Shales In South China Research Articles

Models of shale gas storage capacity during burial and uplift: Application to Wufeng-Longmaxi shales in the Fuling shale gas field

Marine shales in South China, such as the Upper Ordovician Wufeng (O3w) and Lower Silurian Longmaxi (S1l) shales, have generally experienced a long-term burial and rapid uplift process. The variations of temperature and pressure caused by the complex tectonic movements have significant effects on the shale gas storage capacities. Based on the supercritical Dubinin-Radushkevich (SDR) equation and high-pressure methane (CH4) adsorption experiments, equations to calculate the shale gas storage capacities under geological conditions were established in this work. Using the derived equations, the gas storage capacities in two basic geological processes were modeled, namely, the burial process, which has an increased gas content, and the uplift process, which has a constant gas content. The two basic models were applied to investigate the evolution of the shale gas content of the O3w-S1l shales in the JY 1 Well in the Fuling shale gas field, Sichuan Basin, South China. Geological models of the gas storage capacities of the shales during burial show that the storage capacity of adsorbed gas (SCadsorbed gas) initially rapidly increases and then slowly decreases with the increasing burial depth. The storage capacities of free gas (SCfree gas) and total gas (SCtotal gas) increase with the increasing burial depth. Compared with SCadsorbed gas, SCfree gas significantly increases as the pore pressure increases. Such an obvious difference implies that free gas is the predominant gas form in the initial stage of shale gas production. During uplift, the gas content remains unchanged, resulting in a linear decrease in SCfree gas and a linear increase in SCtotal gas. Nonetheless, SCfree gas during uplift is still higher than SCfree gas during burial at the same burial depth. The modeling of the shale gas content evolution of the O3w-S1l shales in the JY 1 shows that from the maximum burial depth of 6200 m to the present-day burial depth of 2415 m, the free gas content declines by 0.89 m3/t, from 2.30 m3/t to 1.41 m3/t, while the adsorbed gas content only increases by 0.58 m3/t, from 0.47 m3/t to 1.05 m3/t. This difference indicates that only a small amount (0.31 m3/t) of shale gas was lost during the uplift and that most (0.58 m3/t) of the reduced free gas was converted into adsorbed gas. A unique aspect of this study is that we illustrate how adsorbed and free gas are converted when the total gas content remains constant. Moreover, the two basic geological models established in this study can also be applied to other shale gas systems with complex tectonic movements.

Mineralogy and geochemical investigation of Cambrian and Ordovician–Silurian shales in South China: Implication for potential environment pollutions

Rapid expansion of shale gas development in China raises environmental and human health concerns. Several studies present related information on these concerns in the United States and Canada, yet they are few in China at present. This paper presents a series of original and published mineralogical and trace elements data from two gas shales (Niutitang shale and Longmaxi shale) considered as producing gas shale in China. Mineralogical and geochemical data surveyed can be applied to evaluate the potential environment pollutions during shale weathering and hydraulic fracturing. After compilations of the mineralogical and trace elements data, we can conclude that (a) there is generally more pyrite compared with carbonate for Niutitang shales, indicating that Niutitang shales are more prone to generate the hydrogen ions than Longmaxi shales; (b) many of the environmental hazardous trace elements considered here show an association with organic matter and/or pyrite, indicating that these trace elements are prone to mobilize and release during shale weathering and hydraulic fracturing; (c) some environmental hazardous elements like As, Ni, and Ba that have extremely high concentrations in some regions as compared with the screening limits for soil and drinking water should attract more attention; and (d) clay‐rich shale with abundant organic matter may be the most favourable shale gas reservoir for the Niutitang shale, especially for those shales in Lower Yangtze region. Moreover, more mineralogical and geochemical data will be required for a comprehensive environmental impacts assessment.

