Organic-rich Black Shales Research Articles

Geochemistry of Organic-Rich Black Shales Overlying the Natural Nuclear Fission Reactors of Oklo, Republic of Gabon

The organic-rich black shales of the Franceville Series' FB Formation overlying the uranium ores and natural nuclear fission reactors of Ohio, Gabon, are not notably metalliferous. Chromium, gold, silver, and barium are slightly enriched in average Ohio black shale (AOK) relative to black shale standard SDO-1. Geochemical variations among the black shale samples of the sedimentary sequence include enrichment in potassium, barium, chromium, and silver in the four lowermost samples, the presence of a bleached zone depleted in organic carbon lowermost in the sequence, and elevated rare earth element (REE) content in samples closest to the Oklo reactor zones. Hydrothermal activity has influenced the geochemistry of the black shale but is evidently not linked to reactor-driven processes. Chondrite-normalized REE patterns of Oklo black shale samples show slight enrichment in light REE and slight depletion in heavy REE, especially in the sample closest to the reactor zone. However, comparison of REE content with various petrographic facies in and near the Oklo reactors shows no apparent enrichment in fission product (intermediate) REE. With few exceptions, reactor facies all contain more REE than AOK The chondrite-normalized REE pattern of AOK resembles that of greywacke-shale turbidites of Archean greenstone belts. The paucity of uranium and manganese in AOK is a curious anomaly in an area of world class uranium and manganese deposits.

Rodinia to Gondwana: the geodynamic map of Gondwana supercontinent assembly : Unrug R., GSA Today, 1997, 7/1 (1–5)

A shallow borehole was drilled in lower Silurian (Rhuddanian–lower Aeronian) Tanezzuft Formation siliciclastic sediments at the eastern margin of the Murzuq Basin. Shale samples were retrieved from the borehole to study their source rock potential and trace element geochemical composition. Thirty shale samples analysed from borehole CDEG-2a reveal three geochemically distinct intervals. Shales from the top (23–38 m) interval are weathered and have negligible organic content. Shales from the middle (38–42 m) interval have moderate to good organic richness (0.6–1.7 wt.% TOC) with Hydrogen Index (HI) values up to 443 mg S2/g TOC. Shales from the bottom (42–51 m) interval have poor to moderate organic richness (0.07–0.8 wt.% TOC) with HI values up to 277 mg S2/g TOC. All samples have limited source rock potential and are immature to early mature for hydrocarbon generation. It is unlikely that they have ever been buried deeper than ∼2 km. Low TOC values (<2 wt.%) together with low concentrations of uranium (<10 ppm) indicate the absence of organic-rich black shale (‘hot shale’) source rocks in the studied interval.Inorganic geochemical redox proxies (Ce-anomaly, authigenic U, Th/U, V/Cr, Ni/Co, and V/Sc) indicate that the lower Silurian Tanezzuft Formation shales from borehole CDEG-2a were deposited under oxic bottom waters, explaining their relatively low TOC values. Deposition took place in a shallow marine environment, in a proximal position with regard to an early Silurian palaeo-shoreline. This combined Rock-Eval pyrolysis and whole-rock geochemical study provides new insights into the Silurian source rock distribution and hydrocarbon prospectivity in central North Africa.

Polycyclic Aromatic Hydrocarbons concentrated in organic rich black shale around Permian/Triassic boundary from Mino belt

Polycyclic Aromatic Hydrocarbons concentrated in organic rich black shale around Permian/Triassic boundary from Mino belt

Kerogen chemical structure and source organisms in a Cenomanian organic-rich black shale (Central Italy) — Indications for an important role of the “sorptive protection” pathway

