Geochemical Evaluation of the Permian Ecca Shale in Eastern Cape Province,South Africa: Implications for Shale Gas Potential

Shale gas has been the exploration focus for future energy supply in South Africa in recent time. Specifically, the Permian black shales of the Prince Albert, Whitehill, Collingham, Ripon and Fort Brown Formations are considered to be most prospective rocks for shale gas exploration. In this study,outcrop and core samples from the Ecca Group were analyzed to assess their total organic carbon(TOC), organic matter type, thermal maturity and hydrocarbon generation potential. These rocks have TOC ranging from 0.11 to 7.35 wt%. The genetic potential values vary from 0.09 to 0.53 mg HC/g,suggesting poor hydrocarbon generative potential. Most of the samples have Hydrogen Index(HI) values of less than 50 mg HC/g TOC, thus suggesting Type-Ⅳ kerogen. Tmax values range from 318℃ to601℃, perhaps indicating immature to over-maturity of the samples. The vitrinite reflectance values range from 2.22% to 3.93%, indicating over-maturity of samples. Binary plots of HI against Oxygen Index(OI), and HI versus Tmax show that the shales are of Type II and mixed Type Ⅱ-Ⅲ kerogen.Based on the geochemical data, the potential source rocks are inferred as immature to over-matured and having present-day potential to produce gas.     

Changes in the composition of bitumen extracts and chemical structure of kerogen during hydrous pyrolysis

The paper presents data on the composition of biomarkers from bitumen extracts and the chemical structure of kerogen from C_org-rich sedimentary rocks before and after hydrothermal treatment in an autoclave at 300°C. Samples selected for this study are kukersite and Ordovician Dictyonema shale from the Baltics, Domanik oil shale from the Ukhta region, Upper Permian brown coal from the Pre-Ural foredeep, carbonaceous shale from the Oxfordian horizon of the Russian plate, and Upper Jurassic oil shales from the Sysola oil shale bearing region. The rocks contain type I, II, III, and II-S kerogens. The highest yield of extractable bitumen is achieved for Type II-S kerogen, whereas Type III kerogen produces the lowest amount of bitumen. The stages of organic matter thermal maturation achieved during the experiments correspond to a transition from PC_2–3 to MC_1–2. The ¹³C NMR data on kerogen indicate that the aromatic structures of geopolymers underwent significant changes.     

Anomalous Enrichment of Silver in Organic Matter of the Songxi Shale-hosted Ag-Sb Deposit in Northeastern Guangdong

The silver contents of organic matter in the host rocks of the Songxi shale-hosted Ag-Sb deposit of northeastern Guangdong, South China, have been directly determined using the electron microprobe technique. The silver contents in two types of organic matter, marine vitrinite and solid bitumen, vary in the range from 100×10-6to 350×10-6, which are from tens of times to thousands of times higher than those of the host rocks in the ore deposit. The average silver content of the organic matter is also several times higher than the pay grade of silver for commercial mining of the ore deposit. It is quite obvious that the organic matter of the host rocks in the ore district is characterized by an anomalous enrichment of silver. The results of this study indicate that the silver anomalies in the organic matter have been derived from both the host rocks and the silver-bearing fluids of the Songxi black shale ore source. In the course of sedimentation and later hydrothermal reworking, organic matter was able     

Solid state C NMR studies of selected oil shales from Queensland, Australia

Solid state ¹³C NMR techniques of cross polarization with magic-angle spinning, and interrupted decoupling have been employed to examine the nature of the organic matter in eight kerogen concentrates representing five Tertiary deposits in Queensland, Australia. The NMR results show that five of the kerogens have high proportions of aliphatic carbon in their organic matter and correspond to Type I–II algal kerogens. Three of the kerogens, derived from carbonaceous shales, have a high proportion of aromatic carbon in their organic matter and correspond to Type III kerogens. The fractions of aliphatic carbon in all the kerogens, regardless of type, are shown to correlate with the conversion characteristics of the corresponding raw shales during Fischer assay. Interrupted decoupling NMR results show the presence of more oxygen-substituted carbon in the carbonaceous shales, which may account for the greater CO₂ evolution and phenolic materials found in the pyrolysis products of the carbonaceous shales.     

