Deposition Of Organic-rich Shales Research Articles

Statistical estimation of the early to middle Ediacaran ocean redox architecture in the Yangtze block of South China

Early to middle Ediacaran organic-rich black shales host several famous fossil biotas and provide key evidence for understanding the coevolution of multicellular organisms and palaeoceanic environment. The water column redox states play critical roles in organic-rich shale deposition. Amongst multiple geochemical redox indexes, iron-speciation chemistry (FeHR/FeT, Fepy/FeHR) in shales constitutes a reliable proxy to reconstruct the first-order redox framework of the ocean basin. However, iron-speciation is a local marine redox proxy, and only compiled data collected from multiple different sedimentary facies record statistically significant changes of oxidation states within a depositional basin. Here we compiled the spatiotemporal distribution of iron-speciation data in early to middle Ediacaran black shales of the Yangtze block in South China. New high-resolution analysis on two outcrops and one drill-core reflects overall ferruginous condition and dominant euxinic condition locally interrupted by oxic states, respectively, generally indicating pervasive anoxic depositional environment for the Member II shales of the Ediacaran Doushantuo Formation in the Lower Yangtze block. Compilation of fourteen Ediacaran sections from shallow-, slope- and deep-water facies demonstrates frequent spatiotemporal oscillation of the redox conditions throughout the Yangtze block. Widespread ferruginous states, accompanied with euxinic zones mainly focusing in lower slope facies, are detected during the early Ediacaran period, in contrast with more widespread euxinic settings in the middle Ediacaran period. Statistical data including new high-resolution results from the Lower Yangtze block show generally low enrichment for redox sensitive elements (RSEs, e.g., Mo, V and U) in the Member II shales probably due to their contemporary low seawater concentrations. An increase of RSEs concentrations occurs in the Member IV shales, which probably reflects more oxidized Earth surface environment during middle Ediacaran period. The increasing Earth oxygenation documents the occurrence of widespread euxinic states during deposition of the Member IV shales. Compilation of spatial distribution of TOC contents in black shales demonstrates their apparent relevance to spatial distribution of the euxinic rather than ferruginous states, which is likely due to the favorable role of sulfurization process on burial and preservation of organic matters. The euxinic states may also facilitate the exceptional preservation of the soft-bodied Ediacaran fossils.

Astronomically paced climate and carbon cycle feedbacks in the lead-up to the Late Devonian Kellwasser Crisis

Abstract. Repeated carbon isotope excursions and widespread organic-rich shale deposition mark the Middle and Upper Devonian series. Various explanations such as extensive volcanism and land plant evolution have been given for these perturbations and the general sensitivity of the Devonian oceans to the development of anoxia, but their repeated nature suggests that astronomical forcing may have controlled their timing. Here, a cyclostratigraphic study of the Kellwasser Crisis at the Frasnian–Famennian stage boundary (ca. 372 Ma) is carried out. The Kellwasser Crisis was one of the most ecologically impactful of the Devonian perturbations and is ranked among the “Big Five” Phanerozoic mass extinctions. The studied site is the Winsenberg roadcut section in the Rhenish Massif, Germany, which represents a quiet tropical shelf basin setting. Centimetre-scale elemental records, generated by portable X-ray scanning, allow for testing of the hypothesis that a 2.4 Myr eccentricity node preceded the Upper Kellwasser event. The study's results are supportive of this hypothesis. We find enhanced chemical weathering (K2O / Al2O3) during the period leading up to the Upper Kellwasser and a peak in distal detrital input (SiO2 / CaO) and riverine runoff (TiO2 / Al2O3) just prior to the start of the Upper Kellwasser event. We interpret this pattern as the long-term eccentricity minimum facilitating excessive regolith build-up in the absence of strong seasonal contrasts. The Earth's system coming out of this node would have rapidly intensified the hydrological cycle, causing these nutrient-rich regoliths to be eroded and washed away to the oceans, where they resulted in eutrophication and anoxia. An astronomical control on regional climate is observed beyond this single crisis. Wet–dry cycles were paced by 405 kyr eccentricity, with both the Lower and Upper Kellwasser events taking place during comparatively drier times. A precession-sensitive monsoonal climate system prevailed on shorter timescales. Intensification of this monsoonal system following the node may have caused the widespread regolith erosion. We estimate the total duration of the Kellwasser Crisis at ca. 900 kyr, with the individual events lasting for ca. 250 and 100 kyr, respectively. If astronomical control indeed operated via regolith development in monsoonal climates, then land plants may have played an important role. This would not have been through evolutionary steps directly triggering Kellwasser perturbations but by gradually strengthening the climatic response to orbital forcing via soil development – creating soils thick enough to meaningfully respond to orbital forcing – and intensifying the hydrological cycle.

