Yangtze Area Research Articles

Relative sea-level changes and organic matter enrichment in the Upper Ordovician-Lower Silurian Wufeng-Longmaxi Formations in the Central Yangtze area, China

Based on the sedimentological and geochemical analysis of total organic carbon (TOC) and major and trace elements in the marine Upper Ordovician-Lower Silurian Wufeng-Longmaxi Formations in Yichang, Central Yangtze area, paleoredox conditions (indicated by U/Th, V/Cr, Ni/Co, TOC-TS-Fe diagram, and Mo–U covariations), paleoproductivity (indicated by Babio, Ba/Al, P/Ti, Ni/Al, Cu/Al, and Zn/Al), terrestrial detrital influx (indicated by Ti and Al), paleoclimate (indicated by chemical index of alteration and Rb/Sr), and upwelling (indicated by Co × Mn) are revealed within the framework of sea-level changes. The influences of these factors on organic matter enrichment are also discussed. The results identify four third-order sequence boundaries in the Wufeng-Longmaxi Formations; each sequence can be divided into one transgressive systems tract (TST) and one regressive systems tract (RST) according to sea-level changes. The high-TOC interval mainly developed in TST1 (average 3.44%) and TST2 (average 3.68%). The analysis of geochemical indicators shows that during the TST1 depositional stage, the climate was dry and cold, the bottom water was in a stable anoxic condition accompanied by rising sea level, and the seawater had moderate-high paleoproductivity. In the RST1 stage, extensive sea-level fall occurred, and the water was anoxic, with decreased productivity and weakened organic matter accumulation. In the TST2 stage, the sea level rise rapidly, extensive anoxicity and high productivity of the water produced the most ideal environment for organic matter enrichment and the highest TOC. In the RST2 stage, sea level slowly fell, the water column gradually became a stable oxidized environment, productivity was low, the terrestrial detrital influx increased and tended to stabilize, and the climate gradually changed to warm and humid conditions, forming relatively organic-poor shale. The upwelling mainly developed during the period of TST1 to TST2, and ceased in the RST2. The correlation analysis between TOC and the above indicators and the Co × Mn versus Cd/Mo diagram show that TOC is chiefly controlled by paleoredox, paleoproductivity, climatic conditions, and terrestrial detrital influx, indicating that preservation conditions are the crucial factor. Sea-level change played a key role in preservation conditions. In the transgression stages, sea-level rise led to the enhancement of reducing water, which controlled the organic matter enrichment in shale.

Late Ordovician paleoceanographic change: Sedimentary and geochemical evidence from Northwest Tarim and Middle Yangtze region, China

Redox variation during the Late Ordovician were an important driver of palaeoceanographic and ecologic changes that may have eventually facilitated biotic extinction in the end of the Ordovician. Previously, the Late Ordovician paleoredox studies have focused mostly on the last 1–2 million years prior to the end of the Ordovician; redox conditions during the Sandbian–Katian time (early–middle Late Ordovician) are poorly understood. In order to fully evaluate the effects of palaeoceanographic variations on biotic evolutionary trends through the entire Late Ordovician, it is important to fully understand redox variations following the Great Ordovician Biodiversification Event (GOBE) and prior to the end-Ordovician mass extinction. The present study reconstructs paleoredox conditions from the Sandbian–Katian successions of the Dawangou section, northwestern Tarim Basin, northwest China and the Puxihe section of the Middle Yangtze region, South China using pyrite framboidal and redox-sensitive element analyses. The distribution of volcanic microspherules, fossil fragment content and elemental analysis were also applied to reveal possible volcanisms and chemical weathering conditions, and to evoluate their effects on palaeoxygenation and biotic variations. The new results indicate three distinct periods of fluctuating paleoredox conditions during the Late Ordovician in the northwest Tarim and the Middle Yangtze regions, China. Overall, redox conditions varied from mainly oxic but punctuated by short-lived anoxic–dysoxic episodes to rapid fluctuations between reducing and oxidizing conditions, to persistent euxinic conditions. The varition of oxygen minimum zone (OMZ) that developed in the midwater column might be the main cause of anoxic events during the Sandbian–early Katian. Moreover, abundant volcanic microspheres, enhanced chemical weathering, decreasing fossil fragments of benthos, and positive europium anomaly from the study sections collectively indicate that intermittent volcanism during the early–middle Katian may have triggered anoxic-euxinic events during this interval. If integrating the newly obtained redox data with previously published data for the Katian to Hirnantian from the Middle Yangtze area, the long-term, stepwisely accelerated oxyen-poor conditions during the Late Ordovician may have facilitated biotic extinctions in the end of the Ordovician.

