Ordovician shale and carbonate units, such as the Wells Creek formation, which rests on the Knox unconformity, and the Rome Formation and Conasauga Group in the Rome trough, are considered to be only local petroleum sources. T max , CAI, and pyrolysis yields from drill-hole cuttings and core indicate that the Utica Shale in eastern and central Ohio is mature with respect to oil generation. Burial, thermal, and hydrocarbon-generation history models suggest that much of the oil was generated from the Utica-Antes source in the late Paleozoic during the Alleghanian orogeny. A pervasive fracture network controlled by basement tectonics aided in the distribution of oil from the source to the trap. This fracture network permitted oil to move laterally and stratigraphically downsection through eastward-dipping, impermeable carbonate sequences to carrier zones such as the Middle Ordovician Knox unconformity, and to reservoirs such as porous dolomite in the Middle Ordovician Trenton Limestone in the Lima-Indiana field. Some of the oil and gas from the Utica-Antes source escaped vertically through a partially fractured, leaky Upper Ordovician shale seal into widespread Lower Silurian sandstone reservoirs.

Ohio, Ontario↗

USGS2015

Detailed description of oil shale organic and mineralogical heterogeneity via fourier transform infrared mircoscopy

Mineralogical and geochemical information on reservoir and source rocks is necessary to assess and produce from petroleum systems. The standard methods in the petroleum industry for obtaining these properties are bulk measurements on homogenized, generally crushed, and pulverized rock samples and can take from hours to days to perform. New methods using Fourier transform infrared (FTIR) spectroscopy have been developed to more rapidly obtain information on mineralogy and geochemistry. However, these methods are also typically performed on bulk, homogenized samples. We present a new approach to rock sample characterization incorporating multivariate analysis and FTIR microscopy to provide non-destructive, spatially resolved mineralogy and geochemistry on whole rock samples. We are able to predict bulk mineralogy and organic carbon content within the same margin of error as standard characterization techniques, including X-ray diffraction (XRD) and total organic carbon (TOC) analysis. Validation of the method was performed using two oil shale samples from the Green River Formation in the Piceance Basin with differing sedimentary structures. One sample represents laminated Green River oil shales, and the other is representative of oil shale breccia. The FTIR microscopy results on the oil shales agree with XRD and LECO TOC data from the homogenized samples but also give additional detail regarding sample heterogeneity by providing information on the distribution of mineral phases and organic content. While measurements for this study were performed on oil shales, the method could also be applied to other geological samples, such as other mudrocks, complex carbonates, and soils.

Energy & Fuels↗

USGS1994

Seawater as the source of minor elements in black shales, phosphorites and other sedimentary rocks

Many of the minor elements in seawater today have a concentration-depth profile similar to that of the biologically essential nutrients, NO-3 and PO3-4. They show a relative depletion in the photic zone and enrichment in the deep ocean. The difference between their surface- and deep-ocean values, normalized to the change in PO3-4, approaches the average of measured minor-element: P ratios in marine plankton, although individual analyses of the latter show extreme scatter for a variety of reasons. Despite this scatter in the minor-element analyses of plankton, agreement between the two sets of data shows unequivocally that an important marine flux of many minor elements through the ocean is in the form of biogenic matter, with a composition approaching that of plankton. This interpretation is further supported by sediment studies, particularly of sediments which accumulate in shelf-slope environments where biological productivity in the photic zone is exceptionally high and organic carbon contents of the underlying sediment elevated. The interelement relations observed for some of these sediments approach the average values of plankton. These same interelement relations are observed in many marine sedimentary rocks such as metalliferous black shales and phosphorites, rocks which have a high content of marine fractions (e.g., organic matter, apatite, biogenic silica and carbonates). Many previous studies of the geochemistry of these rocks have concluded that local hydrothermal activity, and/or seawater with an elemental content different from that of the modern ocean, was required to account for their minor-element contents. However, the similarity in several of the minor-element ratios in many of these formations to minor-element ratios in modern plankton demonstrates that these sedimentary rocks accumulated in environments whose marine chemistry was virtually identical to that seen on continental shelf-slopes, or in marginal seas, of the ocean today. The accumulation of the marine fraction of minor elements on these ancient sea floors was determined largely by the accumulation of organic matter, settling from the photic zone and with a composition of average plankton. A second marine fraction of minor elements in these rocks accumulated through precipitation and adsorption from seawater. The suite of elements in this fraction reflects redox conditions in the bottom water, as determined by bacterial respiration. For example, high Mn, high Cr+V and high Mo concentrations, above those which can be attributed to the accumulation of planktonic matter, characterize accumulation under bottom-water oxidizing, denitrifying and sulfate-reducing conditions, respectively. ?? 1994.

