Ordovician-Lower Silurian Research Articles

Estimation of original total organic carbon content and hydrogen index using major and trace element concentrations in the overmature Upper Ordovician–lower Silurian Wufeng–Longmaxi marine shales, southeast Sichuan Basin, south China

Estimation of original total organic carbon content and hydrogen index using major and trace element concentrations in the overmature Upper Ordovician–lower Silurian Wufeng–Longmaxi marine shales, southeast Sichuan Basin, south China

Upper Ordovician–Lower Silurian deposits in the Zakola section (Ilych River, Northern Urals): Composition, structure, formation environments

Research subject. Section of boundary deposits, exposed in the right side of the Zakola stream (the right tributary of the Ilych River, Northern Urals) with a total thickness of 98 m, presented with rock samples (more than 100 pieces), thin sections (100 pieces), slabs (10 pieces), and chemical analyses (15 pieces). Aim. Detailed description of the most complete section of the Upper Ordovician–Lower Silurian boundary sediments of the Shantym facies complex with a stratigraphic reference of layers, lithological typification of sediments, and restoration of sedimentation conditions. Methods. Conventional lithological and paleontological methods were used to dissect the section. Sampling was carried out after 1 m; however, samples were additionally taken when detailed observations were possible to identify lithological and facial differences. The thin sections were studied by the classic method of optical microscopy. The study of the fauna complex was carried out both by collecting samples of the author’s rocks and by the materials of A.I. Antoshkina. In addition, stock materials were attracted. Results. In the section, 14 packs of rocks were identified, which are represented by 6 lithological types of secondary dolomites. Their distribution along the section, along with the presence of characteristic faunal remains, made it possible to establish three members in the section, which can be correlated with the Katian, Hirnantian, and Ruddanian stages of the General Stratigraphic Scale. Conclusion. The Katian deposits are represented by the Yaptikshor Formation (Strata 1), which was formed under the conditions of the lower sublittoral of a ramp-type carbonate platform with a transgressive carbonate tract and a diverse fauna. The beginning of the Hirnantian (Strata 2) is characterized by shallow water conditions of the upper sublittoral up to the littoral, which reflects a sharp shallowing at the Katian–Hirnantian boundary and corresponds to the beginning of the global early Hirnantian regression. The rocks exposed higher up the section indicate sedimentation with a certain rise in the sea level; however, the correlation of this part of the section with the upper part of the Verkh Ruchey Formation is complicated by the fragmentary nature of the outcrops. The overlying Ruddanian deposits (Strata 3) were accumulated on the outskirts of a ramp-type carbonate platform under the conditions of the lower sublittoral, which corresponds to the beginning of the Early Silurian global transgression.

Rare Earth Element Characteristics of Shales from Wufeng-Longmaxi Formations in Deep-Buried Areas of the Northern Sichuan Basin, Southern China: Implications for Provenance, Depositional Conditions, and Paleoclimate.

To explore the sedimentary environment and the background of the source area of organic-rich shales in the Wufeng-Longmaxi Formations in the northern Sichuan Basin, samples from Well XX1 in the area were subjected to geochemical testing and analysis of organic carbon content, trace elements, and rare earth elements (REEs). The results show that the total content of REE (ΣREE) of the shale in the Wufeng-Longmaxi Formations varied from 183.08 to 234.66 μg/g with an average of 212.59 μg/g, which is significantly higher than the content of the North American shale composite. The fluctuations in the total amount of REEs in the shale of the Wufeng-Longmaxi Formations reflect certain differences in the geochemical conditions of the Upper Ordovician-Lower Silurian shale. The ratios of LREE/HREE, LaN/YbN, LaN/SmN, and GdN/YbN and the distribution of normalized REE patterns indicate that the source supply or sedimentary structural background may have changed during the shale deposition period of the Wufeng Formation, while the shale deposition period of the Longmaxi Formation may be in a relatively stable source supply and sedimentary structural background. There is no significant correlation between δCe and ΣREE, and the obviously negative Eu abnormity and the weak Ce abnormity indicated that the diagenesis had a limited impact on REEs. Geochemical parameters such as values of ∑REE, δEu, δCe, Ceanom, and LaN/YbN indicate that the climate during the Wufeng-Longmaxi Formation shale deposition period was warm and humid, and the shale was deposited mainly in the suboxic-anoxic water environment. The deposition rate was stable and slow, providing good conditions for the production and preservation of organic matter. At the same time, this shows that the water environment of Wufeng Formation is more anoxic and reductive than that of Longmaxi Formation, which is more conducive to the preservation of organic matter. The correlation between ΣREE and the content of Sc, Ti, Cr, Co, Zr, Nb, Th, Hf, Ta, and other elements indicates that the sources of REEs in the shale of Wufeng and Longmaxi Formations in the study area are similar, mainly terrestrial clasts, and some may come from the sea. The REE distribution pattern shows that the shale provenance of the Wufeng-Longmaxi Formations mainly comes from the upper crust. The La/Yb-∑REE diagram shows that the sediment-parent rocks are mainly early sedimentary rocks and these sediment-parent rocks have granite provenance characteristics. Compared to La/Yb, LREE/HREE, LaN/YbN, and other REE characteristic parameters, it is inferred that the tectonic background of the study area is dominated by passive continental margin.