The Bonarelli horizon is an important Cenomanian/Turonian black shale in the Umbria-Marche Appenines of Central Italy. It appears as the final term of a cyclic deposition of ca. 40 thinner, laminated, clayey or siliceous organic-rich levels. The kerogen of the Cenomanian clayey black level located 12 m below the Bonarelli, termed NN 8c, was examined via microscopic, spectroscopic and pyrolytic methods. The chemical structure of this kerogen is based on a network of long aliphatic chains linked via ether bridges. However, in addition to the classical normal chains, NN 8c comprises C 40 chains with a lycopane skeleton. Such chains have only been previously reported in kerogen pyrolysates of two samples derived from the microalga Botryococcus braunii. However, any contribution of B. braunii can be ruled out in the case of NN 8c kerogen since it is nanoscopically amorphous. Based on δ 13C values of their pyrolysis products, a common algal origin is demonstrated for the above two types of chains and hence there is a major algal input to this kerogen. However, some incorporation of bacterial lipids and a low contribution of higher plant material are indicated by the nature of some pyrolysis products. Although a large contribution of pyrite is noted in the NN 8c sample, thus revealing an intense sulphate-reducing activity, no significant sulphur incorporation took place in the organic matter. Accordingly, the vulcanization pathway is not implicated in the formation of NN 8c kerogen. The selective preservation and the degradation-recondensation pathways were possibly involved but they are unlikely to have played a major role. In fact, the “sorptive protection” process, favoured by the clayey nature of the mineral matrix, was probably the main pathway implicated in the formation of NN 8c kerogen and this process may have provided an efficient protection to various diagenetically sensitive constituents, especially lipids.

Formation of the Muzo hydrothermal emerald deposit in Colombia

FOR over 1,000 years, the emerald deposits of Colombia have been the principal source of the world's largest and finest gem-quality emeralds—a variety of beryl containing chromium and vanadium1. Whereas most emerald deposits are found in association with igneous host rocks1, the Colombian deposits occur in organic-rich black shales, and their origin in the absence of any evidence of igneous activity has been a persistent enigma. Here we present evidence from the Muzo mine (located about 100km from Bogota) that hydrothermal brines transported evaporitic sulphate to structurally favourable sites, where it was thermochemically reduced. We suggest that the sulphur generated by this process reacted with organic matter in the shales to release trapped chromium, vanadium and beryllium, which in turn enabled emerald formation.

Organic Geochemistry of Black Shale and Associated Oils of the Pennsylvanian Hermosa Group, Paradox Basin, Utah and Colorado

According to recent estimates, the Paradox basin may contain more than 700 million bbl and 1.3 tcf of producible oil and gas, respectively. Newly developed horizontal drilling methods may increase the potential for oil produced from fractured, organic-rich shale in the Paradox Formation of the Hermosa Group. The purpose of this study was to characterize the organic matter contained in the black shale, interbedded lithologies, and oils produced from associated reservoir rocks. Total organic carbon (TOC) content of the rocks is from less than 0.2 wt.% in evaporites to as much as about 25 wt% in some of the black shales. TOC content generally increases in the order of evaporites < siltstones < carbonates < black shale. Only the carbonate rock and black shale contain sufficient TOC to have generated and expelled petroleum. Varying proportions of organic matter types I, II, and III are present in the organic-rich black shales according to Rock-Eval pyrolysis (hydrogen indexes from [sup [minus]]00 to less than 100 mg/g), visual kerogen analyses (amorphous to structure kerogen), and carbon isotope ratios ([minus]23 to [minus]27% PDB). Analyses of 23 oils from the major producing intervals in the basin indicate variable maturity according to molecular distributions of aromaticmore » hydrocarbons and organosulfur compounds. Source-related differences also are evident among the oils. For example, Cane Creek oils have relatively lower pristane/phytane and tricyclic pentacyclic terpane ratios than Desert Creek and Barker Creek oils. Understanding the distribution and maturity of the organic source facies, coupled with identification of oil-source rock correlations, should help define new areas for exploration in the basin.« less