Distribution laws of large gas fields and further exploration orientation and targets in China

With the rapid expansion and extension of natural gas exploration and development, it is more and more difficult to discover large-scale reserves in China. To intensify the research on new natural gas exploration domains, we reviewed the progress and trend of natural gas exploration and analyzed the main areas with large-scale proved gas in place (GIP). Then, based on a statistic analysis of large gas fields in China as well as their hydrocarbon accumulation characteristics, their genetic systems were classified and each system's distribution law was also identified. Some research results were obtained. First, carbonate paleo-uplifts of cratonic basins, tight sandstones of extensive gentle slopes and thrust structures of foreland basins are the main areas with large-scale proved GIP in China. Second, there are five genetic systems for large gas fields, i.e. cratonic rift and paleo-uplift (A), stable slope of low-angle open lake (B), thrust structure of piedmont fault depression (C), faulted uplift and igneous rock of intracontinental pull-apart fault depression (D), and anticline structure of epicontinental strike–slip fault depression (E), and one genetic system (F) for unconventional gas, i.e. adsorption and accumulation in nano-scale space. Third, there is one core genetic system for large conventional gas fields in each geologic cycle. Fourth, two-level accumulation, i.e. no migration inside the source and large-scale transportation termination, exists in each single genetic system, and sequential accumulation is formed under the control of multiple factors in the areas where multiple genetic systems are superimposed. Fifth, the multi-system superimposed area is rich in large gas fields and the multi-stress hinge zone in the central area is the natural gas convergence zone. Finally, the future orientation and targets of natural gas exploration in China were pointed out. First, the system A includes Sinian–Lower Paleozoic in the Sichuan Basin, Cambrian in the Tarim Basin and Cambrian–Ordovician in the Ordos Basin. Second, the system C includes the transform zone of Kuqa thrust structure, the northwestern Sichuan Basin and the southwestern Tarim Basin. Third, the system E includes the basins in the eastern China seas. Fourth, the system F includes organic-rich shales in South China and deep coal beds in the Ordos Basin in central China.

Volcanic and hydrothermal activities recorded in phosphate nodules from the Lower Cambrian Niutitang Formation black shales in South China

Deposition of black shale of the Lower Cambrian Niutitang Formation (NTT) of the Yangtze Block, South China, in association with the Ediacaran-Cambrian (E-C) transition was accompanied by widespread formation of phosphate nodules. Petrological and geochemical studies of the nodules and host sedimentary rocks were carried out to elucidate hydrographic conditions of the Early Cambrian ocean. Our results reveal that NTT phosphate nodules are composed principally of concentrically banded carbonate fluorapatite (CFA) that likely reflects changing Eh and pH conditions contemporaneous with diagenetic nodule growth. Accumulation of organic-rich sediment and nodule growth may have been induced and sustained by contemporaneous volcanic and hydrothermal activities on the Yangtze Block. The introduction of voluminous nutrients with volcanic ash, including phosphate and aqueous Fe and Si, promoted the bloom of plankton, including sponges and lower algae leading to organic matter enrichment. Productivity may have been further enhanced by upwelling of phosphate-rich bottom water that resulted in widespread phosphogenesis and formation of phosphate nodules. The hyalophane-quartz association documented from phosphate nodules suggests the possible interaction of K-feldspar-rich volcanic ash and Ba-rich hydrothermal fluids at lower pH levels in association with nodule growth. The formation of phosphate nodules of the Niutitang Formation, then, reflects the complex interaction among primary productivity, volcanic and hydrothermal activity, and the accumulation of organic matter at a critical period of Earth history.