Reconfiguring oxygenation at ∼1.4 Ga: New constraints as informed by the ancient oceanic sulfur cycle

The extent of atmospheric and oceanic oxygenation during the Mesoproterozoic remains an area of active debate. Here, we report major and trace elements, organic sulfur (OS) speciation, sulfur isotopes in multiple phases [pyrite (py), OS, and carbonate-associated sulfate (CAS)], and a numerical model of sulfate concentrations from one of the Mesoproterozoic's best-preserved geochemical archives, the Xiamaling Formation (ca. 1.4 Ga) in the Yanliao Basin. These data reveal a previously unrecognized ocean-atmosphere oxygenation process. Starting with Unit 3, intense sulfurization increased organic carbon burial and thus oxygen release, resulting in a deep-water oxidation event within the Yanliao Basin that could be regarded as a transient restricted oxygen oasis based on low pyrite content, elevated kerogen S:C ratios, higher δ34Spy than δ34SOS (Δδ34SOS-py < 0), and a low calculated sulfate concentration of ∼0.07–0.14 mM. During Unit 2, the basin was reconnected to the open ocean with the deposition of organic-rich shale across northern China and northern Australia, accompanied by a shift in Δδ34SOS-py to 5.0‰ and a calculated seawater sulfate concentration of ∼0.1–1.14 mM. Such large-scale burial of organic matter released oxygen into the atmosphere, accompanied by significant sulfate delivery to the basin and euxinic conditions characterized by Mo concentrations and 34S-depleted pyrite at the topmost of Unit 2. A subsequent globally synchronized atmospheric and deep ocean oxygenation event may have occurred during Unit 1. This event was recorded by a progressive increase in Δδ34SOS-py (up to +9.6‰), increased oxidized OS species, and positive Ce anomalies (attributed to the reductive dissolution of the Ce-enriched Mn oxides), all of which were caused by increased sulfate concentrations (calculated to be ∼1.15–2.31 mM) and a spatial descent of the redox interface. The combination of these lines of evidence illustrates the sensitivity of the oceanic sulfur cycle to the ocean-atmosphere system and provides novel constraints on oxygenation at ∼1.4 Ga.

Re-Os geochronology of the Middle to Upper Jurassic marine black shales in the Agardhfjellet Formation, Central Spitsbergen, Svalbard: A cornerstone for global faunal correlation and Os isotopic change

Ammonite provincialism makes it difficult to link the Boreal and Tethyan realms during the Late Jurassic to Early Cretaceous time. Absolute ages through radiometric dating offer new age information that is independent and complementary to relative ages based on biostratigraphy. In this study, we report seven new Re-Os ages for black shales from the Agardhfjellet Formation, Svalbard. We also report pyrolysis results, carbon and sulfur stable isotopic data, and initial 187Os/188Os ratios to evaluate the paleoenvironment during organic-rich shale deposition. Re-Os ages are derived from three Boreal ammonite zones: (1) 159.2 and 160.1 Ma from the shale intervals containing Cadoceras sp.; (2) 149.6, 149.9, 150.8, and 151.9 Ma from the Rasenia cymodoce Zone in the Early Kimmeridgian, which is the equivalent of the Sutneria platynota and Ataxioceras hypselocyclum zones in the Tethyan realm; (3) 146.8 Ma from an upper Volgian interval above the occurrence of Laugeites sp.. Within this 14.5 Myr interval, the initial Os profile shows a gradually increasing trend from 0.335 (one of the lowest shale values throughout the Phanerozoic) to 0.529, consistent with previous initial Os studies and Sr isotopic ratio chemostratigraphic studies. The trend of increasing initial Os ratios indicates an increase in continental runoff relative to unradiogenic Os input from mantle sources, such as seafloor hydrothermal activity or ultramafic source rocks. We provide new Re-Os geochronological anchors in the Late Jurassic with new Re-Os ages and propose correlations of specific Late Jurassic ammonite zones between the Tethyan and Boreal realms. In turn, the gradual increase in seawater Os isotopic ratios may signal climate change in the Late Jurassic, indicating the increase of radiogenic Os or decrease of unradiogenic Os, induced by high continental runoff or less hydrothermal/volcanic activity.