Mössbauer spectroscopic studies of iron in organic material from natural sedimentary environments

⁵⁷Fe Mössbauer spectra have been obtained from samples of humic acid, fulvic acid and kerogen and from the organic material extracted from bituminous chalk with benzene-methanol. The spectra indicate that iron occurs in a trivalent form in the silicate residue of the humic acid fraction, as hydrated ferrous ions associated with the fulvic acid fraction, as pyrite in kerogen and in a form not detectable by Mössbauer spectroscopy in the benzene-methanol extract.     

Gold Content of Mantle-Derived Rocks from Eastern China and Its Implications

We have analyzed the gold content of 65 samples of mantle-derived xenoliths and their host rocks from eastern China, which is found to be inhomogeneous, falling in the ranges of 1.0×109-8.2×109 (averaging 3.8 ×10-9) and 0.2×10-9-5.3×10-9 (averaging 2.7×10-9) in the mantle-derived xenoliths and the host basalts respectively. Except the samples from Wanquan County of Hebei Province and Anding County of Hainan Province, the gold content is the highest on the margins of the North China platform and decreases spatially towards the north and south, and temporally the samples of the Tertiary have a higher gold content than those of the Quaternary. The gold content of the mantle-derived xenoliths and the host Paleozoic kimberlites is 3.8×10-9-180×109and 0.1×10-9-38.0×10-9 respectively, which are higher than that in the mantle-derived xenoliths in basalt and the host Cenozoic basalts. The mantle-derived xenoliths have a higher gold content than the host rocks, but their relativity is not very clear, su     

Early Cretaceous anoxic basin of the Russian Plate: Organic geochemistry

Compositions of organic matter in the bituminous shale accumulated under anoxic conditions and in the underlying clay deposited in aerated basin are noticeably different. Organic matter (OM) of the clay is depleted in hydrogen and kerogen of the clay during pyrolysis produces 2.5 times less aliphatic hydrocarbons than kerogen, which is separated from the bituminous shale, and dozens of times less sulfurous compounds, such as thiophenes and benzothiophenes. Accumulation of OM in the clay was primarily related to the direct inheritance of polymeric lipid biochemical components of the initial OM and lignin included in the higher plants. Changes of gas regime in the basin and sediments, expressed in the origin of anoxic conditions during the accumulation of bituminous shales, intensified the early diagenetic sulfuration and the consequent formation of sulfur- and hydrogen-rich kerogen in the shales. At the same time, less stable lipids and, possibly, carbohydrates were also conserved.Translated from Litologiya i Poleznye Iskopaemye, No. 1, 2005, pp. 25–34.Original Russian Text Copyright 2005 by Bushnev.     

Geochemical Charactristics and Genesis of Topaz—Bearing Porphyries in Yangbin Area of Taishun County,Zhejiang Province

The host rocks of the porphyry tin deposits in the Yangbin area are principally topaz-bearing porphyry dikes about 2 km long and 2–20m wide. Three lithologie types are identified for the dikes: topaz-bearing potassium feldspar granitic porphyry, topaz-bearing monzonitic granitic porphyry and topaz-bearing quartz porphyry. The content of topaz in the rocks ranges from 10 to 20 vol.%. Porphyritic texture is characteristic, with quartz, potassium feldspar and albite as main phenocryst minerals. The phenocryst occupies 10–20 vol% of the rocks. The rock groundmass consists of subhedral topaz, quartz and protolithionite. Topaz has a unit-cell parameter b=8.797 (Å), and F:OH=1.92:0.18, indicating a F-rich variety formed at high temperature. The topaz-bearing porphyries occurring in this area are strongly peraluminous (A/NKC=1.574–12.94), with high ratios of F/Cl (146–303) and Rb/Sr (5–122). They are rich in incompatible elements (Sn, 313 × 10^?6–1042 × 10^?6; W, 6 × 10^?6–218 × 10^?6; Nb, 27 × 10^?6–54 × 10^?6), but poor in compatible elements (Sr, 10 × 10^?6–28 × 10^?6; Ba, 58 × 10^?6–73 × 10^?6; V, 3 × 10^?6–12 × 10^?6, Cl, 150 × 10^?6–226 × 10^?6). The rocks are also characterized by high total REE amount (281.69 × 10^?6–319.76 × 10^?6), with strong Eu depletion (δEu=0.01–0.03) and low ratio of LREE/HREE (0.78–0.84). In summary, the authors propose an idea of S-type genesis for the topaz-bearing porphyries with tin mineralization, instead of I-type.     