Gas Potential of New Albany Shale (Devonian-Mississippian) in the Illinois Basin

A study to update and evaluate publicly available data relating to present and potential gas production from New Albany Shale in the Illinois basin was conducted cooperatively by the Indiana. Illinois, and Kentucky geological surveys (Illinois Basin Consortium), and was partially funded by the Gas Research Institute. Deliverables included a plate of stratigraphic cross sections and six basin-wide maps at a scale of 1:1,000,000. The New Albany Shale is an organic-rich brownish black shale present throughout the Illinois basin. Gas potential of the New Albany Shale may be great because it contains an estimated 86 tcf of natural gas and has produced modest volumes since 1858 from more than 60 fields, mostly in the southeastern part of the basin. Reservoir beds include organic-rich shales of the Grassy Creek (Shale), Clegg Creek, and Blocher (Shale) members. Limited geologic and carbon isotope data indicate that the gas is indigenous and thermogenic. T[sub max] data suggest that the gas generation begins at R[sub o] values of 0.53% and may begin at R[sub 0] values as low as 0.41% in some beds. New Albany Shale reservoirs contain both free gas in open-pore space and gas adsorbed on clay and kerogen surfaces. Natural fracturing ismore » essential for effective reservoir permeability. Fractures are most common near structures such as faults, flexures, and buried carbonate banks. Based on limited data, fractures and joints have preferred orientations of 45-225[degrees] and 135-315[degrees]. Commercial production requires well stimulation to connect the well bore with the natural fracture system and to prop open pressure-sensitive near-borehole fractures. Current stimulations employ hydraulic fracture treatments using nitrogen and foam, with sand as a propping agent.« less

Controls on the origin of the Devonian-Mississippian oil and gas shales, east-central United States

An abundance of organic matter, development of bottom anoxia in a stratified water column, and absence of major clastic dilution are the principal factors controlling the accumulation of organic-rich black shales. During the Devonian and Early Mississippian, such shales became abundant across much of east-central United States, apparently controlled by a unique coincidence of paleoclimatic, paleogeographic and tectonic factors. Like most major black-shale depositional episodes, Devonian-Mississippian deposits were formed during a time of global transgressive and greenhouse states. During this time, the east-central United States was located in the subtropical trade-wind belt where marine and terrestrial organic productivity would have been high and where surface waters would rarely have become cool enough to overturn and disrupt the water stratification necessary to generate bottom anoxia. Location in the trade-wind belt may also have enhanced upwelling which would have increased organic productivity and the likelihood of anoxia. Perhaps most importantly, however, black-shale deposition coincided with early phases of tectonism, when, because of rapid deformational loading in the orogen, subsidence in the adjacent Appalachian foreland basin far outstripped sedimentation. In the resulting deep-water conditions, water stratification and accompanying anoxia were soon established. Because most of the deformational load at this stage was subaqueous, there was no major source of clastic dilution, so that clastic starvation prevailed and largely organic-rich sediments accumulated. Thus hydrocarbon source beds of broad extent like the Devonian-Mississippian black shales, are rarely attributable to a single cause. They typically result from a complex interplay of factors unique to a given time and place.

Mineralogy and geochemistry of the Proterozoic Wafangzi ferromanganese deposit, China

In the Proterozoic Wafangzi deposit, China, sedimentary manganite ores are interstratified with red shale and silty limestone, whereas sedimentary rhodochrosite ores are interbedded with organic-rich black shale. Lateral change in mineralogy of the ores accompanying lateral changes in the host rocks demonstrates a classic example of facies variation in a zoned deposit. The manganite ores show a distinct Ce depletion reflecting a hydrothermal contribution in the seawater from which they precipitated. Absence of volcanics in the sequence suggests that the source of the hydrothermal input should be remote. It is likely that manganite precipitated directly from seawater in these deposits above the redox interface, whereas rhodochrosite was possibly early diagenetic formed in an anoxic environment. Manganite has been converted to braunite through a deoxidation-dehydration reaction during late diagenesis. Both types of ores have been thermally metamorphosed at the contact with igneous intrusions. Four distinct mineral assemblages have been identified in the thermally metamorphosed ores which were produced as a result of variations in bulk compositions (including volatiles) of the premetamorphic ores. In the final stage, the ores have been affected by supergene processes to give rise to pyrolusite-vernadite assemblages in the weathering zone.