Tectonic paleogeography of Late Sinian and its significances for petroleum exploration in the middle-upper Yangtze region, South China

Abstract The paleotectonic pattern, lithofacies paleogeographic features, sedimentary evolution and its controlling effects on hydrocarbon accumulation assemblages during the depositional period of the Sinian Dengying Formation in middle-upper Yangtze region were investigated based on outcrops, drilling, log and seismic data. The study shows that, (1) Affected by the breakup of the Rodinia supercontinent, the middle and upper Yangtze areas were in extensional tectonic environment during the depositional period of Dengying Formation. The carbonate platform was structurally differentiated. Intra-platform depressions controlled by syndepositional faults developed, forming a tectonic-paleogeographic pattern of “three platforms with two depressions”. (2) During the depositional period of the first and second members of the Dengying Formation, rimmed platforms and intra-platform fault depressions developed in upper Yangtze area and isolated platform developed in middle Yangtze area, and there was the Xuanhan-Kaijiang ancient land block in eastern Sichuan. The depositional period of the third member of the Dengying Formation is the transformation period of tectonic-paleogeographic pattern, when a set of shallow water shelf sediment rich in mud was deposited due to transgression on the background of the eroded terrain formed in Episode I of Tongwan Movement. The sediment of the fourth member of the Dengying Formation inherited the paleogeographic pattern of the first and second members of the Dengying Formation in general, but the Deyang-Anyue intra-platform fault depression further expanded, and the middle Yangtze platform evolved into two separated platforms. (3) Tectonic-sedimentary differentiation and evolution of carbonate platform in the Sinian gave rise to two types of accumulation assemblages with wide distribution and great exploration potential, which are platform margin and intra-platform.

Organic-rich siliceous rocks in the upper Permian Dalong Formation (NW middle Yangtze): Provenance, paleoclimate and paleoenvironment

The organic-rich shales of the upper Permian Dalong Formation in the middle Yangtze area are considered an important shale gas exploration horizon. However, the origin of siliceous matter and the mechanism of organic matter enrichment are still controversial. Based on geochemical data, we studied the provenance, paleoclimate, paleoenvironment, primary productivity and hydrological conditions of the Dalong Formation in western Hubei Province and discussed the mechanism of organic matter enrichment. The shales of the Dalong Formation can be divided into two units, organic-rich and organic-lean shales, with average TOC contents of 5.81 wt% and 0.45 wt%, respectively. The western Hubei Basin was a strongly restricted basin during deposition, and the provenance of the Dalong Formation had a continental margin sedimentary background, mainly from mixed felsic and basic sources. Terrigenous clastic quartz, biological quartz and hydrothermal quartz are recognized in the shales of the Dalong Formation. Based on U, V, and Mo concentrations and V/(V + Ni) ratios, the organic-rich shales were deposited in anoxic-sulfurized conditions. In contrast, the organic-lean shales were deposited under oxic-suboxic conditions. The Dalong Formation had moderate to high primary productivity and controls on the redox conditions in the basin. The decrease in primary productivity in organic-lean shales may be related to the transition from a warm and humid climate to a hot and dry climate. The sedimentary model shows that the formation of organic-rich siliceous rocks in the Dalong Formation was controlled by primary productivity. The decrease in primary productivity driven by the hot and dry climate in the latest Permian is the main factor controlling the enrichment of organic matter and the formation of bioquartz.

Study on the Mechanism of Organic Matter Enrichment in Early Cambrian Marine Shales in the Lower Yangtze Area, South China: An Example Using Well JXY1

The abundance of organic matter in shales, which has a direct effect on the hydrocarbon generation potential of shales, is an important organic geochemical parameter for evaluating shale gas reservoirs. The total organic carbon content (TOC content) in shale is controlled by the abundance of original sedimentary organic matter. Therefore, it is very important to study the mechanism of organic matter enrichment in shale. In this paper, the Lower Cambrian marine shales from the Lower Yangtze region are selected as the research subject, most of which originate from a typical area well called Well JXY1. The degree of pyritization (DOP) is used to characterize the redox environment of the water body, while the P/Al ratio is used to analyze the biological productivity of paleoseawater. The paleosalinity of seawater is calculated via carbon and oxygen isotope analysis. In addition, the early Cambrian hydrothermal activities were studied by using core description; Si, Al, Fe, and Mn elemental analysis; and oxygen isotope calculations. The results show that during the early Cambrian Wangyinpu sedimentary period, the seawater was an anaerobic water body with H2S, and the oxygen concentration was approximately 0 mL/L. In the middle stages of the Wangyinpu sedimentary period, the water body had the strongest reducibility and the highest biological productivity. Moreover, the paleoocean in this period between the Yangtze plate and the Cathaysian plate was greatly affected by hydrothermal activities, with temperatures ranging from 90°C to 120°C. Active hydrothermal activities promoted high biological productivity and an anaerobic environment, both of which were conducive to the preservation and enrichment of organic matter, resulting in extremely high TOC content in the Wangyinpu shales (from 6.5% to approximately 16%).