渤海莱州湾凹陷不同类型烃源岩活化能影响因素分析

选取不同干酪根类型的低熟烃源岩作为剖析对象,从干酪根类型、干酪根中有机硫含量、可溶有机质和黏土矿物对烃源岩活化能的影响进行研究,结果表明:Ⅱ₁和Ⅰ干酪根活化能分布范围窄,Ⅱ₂和Ⅲ干酪根活化能分布范围宽,干酪根平均活化能Ⅲ>Ⅱ₂>Ⅰ>Ⅱ₁;有机硫含量高的干酪根平均活化能低,有机硫对烃源岩生烃起催化作用;可溶有机质的存在影响烃源岩平均活化能的大小和分布特征,可溶有机质分布在活化能低值区间,对烃源岩生烃起催化作用;烃源岩黏土矿物中伊/蒙混层含量高,平均活化能高;伊利石含量高,平均活化能低,伊利石对烃源岩生烃起催化作用。      

Gas-generation potential of shales in small and medium-sized basins: a case study from the Xuanhua Basin,north China

Abstract

Small- and medium-sized basins are widely distributed, and some contain commercial gas reservoirs demonstrating their gas-generation potential. The Xuanhua Basin, which is a small-sized coal-bearing basin in north China, includes a promising target for shale-gas exploration in the Xiahuayuan Formation. In this study, we used this basin as a case study to assess the critical geochemical features for small or medium-sized basins to form commercial gas reservoirs. Total organic carbon (TOC) analysis, Rock-Eval pyrolysis, microscopic observation of macerals, vitrinite reflectance measurement and kerogen stable carbon isotope analysis were performed to characterise the organic geochemistry of the Xiahuayuan shales. The original total organic carbon (TOC_o) content and hydrocarbon-generative potential (S_2o) were reconstructed to further evaluate the gas-generation potential of these shales. In addition, geochemical data of shales from other similar-sized basins with gas discoveries were compared. The results showed that the kerogen from the Xiahuayuan Formation is Type III (gas-prone), and macerals are dominated by vitrinite. TOC values showed a strong heterogeneity in the vertical profiles, with most higher than 1.5?wt%. The measured R_o values ranged from 1.4 to 2.0%. However, thermal maturity was not correlated with the present-day burial depth with higher maturity in the wells closest to the diabase intrusion centre. The remaining generation potential (S₂) averaged 0.91?mg HC/g rock, equal to 1.4?cm³ CH₄/g rock, and the average amount of hydrocarbon generated was 4.33?cm³ CH₄/g rock. In small and medium-sized basins, the TOC content of commercially developed gas shales ranged from 0.5 to 2.5?wt%, organic matter was mainly humic (gas-prone), and the burial depth was generally shallow. Biogenic gas reservoirs for commercial exploitation tend to have larger shale thicknesses (120–800?m) than thermogenic gas reservoirs (60–90?m).

1. The Xiahuayuan Formation is a good gas-source rock with gas-prone kerogen type, relatively high TOC values and moderate thermal maturity.

2. The average amount of hydrocarbon generated from the Xiahuayuan shales is about 4.33?cm³ CH₄/g rock, indicating a potential to form a shale gas reservoir.

3. Owing to the influence of diabase intrusions, the Xiahuayuan shales have entered the dry gas window at relatively shallow-buried depths.

4. Small- and medium-sized basins have the potential to generate commercial gas reservoirs with the generated volume mainly a product of the thickness and maturity of black shales.

     

A reinterpretation of the crustal N‐isotope record: evidence for a 15N‐enriched Archean atmosphere?