Discussion on the contribution of graptolite to organic enrichment and gas shale reservoir: A case study of the Wufeng–Longmaxi shales in South China

The graptolitic shale of the Wufeng–Longmaxi Formations is widely deposited across the Ordovician and Silurian transition in South China, which is the target of shale gas exploration and development within China. The contribution of graptolites to organic enrichment and reservoir of gas shale is discussed below based on the statistics of nearly 1000 shale samples from the Wufeng Formation and the bottom part of the Longmaxi Formation in the southern and northern margins of the Yangtze plate. The assessment involves graptolites abundance, the total organic carbon (TOC) content analyses, and the different scales of scanning electron microscopy analyses of related samples. The TOC content of the Wufeng–Longmaxi graptolitic shales (including graptolites and non-graptolites, i.e., the host shale) is mainly controlled by that of its host shale, while less affected by the graptolites abundance, indicating that the graptolites barely influence the organic enrichment. Graptolites consist of a large number of organic matter with reticular biological tissue structure; they account for 20%–50% of the graptolitic area. The aforementioned also developed honeycomb-shaped pores with pore sizes ranging 110 nm-1.7 μm (an average of about 500 nm), which are higher than those of the organic pores in the host shale (108–770 nm, average 330 nm), proving that graptolites have an important contribution to shale gas storage space. Since there are a large number of graptolites within the shales from the Wufeng Formation and the bottom part of the Longmaxi Formation, the laminated and stacked local pattern of their distribution provides abundant storage space for shale gas. Moreover, the feature also serves as the predominant channel for shale gas flow. Therefore, the widely developed graptolites should be considered as one of the essential factors controlling enrichment and high productivity of shale gas in the Wufeng–Longmaxi Formations.

Natural gas potential of Carboniferous and Permian transitional shales in central Hunan, South China

The Carboniferous Ceshui formation and Permian Longtan and Dalong formations were deposited in transitional settings preserved in what is now central Hunan Province, South China, as they are potential natural gas plays. In this study, we analysed the total organic carbon (TOC), vitrinite reflectance (Ro), kerogen type, mineralogy, porosity, permeability, and methane adsorption of representative shale samples from these rock units. Our results indicate that TOC content can be as high as 9.2%, with a mean (x̄) of 3.5%. The Permian shale formations were deposited in more strongly reducing environments than the Carboniferous Ceshui shale. The kerogen composition of the Carboniferous Ceshui shale is dominated by Type III, while both of the Permian shales contain primarily Type II kerogens; Ro values range from 1.1% to 2.4% (x̄ = 1.6%). The organic matter in all the studied shales is in the wet gas window of thermal maturity and is relatively less mature than Lower Palaeozoic marine shales in south China. Mineral compositions are dominated by quartz (x̄ = 53.8%) and clay (x̄ = 35.6%), suggesting a high brittleness index. Porosity ranges from 0.5% to 14% (x̄ = 6.4%), while permeability varies from 0.0026 micro Darcy (mD) to 0.0640 mD (x̄ = 0.0130 mD). The gas adsorption capacity varies from 1.24 to 4.53 cm3/g (x̄ = 2.40 cm3/g). Relatively less mature shale samples (Ro<1.5%) have low methane adsorption capacities, regardless of their TOC values. However, the methane adsorption capacity of more mature (Ro>1.5%) shales samples exhibit a positive correlation with TOC content.

A comparative study of characterization of lower Palaeozoic Niutitang shale in northwestern Hunan, China

The lower Palaeozoic Cambrian Niutitang shale and Silurian Longmaxi shale in the upper Yangtze region are the two most favourable marine shales in south China. Great achievements on shale gas development and production have already been made in the Longmaxi shale in southeastern Sichuan (SES). However, to date, economic shale gas potential has not been proved in northwestern Hunan (NWH), which is still a virgin area for shale gas exploration. Based on detailed outcrop investigation, laboratory testing and numerical simulation, the shale gas potential of Niutitang shale were evaluated in this work. Comparisons were also made through vertically diachroneity contrasting with the Longmaxi shale in the NWH area and horizontally isochronic contrasting with Niutitang shale in the SES area. Through analysing the evolution of the palaeoenvironment and palaeotectonics of the period from Niutitang to Longmaxi, it was found that: (1) the NWH Niutitang shale was developed in the hydrothermal continental shelf paleoenvironment, which provided a favourable environment for shale deposition, so the thicker and higher total organic carbon (TOC) shale was largely developed during this time; (2) By the influence of hydrothermal igneous leaching, the element Si formed quartz, which improved the brittleness of the Niutitang shale; (3) Induced by intense tectonic movement, natural fractures were highly developed in the Niutitang shale, which improved the reservoir connectivity; and (4) By comprehensively considering the TOC, reservoir property, and gas retention conditions, the Niutitang shale is a promising target for shale gas development in this area. The results from this work warrant further exploration work in the Niutitang shale in the NWH area.