Surface Gamma-Ray Logs as a Correlation Tool Between Outcrop and Subsurface: Application to the Silurian-Devonian of West Texas and Southern New Mexico

Outcrop gamma-ray logs are an excellent tool for correlation between surface measured sections and subsurface well logs. The work presented here illustrates the utility of constructing such profiles and the applicability of this technique to carbonate sequences such as those of the Permian basin. Outcrop sections with gamma-ray profiles have been measured over the Silurian-Devonian section in three separate areas. These sections are located in the Sacramento Mountains of southern New Mexico, and the northern Franklin Mountains and Hueco Mountains of west Texas. A hand-held Scintrex Model BGS-4 Digital Scintillometer was used to measure surface gamma radiation while detailed stratigraphic sections were being measured and described. Data were collected at regularly spaced intervals along the section. The scintillometer detects natural radiation emitted by radioactive elements that occur in most clay minerals and generally are more abundant in shales than in sandstones or carbonates. The lithology of poorly exposed or covered units also may be inferred from surface gamma-ray profiles. Organic-rich black shales are particularly radioactive, as are condensed sections. The strength of this method does not lie in the absolute reading of gamma radiation. The value of this tool lies in recognizing patterns within each profile, directly relating these patternsmore » to their associated facies, and correlating them with subsurface profiles.« less

Benthic community analysis and the prediction of organic carbon content in Mesozoic black shale facies

The burial and preservation of significant quantities of marine organic matter in epicontinental basins results from low bottom water oxygen levels, and moderate to low sedimentation rates of fine-grained siliciclastic sediment under at least moderately productive waters. Because benthic biotas are extremely sensitive to such factors as bottom water oxygen, grain size, substrate consistency and sedimentation rate, there is a potentially predictive relationship between biofacies and Corg potential of marine strata. High-resolution biological, sedimentological and geochemical studies of Mesozoic organic-rich black shale facies, comprising major hydrocarbon source rocks, allow characterization of this relationship: a) A series of recurrent, bivalve-dominated benthic communities are defined based on trends in diversity, abundance, equitability and trophic specialization. They comprise biofacies which reflect oxygen and substrate gradients in time and space, and thus the Corg preservation potential of marine sediments/strata. The biotas are surprisingly robust in terms of population size and inferred biomass, and demonstrably colonized the benthic zone (as opposed to pseudoplanktic origins). They include both resident communities of taxa specifically adapted to low oxygen (dysoxic) conditions through physiological and anatomical means (e.g., expanded oxygen adsorptive surfaces), or possibly through bacterial chemosymbiosis, and event communities of less tolerant taxa reflecting short-term improvements in benthic conditions; b) many organic-rich sequences contain common event biofacies, indicating a dynamic benthic environment with frequent short-term fluctuations in oxygen and substrate. These are attributable to mixing during large storms, productivity blooms, rapid deposition from mass flows, or reduced sedimentation during condensation/bypass events. However, increasing frequency of events is not necessarily correlated with decrease in Corg levels; c) comparison of trends in the quantity and quality of Corg to biofacies patterns indicate that the most favorable Corg preservation is associated with “benthic boundary biofacies.” These biofacies represent a redox boundary at or near the sediment-water interface and a predominantly dysoxic to episodically oxic water column. The “benthic boundary” may have been associated with the presence of microbial (mainly sulphur bacterial) mats on the substrate surface, which could have regulated its position and contributed to the pool of sedimented Corg; d) mapping of benthic boundary biofacies in time and space within a basin makes possible correlation of Corg preservation potential to paleogeography and sea level history. Although the results of such analyses from the Greenhorn Formation, Western Interior basin, suggest highest Corg burial and preservation in fine-grained shale facies of distal offshore environments during sea level rise, high Corg is not necessarily associated with an anoxic water column. This suggests the need for reappraisal of stagnant basin scenarios for source rock formation. The most important factors in Corg accumulation and preservation may be the development of benthic boundaries and consequent maintenance of anoxia below the sediment-water interface in fine-grained, organic-rich substrates, and the contribution of bacteria to the flux of Corg into the sediment.