A new compilation of N‐isotope and abundance data for metasedimentary rocks, and hyrdothermal micas that proxy for bulk crust, show systematic patterns. (1) δ¹⁵N values of kerogen in Precambrian cherts are more negative relative to siliciclastic counterparts, probably due to a mantle hydrothermal component. (2) There is a secular trend from average δ¹⁵N 15.3 ± 1.8‰ in Archean shales, through intermediate values in the Proterozoic, to Phanerozoic counterparts where δ¹⁵N averages +3.5‰. (3) Hydrothermal micas in metamorphic hydrothermal systems of Palaeozoic and Mesozoic age that proxy for crust have δ¹⁵N within the range of contemporaneous sedimentary rocks. (4) Hydrothermal micas track the secular trend of δ¹⁵N for kerogen from 2.7 Ga to the Phanerozoic. (5) Within Precambrian datasets δ¹⁵N does not increase with decreasing N content; accordingly, high δ¹⁵N values cannot stem either from metamorphism or form Rayleigh fractionation. (6) Previous studies show isotopic shifts during metamorphism are only +1 to +3‰ up to amphibolite facies. Values of 10–24‰ are attributed to a high δ¹⁵N Archean atmosphere, a residual signature of CI carbonaceous chondrites where δ¹⁵N is +30‰ to + 42‰.     

Geochemical characterisation of organic matter in Keg River Formation (Elk point group, Middle Devonian), La Crete Basin, Western Canada

Carbonates from the Keg River Formation, La Crete Basin, Alberta, western Canada were examined in order to define: (a) oil source rock potential; (b) bulk maceral composition; (c) extract yield; and finally (d) facies variations using PY-GC-MS. Thirty samples from 6 different wells were examined from the lower Keg River member and 4 from the upper Keg River member using conventional geochemical methods. As maturity differences are absent within the sample set, variations in TOC, T_max, hydrogen index, organic petrography and extract yields are caused by variability in organic matter input, which is revealed by molecular characterisation using PY-GC-MS. Lower Keg River member bituminous wackestones are excellent potential source rocks containing Types I–II and Type II organic matter. Types I–II organic matter contains large well preserved (up to 200μm in diameter) thick-walled Tasmanites (10–15% of sample) and akinete algal cells indicative of algal blooms within an amorphous bituminite. Type II organic matter contains a higher proportion of degraded alginites/bituminite relative to well-preserved alginites. Extract yields (mg/g TOC) were seen to increase from Types I–II to Type II organic matter. PY-GC-MS revealed that 1,2,3,4-tetramethylbenzene was a major peak in most samples. This is a pyrolysis product arising from β-cleavage of C₄₀ diaromatic carotenoids incorporated within the kerogen during diagenesis. The source of this compound is thought to be from an unknown diaromatic compound with a 2,3,6-/3,4,5-trimethyl substitution pattern and isorenieratene, which is specific to photosynthetic green sulphur bacteria (Chlorobiaceae) suggesting that the photic zone was at least partially anoxic during deposition of these samples. The relative abundance of this compound/n-C11-alkene and organic sulphur (calculated from the thiophene ratio) both increase from Types I–II to Type II organic matter. This trend was grossly similar to the trend seen in the variability of extract yield with hydrogen index. A similar trend for HI and T_max indicates samples containing a higher proportion of degraded alginites/bituminite relative to well-preserved alginite are more labile than Type I–II organic matter. Upper Keg River member marls contain Type II organic matter, which is characterised by heavily degraded algal material within a bituminous groundmass. Pyrolysates of two of the marl samples contain only low amounts of 1,2,3,4-tetramethylbenzene, in contrast to the bituminous wackestones, indicating that the depositional environment/source input was different during deposition of the marl samples. Although both marls contain similar organic matter, their pyrolysates were significantly different. One marl (1141.9 m) was highly paraffinic containing dominantly short-chain alkene/alkane doublets, while the other marl (1137.6 m) contained a bimodal n-alkane/alkene distribution and high amounts of alkylphenols, which may be derived from preservation of resistant algal polyphenolic molecules or suggest a terrestrial input.     