Role of magmatism in efficient gas accumulation in Upper Ordovician-Lower Silurian shales in the Yangtze plate, South China: Evidence from the calcite U-Pb ages and gas C-N isotopes

Shale gas exploration in the Yangtze plate has been hindered by strong heterogeneity of gas compositions resulting from magmatism. Borehole core and gas samples from Upper Ordovician-Lower Silurian in the Upper Yangtze plate and the Lower Yangtze plate were analyzed to explore the effects of magmatism on gas accumulation. The analyses included nitrogen isotope of gas, pore structure and X-ray photoelectron spectroscopy of shales, and U-Pb ages of fracture-filling calcites. Our results suggest that magmatism caused the development of micro-fractures in shale reservoirs and accelerated the thermal evolution rate of organic matter. Magmatism in the Upper Yangtze plate predates the peak of gas generation of Upper Ordovician-Lower Silurian shales. Generation and expulsion of hydrocarbon at high temperatures removes 12C from carbon reservoir. After the magmatism, the continuous subsidence of the strata led to further hydrocarbon generation of organic matter. Moreover, the self-sealing property of shale reservoirs were enhanced by the closure of micro-fractures, development of graphite structures, and enrichment of 13C in residual methane, which favor the efficient accumulation of shale gas. Conversely, magmatism in the Lower Yangtze plate occurred after the peak of gas generation of Upper Ordovician-Lower Silurian shales. Magmatism compromised shale gas reservoirs by disrupting sealing conditions, facilitating methane diffusion and atmospheric nitrogen influx. Additionally, overburden pressure led to the collapse of graphitized OM-hosted pores, further inhibiting gas accumulation. Our findings underscore the pivotal role of magmatism in various stages of shale gas accumulation and are critical for exploration strategies in sedimentary basins with prevalent magmatic activities.

Geological and geochemical characteristics of Late Ordovician volcanic levels of the Gözdağ Formation in the İstanbul-Zonguldak Tectonic Unit (NW Turkey): Implications for global events during the Ordovician

The İstanbul-Zonguldak Tectonic Unit is regarded as the easternmost fragment of Avalonia-Carolina and stands for Far East Avalonia. Its stratigraphy also is characterized by discontinuous sedimentation from Late Ediacaran to Late Carboniferous. In the western part of the block, known as the İstanbul Terrane, the Gözdağ Formation is represented by lagoonal sedimentary rocks consisting of shale-sandstone with limestone of Middle Ordovician-Lower Silurian age. Here, I report on stratigraphic positions and petrographical and geochemical data of fine- and coarse-grained tuffs and lavas in the Late Ordovician strata of the Gözdağ Formation. The fine- and coarse-grained tuffs have pyroclastic and the lavas have porphyritic, vitrophyric and aphanitic textures. The fine-and coarse-grained tuffs are Sandbian and Katian ages, and the lavas are Hirnantian ages, according to the stratigraphic positions of the Late Ordovician volcanic rocks. Geochemically, the-fine grained tuffs have High-K calc-alkaline character and the coarse-grained tuffs and lavas have calc-alkaline affinity. They are devoid of noticeable with-in plate components, as deduced by the presence of obvious negative Nb anomalies, and they have subduction signatures. In conjunction with the data from literature, the Sandbian fine-grained tuffs were deposited in a lagoonal depocenter in the İstanbul-Zonguldak Tectonic Unit during the Taconic orogeny formed in the earliest late Ordovician between the Piedmont Terrane and Laurentia. The Katian coarse-grained tuffs were deposited in a shallow marine in it during the last phase of the consumption of the Teisseyre-Tornquist Ocean from volcanoes occurred in volcanic arc-related settings. Besides, I the Hirnantian lavas flowed in a lagoonal depocenter in it during the soft-docking of Avalonia and Baltica, known as the pre-Caledonian orogeny.