Constraints on the accumulation of organic matter in Upper Ordovician-lower Silurian black shales from the Lower Yangtze region, South China

The Late Ordovician to early Silurian (O/S) was an important period in which many significant geological events occurred. During the O/S transition, organic-rich black shales were widely deposited and became important unconventional energy sources. Paleoproductivity, detrital input, and redox conditions are the three major factors controlling organic matter (OM) accumulation. Although the effect of each factor is relatively clear, the complex influences of these factors remain poorly understood. Specifically, the evolution of these factors and the corresponding influence on the accumulation of OM during the O/S transition has not yet been systematically investigated. This paper investigates the constraints on OM accumulation in O/S transition black shales from the Lower Yangtze region, based on graptolite fossil identification, zircon U–Pb dating, and geochemical analyses of a well sequenced drilling profile. The results show that (i) the SY-1 core sequence was well constrained from graptolite zones WF2 (447.62 Ma) to LM5 (441.57 Ma) across the O/S boundary, with decreasing sedimentation rates (SRs) in the upper Katian (WF2 to WF3) and increasing SRs in the lower Rhuddanian; (ii) complementary volcanic inputs (Katian and Rhuddanian) and upwelling events (Hirnantian) exerted strong control over paleoproductivity; (iii) orogenic processes in South China controlled detrital inputs and redox conditions; (iv) OM accumulation in the Lower Yangtze area across the O/S transition occurred in three stages. In stage 1 (WF2–WF3), strong volcanism boosted productivity, whereas OM preservation was not favored in the oxic environment. In stage 2 (WF4–LM3), upwelling and volcanism resulted in high productivity and provided the optimal configurations for OM enrichment. In stage 3 (LM4 and later), rare OM was preserved due to terrestrial dilution and unfavorable preservation conditions. Eventually, WF4–LM3 became the prominent OM-rich biozones due to the effective coordination of the controlling factors, namely upwelling, volcanism, and redox conditions.

A new Changhsingian (Lopingian) brachiopod fauna of the shallow‐water clastic shelf facies from Fujian Province, south‐eastern China

AbstractAlthough much attention has been paid to Changhsingian brachiopods in South China, there are only two Chinese studies, published in 1979 and 1990, on Changhsingian brachiopods from the south‐eastern part of South China. Based on systematically collected fossil material, this paper describes and quantitatively analyses a Changhsingian brachiopod fauna from the Luokeng Formation at the Hongtian section, Fujian Province, south‐eastern China, for the first time. Of the brachiopods (30 species in 19 genera) described and illustrated, Cathaysia yongdingensis Liao and an endemic genus of South China (Qinglongia) are reported in English for the first time. Two new species, Neochonetes (Sommeriella) longa and Neochonetes (Sommeriella) transversa, are also proposed. The Hongtian fauna is mainly composed of Cathaysia (29.4%), Neochonetes (19.3%), Linoproductus (13.5%), and Orthothetina (11.7%), and is correlated with the brachiopod fauna from the lower part of the Luokeng Formation in Fujian Province. Based on new and previously published quantitative data, we suggest that the Cathaysia–Neochonetes–Orthothetina Assemblage should represent the early Changhsingian brachiopod fauna in the shallow‐water clastic shelf of Fujian Province. After comparing this fauna with contemporaneous brachiopod fauna from the shallow‐water clastic shelf in the Yangtze area, we find that Neochonetes is a common genus in both areas, but that Fusichonetes is predominant in the Yangtze, while Cathaysia–Orthothetina are the additional main genera in south‐western China.