Differences in and factors controlling organic matter enrichment in the Ziliujing Formation shale in the Sichuan Basin

Lacustrine shale oil and gas are important fields for unconventional exploration and development in China, and organic-rich shale deposition lays down the critical foundation for hydrocarbon generation. There are two sets of shale, the Dongyuemiao and Da’anzhai Members, in the Ziliujing Formation in the Sichuan Basin. To identify the differential enrichment characteristics of organic matter and clarify its controlling factors, geochemical analyses of organic and inorganic geochemical analyses were performed. The results showed that the total organic carbon content of the Dongyuemiao shale (1.36%) is slightly higher than that of the Da’anzhai shale (0.95%). The enrichment of organic matter in the two shales resulted from the comprehensive controls of paleoproductivity, paleoenvironment, and terrigenous input, but different factors have different effects. In addition, driven by climate, the change in the sulfate concentration in the bottom water further led to the different intensities of bacterial sulfate reduction in early diagenesis. This made a great difference regarding organic matter accumulation in the two members. In general, climate may have played a dominant role in organic matter enrichment in the two sets of shale.

Anomalous weathering trends during the Early Silurian warming: Implications for the biotic crisis and recovery

The Late Ordovician mass extinctions were divided into two distinct stages, with the second stage occurring during the Late Hirnantian. However, biotic recovery was delayed until the Aeronian and the triggering factors are still unknown. To comprehend the connections between paleoclimate, marine redox states, and biotic recovery in the Early Silurian, we analyzed temporal variations in chemical weathering intensity using multi-geochemical proxies for the shales of the Longmaxi Formation in South China. The results reveal a significant increase in chemical weathering intensity from the Rhuddanian warming phase to the Aeronian cooling phase. This increase is anomalous, considering it coincides with the Early Silurian's climate cooling. This study posits that Rhuddanian sediments exhibited low chemical weathering intensity and more congruent weathering, while Aeronian sediments were characterized by intermediate chemical weathering intensity and incongruent weathering. Given that the gradual enrichment of oxygen within bottom waters can be supported by Corg/P and Corg-S-Fe, this study suggests that congruent weathering-induced high paleoproductivity is the primary factor accelerating anoxia in the Rhuddanian. In summary, the persistent marine anoxia of seawater thus triggered the deposition of organic-rich shales and delayed biotic recovery in the Rhuddanian.

Sequence stratigraphic analysis of Devonian organic-rich shales in northern Guangxi

Deep-water sediments are widely developed in Devonian sedimentary strata around the world. The Devonian's unique climate change, relative sea-level fluctuations, biological evolutionary stages and tectonic activities resulted in the widespread deposition of organic-rich shales across the globe in this era. The Devonian deep-water facies shales in northern Guangxi are widely distributed along the syn-sedimentary faults, such as the Ziyun–Luodian–Nandan–Du'an ancient fault zone and the Yishan fault, forming a series of special depositional successions under the deep-water system. Based on the stratigraphic cyclicity and spatial distribution of the Devonian strata, in conjunction with the total organic carbon content (TOC) of the dark shales at the Tonggong, Luzhai, Dongxing and Shuiyuan sections, it is determined that the Devonian system in northern Guangxi can be divided into 13 third-order sequences. Additional, the organic-rich shales, which are mainly composed of carbonaceous Tentaculitic shales in SQ5–SQ11 (1) developed in the TSTs of SQ5–SQ9 and mainly distributed in the Nandan–Hechi region in the west of the study area, and (2) developed in the HSTs of SQ10–SQ11 and extended along the fault zone from Nandan to the Luzhai region in the east of the study area. The spatial and temporal distribution of these Devonian organic-rich shales indicate that their formation is controlled by a relative sea-level change. The current study provides important clues for future unconventional oil and gas exploration and development in northern Guangxi.