Franklinian Composite Tectono-Sedimentary Element, Canadian Arctic Islands

Abstract The Neoproterozoic–Devonian strata of the Canadian Arctic Islands record three Tectono-Sedimentary Elements (TSEs): (i) the Neoproterozoic Synrift TSE; (ii) the Neoproterozoic–Late Devonian Franklinian Margin TSE composed of Cambrian–Ordovician carbonate-dominated passive-margin strata and latest Ordovician, Silurian and Lower Devonian mixed carbonate and clastic strata deposited on an unstable margin; and (iii) Middle–Upper Devonian Foreland Basin TSE deposited in an expanding foreland basin. These three TSEs comprise the Franklinian Composite Tectono-Sedimentary Element (CTSE). In the southern parts of the Arctic Islands, strata are flat lying but there is localized faulting and folding in the central Arctic Islands and towards the northern end of the Franklinian CTSE. These strata of the Franklinian Margin and Foreland Basin TSEs were broadly folded over deeply seated reverse faults localized along Late Silurian–Early Devonian contractional uplifts or deformed by widespread Late Devonian thin-skin folds and thrusts related to the Ellesmerian Orogeny. The parts of the CTSE overprinted by Late Silurian–Early Devonian north–south-trending structures are grouped as Boothia Structural Domains, and the area of the CTSE overprinted by Ellesmerian structures is termed the Ellesmerian Structural Domain. The major hydrocarbon source rock is Upper Ordovician–Lower Silurian shale but maximum thermal maturity over much of the area was achieved by the Late Devonian, prior to the formation of large folds. The complex geological history of the area since the Late Devonian reduces the chances of hydrocarbon preservation.

Differential sedimentary mechanisms of Upper Ordovician-Lower Silurian shale in southern Sichuan Basin, China

The Wufeng-Longmaxi Formations are the focus of shale gas exploration in China. Their sedimentation is affected by global and regional geological events, and the resulting heterogeneity hinders the expansion of exploration results. This study combines logging, mineralogy, and geochemistry to reconstruct the sedimentary mechanisms of shale in the southern Sichuan Basin and discusses its implications for exploration. The shale can be divided into third-order sequences, in which the Wufeng Formation and lower Long-1 Member are transgressive systems tracts (TST1 and TST2), and the Guanyinqiao Member and the middle-upper Long-1 Member are highstand systems tracts (HST1 and HST2). Volcanic eruption and glaciation evolution are the driving factors of the sedimentary environment during the TST. The area is in a cold-warm climate, characterized by anoxic conditions, slow sedimentation, booming productivity, and low terrigenous influx, thereby depositing four types of siliceous shale. The siliceous shale is rich in biogenic microcrystalline quartz and high TOC, which are the keys to becoming the “sweet spot” of shale gas. Microcrystalline quartz fills the intergranular pores to support the entire pore network framework. The organic matter undergoes thermal evolution to produce abundant organic matter pores, which constitute the main place for adsorbed gas and free gas. For the highstand systems tract, HST1 is the product of the peak of glaciation, mainly composed of calcareous shale and carbonate rocks. The sedimentation of HST2 is affected by turbidity currents and violent orogeny. The area is in a warm climate, characterized by high terrigenous influx, dysoxic-oxic conditions, medium productivity and rapid sedimentation rates. The lithofacies gradually changed from siliceous shale to mixed shale and argillaceous shale. Excessive clay minerals and low TOC make it difficult for HST2 to realize the commercial development of shale gas.