Chemical Geology↗

USGS2006

Unusual Holocene and late Pleistocene carbonate sedimentation in Bear Lake, Utah and Idaho, USA

Bear Lake (Utah-Idaho, USA) has been producing large quantities of carbonate minerals of varying mineralogy for the past 17,000 years. The history of sedimentation in Bear Lake is documented through the study of isotopic ratios of oxygen, carbon, and strontium, percent organic carbon, percent CaCO3, X-ray diffraction mineralogy, HCl-leach inorganic geochemistry, and magnetic properties on samples from three piston cores. Historically, the Bear River, the main source of water for Great Salt Lake, did not enter Bear Lake until it was artificially diverted into the lake at the beginning of the 20th century. However, during the last glacial interval, the Bear River did enter Bear Lake depositing red, calcareous, silty clay. About 18,000 years ago, the Bear River became disconnected from Bear Lake. A combination of warmer water, increased evaporation, and increased organic productivity triggered the precipitation of calcium carbonate, first as calcite. As the salinity of the lake increased due to evaporation, aragonite began to precipitate about 11,000 years ago. Aragonite is the dominant mineral that accumulated in bottom sediments of the lake during the Holocene, comprising an average of about 70 wt.% of the sediments. Aragonite formation in a large, cold, oligotrophic, high latitude lake is highly unusual. Lacustrine aragonite usually is found in small, saline lakes in which the salinity varies considerably over time. However, Bear Lake contains endemic ostracodes and fish, which indicate that the chemistry of the lake has remained fairly constant for a long time. Stable isotope data from Holocene aragonite show that the salinity of Bear Lake increased throughout the Holocene, but never reached highly evolved values of ??18O in spite of an evaporation-dominated water balance. Bear Lake hydrology combined with evaporation created an unusual situation that produced large amounts of aragonite, but no evaporite minerals.

Sedimentary Geology↗

USGS1994

The geochemical evolution of low-molecular-weight organic acids derived from the degradation of petroleum contaminants in groundwater

The geochemical evolution of low-molecular-weight organic acids in groundwater downgradient from a crude-oil spill near Bemidji, Minnesota, was studied over a five year period (1986–1990). The organic acids are metabolic intermediates of the degradation of components of the crude oil and are structurally related to hydrocarbon precursors. The concentrations of organic acids, particularly aliphatic acids, increase as the microbial alteration of hydrocarbons progresses. The organic-acid pool changes in composition and concentration over time and in space as the degradation processes shift from Fe(III) reduction to methanogenesis. Over time, the aquifer system evolves into one in which the groundwater contains more oxidized products of hydrocarbon degradation and the reduced forms of iron, manganese, and nitrogen. Laboratory microcosm experiments with aquifer material support the hypothesis that organic acids observed in the groundwater originate from the microbial degradation of aromatic hydrocarbons under anoxic conditions. The geochemistry of two other shallow aquifers in coastal plain sediments, one contaminated with creosote waste and the other with gasoline, were compared to the Bemidji site. The geochemical evolution of the low-molecular-weight organic acid pool in these systems is controlled, in part, by the presence of electron acceptors available for microbially mediated electron-transfer reactions. The depletion of electron acceptors in aquifers leads to the accumulation of aliphatic organic acids in anoxic groundwater.