Multi-Proxies Analysis of Organic Matter Accumulation of the Late Ordovician–Early Silurian Black Shale in the Lower Yangtze Region, South China

The evolutional process of palaeoceanic environment and its effect on the accumulation of organic matter during the Ordovician–Silurian transition in Lower Yangtze region has been overlooked compared to that in Upper Yangtze region of South China, although their paleogeographic settings were expected to be discrepant. This paper documents the marine depositional environment, paleoclimate, and sediment supply changes, and discusses their roles in controlling the organic matter enrichment in sedimentary rocks within the Ordovician–Silurian transition of the Lower Yangtze region, using the latest geochemical data of the continuous drilling core. The stratigraphic framework of the Ordovician–Silurian transition in the Lower Yangtze region is composed of two third-order sequences, each of which can be subdivided into a lower TST (transgressive systems tract) and an upper RST (regressive systems tract). TST1 represented an evident depositional transition stage which was marked by the ending of the underlying carbonate sediments and the initiation of the terrigenous clastic-dominated sediments. Geochemical proxies indicate that the relatively low productivity, dysoxic water column condition, and high sediment supply flux collectively resulted in inadequate organic matter hosted in deposits of the TST1. During the depositional period of RST1, the global sea level declined due to the Hirnantian glaciation age. The icehouse also caused the decrease in overall river flux and, thus, the terrigenous clastic sediment supply. The icehouse also strengthened the upwelling that occurred in the Lower Yangtze sea. The upwelling boosted the marine algae explosion through the delivery of abundant nutrients, which not only enhanced paleoproductivity but also led to an anoxic environment by oxygen consumption. Such high paleoproductivity, anoxic water column environment, and low sediment supply flux caused the deposition of organic-rich shale. The sea level rose during the TST2 due to the ending of an ice age. The relatively large water depth and high paleoproductivity associated with volcanic eruptions are the main factors that caused the enrichment of organic matter during this stage. During the deposition of RTS2, the increase of sediment supply flux resulted in a decrease in accommodation space and water depth and the dilution of organic matter in deposits, which was the primary constraint of organic matter accumulation.

Controls on Organic Matter Accumulation from an Upper Slope Section on the Early Cambrian Yangtze Platform, South China

The early Cambrian witnessed profound environmental changes and biological evolution in Earth’ history. During this period, organic-rich shales were widely distributed over almost the entire Yangtze Block. However, the dominant factor that drove the significant accumulation of organic matter (OM) remains controversial and is still debated. Here, we analyzed TOC, organic carbon isotopes, iron speciation, major and trace elements for the lower Cambrian Niutitang Formation in the upper slope Meiziwan section, to investigate the dominant factor controlling OM accumulation. High contents of TOC and Baxs reveal an OM-enriched feature of the Niutitang Formation, and the coupled relationship between them suggest a strong production control on OM accumulation at Meiziwan. Meanwhile, negative relationships between TOC and chemical index of alteration (CIA) values as well as Al contents suggest that influence of chemical weathering and terrestrial input on OM accumulation were limited. Fairly low CoEF × MnEF values provide strong evidence that the deposition of organic-rich shales was under the control of oceanic upwelling event. The upwelling event would bring nutrient-rich deep waters into surface water, stimulating phytoplankton bloom and primary productivity in surface water and facilitating OM enrichment. Meanwhile, enhanced accumulation of OM would have promoted subsequent bacterial sulfate reduction, leading to the occurrence of occasional euxinia (evidenced by iron speciation and redox-sensitive trace element data) and promoting preservation of OM. Taken together, our results shed light on the critical role of oceanic upwelling on the marine primary productivity on the earliest Cambrian Yangtze Platform.

Shale gas accumulation patterns in China

Although significant progress has been made in the geological theory and exploration and development of shale gas in China, the research on the geological particularity and enrichment regularity associated with shale gas accumulation remains weak. With this in mind, we analyzed the shale gas accumulation patterns in China. The findings show that: (1) A variety of organic-rich shales were deposited in the continental margin-type marine and transitional environments during the Pre-Mesozoic period and the basin-type terrestrial environment during the Mesozoic and Cenozoic eras. (2) The basins dominated by Mesozoic and Cenozoic terrestrial sediments superimpose on the Pre-Mesozoic continental margin-type marine–or transitional-facies sediments. The former developed the terrestrial shales with large thickness, high organic matter abundance, new age, and low thermal maturity; meanwhile, the deep parts of the basins are favorable for shale gas enrichment. However, the latter preserves the shale with old age, high thermal maturity, and undergoing multiphase tectonic movements; and the tectonically stable regions are favorable for shale gas enrichment. (3) The aulacogens mainly formed in the Meso-Neoproterozoic, Sinian-Ordovician, and Devon-Triassic periods are favorable for marine organic-rich shale deposition; moreover, this type of shale and other types including transitional facies shales with favorable reservoir-forming assemblages, coal-measure shales in small and medium-sized basins, and low-medium mature shales affected by magma intrusion, have jointly constituted the various shale gas accumulation patterns in China. (4) Based on the drilling discoveries in China, shale gas accumulation patterns can be categorized into two types: tectonic-controlled and stratigraphic-controlled. The former is mainly related to structural features, whereas the latter is mainly related to the variation of lithology and strata. (5) Shale development and gas content are mainly affected and controlled by geological factors including sedimentation, diagenesis, and later tectonic movements. Favorable sedimentary environments, appropriate thermal maturity, good sealing capacity of caprock, and excellent structural preservation are the necessary conditions for shale gas enrichment.