The Antrim Shale: Structural and Stratigraphic Influences on Gas Production

The Antrim Shale of the Michigan basin is one of the most actively drilled gas plays in the United States. Core analysis, geologic mapping, and core to log correlations of a 9 mi{sup 2} study area in the middle of the present play have defined geologic influences on the location and productivity of Antrim reservoirs. Application of these factors in the design of exploration and development strategies could improve gas recovery from the Antrim Shale. The lower section of the Antrim Shale, containing the present producing horizons, is composed of four lithologies that subdivide the Antrim into facies and parasequences based upon their mineralogy and textural characteristics. The black shales of the producing horizons are characterized by high but variable quartz contents and an extremely fine-grained matrix of muscovite and clays. The black shales are surrounded by two types of gray shale, differentiated by amount and form of carbonates, and a green shale. The type of shale bounding the productive, organic-rich black shales may affect stimulation strategies and their effectiveness. These black shales average 10% but can be as high as 20% TOC by weight. The organic contents impart a distinctive signature to gamma ray logs that enabled isopach, lithofacies,more » and structural mapping of the Antrim. Correlated with available production data, the maps reveal distinct trends suggesting that well performance is influenced by both structural and stratigraphic controls.« less

Controls of porphyrin concentrations of Pennsylvanian organic-rich shales, western U.S.A

Organic-rich black shales of Middle Pennsylvania (Desmoinesian) age occur over much of the central U.S. and as far west as the northern Denver and southeastern Powder River basins. Total organic carbon contents (C org ) are commonly greater than 10 wt %. Porphyrin concentrations (vanadyl + nickel) are as high as 40000 ppm relative to extractable bitumen. In bulk, the organic matter contained in the shales is mostly type II and III (Rock-Eval hydrogen indexes 200-400 mg of hydrocarbons/g of C org ). The finding of high porphyrin concentrations in type III organic matter is unusual but can be explained by a depositional model wherein high preservation of primary organic production (water column photosynthesis) is combined with substantial input of allochthonous organic matter. The allochthonous organic matter (low porphyrin concentration) may come from erosion during advance of the sea across the area or from fluvial transport from shore

Organic geochemistry of a lower jurassic synrift lacustrine sequence, Hartford Basin, Connecticut, U.S.A.