Geochimica et Cosmochimica Acta↗

USGS1998

Influence of an igneous intrusion on the inorganic geochemistry of a bituminous coal from Pitkin County, Colorado

Although the effects of igneous dikes on the organic matter in coal have been observed at many localities there is virtually no information on the effects of the intrusions of the inorganic constituents in the coal. Such a study may help to elucidate the behavior of trace elements during in situ gasification of coal and may provide insights into the resources potential for coal and coke affected by the intrusion. To determine the effects of an igneous intrusion on the inorganic chemistry of a coal we used a series of 11 samples of coal and natural coke that had been collected at intervals from 3 to 106 cm from a dike that intruded the bituminous Dutch Creek coal in Pitkin, CO. The samples were chemically analyzed for 66 elements. SEM-EDX and X-ray diffraction analysis were performed on selected samples. Volatile elements such as F, Cl, Hg, and Se are not depleted in the samples (coke and coal) nearest the dike that were exposed to the highest temperatures. Their presence in these samples is likely due to secondary enrichment following volatilization of the elements inherent in the coal. Equilibration with ground water may account for the uniform distribution of Na, B, and Cl. High concentrations of Ca, Mg, Fe, Mn, Sr, and CO2 in the coke region are attributed to the reaction of CO and CO2 generated during the coking of the coal with fluids from the intrusion, resulting in the precipitation of carbonates. Similarly, precipitation of sulfide minerals in the coke zone may account for the relatively high concentrations of Ag, Hg, Cu, Zn, and Fe. Most elements are concentrated at the juncture of the fluidized coke and the thermally metamorphosed coal. Many of the elements enriched in this region (for example, Ga, Ge, Mo, Rb, U, La, Ce, Al, K, and Si) may have been adsorbed on either the clays or the organic matter or on both.Although the effects of igneous dikes on the organic matter in coal have been observed at many localities there is virtually no information on the effects of the intrusions on the inorganic constituents in the coal. Such a study may help to elucidate the behavior of trace elements during in situ gasification of coal and may provide insights into the resource potential of coal and coke affected by the intrusion. To determine the effects of an igneous intrusion on the inorganic chemistry of a coal we used a series of 11 samples of coal and natural coke that had been collected at intervals from 3 to 106 cm from a dike that intruded the bituminous Dutch Creek coal in Pitkin, CO. The samples were chemically analyzed for 66 elements. SEM-EDX and X-ray diffraction analysis were performed on selected samples. Volatile elements such as F, Cl, Hg, and Se are not depleted in the samples (coke and coal) nearest the dike that were exposed to the highest temperatures. Their presence in these samples is likely due to secondary enrichment following volatilization of the elements inherent in the coal. Equilibration with ground water may account for the uniform distribution of Na, B, and Cl. High concentrations of Ca, Mg, Fe, Mn, Sr, and CO2 in the coke region are attributed to the reaction of CO and CO2 generated during the coking of the coal with fluids from the intrusion, resulting in the precipitation of carbonates. Similarly, precipitation of sulfide minerals in the coke zone may account for the relatively high concentrations of Ag, Hg, Cu, Zn, and Fe. Most elements are concentrated at the juncture of the fluidized coke and the thermally metamorphosed coal. Many of the elements enriched in this region (for example, Ga, Ge, Mo, Rb, U, La, Ce, Al, K, and Si) may have been adsorbed on either the clays or the organic matter or on both.

International Journal of Coal Geology↗

USGS2010

Geochemistry of Eagle Ford group source rocks and oils from the first shot field area, Texas

Total organic carbon, Rock-Eval pyrolysis, and vitrinite reflectance analyses performed on Eagle Ford Group core and cuttings samples from the First Shot field area, Texas demonstrate these samples have sufficient quantity, quality, and maturity of organic matter to have generated oil. Furthermore, gas chromatography and biomarker analyses performed on Eagle Ford Group oils and source rock extracts as well as weight percent sulfur analyses on the oils indicate the source rock facies for most of the oils are fairly similar. Specifically, these source rock facies vary in lithology from shales to marls, contain elevated levels of sulfur, and were deposited in a marine environment under anoxic conditions. It is these First Shot Eagle Ford source facies that have generated the oils in the First Shot Field. However, in contrast to the generally similar source rock facies and organic matter, maturity varies from early oil window to late oil window in the study area, and these maturity variations have a pronounced effect on both the source rock and oil characteristics. Finally, most of the oils appear to have been generated locally and have not experienced long distance migration.