Isotopic evidence for changes in the mercury and zinc cycles during Oceanic Anoxic Event 2 in the northwestern Tethys, Austria

The Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE 2, ca. 94 Ma) was one of the most extreme carbon cycle and climatic perturbations of the Phanerozoic Eon. Widespread deposition of organic-rich shales during OAE 2 has been attributed to a rapid rise in atmospheric CO2, global heating, and marine anoxia triggered by intense large igneous province (LIP) volcanism. Here, we present new Hg and Zn elemental and isotopic analyses from samples spanning OAE 2 in a hemipelagic section from Rehkogelgraben, Austria, which was part of the north-western Tethys. We compare our data to existing records from a range of sites to constrain the relative timing, magnitude and geographic extent of the perturbation. We find a prominent Hg concentration peak and an overall positive Δ199Hg excursion, with no correlation between Hg content and organic matter (OM), Mn-Fe-oxyhydroxides, and/or clay minerals. We interpret this to indicate a terrestrial volcanic origin of Hg. The Hg excursion is coincident with an osmium (Os) isotope excursion, and together, this supports a global period of intense LIP volcanism. The δ66Zn record from the Rehkogelgraben section decreases abruptly by ~0.5‰ prior to the onset of OAE 2, a change recorded consistently among all reference sections. Combined with the Hg data, we interpret this to result from isotopically light Zn sourced from LIP activity. However, the second negative excursion in δ66Zn during the Plenus Cold Event (PCE), which is recorded in the proto-North Atlantic and adjacent areas and has been attributed to Zn released from OM during re‑oxygenation, is not recorded in this section. We suggest that the cool, oxygenated deep water mass did not invade the Penninic Ocean in the northwestern Tethys. Alternatively, this excursion could be missing in our section due to the presence of carbonate-free sediments during the PCE. After the PCE, the positive excursion in δ66Zn recorded in all sections reveals a recovery of the atmosphere-ocean system. Our findings highlight the significance of spatial and temporal variations in Hg and Zn isotopes during OAE 2.

New high-precision U-Pb zircon age of the Irati Formation (Paraná Basin) and implications for the timing of the Kungurian anoxic events recorded in southern Gondwana

The early Permian Irati Formation is well known for its black shale oil and gas reserves and its widespread lateral correlation throughout the entire Paraná Basin. This unit records an effective water circulation restriction between the Whitehill-Irati Sea and the Panthalassa Ocean, which along with other factors, controlled the organic-rich shale deposition in several southern basins of Gondwana. The Permian was also a period of intense magmatism in this region due to the Terra Australis Orogen (Gondwanides Orogeny). Several volcanoclastic horizons are preserved within the sedimentary record, and in this study, a high-precision chemical abrasion isotope dilution U-Pb zircon age of 275.75 ± 0.29 Ma for a volcanic tuff in the Irati Formation of the southern Paraná Basin is presented. The Kungurian age agrees with previous published radioisitopic and relative ages for the Assistência Member (upper Irati Formation) and Mangrullo Formation, a stratigraphical correlated unit in Uruguay. In addition, zircon trace elements and whole-rock geochemistry allow the constraining of the magmatic characteristics of the volcanic source and its similarity with the lower Choiyoi magmatism (Argentina). Therefore, this study contributes to the understanding of the timing of black shale-bearing rocks of the Irati Formation and the anoxic events recorded in the southern Paraná Basin.