Late Ordovician-Early Silurian extension of the northern margin of the Upper Yangtze Platform (South China) and its impact on organic matter accumulation

Breakthroughs in shale gas exploration in the Upper Ordovician-Lower Silurian strata of the Upper Yangtze Platform have attracted interest in its sedimentary-tectonic evolution, but the tectonic background of the northern margin of the Upper Yangtze Platform remains unclear. In this paper, the Wufeng-Longmaxi formations on the northern margin of the Upper Yangtze Platform were investigated. Based on geochemical and mineralogical analyses of the tuffs/K-bentonites of the Wufeng Formation and the barite in the Longmaxi Formation, as well as previous research results, it was concluded that the northern margin of the Upper Yangtze Platform was in an extensional tectonic background during the Late Ordovician-Early Silurian. Detailed analysis revealed that, (1) the U–Pb zircon age of the tuff in the Bajiaokou section in South Qinling is 443.91 ± 0.92 Ma. The Zr/TiO2–Nb/Y diagram of the tuffs/K-bentonites indicates that their protoliths were alkaline-subalkaline basalt and andesite series rock. Based on the Th–Hf/3-Ta, Th–Tb*3-Ta*2, and TiO2–Nb/3-Th diagrams, there are undiscovered intraplate tension calc-alkaline basalts in the northern Yangtze Platform or the southern Qinling region, which provided volcanic clastic materials to the Ziyang, Lan'gao, Chengkou, Yichang and other regions. (2) Scanning electron microscopy revealed that the barite crystals in the Longmaxi Formation exhibit dissolution features and have a large particle size. Energy spectrum analysis of these barite crystals revealed that they have C, O, S, and Ba contents of 8.48 wt%, 22.98 wt%, 13.09 wt% and 55.44 wt%, so they are speculated to have been formed via cold methane seep genesis in a weak extensional tectonic setting. The 87Sr/86Sr ratios of the barite revealed that different types of barite were simultaneously formed in this area under the influences of hydrothermal and cold methane seeps. (3) The analysis of the heavy minerals in the Lower Silurian strata in the Bajiaokou section revealed that the provenance in the South Qinling area changed significantly during the late Early Silurian. Based on the above analyses, the northern margin of the Upper Yangtze Platform was in an extensional tectonic setting during the Late Ordovician-Early Silurian. The distribution of the total organic carbon content indicated that the extensional tectonic background provided good conditions for the enrichment and preservation of organic matter. The results of this study provide an understanding of the regional sedimentary-tectonic pattern and evolution of the Yangtze Platform during this period, as well as a reference for future shale gas exploration in this region.

Petrological and geochemical characteristics of the Ordovician–Silurian black shale in eastern Sichuan and western Hubei, South China: Differential sedimentary responses to tectonism and glaciation

During the Ordovician–Silurian transition period, the Yangtze area experienced significant environmental changes, which were recorded by the widely distributed black shale of the Upper Ordovician Wufeng and Lower Silurian Longmaxi formations. However, the petrological and geochemical analyses of the Upper Ordovician-Lower Silurian black shale in Well JY1 (eastern Sichuan Basin) and Well YY2 (western Hubei Province) drill cores show that there are some significant differences with respect to lithology, thickness, and geochemical characteristics despite the long-term similar variation trend in petrology and geochemistry. This may suggest some major environmental changes and their differential influence on black shale deposition in the Yangtze area. Based on the analyses of lithology, total organic carbon (TOC), MoEF, and UEF, four long-term sedimentary cycles (one for the Wufeng Formation and three for the Longmaxi Formation) were identified in Well JY1. In Well YY2, only three long-term sedimentary cycles can be recognized (one for the Wufeng Formation and two for the Longmaxi Formation). Different from the gradual increase of the geochemical proxies (such as TOC, MoEF, and NiEF) followed by an abrupt decrease in the Wufeng Formation, these proxies of the Silurian sedimentary cycles in both wells exhibit an abrupt increase at the base followed by a gradual decrease, showing that controlling mechanisms for the Lower Silurian black shale deposition in the study areas were different from those of the Upper Ordovician black shale. However, the black shale of each Silurian cycle in Well JY1 is much thicker than that in Well YY2. Compared with the thick silty mudstone developed in Well JY1 during the Rhuddanian–Aeronian transition, there is a stratigraphic gap in Well YY2 except for a thin calcareous dolostone formed in a strongly restricted environment. These differences indicate that tectonism and glacial eustasy should be the key factors for black shale development, but their influences on the deposition of the Lower Silurian black shale varied between eastern Sichuan Basin and western Hubei province. Since the early Rhuddanian stage, continuous tectonic subsidence in eastern Sichuan Basin promoted the development of thick black shale, while a persistent tectonic uplift in western Hubei Province led to the formation of thin black shale. Meanwhile, the study areas experienced a significant sea-level drop related to intensified glaciation during the Rhuddanian–Aeronian transition, which is recorded by the low chemical index of alteration (CIA) values in both wells. This resulted in the formation of a stratigraphic gap in western Hubei Province and the deposition of thick silty mudstone in eastern Sichuan Basin. Moreover, abrupt increases in CIA, TOC, MoEF, and NiEF observed in the lower part of the Aeronian black shale in the two wells suggest that eastern Sichuan Basin and western Hubei Province experienced a significant sea-level rise during the early Aeronian stage, which may be related to a global warming event. Subsequently, eastern Sichuan Basin and western Hubei Province both suffered a significant tectonic uplift related to the Kwangsian Movement leading to the termination of the Aeronian black shale formation. However, the thickness variations of the Aeronian black shale in the two study areas indicate that remarkable geomorphological differences persisted in these two areas until the middle Aeronian stage. Thus, this study not only reveals differences of the Upper Ordovician-Lower Silurian black shale in eastern Sichuan Basin and western Hubei Province, but also interprets the causes of these differences, which provide new data and perspectives for the understanding of global paleoclimate and sea-level changes during the Ordovician–Silurian transition.