Guadalupian (Middle Permian) δ13Corg changes in the Lower Yangtze, South China

The Middle Permian Guadalupian witnessed significant environmental changes in the Phanerozoic such as large-scale sea-level drop, supercontinent Pangaea assembly, and transition from Early Permian glaciation to Late Permian non-glacial intervals. Carbonisotope tracers can provide insights for these environmental changes. The δ13C studies of the entire Guadalupian epoch are rare, and most of them has focused on near the end of Guadalupian and carbon isotopes of inoragnics. Here, we present carbon isotopic compositions of organic matters in the Guadalupian from two sections (Chaohu and Xiaolao) in the Lower Yangtze area, South China. Our results show that δ13Corg profiles in the Guadalupian show a peak in the Roadian and a gradual negative shift from the Roadian to the middle Capitanian. These trends can be matched by δ13C changes of carbonate rocks or organic matter in South China and other places in the world, representing a global carbon cycle signal. The Roadian positive peak was probably due to high productivity which was caused by upwelling during cooling time. The gradual negative shift of δ13C was caused mainly by a decrease of organic matter burial on land and in the ocean, resulting from global sea-level drop and anoxia-caused benthos decline, respectively. The less important causes for the gradual δ13C negative shift are volcanic-gases releasing, decreased mountain belts, and the resultant reduced silicate weathering-consumption of CO2. The gradual negative shift of δ13C coincides with the gradual extinction in the Guadalupian. Therefore, global sea-level drop and marine reducing conditions may be the main causes of the gradual extinction in the Guadalupian.

The depositional mechanism of organic-rich siliceous shales in Upper Yangtze area: Response to the Kwangsian Orogeny in South China

The organic-rich siliceous shale in Upper Ordovician is the major source rock for shale gas in the South China, while the deposition mechanism of these rocks is still under debated. Here we present systematic geochemical and petrographic characteristics of the Upper Ordovician sediments to evaluate the redox conditions, paleo-productivity, terrestrial input, and to investigate the depositional mechanisms of these organic rich siliceous shales in the Wufeng Formation. In shallow water areas, the Linxiang Formation is dominantly characterized by grey thick Limestone. The Upper Daduhe Formation is composed of calcareous mudstone intercalated with argillaceous limestone, while the Lower Daduhe Formation consists of limestone interbedded with mudstone. At deep water area, the Linxiang Formation is dominated by limestone and muddy limestone. The lower and upper Wufeng Formation are mainly characterized by grey argillaceous shale and organic-rich siliceous shale respectively. It is suggested that the lithological and environmental changes in the Yangtze area were the response to the development of the Kwangsian Orogeny. During the late Ordovician, the Yangtze basement constantly descended and the sea level rose relatively due to the collision between the Yangtze and Cathaysia blocks. Thus, the oxygen concentration decreased with the rising sea level according to the UEF values and Ce/Ce* ratios, which resulted in anoxic water condition and promoted the preservation and enrichment of organic matter in marine sediments. Meanwhile, uplifts (e.g., Jiangnan - Xuefeng, Qianzhong and Chuanzhong) constantly expanded around the Sichuan Basin, leading to high terrigenous input that is reflected in geochemical and Nd isotopic compositions. Additionally, primary productivity gradually increased, because the Si/Al ratios exhibit an increasing trend in both shallow and deep water areas, and it may be related to the high clastic influx which provided adequate nutrients for siliceous organisms. Overall, anoxic water condition and high biogenic Si influx caused by Kwangsian Orogeny are considered as the most important factors controlling the organic-rich siliceous shale in the Wufeng Formation.

Lithofacies and depositional mechanisms of the Ordovician–Silurian Wufeng–Longmaxi organic-rich shales in the Upper Yangtze area, southern China

The Ordovician–Silurian organic-rich shales in the Wufeng and Longmaxi Formations are widespread in southern China and are the most important shale gas producers in China. The shales display varying total organic carbon content ranging from 0.2% to 6% and are composed of black siliceous mudstone, shelly limestone, siliceous clay-rich mudstone, gray silty–shaly interlaminated mudstone, gray argillaceous mudstone, and calcareous or dolomitic mudstone. Detailed stratigraphic correlation of these formations was achieved, and a high-resolution sequence stratigraphy was constructed based on the integration of well-log data, mineralogical, geochemical, and petrophysical analyses. The Wufeng Formation and the Guanyinqiao Bed were interpreted to be a condensed section with four cycles of sea-level fluctuation. The Longmaxi Formation consists of three members. The Long-1 Member was interpreted to be a third-order depositional sequence that can be subdivided into a transgressive systems tract, an early highstand systems tract, and a late highstand systems tract. Within the constructed isochronal stratigraphic framework, the characteristics and genetic mechanism of different shale lithofacies were studied. The deposition of black siliceous mudstone of high total organic carbon in the Wufeng Formation and the Long-1 transgressive systems tract was likely influenced by volcanic activity. The Guanyinqiao shelly limestone or calcareous mudstone was associated with the Ordovician–Silurian transition when glaciation and mass extinction took place. The gray silty–shaly interlaminated mudstone of moderate total organic carbon in the Long-1 early highstand systems tract was the product of bottom flow intrusion. The deposition of gray argillaceous mudstone and calcareous or dolomitic mudstone with low total organic carbon in the Long-1 late highstand systems tract resulted from enhanced terrigenous input from peripheral uplifts. The distribution pattern of different lithofacies varies greatly in the Upper Yangtze area, and this difference is a result of dissimilar sediment provenances and variable hydraulic restriction levels that were controlled by basin geometry. A better understanding of the distribution pattern and depositional history of the Wufeng and Longmaxi Formations might provide guidance for future profitable shale gas exploration.