Basic characteristics of key interfaces in Upper Ordovician Wufeng Formation – Lower Silurian Longmaxi Formation in Sichuan Basin and its periphery, SW China

Based on anatomy of key areas and data points and analysis of typical features of shell layer in Guanyinqiao Member, basic characteristics of key interfaces, mainly bentonite layers, in the Upper Ordovician Wufeng Formation—Lower Silurian Longmaxi Formation in the Sichuan Basin and its surrounding areas and the relationship between these key interfaces with the deposition of organic-rich shale have been examined systematically. The Wufeng Formation—Longmaxi Formation has four types of marker beds with interface attributes, namely, the characteristic graptolite belt, Guanyinqiao Member shell layer, section with dense bentonite layers, and concretion section, which can be taken as key interfaces for stratigraphic division and correlation of the graptolite shale. The shell layer in Guanyinqiao Member is the most standard key interface in Wufeng Formation—Longmaxi Formation, and can also be regarded as an important indicator for judging the depositional scale of organic-rich shale in key areas. There are 8 dense bentonite sections of two types mainly occurring in 7 graptolite belts in these formations. They have similar interface characteristics with the shell layer in Guanyinqiao Member in thickness and natural gamma response, and belong to tectonic interfaces (i.e., event deposits). They have three kinds of distribution scales: whole region, large part of the region, and local part, and can be the third, fourth and fifth order sequence interfaces, and have a differential control effect on organic-rich shale deposits. The horizon the characteristic graptolite belt occurs first is the isochronous interface, which is not directly related to the deposition of organic-rich shale. Concretions only appear in local areas, and show poor stability in vertical and horizontal directions, and have no obvious relationship with the deposition of the organic-rich shale.

Characteristics of rock types and exploration significance of the shale strata in the Chang 73 sub-member of Yanchang Formation, Ordos Basin, China

For a long time, the Chang 73 sub-member of the Ordos Basin was mostly investigated as a single source rock, with hardly any investigation into the characteristics of various rock types or the integration of source and reservoir. Based on correlation analysis of lithology and physical properties combined with a fine interpretation of logging, the characteristics of the Chang 73 sub-member are discussed in detail, and the reservoir characteristics and exploration significance of different types of rocks are analyzed. The results show that: (1) There are five types of lithology in the Chang 73 sub-member: black shale, dark mudstone, fine sandstone, siltstone, and tuff. (2)The storage space of sandy rocks is primarily made up of feldspar dissolution pores and residual intergranular pores, which are held together by rigid grains. The sandstone with high feldspar and quartz mineral content, low carbonate cementation, low clay mineral content, and a large accumulated thickness is the optimal shale oil reservoirs of Chang 73 sub-member. (3) The Chang 73 sub-member has the most organic-rich shale deposits. Clay mineral intercrystalline pores, pyrite intercrystalline pores, and clastic grain intergranular pores make up the majority of the reservoir space. Although organic matter pores are largely undeveloped, organic matter contraction fractures are prevalent. Despite the porosity and pore size of shale in the Chang 73 sub-member being microscopic, it possesses specific reservoir performance and hydrocarbon fluid mobility. It is estimated that the total amount of retained hydrocarbon resources in Chang 7 Member shale is about 15 × 109 t, with movable hydrocarbon resources of about 6 × 109 t; (4)The oil saturation of Chang 73 sub-member is generally higher than that in Chang 71 and Chang 72 sub-members. Several factors play a key role in rapidly increasing shale oil production, including further optimizing the vertical and plane distribution characteristics of sand bodies, improving the drilling encounter rate of sandstone in horizontal wells, strengthening the research of pilot production scheme, and ensuring sufficient oil-water infiltration and replacement. (5) Optimizing black shale with high brittleness index and oil content, developing high-efficiency permeability improvers, adopting pressure drive production integrated design, increasing fracture width and height, and improving the conductivity of artificial fractures are all important technical means to realize shale oil production.

Paleoenvironment and Organic Matter Accumulation Mechanism of Marine–Continental Transitional Shales: Outcrop Characterizations of the Carboniferous–Permian Strata, Ordos Basin, North China

In the Carboniferous–Permian period, several organic-rich black shales were deposited in a marine–continental transitional environment in the Linfen area on the eastern margin of the Ordos Basin. Integrated sedimentological and organic geochemical analyses are performed on an outcrop in order to clarify the relationship between paleoenvironment and organic matter accumulation. The results of this study show that the marine–continental transitional strata of the Upper Carboniferous Benxi Formation to Lower Permian Taiyuan and Shanxi Formation exposed in the Linfen area are composed of sandstone, shale, coal, and limestone. Total organic carbon (TOC) contents of the studied samples were mainly distributed in the range of 0.59%–35.4%, with an average of 7.32%. From Benxi Formation to Shanxi formation, the humidity gradually increased, and the climate gradually changed from hot and humid to warm and humid during Carboniferous to Permian. The deposition of the Shanxi Formation ended with the climate returning to hot and humid, having an oxic-suboxic conditions and a high paleoproductivity. Paleoredox conditions and paleoproductivity are the two vital factors controlling the formation of organic matter in black shales. The transitional environment characterized by oxic-suboxic, relatively high deposition rate, and various source of organic matter, although different from the marine environment, provides a good material basis for the deposition of organic-rich shales.