Mineralogy and Geochemistry of the Upper Ordovician and Lower Silurian Wufeng-Longmaxi Shale on the Yangtze Platform, South China: Implications for Provenance Analysis and Shale Gas Sweet-Spot Interval

The sediment provenance influences the formation of the shale gas sweet-spot interval of the Upper Ordovician–Lower Silurian Wufeng–Longmaxi shale from the Yangtze Platform, South China. To identify the provenance, the mineralogy and geochemistry of the shale were investigated. The methods included optical microscopy analysis, X-ray diffraction testing, field-emission scanning electron imaging, and major and trace element analysis. The Wufeng–Longmaxi shale is mainly composed of quartz (avg. 39.94%), calcite (avg. 12.29%), dolomite (avg. 11.75%), and clay minerals (avg. 28.31%). The LM1 interval is the shale gas sweet-spot and has the highest contents of total quartz (avg. 62.1%, among which microcrystalline quartz accounts for 52.8% on average) and total organic carbon (avg. 4.6%). The relatively narrow range of TiO2–Zr variation and the close correlation between Th/Sc and Zr/Sc signify no obvious sorting and recycling of the sediment source rocks. Sedimentary sorting has a limited impact on the geochemical features of the shale. The relatively high value of ICV (index of compositional variability) (1.03–3.86) and the low value of CIA (chemical index of alteration values) (50.62–74.48) indicate immature sediment source rocks, probably undergoing weak to moderate chemical weathering. All samples have patterns of moderately enriched light rare-earth elements and flat heavy rare-earth elements with negative Eu anomalies (Eu/Eu* = 0.35–0.92) in chondrite-normalized diagrams. According to Th/Sc, Zr/Sc, La/Th, Zr/Al2O3, TiO2/Zr, Co/Th, SiO2/Al2O3, K2O/Na2O, and La/Sc, it can be inferred that the major sediment source rocks were acidic igneous rocks derived from the active continental margin and continental island arc. A limited terrigenous supply caused by the inactive tectonic setting is an alternative interpretation of the formation of the sweet-spot interval.

Paleodepositional conditions and organic matter accumulation mechanisms in the Upper Ordovician-lower Silurian Wufeng-Longmaxi shales, Middle Yangtze region, South China

The deposition of black shales has been widely attributed to primary productivity, preservation controlled by redox conditions, and detrital dilution controlled by sedimentation rates. It is difficult to distinguish the relative importance of productivity and redox conditions in organic matter accumulation. On the Upper Ordovician-Lower Silurian Wufeng-Longmaxi black shales, we combined petrographic features, major and trace elemental proxies, and organic carbon isotopic analysis to determine the relative importance of productivity versus anoxia on organic matter accumulation and the evolution of factors controlling organic matter accumulation across the Ordovician-Silurian boundary in the Middle Yangtze region. Our research also sheds light on the temporal variations in redox conditions that caused the Late Ordovician mass extinction.Productivity, redox conditions, detrital dilution, and basin restriction changes over time as the Wufeng-Longmaxi Formations were deposited (from upper Katian to Rhuddanian stage). Based on the stratigraphic profiles of productivity (biogenic Si and P/Al), redox conditions (Mo/Al, U/Th ratios, and DOPT values), detrital fraction flux (Al2O3), and the cross-plots of biogenic Si versus Co*Mn values, our findings indicate that organic matter accumulation was a combined effect of productivity and anoxia; however, the elevated productivity caused by upwelling that introduced plenty of nutrients to the oceans was the direct factor driving organic matter accumulation. Previous research found a simultaneous variation in stratigraphic profiles of productivity and redox proxies; however, in our datasets, we found an offset between productivity geochemical proxies (biogenic Si content) and redox conditions (Mo/Al). Biogenic Si profile peaks occur stratigraphically beneath Mo/Al profile peaks. Anoxia does not coexist with increased paleoproductivity across the Ordovician-Silurian boundary in the Middle Yangtze region, and redox conditions may be disrupted by local geological events.