Organic Geochemistry of Organic-rich Cherts in the Middle Permian Gufeng Formation in Chaohu, Lower Yangtze Area

Organic Geochemistry of Organic-rich Cherts in the Middle Permian Gufeng Formation in Chaohu, Lower Yangtze Area

Oceanic environment changes caused the Late Ordovician extinction: evidence from geochemical and Nd isotopic composition in the Yangtze area, South China

AbstractThe Ordovician–Silurian (O–S) transition was a critical interval in geological history. Multiple geochemical methods are used to explore the changes in oceanic environment. The Nd isotopic compositions in the Yangtze Sea are controlled by two sources: the continental erosion and the Panthalassa Ocean. HighεNd(t) values during the Katian, late Hirnantian and Rhuddanian intervals are associated with the high sea level, which resulted in less terrestrial input based on the low Ti/Al and Zr/Al ratios. In contrast, lowεNd(t) values during the early Hirnantian interval are related to the sea-level fall; in this case, the exposure of submarine highs and the growth of Yangtze Oldlands could lead to more continental materials being transported into the Yangtze Sea based on high Ti/Al and Zr/Al ratios. In addition, the negativeεNd(t) excursion can also be attributed to the weak circulation between the Yangtze Sea and Panthalassa Ocean when sea level was low. Furthermore, the sea-level eustacy plays a significant role in the changes in redox water conditions. The redox indices, mainly UEF, Ce/Ce* and Corg/PT, across the O–S transition show a predominance of anoxic ocean over the Yangtze Sea during the Katian, late Hirnantian and Rhuddanian intervals, and an oxygenated episode was briefly introduced during the early Hirnantian period because of the fall in sea level. The Late Ordovician biotic crisis was marked by two-phase extinction events, and the change in sea level and redox chemistry may be the important kill mechanisms.

Experimental study on the existence of nano-scale pores and the evolution of organic matter in organic-rich shale

Abstract To explore the characteristics of nanopores in organic-rich shale, the shale of the Upper Yangtze area under different maturity are studied. Scanning electron microscopy (SEM), low temperature nitrogen experiment (LP-N2-GA) and the Frenkel-Halsey-Hill (FHH) fractal model were used. Results show that the intergranular pores and organic pores are most developed, and the pore size is distributed from a few nanometers to several tens of micrometers. The SEM gray scale images make the pore development characteristics of shale be observed more intuitively. Based on the results of low temperature nitrogen adsorption test and FHH fractal model, the results show the fractal dimension of shale is positively correlated with TOC and quartz, but negatively related to clay minerals and feldspar. There is a good relation between maturity and pore volume.With releasing of pressure, the pore evolution gradually returns to the normal compaction trend, and the pore volume of the shale decrease. Graphical abstract: There are many nano-scale pores in the shale, and the pore characteristics determine the characteristics of shale reservoirs. At the initial stage of sedimentary burial, when Ro is less than 0.7%, the pore volume and specific surface area of the initial shale are large. At the medium maturity stage (0.7%<Ro<1.3%), the pore volume and pores caused by mechanical compaction to some extent. As the degree of thermal evolution deepens to the high maturity stage (1.3% <Ro < 2.6%), the shale is generating large amount of gas. The shale belongs to the low-porosity and low-permeability reservoirs, and the generated oil and gas cannot be effectively transported in a short period of time. The overpressure of the reservoir and the inter-mineral pore characteristics caused by the organic pores and organic acids formed by the hydrocarbon generation are more complicated. When shale is in the stage of over-maturation (Ro>2.6%), as the increase of buried depth, the end of hydrocarbon generation process and long-term pressure release, the under-compacting effect gradually disappear. Porosity evolution gradually returns to normal compaction trend in the shale.