Synrift terrestrial strata of the Lower Jurassic East Berlin Formation (Hanford basin, Connecticut, U.S.A.) record cyclical expansion and contraction of major lakes, six of which were deep enough to develop anoxic bottom waters. We have studied one representative lacustrine sequence in detail, sampling a new roadcut near the village of East Berlin. The section examined is 4 m thick, with a gray silstone at the base, deposited in shallow water, overlain by an organic-rich black shale (deep water), succeeded in turn by another gray siltstone, deposited as the lake waters gradually receded. The upper gray siltstone is chemically distinct from the lower siltstone, as it contains small amounts of corrensite, analcime and gypsum, reflecting the increasing salinity and alkalinity of the contracting lake. The samples in the center of the black shale unit contain laminae of thermally-altered, yellowish orange-fluorescing, mottled telalginite. The fluorescence properties indicate a peak oil generation maturity level, confirmed by a vitrinite reflectance of 1.13% and a Methylphenanthrene Index of 1.08. The other samples have less organic matter, becoming increasingly lean towards the top and bottom of the sequence. Samples in the middle of the black shale unit are distinguished by the presence of an homologous series of tricyclic terpanes extending from C 20 to at least C 41, and by the near absence of hopanes and steranes. Moving upsection into the gray siltstone, the samples contain markedly lessextractable organic material (EOM) and the concentration of tricyclic terpanes relative to hopanes steadily decreases. In the uppermost sample, hopanes are the predominant terpanes. Moving downsection from the black shale into the lower gray siltstone, EOM and the ratio of tricyclic terpanes also decrease, except in the lowermost samples, which contain terpane distributions like those of the middle part of the black shale. This likely is migrated material, because bitumen fills microfractures and extensive megafractures. The lack of hopanes and steranes in the black shales cannot simply be a maturation effect, as these biomarkers appear in the adjacent beds. Instead, the manual terpane distributions may indicate a changing depositional environment, documenting the geochemical evolution of the lake. Or, more likely, they may result from fractionation during expulsion of petroleum from these mature source rocks.

Evolution of the San Jorge Basin, Argentina (1)

The San Jorge basin, although small, is the most important hydrocarbon-producing basin in Argentina. Remaining untested potential is high because of the presence of good source rock, favorable structural complexity, and multiple reservoirs. Reservoir quality is commonly low because of the highly tuffaceous sandstones. The sedimentary fill of the basin is closely related to its tectonic history. Northwest-southeast-trending grabens formed and filled during a Triassic and Early Jurassic early rift phase, climaxing with a pervasive Middle Jurassic volcanic episode; continued growth and filling of the basin occurred during a Late Jurassic-earliest Cretaceous late rift phase and Cretaceous early and late sag phases. Late Cretaceous-early Tertiary extension set up many of the present-day structural traps along normal faults. Middle Tertiary Andean compression produced the narrow, north-south San Bernardo structural belt, which exhibits reversed movement on older, normal, graben-bounding faults and on local, low-angle thrust faults. Marked early to middle Tertiary erosion produced a significant unconformi y within Cretaceous beds around basin margins. Origin of Upper Jurassic and lowermost Cretaceous sedimentary fill is primarily lacustrine or fluvial in origin. Lacustrine, organic-rich black shales are fringed by oolitic and other limestones and fluvial-deltaic sandstones derived mostly from the north. A significant southern source of sand existed during the Valanginian. Interbedded marine shales occur mostly to the west toward a presumed marine seaway connection to the northern Magallanes basin. Middle to Upper Cretaceous sedimentary rocks, sourced mostly from the north, are mainly fluvial sandstone-shale successions with some minor lacustrine influence. Reservoir-quality glauconitic sands were deposited during a Late Cretaceous-early Tertiary marine incursion from the Atlantic. After development of a lower Tertiary regional unco formity, relatively undisturbed younger Tertiary sediments completed the basin fill. Lower Cretaceous lacustrine shales of the D-129 Formation are the best source rocks yet identified. However, most hydrocarbon production occurs above the source in middle and Upper Cretaceous sandstones along the basin flanks. The most obvious hydrocarbon emplacement mechanism is vertical migration along reactivated graben faults.

Stratigraphic Evolution of Mesozoic Continental Margin and Oceanic Sequences Northwest Australia and North Himalayas: ABSTRACT