Geochemical Characteristics and Origin of Shale Gases From Sichuan Basin, China

Natural gases from the Taiyang (shallow), Jiaoshiba (middle), and Weirong (deep) shale gas fields in the southern Sichuan Basin were analyzed for molecular and stable carbon isotopic compositions to investigate the geochemical characteristics and gas origins. All the gases belong to shale gas from the Upper Ordovician–Lower Silurian shale and are dominated by methane with gas wetness generally less than 0.83%. The δ13C1 values are −28.5‰, −30.3‰, and −35.2‰ in Taiyang, Jiaoshiba, and Weirong shale gas fields, respectively. The extremely high thermal maturity is the controlling factor for the enrichment of 13C in methane, with a minor contribution from the heavy carbon isotope of the organic matter in the Ordovician Wufeng Formation. Fischer–Tropsch-type synthesis of hydrocarbon gas from CO2 and H2 contributes to the increase of wet gas, which results in the offset from the δ13C1∼wetness linear trend in the Taiyang and Jiaoshiba gas fields. Methane, ethane, and propane in the Taiyang shale gas field have increasing δ13C values with increasing burial depth, which is mainly caused by diffusive migration. All gases are characterized by a complete carbon isotopic reversal trend (δ13C1 > δ13C2 > δ13C3), and it is mainly caused by the reversible free-radical reactions with the conversion from alkane to alkyl groups, with some contribution from the Fischer–Tropsch-type synthesis. The results of this study will improve our understanding of the geochemical characteristics of shale gases from different burial depths and have important implications for future shale gas exploration in the deep and shallow layers.

Organic Matter Enrichment of Black Shale at the Turn of Ordovician-Silurian in the Paleosedimentary Center in Southern Sichuan Basin, Upper Yangtze Area

Abstract Herein, integrated vertical variation characteristic analysis was conducted by measuring total organic carbon (TOC), major and trace elements in Upper Ordovician-Lower Silurian (Wufeng-Longmaxi formations) black shale in Weiyuan, Luzhou, and Changning areas, southern Sichuan Basin to clarify the control of organic matter (OM) enrichment in different sublayers. According to the cycle boundary, it was divided into member 1 (submember 1 and submember 2) and member 2 from early to late Longmaxi formation, while it was divided into 4 sublayers from early to late during submember 1 depositional period. Under the warm and humid paleoclimate, high paleoproductivity, and gradually oxic paleoredox conditions, the TOC content (0.22-3.27 wt% in Luzhou area) in Wufeng formation increased gradually from early to late under the cocontrolling of tectonic framework and stagnant sea. After the glacial period, the warm and humid climate of Wufeng formation continued in Longmaxi formation, and TOC content reached the highest in sublayer 1 (2.02-4.02 wt%, 4.78 wt%, and 6.45 wt% in Weiyuan, Luzhou, and Changning areas, respectively), due to the high paleoproductivity and high sea level caused by melting glaciers, causing the extremely anoxic environment. The OM was preserved best. It remains higher in sublayer 2 (2.39 wt%, 3.69-4.18 wt%, and 3.5-3.86 wt% in three areas, respectively) because of the anoxic environment caused by deep water and the high paleoproductivity, but the temporary hot and dry environment has a certain negative impact on the OM preservation. Under the stable paleoproductivity, sublayer 3 has a character of water decreasing slowly, and the suboxic environment led to the decrease of TOC content (3.82-5.28 wt% and 3.77-4.13 wt% in Luzhou and Changning areas, respectively). Meanwhile, that in the yuan area became deeper and the TOC content became relatively higher (1.92-3.33 wt%). Under the open environment, sublayer 4 was controlled by lower sea level caused by regression, resulting in lower TOC content (1.31-4.57 wt%, 1.64-3.52 wt%, and 3.88-4.49 wt% in three areas, respectively). However, the global regression of the submember 2 period, high terrigenous debris dilution, and oxic environment lead to the no preservation of OM. Generally speaking, the enrichment of OM is not only affected by the changes of global sea level and paleoproductivity to some extent (Longmaxi shale) but also controlled by the tectonic framework in Sichuan Basin (Wufeng shale). The findings of this study can help for a better understanding of the OM enrichment mechanism and provide a theoretical basis for the evaluation of high-quality source rocks.