Combining Isotopic Geochemical Data and Logging Data to Predict the Range of the Total Gas Content in Shale: A Case Study from the Wufeng and Longmaxi Shales in the Middle Yangtze Area, South China

The marine shales of the lower Silurian Longmaxi formation (S1l) and the upper Ordovician Wufeng formation (O3w) in the middle Yangtze area of southern China have been recognized as effective gas s...

Ediacaran, Cambrian, Ordovician, Silurian and Permian shales of the Upper Yangtze Platform, South China: Deposition, thermal maturity and shale gas potential

For many years, the Yangtze Platform has been successfully explored and exploited for petroleum. The Lower Silurian Longmaxi Shale proved to contain commercial quantities of gas, but several other formations might also have a high potential for shale-hosted gas exploration. This paper comprises of geochemical, petrographical, petrophysical and mineralogical information on the major Proterozoic and Paleozoic gas shales in the Upper Yangtze area, which provides insights on the depositional environment, the thermal maturity of organic matter, shale gas storage capacity and the fracability of these rocks. The total organic carbon (TOC) content varies from 0.1 to 22.5% with an average value of 2.7% and most of these shales were deposited in an oxygen-depleted marine environment. Equivalent vitrinite reflectance (VRr) values range from 2.20 to 4.25%, indicating that all samples are in the gas generation window and lost a large amount of primary organic carbon. Porosity varies between 1.5 and 13.6% with an average value of 5.8%, while the excess sorption capacity ranges from 0.10 to 0.22 mmol/g rock. Minerology data show that quartz is the predominant mineral except for the Upper Permian Longtan Formation, in which clay minerals account for 29 to 84%. New data were combined with those from other publications and compared to information on other, well-studied shale-hosted gas systems.

Tectono-depositional pattern and evolution of the middle Yangtze Platform (South China) during the late Ediacaran

The depositional environments and evolution of the Yangtze Block during the Ediacaran–Cambrian transition remain uncertain, preventing a thorough understanding of the coeval ocean stratification, redox heterogeneity and organism distribution. In this study, detailed facies and sequence stratigraphic analyses were carried out on the Dengying Formation and its correlatives to better constrain the tectono-depositional pattern and evolution of the middle Yangtze area in the late Ediacaran. Twenty-five lithofacies were identified in nineteen sections and grouped into six depositional facies: intrashelf basin, intrashelf shoal, restricted platform interior, platform margin, upper-middle slope, and lower siliceous slope-basin. The spatiotemporal distributions of these depositional facies reveal an isolated, rimmed middle Yangtze carbonate platform embayed with a westward-opening intrashelf basin. This intrashelf basin gradually became wider and deeper westwards, linking to a NNE-trending basinal seaway that separated the middle Yangtze Platform from its upper Yangtze counterpart. The enclosure of the marginal shoal-reef complex with an intrashelf shoal chain formed a large-scale restricted lagoon and associated tidal flat in the platform interior. Based on key sequence boundaries and cycle stacking patterns, two and a half transgressive-regressive sequences were distinguished in the Dengying Formation and its correlatives in the twelve representative sections. Furthermore, these sequences can be correlated well with those identified in other upper Ediacaran successions elsewhere in upper Yangtze, Namibia, India and Oman, indicating a eustatic origin for these sequences. However, the complex platform-basin configuration, with significant facies and thickness variations, reflects a transtensional tectonic regime in the middle Yangtze area during the late Ediacaran. The abrupt changes in lithofacies and thickness across the southern principal platform margin and those from sandy shoals/banks to storm-influenced shelf along the northern portion of the intrashelf basin in the Three Gorges (western Hubei) suggest synthetic extensional faulting across the margins during deposition. The two synthetic extensional fault zones might have been induced by NNE-trending transtensional faulting along the eastern margin of the upper Yangtze Platform. This structural pattern fundamentally controlled the depositional differentiation of the middle Yangtze Platform during the late Ediacaran and also laid the foundation for the subsequent facies distribution and depositional evolution during the early Cambrian Terreneuvian Epoch.