Controls on silica enrichment of lower cambrian organic-rich shale deposits

The factors controlling silica enrichment of the Lower Cambrian sedimentary succession have been disputed for a long time. This paper describes results of a petrological and geochemical investigation of Lower Cambrian shale deposits of the intraplatfrom basin of the Yangtze Block, South China, with the goal of elucidating potential controls on silica enrichment of these deposits. Excess silica (Sixs) concentrations in the studied deposits suggest that silica was generally enriched in Lower Cambrian units, including the Maidiping Formation (MDP) and the overlying Qiongzhusi Formation (QZS). Greater excess silica concentrations in the MDP relative to the QZS may reflect the influence of three factors: continental weathering, aeolian input, and biological activity. Corrected chemical index of alteration (CIAcorr) values and Ti/Al ratios of the MDP suggest that strong chemical weathering increased the delivery of dissolved silica to the global ocean during the episodes of warm, humid paleo-climate conditions. This, in tandem with diminished aeolian input that accompanied the humid climate, favored establishment of a silica-rich ocean during Fortunian and early Stage 2 time. The presence of coeval massive chert-phosphorite deposits may be attributed to blooms of phytoplankton and siliceous organisms (mainly sponge spicules) induced by upwelling of nutrient-rich bottom waters (e.g., Si and P). However, the abundance of low-diversity sponge spicules was likely insufficient to achieve silica supersaturation of seawater by itself so that direct precipitation of aqueous silica from Si-enriched seawater and pore water may have partly contributed to silica enrichment of the MDP. Diminished CIAcorr values and elevated Ti/Al ratios of the QZS suggest the increased terrestrial and aeolian inputs into the global ocean under a relatively cool, arid paleo-climate condition during middle Stage 2 to the Stage 3 time thereby reducing aqueous silica concentrations. The present study suggests that the interaction among paleo-climate fluctuations, increased delivery of nutrients to the ocean, and redox variability was critical to the diversification of sponge spicule and the appearance of radiolarian during the Cambrian Explosion.

INSIGHTS OF SHALE GAS EXPLORATION AND DEVELOPMENT IN INDIA: REVIEW

Encouraging results of shale gas production in different countries viz. USA, China and Australia, India is also inspired to explore shale gas resources, which may not only support meeting the growing energy demand but also assist to reduce the import of hydrocarbons. This paper aims to review the current status of shale gas exploration programs in various sedimentary basins in India for a better understanding of the prospect of shale gas. The signicant thickness of organic-rich shale deposits is reported in various Gondwana and Tertiary sedimentary basins including Damodar valley, Krishna-Godavari, Cauvery, Cambay, Assam-Arakan, Ganga and Vindhayan basins. The geochemical properties like TOC and thermal maturity, Organo-inorganic mineral compositions, pore-network system, and depth of shale deposits are favoring the potential prospect of shale gas in India. Several agencies namely EIA, ONGC, USGS, Schlumberger, and CMPDI reported enormous resources of shale gas in its various sedimentary basins.

The impact of volcanic activity on the deposition of organic-rich shales: Evidence from carbon isotope and geochemical compositions

The organic-rich black shales from the Late Ordovician and early Silurian were identified as essential source rocks for shale gas exploration and development in South China. To investigate the influence of volcanic activity on the deposition of organic-rich shales in the Wufeng and Longmaxi formations, this study presents the carbon isotope and geochemical compositions of the Upper Ordovician-lower Silurian succession of the Yangtze Platform. The organic-rich Wufeng and lower Longmaxi shales are characterized by isotopically negative carbon shifts; however, obviously high δ13Corg values are shown in the organic-poor shales of the upper Longmaxi Formation. The negative δ13Corg excursion is attributed to 12C-depleted CO2 sourced from volcanic activity. Furthermore, the positive δ13Corg excursion in the upper Longmaxi Formation occurs because a large amount of 12C was removed from the ocean-atmosphere system through continuous burial of organic carbon. The biogenic silica (Sibio) contents and (Cu + Ni)/Al ratios in the lower Longmaxi Formation display higher values than those in the upper Longmaxi Formation, suggesting that the primary productivity gradually decreased from the Wufeng Formation to the Longmaxi Formation. This study proposes that high primary productivity was related to accelerated chemical weathering and release of micro-nutrients. The former emphasizes that high pCO2 and globally high temperatures can enhance chemical weathering, so that more nutrients are transported from continents to oceans (Yan et al., 2010, 2019). The latter highlights that volcanic material input can act as fertilizer by releasing many nutrients. Moreover, the values of UEF and MoEF are higher in the Wufeng and lower Longmaxi formations than in the upper Longmaxi Formation, suggesting that the oxygen concentration increased upward through the section. Given the positive correlations between indicators of productivity and redox conditions, the constantly high primary productivity would consume much O2, resulting in anoxic conditions in the Wufeng and lower Longmaxi formations. Overall, this study considers that volcanic activity-driven primary productivity is the first-order control for organic matter (OM) enrichment.