Geochemical investigation of the lower Cambrian mineralised black shales of South China and the late Devonian Nick deposit, Canada

Lower Cambrian (Tommotian) black shales of the Niutitang Formation, South China, host a thin accumulation (5–20 cm) of Ni, Mo, platinum group elements (PGE)-Au, Ni, As, Zn, Cu, V and rare earth elements (REE). Of all known deposits, the late Devonian Ni-Mo-PGE sulphide horizon in black shales of the Nick deposit, Selwyn Basin in Canadian Yukon, presents the strongest similarity with the Niutitang Formation polymetallic layer. In the present study, samples of the mineralised layer and host shales in South China from two mine sites (Zunyi and Sancha) were investigated in order to further characterise the prevailing redox conditions and mechanisms involved in the metal concentration process at both sites. Additional comparison with the Nick deposit mineralised layer was also undertaken. Bulk geochemical analysis (including PGE analysis) were conducted on all samples, along with μm-scale investigation of metal distributions and associations by X-ray fluorescence (XRF) mapping and Synchrotron XRF mapping. The comparison of the two sites from South China highlights strong similarities in metal enrichment factors. PGE and REE distribution patterns are also highly similar and suggest a source from seawater. Strong variations in elemental distributions at µm-scale were observed at both sites, most likely related to intense variations in redox conditions in the sediment. The comparison with the Nick deposit highlights a stronger enrichment in Ni and Zn in the Nick deposit and a greater variety of minerals in the Niutitang shales. The PGE and REE distributions of both the Nick deposit and the Niutitang shales, however, present highly similar patterns, and support an origin from seawater. The present study provides further insight into mineralisation style and processes in these mineralised black shales, highlighting the importance of redox conditions and re-emphasizing the role of organic matter in the formation of these mineralised layers.

Pore connectivity and tracer migration of typical shales in south China

Pore connectivity of shale is a critical factor in shale gas migration and production. In order to investigate the pore connectivity from three typical shale formations in south China (Longmaxi, Niutitang, and Longtan shales), we conducted complementary analyses with field emission-scanning electron microscopy (FE-SEM), mercury intrusion capillary pressure (MICP), gas (CO2 and N2) sorption isotherm, contact angle, spontaneous imbibition, as well as saturated diffusion with tracer distribution mapped with laser ablation-inductively coupled plasma-mass spectrometry. The use of water and n-decane fluids enables us to examine the connectivity of hydrophilic and hydrophobic pore networks of the shale, and the resultant imbibition slopes demonstrate that Longmaxi shale has a high connectivity from its high-development of both well-connected organic and inorganic pores. Limited diffusion distances and associated high tortuosity (from both diffusion and MICP analyses) are consistent with the predominant presence of nano-sized pore throats (3–50nm) from gas sorption isotherm and MICP tests. Pore types, development degree, and connection are directly supported by FE-SEM-imaging. Overall, the variable and limited pore connectivity of shale samples will affect hydrocarbon preservation and recovery.