Texas↗

USGS2009

Weathering of the New Albany Shale, Kentucky, USA: I. Weathering zones defined by mineralogy and major-element composition

Comprehensive understanding of chemical and mineralogical changes induced by weathering is valuable information when considering the supply of nutrients and toxic elements from rocks. Here minerals that release and fix major elements during progressive weathering of a bed of Devonian New Albany Shale in eastern Kentucky are documented. Samples were collected from unweathered core (parent shale) and across an outcrop excavated into a hillside 40 year prior to sampling. Quantitative X-ray diffraction mineralogical data record progressive shale alteration across the outcrop. Mineral compositional changes reflect subtle alteration processes such as incongruent dissolution and cation exchange. Altered primary minerals include K-feldspars, plagioclase, calcite, pyrite, and chlorite. Secondary minerals include jarosite, gypsum, goethite, amorphous Fe(III) oxides and Fe(II)-Al sulfate salt (efflorescence). The mineralogy in weathered shale defines four weathered intervals on the outcrop-Zones A-C and soil. Alteration of the weakly weathered shale (Zone A) is attributed to the 40-a exposure of the shale. In this zone, pyrite oxidization produces acid that dissolves calcite and attacks chlorite, forming gypsum, jarosite, and minor efflorescent salt. The pre-excavation, active weathering front (Zone B) is where complete pyrite oxidation and alteration of feldspar and organic matter result in increased permeability. Acidic weathering solutions seep through the permeable shale and evaporate on the surface forming abundant efflorescent salt, jarosite and minor goethite. Intensely weathered shale (Zone C) is depleted in feldspars, chlorite, gypsum, jarosite and efflorescent salts, but has retained much of its primary quartz, illite and illite-smectite. Goethite and amorphous FE(III) oxides increase due to hydrolysis of jarosite. Enhanced permeability in this zone is due to a 14% loss of the original mass in parent shale. Denudation rates suggest that characteristics of Zone C were acquired over 1 Ma. Compositional differences between soil and Zone C are largely attributed to illuvial processes, formation of additional Fe(III) oxides and incorporation of modern organic matter.

Applied Geochemistry↗

USGS2009

Flow and geochemistry of groundwater beneath a back-barrier lagoon: The subterranean estuary at Chincoteague Bay, Maryland, USA

To better understand large-scale interactions between fresh and saline groundwater beneath an Atlantic coastal estuary, an offshore drilling and sampling study was performed in a large barrier-bounded lagoon, Chincoteague Bay, Maryland, USA. Groundwater that was significantly fresher than overlying bay water was found in shallow plumes up to 8 m thick extending more than 1700 m offshore. Groundwater saltier than bay surface water was found locally beneath the lagoon and the barrier island, indicating recharge by saline water concentrated by evaporation prior to infiltration. Steep salinity and nutrient gradients occur within a few meters of the sediment surface in most locations studied, with buried peats and estuarine muds acting as confining units. Groundwater ages were generally more than 50 years in both fresh and brackish waters as deep as 23 m below the bay bottom. Water chemistry and isotopic data indicate that freshened plumes beneath the estuary are mixtures of water originally recharged on land and varying amounts of estuarine surface water that circulated through the bay floor, possibly at some distance from the sampling location. Ammonium is the dominant fixed nitrogen species in saline groundwater beneath the estuary at the locations sampled. Isotopic and dissolved-gas data from one location indicate that denitrification within the subsurface flow system removed terrestrial nitrate from fresh groundwater prior to discharge along the western side of the estuary. Similar situations, with one or more shallow semi-confined flow systems where groundwater geochemistry is strongly influenced by circulation of surface estuary water through organic-rich sediments, may be common on the Atlantic margin and elsewhere.