Ediacaran (Sinian) palaeogeographic reconstruction of the Upper Yangtze area, China, and its tectonic implications

ABSTRACT The Ediacaran (ca. 635–541 Ma) was a critical period in Earth history, which saw a dramatic transformation in global biological, chemical and climatic conditions, making it one of the most intensively investigated time intervals worldwide. As the first mixed clastic–carbonate succession deposited on the Upper Yangtze Craton, the Ediacaran succession overlies Neoproterozoic continental rifts, and records a complex basin infill history. In this study, we focus on the tectonostratigraphic environment and basin infill process, integrating multiple data sets, including boreholes, outcrops, well correlations, seismic data, isopach maps and the spatial distribution of sedimentary facies. The Ediacaran System includes the Doushantuo Formation and overlying Dengying Formation which can be subdivided into four lithostratigraphic members (D1–D4). The Doushantuo Formation and D3 Member consist of mixed clastic–carbonate sediments, while carbonates predominate in the D1, D2 and D4 members. Intracratonic depressions and continental margin rifts dominated the Upper Yangtze area during the Ediacaran. During deposition of the Doushantuo Formation, the Upper Yangtze area contained a variety of environments, including terrigenous shorefaces, mixed tidal flats, lagoons, shelves, slopes and bathyal seas. A rimmed carbonate platform developed during deposition of the D1 and D2 members. The Deyang–Anyue Trough, which is related to post-rift subsidence in the intracratonic basin, began to develop in this interval. A relatively intense extension during deposition of the D3 Member resulted in intensification of intracratonic and continental margin rifting, creating distinct tidal flat, deep-water shelf, slope and bathyal environments. With the expansion of the Deyang–Anyue Trough during deposition of the D4 Member, the rimmed carbonate platform was divided into two segments. We suggest that the Deyang–Anyue Trough was a structurally controlled intracratonic rift-sag and likely arose from multiple episodes of rifting in the cratonic basement, as well as differential uplifting and exposure caused by the ongoing Tongwan Movement. Our model highlights the role of basement heritage on the structure and evolution of intracratonic rift-sag.

Mechanism Analysis of Organic Matter Enrichment of Upper Ordovician-Lower Silurian Shale in the Upper Yangtze Area: Taking Jiaoye-1 Well in the Jiaoshiba Block as an Example

Organic matter is the material basis of shale hydrocarbon generation. The current organic matter content in shale is controlled by the original sedimentary organic matter abundance. Therefore, the study of the enrichment mechanism of sedimentary organic matter in shale has become an important issue to be solved. The Upper Yangtze area is the important exploration and exploitation area of marine shale gas in China. The shale of the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in the Yangtze area is the research object. Choosing redox indicator and biological productivity indicator, the study explores the enrichment mechanism of sedimentary organic matter from two aspects, sealing of water and volcanic activity. The results show that excess siliceous mineral in the shale of the Wufeng Formation-Longmaxi Formation in the Upper Yangtze area is bioorigin. Excess siliceous mineral can be used as one of the indicators of biological productivity. On the one hand, layer phenomenon occurred since the strong water sealing during the sedimentary period of Wufeng and the lower section of the Longmaxi Formation, which results in the high content of oxygen in surface water. On the other hand, the active volcanic activity brought volcanic ash which was beneficial to biological reproduction. Both of these factors led to higher biological productivity during this period. At the same time, the strong sealing of water made the lower layer of the water more reductive, and the active volcanic activity caused climate change, enhancing the reduction of the lower layer of the water, which made the rich organic matter deposited from the surface water well preserved. In the sedimentary period of the upper section of the Longmaxi Formation 1st member in the Upper Yangtze area, on the one hand, due to the weakened sealing of water, the oxygen content of the upper water decreased. On the other hand, the volcanic activity weakened until it stopped, and the source of volcanic ash rich in nutrient elements decreased. These two aspects led to lower biological productivity during this period. At the same time, the weaker water sealing could lead to a decrease in the reduction of the lower layer of the water, and the gradual cessation of volcanic activity no longer affected the climate, causing the destruction of sedimentary organic matter by oxidation.