Origin of early Cambrian black-shale-hosted barite deposits in South China: Mineralogical and geochemical studies

The barite deposits hosted in early Cambrian black shales in South China are a classic example of sediment-hosted stratiform barite deposits, and their origin is controversial. In this paper, we address the origin of these deposits by a comprehensive integrated study of mineralogy and geochemistry, based on a case study of the Dahebian deposit in Tianzhu County, Guizhou Province, which contains the largest global reserve of barite. Particular attention was paid to a comparison between ore and non-ore horizons, as this has not been fully addressed previously. The results indicate that the ore horizon is dominated by barite with intergrown hyalophane, quartz and pyrite. The hyalophane is zoned, implying hydrothermal activity. In contrast, the non-ore horizons contain minor amounts of granular barite and are free of hyalophane, suggesting that these horizons experienced weaker hydrothermal activity. The contrasting trace and rare earth element compositions of the ore and non-ore samples indicate dilution of complex geochemistry with multiple ore-forming processes and materials during barite mineralization. The barite mineralization was rapid, as demonstrated by the relatively low terrigenous input in the ore samples compared with the non-ore samples. The ore samples have positive Eu anomalies (>3.0), indicative of hydrothermal activity during mineralization. Organic geochemical analyses provide evidence of the development of abundant organic matter and hydrothermal activity. The sulphur isotopic values (δ34S) of the ore samples show an enrichment relative to contemporaneous early Cambrian seawater and substantial variation (+36.7‰ to +43.8‰), indicative of sulphur derived from seawater that was affected by sulphate-reducing bacteria and hydrothermal activity in a restricted marine environment. To summarize, the barite deposit in the Dahebian site was most likely formed in a generally restricted and reducing marine basin environment with rapid sedimentation during a period of hydrothermal activity. These results have general implications for the future study of other black-shale-hosted deposits worldwide.

Geochemistry of major and trace elements and Pb–Sr isotopes of a weathering profile developed on the Lower Cambrian black shales in central Hunan, China

This paper reports a geochemical study on the major and trace elements and Pb–Sr isotopes of a weathering profile developed in the Lower Cambrian black shales in central Hunan (China). Six weathering horizons were identified and sampled vertically throughout the profile. The chemical composition of the profile consists of variable concentrations of the major elements Fe2O3, FeO, MnO, MgO, CaO, Na2O, and P2O5 and of less variable concentrations of SiO2, TiO2, Al2O3, and K2O. The chemical change caused by weathering is estimated by mass-balance calculations, and the results show that the element mobility is characterised by substantial loss of SiO2, FeO, CaO, K2O, Na2O, LOI, Cr, V, Ba, Cs, Rb, Sr, U, and Th, and moderate loss of Al2O3, MgO, Fe2O3, Ni, Cu, Pb, Tl, Sn, Sc, Ge and REE (Y). The high field strength elements TiO2, Sn, Sc, U, Ga, Ge, Zr, Hf, Nb, and Ta were immobile during weathering. The chemical changes and the Pb–Sr isotopic data suggest that four types of chemical reactions occurred: the oxidation of sulphide minerals (e.g., pyrite) and organic carbon (OS), the dissolution of less resistant clinochlore-Ia, calcite, and P-bearing minerals (DL), the dissolution of detrital albite and microcline (DA), and the transformation of clay (TC) minerals (e.g., muscovite and illite–smectite). These chemical reactions then led to two stages of geochemical processes, an early stage of chemical differentiation and a later stage of chemical homogenisation. The chemical differentiation dominated by the OS, DL, and DA reactions, led to the leaching of mobile elements (e.g., MgO, Na2O, K2O, P2O5, Sr, and REE) and the redistribution of some less mobile elements (e.g., SiO2 and Al2O3). In contrast, the chemical homogenisation, which was caused by TC reactions, led to the leaching of both mobile and less mobile elements from the system and ultimately transformed the weathered black shales into soil. Soils derived from black shales in South China might result from the above two geochemical processes.