Marine Chemistry↗

USGS2010

Geochemical constraints on the origin and volume of gas in the New Albany Shale (Devonian-Mississippian), eastern Illinois Basin

This study involved analyses of kerogen petrography, gas desorption, geochemistry, microporosity, and mesoporosity of the New Albany Shale (Devonian-Mississippian) in the eastern part of the Illinois Basin. Specifically, detailed core analysis from two locations, one in Owen County, Indiana, and one in Pike County, Indiana, has been conducted. The gas content in the locations studied was primarily dependent on total organic carbon content and the micropore volume of the shales. Gas origin was assessed using stable isotope geochemistry. Measured and modeled vitrinite reflectance values were compared. Depth of burial and formation water salinity dictated different dominant origins of the gas in place in the two locations studied in detail. The shallower Owen County location (415-433 m [1362-1421 ft] deep) contained significant additions of microbial methane, whereas the Pike County location (832-860 m [2730-2822 ft] deep) was characterized exclusively by thermogenic gas. Despite differences in the gas origin, the total gas in both locations was similar, reaching up to 2.1 cm3/g (66 scf/ton). Lower thermogenic gas content in the shallower location (lower maturity and higher loss of gas related to uplift and leakage via relaxed fractures) was compensated for by the additional generation of microbial methane, which was stimulated by an influx of glacial melt water, inducing brine dilution and microbial inoculation. The characteristics of the shale of the Maquoketa Group (Ordovician) in the Pike County location are briefly discussed to provide a comparison to the New Albany Shale. Copyright ??2010. The American Association of Petroleum Geologists. All rights reserved.

American Association of Petroleum Geologists Bulle↗

USGS2020

Compositional analysis of formation water geochemistry and microbiology of commercial and carbon dioxide-rich wells in the southwestern United States

Studies of naturally occurring subsurface carbon dioxide (CO 2 ) accumulations can provide useful information for potential CO 2 injection projects; however, the microbial communities and formation water geochemistry of most reservoirs are understudied. Formation water and microbial biomass were sampled at four CO 2 -rich reservoir sites: two within Bravo Dome, a commercial CO 2 field in New Mexico; one northwest of Bravo Dome in Colorado (Oakdale Field); and one southwest of Bravo Dome in New Mexico (Rafter “K” Ranch). Aside from the Rafter “K” Ranch site, minor differences were observed in the geochemistry of formation water collected from these sites compared to historical data. No organisms were significantly associated with Oakdale Field compared to the other three sites, nor were any hydrogeochemical or gas geochemical parameters (for example, CO 2 concentration) found to have significant associations with the microbial ecology of these four sites. Microorganisms from these sites were metabolically diverse and had the potential to (1) generate methane, (2) produce corrosive hydrogen sulfide (H 2 S), and (3) rapidly biofoul and (or) clog pore spaces by shifting microbial communities with changes in salinity or nutrient supply. This study demonstrates that high concentrations of CO 2 in subsurface reservoirs apparently have not imparted a distinct geochemical or microbiological signature on the associated formation waters and that the microorganisms in these reservoirs are metabolically diverse and could adapt to geochemical changes in the subsurface.

Colorado, New Mexico, Texas, Oklahoma↗

USGS2000

Chronology and geochemistry of late Holocene eolian deposits in the Brandon Sand Hills, Manitoba, Canada