The Lower Yangtze area: A next shale gas block for China? Preliminary potential assessment from some geology and organic geochemistry information

In the last several years, the massive success of the Fuling shale gas field in south‐western China has shown great potential in finding giant shale gas fields in the Upper Yangtze area, and it has also attracted the increasing attention from geologists to find more shale gas reservoirs around the world. However, there is still a great dispute as to whether the neighbouring Lower Yangtze area is a next potential shale gas block in China. In this study, we evaluate preliminarily the shale gas potentials of five important sets of marine shales based on systematic analyses for the sedimentary environment, thickness, total organic carbon (TOC), thermal maturity, and other organic geochemistry parameters of both geological records from outcrops and wells in the Lower Yangtze area. The first two sets of marine shales occur in the Lower Cambrian and the Ordovician, and they all are of over–high organic maturity due to strong effects from regional tectonic movements and thus have poor potential. Although the Lower Silurian shales are widely distributed in the Lower Yangtze region, a terrigenous delta, a low amount of organic matter (TOC < 1.2%), and late tectonic activity together created unfavourable conditions for the generation and accumulation of the Lower Silurian shale gas. The Permian black shales occur in the Middle and Upper Permian. Several proxies, such as the appropriate maturity (vitrinite reflectance Ro of approximately 2.0%), abundant organic matter (TOC of approximately 4 wt%), good total hydrocarbon potential (S1 + S2), and high level of hydrocarbon conversion have resulted in good conditions for shale gas generation. However, the relative thinness (only 30 m), severe tectonic modification, and regional cap rock are not favourable for shale gas preservation. The Lower Yangtze area therefore is not an excellent block for shale gas exploration compared to its neighbouring Upper Yangtze area. However, in some areas with less structural deformation, the possibility of finding shale gas reservoirs cannot be ruled out, especially for the Permian shales, which had a greater opportunity to generate and accumulate shale gas than the other marine shales. In addition, for new shale gas exploration breakthroughs in the future, structural studies are essential in offering better understanding of the preservation conditions in the Lower Yangtze area.

Pyrite Morphology as an Indicator of Paleoredox Conditions and Shale Gas Content of the Longmaxi and Wufeng Shales in the Middle Yangtze Area, South China

Pyrite is the most common authigenic mineral preserved in many ancient sedimentary rocks. Pyrite also widely exists in the Longmaxi and Wufeng marine shales in the middle Yangtze area in South China. The Longmaxi and Wufeng shales were mainly discovered with 3 types of pyrites: pyrite framboids, euhedral pyrites and infilled framboids. Euhedral pyrites (Py4) and infilled framboids (Py5) belong to the diagenetic pyrites. Based on the formation mechanism of pyrites, the pyrites could be divided into syngenetic pyrites, early diagenetic pyrites, and late diagenetic pyrites. Under a scanning electron microscope (SEM), the syngenetic pyrites are mostly small framboids composed of small microcrystals, but the diagenetic pyrites are variable in shapes and the diagenetic framboids are variable in sizes with large microcrystals. Due to the deep burial stage, the pore space in the sediment was sharply reduced and the diameter of the late diagenetic framboids that formed in the pore space is similar to the diameter of the syngenetic framboids. However, the diameter of the syngenetic framboid microcrystals is suggested to range mainly from 0.3 µm to 0.4 µm, and that of the diagenetic framboid microcrystals is larger than 0.4 µm in the study area. According to the diameter of the pyrite framboids (D) and the diameter of the framboid microcrystals (d), the pyrite framboids could be divided into 3 sizes: syngenetic framboids (Py1, D < 5 µm, d ≤ 0.4 µm), early diagenetic framboids (Py2, D > 5 µm, d > 0.4 µm) and late diagenetic framboids (Py3, D < 5 µm, d > 0.4 µm). Additionally, the mean size and standard deviation/skewness values of the populations of pyrite framboids were used to distinguish the paleoredox conditions during the sedimentary stage. In the study area, most of the pyrite framboids are smaller than 5 µm, indicating the sedimentary water body was a euxinic environment. However, pyrite framboids larger than 5 µm in the shales indicated that the sedimentary water body transformed to an oxic-dysoxic environment with relatively low total organic carbon (TOC: 0.4–0.99%). Furthermore, the size of the framboid microcrystals could be used to estimate the gas content due to thermochemical sulfate reduction (TSR). The process of TSR occurs with oxidation of organic matter (OM) and depletes the H bond of the OM, which will influence the amount of alkane gas produced from the organic matter during the thermal evolution. Thus, syngenetic pyrites (d ranges from 0.35 µm to 0.37 µm) occupy the main proportion of pyrites in the Wufeng shales with high gas content (1.30–2.30 m3/t), but the Longmaxi shales (d ranges from 0.35 µm to 0.72 µm) with a relatively low gas content (0.07–0.93 m3/t) contain diagenetic pyrites. Because of TSR, the increasing size of the microcrystals may result in an increase in the value of δ13C1 and a decrease in the value of δ13C1-δ13C2. Consequently, the size of pyrite framboids and microcrystals could be widely used for rapid evaluation of the paleoredox conditions and the gas content in shales.