The effect of organic matter type and organic carbon content on Rock-Eval hydrogen index in oil shales and source rocks

The amount of “gas-prone” kerogen (woody, fungal and “inert”) and the organic carbon content (TOC) are the two predominant factors affecting the hydrogen index (HI) in the 226 samples of lacustrine and marine oil shales and source rocks studied. HI decreases as a function of the amount of “gas-prone” kerogen and increases as a function of TOC. In addition, the type of amorphous kerogen influences the hydrogen index, and this can be roughly estimated from the fluorescence intensity of the amorphous kerogen. Nearly eighty percent of the variation in HI in these samples can be accounted for by the percentage of “gas-prone” kerogen, the TOC content, and the fluorescence of the amorphous kerogen in a multiple regression analysis.Hydrogen index increases as a function of TOC up to about 10% TOC (the relationship can be approximated by a quadratic equation) and then levels off. A possible explanation for this is that the capability of a rock to generate and expel hydrocarbons during pyrolysis increases with TOC. When the retention capacity of the rock matrix is saturated (at about 10% TOC) further increases in TOC have no effect on HI. It is also possible that the quality (i.e. oil-proneness) of the amorphous kerogen is poorer in low TOC samples than in high TOC samples.The samples came from the following oil shales and source rocks: Rundle (Queensland Eocene-Miocene), Mae Sot (northwestern Thailand, Eocene-Pliocene), River River (northwestern Colorado, Eocene), Toolebuc (western Queensland, Late Albian), the “Posidonienschiefer” (southwestern Germany, Toarcian), an Argentinian lacustrine deposit (Eocene-Miocene), the Kimmeridgian sections from four North Sea wells (blocks 21, 30, and 210), Monterey Shale (California, Miocene), and sections from six wells from the Alaskan Tertiary (North Slope, North Aleutian Shelf, Navarin Basin, Norton Sound). Most samples appear to be thermally immature (T.A.I. less than 1.8; R_o less than 0.6%) so they should be considered only potential source rocks.The lacustrine oil shales have a higher conversion ratio (yeild/TOC or S₁ + S₂/TOC) than do the marine oil shales in samples with only amorphous and algal kerogen. These, in turn, have a higher conversion ratio than the marine source rocks. These differences are roughly reflected in the fluorescence intensity of the amorphous kerogen. Free hydrocarbons are higher in the marine source rocks than in the marine oil shales, and are lowest in the lacustrine oil shales.     

In, Sn, Pb and Zn Contents and Their Relationships in Ore-forming Fluids from Some In-rich and In-poor Deposits in China

All the indium-rich deposits with indium contents in ores more than 100×10- 6 seems to be of cassiterite-sulfide deposits or Sn-bearing Pb-Zn deposits, e.g., in the Dachang Sn deposit in Guangxi, the Dulong Sn-Zn deposit in Yunnan, and the Meng'entaolegai Ag-Pb-Zn deposit in Inner Mongolia, the indium contents in ores range from 98×10-6 to 236×10-6 and show a good positive correlation with contents of zinc and tin, and their correlation coefficients are 0.8781 and 0.7430, respectively. The indium contents from such Sn-poor deposits as the Fozichong Pb-Zn deposit in Guangxi and the Huanren Pb-Zn deposit in Liaoning are generally lower than 10×10-6, i.e., whether tin is present or not in a deposit implies the enrichment extent of indium in ores. Whether the In enrichment itself in the ore -forming fluids or the ore-forming conditions has actually caused the enrichment/depletion of indium in the deposits? After studying the fluid inclusions in quartz crystallized at the main stage of mineralization of several In-rich and In-poor deposits in China, this paper analyzed the contents and studied the variation trend of In, Sn, Pb and Zn in the ore-forming fluids. The results show that the contents of lead and zinc in the ore-forming fluids of In-rich and -poor deposits are at the same level, and the lead contents range from 22×10-6 to 81×10-6 and zinc from 164×10-6 to 309×10-6, while the contents of indium and tin in the ore-forming fluids of In-rich deposits are far higher than those of In-poor deposits, with a difference of 1-2 orders of magnitude. Indium and tin contents in ore-forming fluid of In-rich deposits are 1.9×10-6-4.1×10-6 and 7×100-6-55×10-6, and there is a very good positive correlation between the two elements, with a correlation coefficient of 0.9552. Indium and tin contents in ore-forming fluid of In-poor deposits are 0.03×10-6-0.09×10-6 and 0.4×10-6--2.0×10-6, respectively, and there is no apparent correlation between them. This indicates, on one hand, that In-rich ore-forming fluids are the material basis for the formation of In-rich deposits, and, on the other hand, tin probably played a very important role in the transport and enrichment of indium.     