Accelerator mass spectrometry and conventional radiocarbon age determinations of organic matter from paleosols indicate that the Brandon Sand Hills area of southern Manitoba has been subjected to recurrent intervals of eolian activity in the past 5000 years. Although precise regional correlations are precluded by dating uncertainties, periods of most notable paleosol development occurred around 2300 to 2000, 1400 to 1000, and 600 to 500 cal yr BP with eolian activity occurring before and after each of these periods. Episodes of eolian activity may correspond to periods of regional drought, whereas paleosols mark periods of increased moisture availability and stabilization by vegetation. The geochemistry of the eolian sands, paleosols and source sediments indicates that partial leaching of carbonates occurs from pedogenesis during humid climatic phases, and that this is probably the primary mechanism of carbonate depletion of eolian sands in this area. Recent trends in sand dune activity from historic aerial photography and early explorers' accounts indicate that the few active dunes that presently exist have stabilized at a rate of 10-20% per decade, despite several severe droughts in the 20th century. This may be attributed to pre-settlement droughts that were more severe than those in historic times although regional dune stabilization may also be related, in part, to the spread of forest cover in the past few hundred years. Crown copyright (C) 2000 Published by Elsevier Science Ltd. All rights reserved.

Quaternary International↗

USGS1996

Geohydrology and geochemistry near coastal ground-water-discharge areas of the Eastern Shore, Virginia

Local and regional patterns in the organic content of sediments in the surficial aquifer, as reflected in topography and land use, control dissolved oxygen and nitrate concentrations in ground water that recharged through agricultural fields and flowed beneath riparian woodlands. Dissolved oxygen and nitrate concentrations decreased beneath the woodlands as a result of changes in the organic content of the sediments that resulted from deposition of the sediments, not the current presence of riparian woodlands.

Water Supply Paper↗

USGS2025

Sulfur-to-iron ratio as a proxy for degree of organic sulfurization

The degree of organic sulfurization is broadly relevant yet underreported. We present a statistically significant correlation between whole rock S/Fe and the measured degree of organic sulfurization in the thermally immature Cenomanian–Turonian Eagle Ford Group. This relationship shows a sink switch for sulfur from pyrite to organic matter. Excess iron and excess sulfur relative to pyrite, which are mathematically related to S/Fe, provide better insights into organic sulfurization than previous approaches that calculate excess iron relative to detrital iron based on aluminum concentrations. Organic sulfurization and S/Fe are tightly coupled in the Eagle Ford partially due to limited sulfur- and iron-bearing components. Similar relationships could exist in other thermally immature, organic-rich, anoxia-prone, calcareous mudstones. The degree of organic sulfurization was estimated from S/Fe, which was used to map stratigraphic and regional variations of organic sulfurization across the Eagle Ford and to investigate how organic sulfurization relates to organic enrichment, organic preservation, and depositional redox chemistry. The extent of organic sulfurization is more tightly linked to organic preservation than enrichment. Together, organic sulfurization and Mo provide concordant evidence for depositional euxinia. The relationship between Mo and degree of organic sulfurization could indicate that sulfurized organic matter provides a pathway for Mo enrichment, but future work needs to disentangle direct mechanisms from indirect covariations between Mo, organic content, and degree of organic sulfurization. Whole rock elemental chemistry and programmed pyrolysis provide insights into organic sulfurization variations that can be upscaled and can guide subsequent detailed organic sulfur analyses.

Arkansas, Louisiana, Oklahoma, Texas↗

USGS2016

Environmental drivers of differences in microbial community structure in crude oil reservoirs across a methanogenic gradient

Stimulating in situ microbial communities in oil reservoirs to produce natural gas is a potentially viable strategy for recovering additional fossil fuel resources following traditional recovery operations. Little is known about what geochemical parameters drive microbial population dynamics in biodegraded, methanogenic oil reservoirs. We investigated if microbial community structure was significantly impacted by the extent of crude oil biodegradation, extent of biogenic methane production, and formation water chemistry. Twenty-two oil production wells from north central Louisiana, USA, were sampled for analysis of microbial community structure and fluid geochemistry. Archaea were the dominant microbial community in the majority of the wells sampled. Methanogens, including hydrogenotrophic and methylotrophic organisms, were numerically dominant in every well, accounting for, on average, over 98% of the total Archaea present. The dominant Bacteria groups were Pseudomonas, Acinetobacter , Enterobacteriaceae, and Clostridiales, which have also been identified in other microbially-altered oil reservoirs. Comparing microbial community structure to fluid (gas, water, and oil) geochemistry revealed that the relative extent of biodegradation, salinity, and spatial location were the major drivers of microbial diversity. Archaeal relative abundance was independent of the extent of methanogenesis, but closely correlated to the extent of crude oil biodegradation; therefore, microbial community structure is likely not a good sole predictor of methanogenic activity, but may predict the extent of crude oil biodegradation. However, when the shallow, highly biodegraded, low salinity wells were excluded from the statistical analysis, no environmental parameters could explain the differences in microbial community structure. This suggests that the microbial community structure of the 5 shallow, up-dip wells was different than the 17 deeper, down-dip wells. Also, the 17 down-dip wells had statistically similar microbial communities despite significant changes in environmental parameters between oil fields. Together, this implies that no single microbial population is a reliable indicator of a reservoir's ability to degrade crude oil to methane, and that geochemistry may be a more important indicator for selecting a reservoir suitable for microbial enhancement of natural gas generation.