Fluctuation of shelf basin redox conditions in the early Ediacaran: Evidence from Lantian Formation black shales in South China

Finely laminated, pyrite- and organic-rich black shales that bear a euxinic interpretation dominate the Ediacaran Lantian Formation in southern Anhui, South China. However, these black shales preserve benthic sessile algae and possible metazoans in situ, suggesting that the Lantian shelf basin may have had at least episodically oxidized bottom water during the early Ediacaran Period. Redox conditions of the fossiliferous Lantian black shales were examined by petrological analysis, pyrite framboid and δ34S measurements, and redox-sensitive trace element (RSTE) analysis. Lamina-by-lamina pyrite framboid measurements show that Lantian black shales were deposited under fluctuating redox conditions. The δ34S values of pyrites range from −20.5‰ to 2.3‰ for different laminae and display less negative values than the earliest Ediacaran black shales, supporting a return to a relatively low seawater sulfate concentration after the early Ediacaran oxidation event. Similarly, the Lantian black shale RSTE concentrations (e.g., Mo, U, and V) are low compared to previously published results from the early Ediacaran and constant from lamina to lamina, suggesting a drop in ocean wide RSTE concentrations occurring independently of frequent local redox condition changes. These frequent redox condition changes in the Lantian shelf basin may have facilitated the productivity and preservation of macroscopic Lantian biota.

Affirming life aquatic for the Ediacara biota in China and Australia

The Ediacara biota has been long championed as a snapshot of the marine ecosystem on the eve of the Cambrian explosion, providing important insights into the early evolution of animals. Fossiliferous beds in the eponymous Ediacara Member of South Australia have been recently reinterpreted as paleosols and Ediacara fossils as lichens or microbial colonies that lived on terrestrial soils. This reinterpretation, here dubbed the terrestrial Ediacara hypothesis, would fundamentally change our views of biological evolution just prior to the Cambrian explosion. We take a comparative paleobiology approach to test this hypothesis. The Ediacara Member shares a number of forms with assemblages in Ediacaran marine black shales in South China, shales that show no evidence of pedogenesis. Thus, the shared Ediacara fossils, and by extension other co-occurring fossils, are unlikely to have been terrestrial organisms. A terrestrial interpretation is also inconsistent with functional morphological evidence; some of the shared forms are not morphologically adapted to address the most critical challenges for terrestrial life (e.g., mechanical support and desiccation). Thus, the terrestrial Ediacara hypothesis can be falsified on comparative paleobiological and functional morphological grounds, and we urge paleopedologists to critically reevaluate evidence for pedogenesis in the Ediacara Member and other Ediacaran successions.

Mo isotope and trace element patterns of Lower Cambrian black shales in South China: Multi-proxy constraints on the paleoenvironment

The Precambrian/Cambrian boundary marks a critical interval in Earth history. Paleoenvironmental changes of the oceans have been suggested as a possible explanation for some significant events such as the marine biological evolution (“Cambrian explosion”). Here we report a remarkable variation range in trace element, iron speciation and molybdenum isotope signatures of black shales from the Early Cambrian Dingtai profile (Guizhou province) and three synsedimentary polymetallic Ni–Mo–(PGE–Au) sulfide ore deposits (Dazhuliushui and Maluhe in Guizhou province, and Sancha in Hunan province) on the Yangtze Platform, South China. Based on the (isotope) geochemical features, we classify the Dingtai profile into three intervals: interval 1 (0–1m) has extreme metal and TOC enrichment and heavy δ98/95Mo values. This unit has all features of a euxinic environment, and is then followed by interval 2 (1–11m) with variable δ98/95Mo composition. Elevated contents of redox-sensitive elements and their ratios, and relatively high FeHR/FeT ratios (“reactive iron”) suggest anoxic conditions. The overlying interval 3 (11–33m) reveals oxic to suboxic conditions with generally light δ98/95Mo values in the lowermost part and fluctuant δ98/95Mo values in the uppermost part, and relatively low redox-sensitive element contents. We propose a model of a partly closed ocean basin with periodic replenishment together with changing redox conditions for the remarkable variability of δ98/95Mo values. Light δ98/95Mo values indicate restricted seawater circulation dominated by local continental input, periodically replenished by fresh seawater with heavy δ98/95Mo.A global compilation of Mo isotope data from black shales indicates that the Mo isotopic variations have a consistent temporal trend, reflecting the oxygen evolution of the oceans. Relatively light δ98/95Mo values of the Lower Cambrian black shales compared to those of modern euxinic sediments are indicative of more widespread anoxic marine environments during the Early Cambrian.