The authors are investigating continental margin to ocean sequences of the incipient Indian Ocean as it replaced central Tethys. Objectives of this study are the dynamic relation between sedimentation, tectonics, and paleogeography. Principal basins formation along the northern edge of eastern Gondwana started in the Late Permian to the Triassic. By the Late Triassic-Early Jurassic, platform carbonates with thin, organic-rich lagoonal shales were laid down in a subtropical climate. This unit, which harbors some of the oldest known nannofossils, shows repeated shallowing-upward sequences. Subsequent southward drift of the Gondwana margin during the Middle Jurassic increased siliciclastic input in Nepal, when widespread sediment starvation or erosion during local uplift took place off parts of northwest Australia. A middle Callovian-early Oxfordian hiatus in Nepal is submarine and appears global in extent. The overlying 250-m-thick organic-rich black shales, correlative to the Oxford/Kimmeridge clays of circum-Atlantic petroleum basins, may be traced along the northern Himalayan Range, and probably represent an extensive continental slope deposit formed under an oxygen minimum layer in southern Tethys. The deposit's diverse foraminiferal microfauna was previously only known from boreal Laurasia. The Callovian breakup unconformity, off northwest Australia, precedes onset of sea-floor spreading at least 15-25 Ma. Sea-floor spreading, leadingmore » to the present Indian Ocean started in the Argo Abyssal Plain around 140 Ma, at the end of the Jurassic, was about 15 m.y. later than previously postulated. Australia and Greater India separated as early as the Late Valanginian, about 130 Ma. Mafic volcaniclastics in Nepalese deltaic sediments probably testify to concurrent continental margin volcanic activity, which may be a precursor to the slightly younger Rajmahal traps in eastern India.« less

Application of Porphyrin and Other Biological Marker Geochemistry to Paleoenvironmental Assessment--Pennsylvanian Black Shales, Northern Denver Basin: ABSTRACT

Thin, Middle Pennsylvanian (Desmoinesian) organic-rich black shales underlie much of the northern Denver basin. During the Desmoinesian, the present-day area of the Nebraska panhandle was a shallow, at times highly saline restricted sea. In contrast, black shale deposition occurred in a marine environment with normal salinity (open marine) in northeastern Colorado. About eight individual black shale beds are present in the Desmoinesian section in both the restricted basin and open-marine settings, separated vertically by carbonate-evaporate rocks in the restricted basin and by predominantly carbonate rocks in the open-marine sequence. Preservation of high amounts of organic matter occurred in black shales deposited in both the restricted basin and open-marine settings. Chloroform-extractable organic matter (bitumen) concentrations are typically in the range of 4,000 to 8,000 ppm, and total organic carbon content (TOC) is as high as 20 wt. %. However, the composition of the bitumen contained in stratigraphically equivalent black shales is variable and is correlative to differences in paleoenvironmental conditions during sedimentation in the restricted and open-marine basins. Total porphyrin concentration (ppm relative to bitumen) is typically 10-100 times greater in the restricted basin compared to the open marine. The total porphyrin concentration (vanadyl + nickel + carboxyl) correlates with totalmore » sulfur content in the rocks (normalized to TOC) and C{sub 14}-C{sub 20} aryl isoprenoid content. The coincidence of high porphyrin and aryl isoprenoid concentration, along with high S/C ratios, is explained by differences in water column chemistry of the restricted basin and open-marine environments during sedimentation. The restricted basin was characterized by a stratified water column with euxinic conditions below the chemocline and a thin overlying oxic water column.« less

Accumulation and Preservation of Organic Carbon in Marine Sediments: The Roles of Anoxia vs. Production: ABSTRACT

Organic carbon enrichments in marine sediments and sedimentary rocks commonly are explained by the preferential preservation of the deposited organic matter under anoxic conditions; the role of primary organic (plankton) production is seldom considered. A review of the available information shows that modern marine sediment accumulating in oxic and anoxic basins in similar topographic and sedimentary settings have very similar carbon contents. On continental slopes, carbon maxima are apparently produced by the complex interplay between the supply of carbon to the sea floor, the texture of the sediment, the dilution of carbon by other sediment components, and the decreasing settling flux of carbon in the deeper waters of the open ocean. Contrary to contemporary thought, there is no causal relationship between such maxima and the position of the oxygen minimum. The degradation of sedimentary organic matter by aerobes and by sulfate reducers is very similar where the supply of fresh organic matter to the sea floor is similar. Hence, there is no evidence for the preferential preservation of organic matter under anoxic conditions. Terrestrial organic matter, however, appears to be degraded to a lesser extent by sulfate reducers. The burial of carbon below the surficial, oxygenated horizons of a sedimentmore » removes the easily oxidized fractions leaving material that may be less susceptible to attack by sulfate reducers. Sedimentary carbon maxima in Pleistocene glacial horizons are due to the increased settling flux of organic matter brought about by climatically induced increases in upwelling in the equatorial and marginal areas of the ocean. Changes in bottom water oxygen levels during these periods plays a minor role in producing these signals. Previous work that claimed that anoxic bottom waters were prevalent during the accumulation of organic-rich black shales in the geological record should be reevaluated.« less