Continuous record of Upper Ordovician (Katian) to lower Silurian (Telychian) global δ13C excursions in the Williston Basin

AbstractThe Williston Basin retained a high‐resolution record of global δ13C fluctuations during the ~13 Ma duration Upper Ordovician–lower Silurian, and is a possible missing link in correlations between Laurentian margins and its interior. This study integrates δ13C and conodont biostratigraphy to yield a high‐resolution, continuous composite spanning from the Upper Ordovician Aphelognathus grandis Zone into the lower Silurian Distomodus staurognathoides Zone. From oldest to youngest, six global δ13C excursions are identified: (1) Whitewater (=Moe) and (2) Elkhorn (=Paroveja) in the Amorphognathus divergens Zone; (3) LHICE (KaH?) in the Aphelognathus shatzeri Zone; (4) HICE in the Ozarkodina hassi Zone; (5) Early Aeronian within the Distomodus kentuckyensis–Pranognathaus tenuis zones and (6) late Aeronian within the Psuedolonchodina expansa–D. staurognathoides zones. This δ13C record helps to alleviate some of the uncertainties in determining the precise order, duration and timing of events influencing the biotic, climatic, oceanic and sea‐level evolution of the Upper Ordovician–lower Silurian successions across Laurentia.

The Volume and Geochemical Characteristics of Desorption Gases From Wufeng–Longmaxi (O3w-S1l) Shale in the Xishui Area, North Guizhou, China

The Upper Ordovician–Lower Silurian Wufeng–Longmaxi (O3w-S1l) shale is widely distributed in the Xishui area, north Guizhou, China. However, the potential and characteristics of shale gases in the Xishui area are not well evaluated, which has limited the exploration of shale gas in this area. In this study, we collected 14 fresh core samples of Wufeng–Longmaxi Formations from Well XK-3 and conducted gas desorption experiments to evaluate the potential of shale gas resources in the Xishui area. After analyzing the chemical and isotopic composition of desorption gases, the origin and genesis of the shale gas were systematically studied. Our results show that the volume of desorption gases varies from 2.14 m3/t to 6.01 m3/t, with a mean value of 4.12 m3/t, showing great potential for shale gas in this area. The positive correlation between total organic carbon (TOC) and desorption gas volume indicates that organic matter provides the main pore volume for shale gas preservation. The desorption gases are very dry, which are mainly composed of methane (99.64‰–99.74%) with a very low content of ethane, and no nitrogen, carbon dioxide, and hydrogen sulfide are detected. Based on geochemical diagrams, the desorption gases are identified as oil-associated thermogenic gases. The carbon isotopic values of methane and ethane display reversal distribution, due to the mixing of primary gases from kerogen cracking and the secondary gases from retained oil cracking and further possible cracking of heavy gaseous hydrocarbons (C3H8—C5H12) at higher thermal maturity (Ro > 2.00%). After comparing with Jiaoshiba, Changning, Weiyuan, and Dingshan areas, the carbon isotopic values of shale gases of Wufeng–Longmaxi shale also show different features at different areas, which are mainly controlled by the thermal maturity levels of the shale.

Source of silica and its implications for organic matter enrichment in the Upper Ordovician-Lower Silurian black shale in western Hubei Province, China: Insights from geochemical and petrological analysis

To improve the understanding of the relationship between silica source and organic matter accumulation, the origin of silica and its implications for organic matter enrichment in the Upper Ordovician-Lower Silurian (O 3 w -S 1 l ) black shale in western Hubei Province in the middle Yangtze area, were investigated through geochemical and petrological analyses. The results show that the O 3 w -S 1 l black shale is mainly composed of five lithofacies with varying graptolite abundance, total organic carbon (TOC), and silica contents. Biogenic silica and terrigenous siliciclastic input constitute the main silica sources in O 3 w -S 1 l black shale and they exhibit an upward inverse variation trend interpreted to be related to sea-level changes. Moreover, with the increase in biogenic silica content or decrease in terrigenous siliciclastic input, TOC values in black shale initially rise and then fall, which is different from originally expected simple linear relationship. We infer that organic matter enrichment and the distribution of silica from different origins are controlled by sea-level changes and variations in terrigenous input in a continental shelf with little hydrothermal influence. An environment with appropriate sea level and terrigenous input should be most favorable for organic matter accumulation, rather than one with excessive high sea level and less terrigenous input.