The shale gas potential of Tournaisian, Visean, and Namurian black shales in North Germany: baseline parameters in a geological context

Carboniferous black mudrocks with known petroleum potential occur throughout Northern Germany. However, despite numerous boreholes exploring for conventional hydrocarbons, the potential for shale gas resources remains uncertain. Therefore, an integrated investigation was conducted to elucidate the shale gas potential for three different Carboniferous facies incorporating baseline parameters from sedimentological and organic-geochemical analyses. Tournaisian–Namurian fine-grained rocks of the Culm-facies, with Type II + III kerogen were deposited in the basin center. TOC contents of up to 7 % occur in the Lower Alum Shale (3.6 % VRr) and up to 6 % in the Upper Alum Shale (4.4 % VRr). Bands of organic-rich black shales, reflecting sea-level variations controlled by global eustatic cycles, occur within the Tournaisian–Visean “Kohlenkalk”-facies north of the Rhenish Slate Mountains and in the Rügen island area. In both areas the organic matter is characterized by a kerogen Type II + III with TOC contents of up to 7 % and maturities of up to 4.2 and 1.8 % VRr, respectively. Black hemipelagites intercalated with coarser-grained silt- and sandstones occur in the Synorogenic Flysch Formation of the Namurian A along the southern basin margin. TOC contents vary from 0.5 to 2.0 % with Type III kerogen dominated organic matter and maturities of up to 2.5 % VRr. The baseline parameters presented in this paper indicate a shale gas potential for the sediments of the Culm-facies on the southern basin margin and of the “Kohlenkalk”-facies in the Rügen area.     

Comparison of oil shales and kerogen concentrates by 13C nuclear magnetic resonance

Six oil shales and their kerogen concentrates have been studied using ¹³C CP/MAS NMR techniques to study the distribution of organic carbon species. It is found that if the aromatic and aliphatic regions are divided at about 80 or 100 ppm, the apparent aromaticities of a raw shale and its kerogen concentrate are in good agreement. The presence of oxygen-substituted carbons in the raw shales and their depletion in the kerogen concentrates are observed and discussed.     

Comparison of oil shales and kerogen concentrates by C nuclear magnetic resonance

Six oil shales and their kerogen concentrates have been studied using ¹³C CP/MAS NMR techniques to study the distribution of organic carbon species. It is found that if the aromatic and aliphatic regions are divided at about 80 or 100 ppm, the apparent aromaticities of a raw shale and its kerogen concentrate are in good agreement. The presence of oxygen-substituted carbons in the raw shales and their depletion in the kerogen concentrates are observed and discussed.     

Organic geochemistry and petrology of barren and Mo–Ni–PGE mineralized marine black shales of the Lower Cambrian Niutitang Formation (South China)

Marine black shales of the Lower Cambrian Niutitang Formation in southern China host Mo–Ni–platinum group elements (PGE) mineralization confined to a phosphate- and pyrite-rich stratiform body (max. 20-cm thick). The H/C atomic ratio, carbon isotopic composition, FTIR spectra of bulk organic matter, and spectra of extractable part of organic matter indicate similar sources and thermal evolution of organic matter in barren and mineralized black shales.The morphology and relative abundance of organic particles in barren and mineralized shales are different. In barren black shales, organic particles comprise only elongated bodies and laminae 2–10 μm across or elongated larger bodies (> 10 μm) with R_max = 2.96–5.21% (Type I particles). Mineralized black shales contain Type I particles in rock matrix (90–95 vol%), small veinlets or irregular organic accumulations (Type II particles, 1–5 vol%) that display weak to well developed mosaic texture and a variable reflectance (R_max = 3.55–8.65%), and small (< 1 to 5 μm) rounded or irregular Type III organic particles (1–4 vol%) distributed within phosphate nodules and sulphide rip-up clasts. Type III particles show similar reflectance as particles of Type I in rock matrix. Type I particles are interpreted as remnants of in situ bacterially reworked organic matter of cyanobacteria/algal type, Type II as solidified products or oil-derived material (migrabitumen), and Type III particles as remnants of original organic matter in phosphatized or sulphidized algal/microbial oncolite-like bodies. Equivalent vitrinite reflectances of Type I and III particles in barren and mineralized rocks are similar and correspond to semi-anthracite and anthracite. Micro-Raman spectra of organic particles in rocks display a wide belt in the area of 1600 cm^− 1 (G belt) and approximately the same belt in the area of 1350 cm^− 1 (D belt). The ratio of integrated areas of the two belts correlate with R_max values.The Mo–Ni–PGE mineralized body is interpreted as to represent a remnant of phosphate- and sulphide-rich subaquatic hardground supplied with organic material derived from plankton and benthic communities as well as with algal/microbial oncolite-like bodies that originated in wave-agitated, shallow-water, nearshore environment.     