Louisiana↗

USGS1989

Can we determine the biological availability of sediment-bound trace elements?

It is clear from available data that the susceptibility of biological communities to trace element contamination differs among aquatic environments. One important reason is that the bioavailability of metals in sediments appears to be altered by variations in sediment geochemistry. However, methods for explaining or predicting the effect of sediment geochemistry upon metal bioavailability are poorly developed. Experimental studies demonstrate that ingestion of sediments and uptake from solution may both be important pathways of metal bioaccumulation in deposit/detritus feeding species. Relative importance between the two is geochemistry dependent. Geochemical characteristics of sediments also affect metal concentrations in the tissues of organisms collected from nature, but the specific mechanisms by which these characteristics influence metal bioavailability have not been rigorously demonstrated. Several prerequisites are necessary to better understand the processes that control metal bioavailability from sediments. 1) improved computational or analytical methods for analyzing distribution of metals among components of the sediments; 2) improved computational methods for assessing the influences of metal form in sediments on sediment-water metal exchange; and 3) a better understanding of the processes controlling bioaccumulation of metals from solution and food by metazoan species directly exposed to the sediments. Such capabilities would allow mechanistic explanations essential to the development of practical tools sought for determining sediment quality criteria for metals.

Hydrobiologia↗

USGS2021

Effect of an algal amendment on the microbial conversion of coal to methane at different sulfate concentrations from the Powder River Basin, USA

Biogenic methane is estimated to account for one-fifth of the natural gas worldwide and there is great interest in controlling methane from different sources. Biogenic coalbed methane (CBM) production relies on syntrophic associations between fermentative bacteria and methanogenic archaea to anaerobically degrade recalcitrant coal and produce methanogenic substrates. However, very little is known about how differences in geochemistry, hydrology, and microbial community composition influence subsurface carbon utilization and CBM production. The addition of an amendment consisting of microalgal biomass has previously been shown to increase CBM production while providing the possibility of a closed-loop fossil system where waste (production water) is used to grow algae to ultimately produce energy (methane). However, the efficiency of enhancing CBM production under different redox conditions remains unresolved. In this study, we focused on the U.S. Geological Survey's Birney test site (Montana, USA) that has nine wells vertically accessing four coal seams with varying geochemistry (low and high sulfate (SO 4 2− )) and methane production rates. We used organic matter (OM) in the form of algal biomass to discern the effect of this amendment on OM degradation and microbially enhanced CBM production potential under different geochemical constraints. We tracked changes in community composition, OM composition, organic carbon (OC) concentration, methane production, and nutrients in batch systems over six months. Methane production was detected only in microcosms from low SO 4 2− wells (168 to 800 μg methane per gram of coal). The OC consumption varied across time for all wells and the variation was greatest for the low SO 4 2− wells. Different groups of syntrophic bacteria were associated with net‑carbon consuming microcosms, and specifically Syntrophorhabdus was identified with several different statistical methods as a potentially important coal degrader. Results from this study provide insight into potential coal-degraders, the compositional changes in some of the different OM fractions, and trends in carbon consumption related to methane production across coal seams along the vertical SO 4 2− gradient.

Montana, Wyoming↗