Biostratigraphy and geography of the Ordovician-Silurian Lungmachi black shales in South China

Based on the new material of seven Ordovician-Silurian boundary sections investigated recently, together with previously published data, we analyze the temporal and spatial distributions of the Lungmachi black shales, a key petroleum source bed widely distributed in South China. The Lungmachi black shales range in age from the Normalograptus persculptus Biozone of the uppermost Ordovician to the Spirograptus guerichi Biozone of the lower Telychian, and ten graptolite biozones can be recognized within this unit. The basal and upper contacts of the Lungmachi black shales are diachronous. The basal contact ranges from the N. persculptus to the C. cyphus biozones, a span of five graptolite biozones over two stages. The upper contact ranges from the D. pectinatus-M. argenteus Biozone to the Spirograptus guerichi Biozone, which spans four graptolite biozones over two stages. The Yichang Uplift resulted in the formation of the Hunan-Hubei Submarine High in the border area of Hubei, Hunan, and Chongqing. This is supported by a break in sedimentation in this area spanning all or part of the Hirnantian, and in many areas extending into the underlying Katian and overlying Rhuddanian. Comparison of the distribution of the Katian to Rhuddanian strata in this area indicates a growth and subsequent reduction in area of the Hunan-Hubei Submarine High particularly in the Hirnantian to early Rhuddanian. This may partly represent the influence of the process of formation and melting of ice sheet in Ordovician South Pole and consequent sea level change.

Tectonic and eustatic control on the distribution of black-shale source beds in the Wufeng and Longmaxi formations (Ordovician-Silurian), South China

It is known that high-quality, black-shale source rocks occur in the uppermost Ordovician Wufeng Formation and in the lowermost Silurian Longmaxi Formation in South China. Hence, it is important to understand their lithostratigraphy and the controls on their deposition. A review of lithostratigraphic criteria for subdividing the two adjacent formations provides new regional correlations between the formations and related stratigraphic successions and facies. Both the black shales and the related, overlying flysch deposits at the Ordovician-Silurian transition in South China appear to have migrated northwestward in time and space, reflecting probable flexural control in a foreland basin that developed in response to subduction-type orogeny southeast of the Yangtze block. The black shales also contain K-bentonites from explosive, felsic-intermediate volcanism, the distribution of which also supports orogeny to the southeast. Finally, the analysis of sequence stratigraphy, which shows that the initiation of transgressive system tracts (TST) and condensed section (CS) in the related third-order sequences coincided with the two black-shale horizons respectively, indicates that the main controlling factors for the deposition of the Ordovician-Silurian black shales in South China are (1) northwestwardly migrating, foreland-basin subsidence caused by deformational loading related to episodic accretion of the Cathaysia block to the Yangtze block during this period, and (2) the anoxic, sediment-starved water column caused by rapid rise of the sea-level during the two successive phases of third-order global sea-level rise near the Ordovician-Silurian transition in South China. In future exploration for hydrocarbon source rocks in the area, it is important to consider likely flexural and eustatic causes for subsiding, deep, anoxic seas in recognizing other source rock intervals, and our understanding of the Wufeng and Longmaxi formations may serve as models for future source rock exploration.