Geochemical and stable isotope resetting in shear zones from Täschalp: constraints on fluid flow during exhumation in the Western Alps

Fluid flow at greenschist facies conditions during exhumation of the western Alps occurred in several penecontemporaneous systems, including shear zones at lithological contacts, deformed contacts between serpentinite bodies and metabasalts, albite veins within metabasalts, and calcite + quartz veins within calcareous schists. Fluid flow in shear zones that juxtapose metasediments and ophiolitic rocks within the Piemonte Unit reset O and H isotope ratios. δ¹⁸O values are buffered by the wall rocks; however, calculated fluid δ²H values are similar within all the shear zones suggesting that they formed an interconnected network. The similarity of δ²H values of the sheared rocks and those of unsheared calcareous schists suggests that the fluids were derived from, or had equilibrated with, the schists that envelop the ophiolite rocks. Time‐integrated fluid fluxes at the sheared contacts estimated from changes in Si in metabasalts were up to 10⁵ m³ m^?2, with the fluid flowing up temperature driven either by topography or seismic pumping. Individual shear zones were active for c. 2–3 Myr, implying average fluid fluxes of up to 10^?9 m³ m^?2 s^?1. Rocks in shear zones within the ophiolite away from contacts with the metasediments show much less marked isotopic and geochemical changes, implying that fluid volumes decreased into the ophiolite unit, consistent with the source of fluids being the metasediments. Fluids were generated by dehydration reactions that were intersected during exhumation and, while many rocks show the affects of fluid–rock interaction, large‐scale fluid flow between major units was not common.     

Environmental control of carbon isotope variations in Pennsylvania black-shale sequences, Midcontinent, U.S.A.

A reversal of the conventional carbon isotope relationship, “terrestrial-lighter-than-marine” organic matter, has been documented for two Pennsylvanian (Desmoinesian) cyclothemic sequence cores from the Midcontinent craton of the central United States. “Deep” water organic-rich phosphatic black shales contain a significant proportion of algal-derived marine organic matter (as indicated by organic petrography, Rock-Eval hydrogen index and ratios) and display the lightest δ¹³C-values (max −27.80‰ for kerogen) while shallower water, more oxic facies (e.g. fossiliferous shales and limestones) contain dominantly terrestrial organic matter and have heavier δ¹³C_kerogen-values (to −22.87‰ for a stratigraphically adjacent coal). δ¹³C-values for extract fractions were relatively homogeneous for the organic-rich black shales with the lightest fraction (often the aromatics) being only 1‰, or less, more negative than the kerogen. Differences between extract fractions and kerogens were much greater for oxic facies and coals (e.g. saturates nearly 5‰ lighter than the kerogen).A proposed depositional model for the black shales calls upon a large influx of nutrients and humic detritus to the marine environment from the laterally adjacent, extremely widespread Pennsylvanian (peat) swamps which were rapidly submerged by transgression of the epicontinental seas. In this setting marine organisms drew upon a CO₂-reservoir which was in a state of disequilibrium with the atmosphere, being affected by isotopically light “recycled-CO₂” derived from the decomposition of peaty material in the water column and possibly from the anoxic diagenesis of organic matter in the sediments.