Sedimentary Environment and Organic Matter Accumulation of Wufeng-Longmaxi Shales, Southwest Yangtze Platform: Insights from Geochemical and Petrological Evidence

Upper Ordovician-Lower Silurian Wufeng-Longmaxi Foration is the most developed strata of shale gas in southern China. As several shale gas wells were drilled in the mountain-basin transition zone in the Southwest Sichuan basin, the attraction was drawn by this area. While, due to the complex sedimentary environment adjacent to the Kangdian Uplift, the area favorable for organic-rich shale development is still in fog. Further study is necessary to obtain a more complete understanding of organic matter enrichment in the marginal region around Sichuan Basin. We therefore focus on the mechanism of accumulation of organic matter and the characterization of the sedimentary environment of the Wufeng-Longmaxi Shales. In this study, a combination of methods was applied, including thin section identification, scanning electron microscopy (SEM) observations and X-ray diffraction (XRD), and elemental analysis on outcrop samples. According to the mineralogical and petrological analyses, five lithofacies have been defined, including mudstone, bioclastic limestone, silty shale, dolomitic shale and carbonaceous siliceous shale. And for the geochemical characterization, the detrital influx, redox conditions and paleo-productivity have been studied. The results showed that Wufeng period is generally a suboxic environment and the early Longmaxi period is a reducing environment.

Shale Gas Reservoir Evaluation by Geophysical Measurements: A Case Study of the Upper Ordovician–Lower Silurian in the Fenggang Block, Northern Guizhou Province

AbstractWith the aid of geophysical measurements, including seventeen two‐dimensional (2D) seismic lines and the well logging curves of well FGY1, the structure and reservoir characteristics of the Upper Ordovician–Lower Silurian strata in the Fenggang block, northern Guizhou Province, were analyzed thoroughly to identify desert areas and favorable intervals. The results show that Longmaxi‐Wufeng is the most prospect‐rich formation, consisting of a thick succession of overmature black shale, this formation remaining partially in the Suiyang, Fenggang and Jianchaxi synclines. The Longmaxi‐Wufeng shale, especially the lower member, was deposited in a reducing low‐energy environment with relatively high U content and a low Th/U value. In this shale, the organic matter type (sapropelic and humic‐sapropelic), total organic carbon (TOC) content, gas content, gas adsorption capacity, vitrinite reflectance and brittle mineral content are profitable for shale gas preservation and development. The fractures of this shale were closed because of its high overburden pressure. The gas adsorption capacity of this shale increases with increasing TOC content and Ro. In the Longmaxi‐Wufeng Formation at well FGY1, the most favorable intervals are in the depth ranges of 2312.4–2325.1 m and 2325.8–2331.1 m.

Foxe Platform and Basin Tectono-Sedimentary Element, Northeast Canada

Abstract The Foxe Platform and Basin Tectono-Sedimentary Element (TSE) is an ovoid-shaped, predominantly marine basin located in the Canadian Arctic. The Paleozoic sedimentary succession (Cambrian–Silurian) unconformably overlies the Precambrian basement and reaches a maximum measured thickness of slightly over 500 m in the only exploration well drilled in this basin. The Lower Paleozoic Foxe Platform and Basin TSE is surrounded by Precambrian basement, and by the Paleozoic Arctic Platform to the north and by the Paleozoic–Mesozoic (?) Hudson Bay Strait Platform and Basin to the south. The Paleozoic succession consists of a Cambrian clastic-dominated interval overlain by Ordovician–lower Silurian predominantly shallow-marine carbonate. Other than a single well drilled in the northern part of the basin, no subsurface information is available. Thermally immature Upper Ordovician organic-matter-rich calcareous black shales have been mapped on the onshore extension of the basin to the SE. Potential hydrocarbon reservoirs consist of Cambrian porous coarse-grained clastics, as well as Upper Ordovician dolostones and reefs.

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.