A C/MoS2mixed-layer phase (MoSC) occurring in metalliferous black shales from southern China, and new data on jordisite

A new phase composed mainly of Mo, S, and C and referred to herein as MoSC occurs widely in organic-rich, metalliferous Cambrian black shales in south China. MoSC, which has previously been referred to as jordisite, has been studied by scanning electron microscopy (SEM), electron microprobe analysis (EMPA), transmission electron microscopy (TEM), powder X-ray diffraction, (XRD), extended X-ray absorption fine structure (EXAFS), and catalytic activity. TEM data show MoSC to have a layered structure, with packets resembling molybdenite and graphite-like carbon that average five layers in thickness. Analytical data are consistent with an idealized formula of Mo3S6C10 but it commonly contains 1–3 wt% each of Fe, Ni, and As so that its composition may be better approximated by the formula (Mo,Fe,Ni)3(S,As)6C10. Selected area electron diffraction (SAED) patterns show a small number of broad, inhomogeneous rings corresponding to randomly oriented layers arranged in subspherical cells. A single broad, weak peak corresponds to a 10–11 A layer spacing in powder XRD diffraction patterns. Pseudomorphism after fossil bacteria implies an origin by replacement of sedimentary organic material. In its chemical properties and structure, MoSC resembles synthetic compounds used as catalysts for hydrodesulfurization (HDS) in the petrochemical industry. The large surface-to-volume ratio for MoSC may be an important factor in its relatively strong HDS catalytic activity. Cotype samples of jordisite from Germany, previously thought to be amorphous MoS2, were also studied by SEM and TEM. Jordisite occurs as sequences of a few curved layers that form sub-spherical units, with an appearance remarkably like that of MoSC. However, the layer spacing is ~6 A, like that of molybdenite. The ratio of Mo:S is ~1:2, and no carbon was detected, although it coexists with kerogen. Jordisite is thus confirmed to be a form of MoS2, but because powder diffraction-like SAED patterns were obtained, it is not amorphous.

Submarine-hydrothermal exhalative ore layers in black shales from South China and associated fossils — insights into a Lower Cambrian facies and bio-evolution

Early Cambrian (Tommotian) black shales in South China contain submarine-hydrothermal exhalative sulfide ore layers within an arc-parallel linear belt extending more than 1600 km. Evidence for an Early Cambrian hydrothermal venting is given by the records of main and trace elements, rare earth elements (REE), sulfur isotopes and the petrological and geological investigations. Mass occurrences of arthropods, sponges and undetermined shelly remains co-occur with hydrothermal vent deposits in the basal Niutitang Formation at the Sansha (Hunan Province) and Zunyi sections (Guizhou Province). This fauna, although not considered as vent fauna sensu stricto, appears linked to the hydrothermal vents via primary and secondary consumption on lithochemotrophic primary producers. Mats of sheathed sulfur bacteria are common in contemporaneous black shales nearby the vent localities. The ‘Cambrian explosion’, an event of probably rapid biological diversification, is discussed here in context of adaptation to shallow water habitats and major changes in trophic resources. According to our new hypothesis metazoans may have already existed in deep-sea hot vent areas during the Precambrian. This unique ecological niche also supplied favourable habitats for metazoans during the Neoproterozoic supposedly world-wide Varanger glaciation. Cessation of deep ventilation and expansion of dysoxic conditions into shelf areas may have driven the migration of metazoans from hydrothermal vents to shallow water areas in the earliest Cambrian thus enhancing both evolutionary opportunities and the chance of preservation in the geologic record.

Early Paleozoic whole-rock Ce anomalies and secular eustatic changes in the Upper Yangtze region

The 65 whole-rock REE samples studied come from the classic sections of Early Cambrian to Early Silurian in the Yichang area, Hubei Province. REE concentrations tested on ICP-AES are normalized to the North American Shale Composite (NASC), and Ce anomaly values are calculated based on log[3Ce n /(2La n +Nd n )]. With the whole-rock Ce anomaly used as a quantitative index of sea-level changes, 5 prominent eustatic circles are recognized, and a good corresponding relationship is observed between the black shale episodes and the transgressive events. On account of the global distributions of coeval black shales and the specified paleogeographic background of South China, a genetic hypothesis based on the model of O 2 - minimum zone expansion is suggested for the Early Paleozoic multi-episode black shales in South China.