Biological markers in Lower Jurassic synrift lacustrine black shales, Hartford basin, Connecticut, U.S.A.

The East Berlin Formation (Lower Jurassic, Hartford basin, Connecticut, U.S.A.) is distinctive for its six cyclic units of lacustrine black shale and gray mudstone, separated by playa and fluvial redbeds. The black shales are each about a meter thick and were deposited in subtropical, thermally strastified, oligomictic lakes, the youngest of which (lakes 3 through 6) were large enough to flood most of the basin and attained depths of several tens of meters.The saturate fractions of solvent extracts of organic-rich black shales from each of the six lakes, collected at fresh roadcuts near East Berlin, are dominated by extended homologous series of n-alkanes, alkylcyclohexanes, and branched chain alkanes. A striking feature of the black shales is the presence of a series of extended tricyclic terpanes from C20 to at least C41. Hopanes are either not detectable or present only in subordinate quantities relative to the tricyclic terpanes. The samples are depleted in hopanes in part because of the elevated maturity level (mid to late oil window). Tricyclic terpane concentrations may also have been enhanced by fractionation effects related to oil expulsion out of the black shales. In addition, the original organic matter may have been exceptionally rich in tricyclic terpane precursors, i.e. fossil lipids of prokaryotes present in anoxic, moderately saline, alkaline lakes.

The sedimentology of Lower Palaeozoic black shales from the shallow wells Skelbro 1 and Billegrav 1, Bomholm, Denmark

Previous studies of the Lower Palaeozoic shales on Bornholm have mainly been based on the outcrops along the streams. The outcrops provide data on the lateral continuity of the facies while the selection of (2-3 cm wide) cores for the present study focused the attention on the vertical sequence of facies. Cores of high quality were obtained and have been found to provide an excellent basis for a study of structures, sediment composition and diagenesis. In the outcrops of shale fossils are locally present in high numbers and such levels are also recognized in the cores. Seven sedimentary facies, ranging from black mudshale to greyish mudstone and silt-streaked shale, are distinguished in the Middle Cambrian to lowermost Silurian shales. Gamma-ray logs were run in the two wells and a convincing correlation to the cored sequence can be demonstrated. Gamma-ray logs provide thus a good means of correlation to wells where no cores have been cut. The shales are all interpreted as deposited in an epicontinental sea due to their geological setting i.e. the association with shallow water carbonates (Andrarum Limestone and Komstad Limestone) and their stratigraphical position above the shelf to shoreface sandstones of the Lresa formation. Three facies associations are distinguished: The mudshale association comprises black organic-rich shales which represent a low-energy anoxic depositional environment which prevailed in the Middle Cambrian to Lower Ordovician. The mudstone association is typical of the Middle and Upper Ordovician and represents a continuation of low-energy environments though mottling indicates that ventilation improved in certain periods. The siltshale association represents higher energy environments which were dominant in the lowermost Silurian. A well defined Upper Ordovician CU sequence probably reflects the global eustatic fall in sea level caused by the extensive glaciation in Gondwanaland. In the late Silurian the average rate of deposition increased in response to the approaching Caledonian orogeny.