Upper Cretaceous paleoenvironmental changes and petrophysical responses in lacustrine record (Songliao Basin, NE China) and marine sedimentary deposit (Goban Spur Basin, NW Europe)

AbstractThe Cretaceous interval is marked by several important geological changes whose prints are buried in both continental and marine sytems. Although significant paleoenvironmental details of this period have been inferred from biological and geochemical indicators, little is known about the physical proxies. Through scientific borehole data, petrophysical properties of Upper Cretaceous Songliao Basin (SB) in NE China and Goban Spur Basin (GSB) in NW Europe were intercorrelated to investigate the critical geological paleoenvironmental shifts and their petrophysical responses, through statistical, wavelet and spectral approaches. The results demonstrated that petrophysical features, particularly gamma-ray and resistivity reactivities, were responsive to past environmental changes in both terrestrial and marine systems. Shifts in organic-rich shale deposition and brine bearing shale showed a correlation to a probable period of seawater incursion in SB, while the gamma log, resistivity and density reactivities were interrelated to the basin paleo-structuration. At GSB, the gamma-ray and resistivity reactivities are tied-up to the Mid-Atlantic seabed motion, marine-water level shifts and paleoceanographic instabilities. In both paleo-basins, a decrease in the gamma-ray reactivity occurred from Turonian to Maastrichtian and is consistent with a regional or global increase in hydrodynamic energy. The oceanic/lacustrine anoxic events related to low sedimentation rate occurred in both basins and are associated with high gamma-ray and resistivity signals (SB); high gamma-ray and low resistivity signals (GSB). These changes correlated with geochemical evidence, suggesting that gamma-ray and resistivity can represent alternative means for marine and continental paleoenvironmental comparison.

The palaeoenvironmental context of Toarcian vertebrate-yielding shales of southern France (Hérault)

AbstractThe Early Jurassic was marked by several episodes of rapid climate change and environmental perturbation. These changes culminated during the Toarcian Oceanic Anoxic Event (T-OAE), an episode of global warming that led to the widespread deposition of organic-rich shales. The Toarcian shales of NW Europe have also yielded exceptionally preserved fossils of marine vertebrates and invertebrates, but the potential links between the occurrences of these exceptionally preserved fossils and the T-OAE remain poorly investigated. Palaeontological excavations realized in Toarcian strata near Lodève (Hérault, southern France) have yielded several specimens of marine vertebrates and abundant invertebrate fauna. We have developed a multiproxy approach (ammonite biostratigraphy, X-ray diffraction-bulk mineralogy, Rock-Eval pyrolysis, stable isotopes, trace element, phosphorus and mercury contents) to place these findings in a well-defined temporal and palaeoenvironmental context, and hence constrain the factors that led to their remarkable preservation. The Jenkyns Event interval, unambiguously identified at the base of the Toarcian organic-rich shales by a 5‰ negative carbon isotope excursion, records higher mercury fluxes, which suggest a causal link with intense volcanic activity of the Karoo–Ferrar large igneous province. This interval is very condensed and unfossiliferous, and might have been deposited under abnormally low-salinity conditions. Our data show that the deposition of the vertebrate-yielding horizons post-dated the T-OAE by several hundreds of ka, and took place during a prolonged period of widespread oxygen-deficiency and elevated carbon burial. Our results indicate that the unusual richness in vertebrates of the studied site can be explained by a combination of regional factors such as warming-induced, prolonged seafloor anoxia, and more local factors, such as extreme condensation owing to reduced dilution by carbonate and detrital input.