Silurian Organic-rich Shales Research Articles

Enrichment mechanism of organic matter and silicon in lower Cambrian shale of the Yangtze Platform

Early Cambrian time witnessed accumulation of high-quality organic-rich siliceous shale deposits that comprise an especially promising shale gas exploration target, especially in China. The present study of the Cambrian Niutitang Formation of the shelf area of the Upper Yangtze Platform utilizes mineralogical and elemental geochemical data to aide in the paleoenvironment reconstruction of these organic-rich deposits. We are especially interested in elucidating organic carbon and silicon enrichment mechanisms of the Niutitang Formation that will benefit shale gas exploration and development of these deposits. The Niutitang Formation can be subdivided into three intervals based on lithology, gamma ray (GR) signature, and TOC abundance. Enriched TOC (average = 6.7%) and nano-size authigenic microcrystalline quartz (Qn) contents of Interval I deposits likely reflect deposition in an environment that experienced persistent euxinic and elevated organic matter delivery associated with enhanced paleoproductivity driven by hydrothermal activity. Interval II shale is interpreted to have accumulated under euxinic-ferruginous conditions that experienced the effects of robust paleoproductivity and diminished terrigenous input related to persistently warm, humid climatic conditions. Enriched TOC (average = 4.9%) and moderate microcrystalline quartz content of these deposits were likely the result of redox conditions and elevated paleoproductivity. Interval III strata appears to have accumulated under oxygenated bottom conditions that experienced moderate paleoproductivity, brackish water and elevated terrigenous detritus input driven by the establishment of cold, dry climatic conditions. The combined results of these conditions are diminished TOC content (average = 0.7%) and enrichment of the sediment in terrestrial quartz. The elevated content of authigenic quartz appears to have diminished the reservoir capacity of Niutitang Formation shale. Interval II organic-rich siliceous shale deposits of Upper Yangtze Platform shelf areas, which have the comparable authigenic microcrystalline quartz content, reservoir performance, and greater organic matter abundance as the rift trough, appear to comprise the stratigraphic horizon with the greatest exploration and development potential. Results of the present study offer theoretical guidance for future shale gas exploration and development efforts in geologically similar regions.

Pore Structure and Factors Controlling Shale Reservoir Quality: A Case Study of Chang 7 Formation in the Southern Ordos Basin, China

The lithofacies types, pore structure differences, and main controlling factors on the shale reservoirs are vital problems that need to be addressed in the process of shale oil exploration and development. This study explores the Luohe oilfield in the southern Ordos Basin, which is composed of organic-rich shale in the Chang 7 member, to clarify the reservoir properties and analyze major factors affecting the reservoir quality. The shale reservoir can be divided into five lithofacies using ternary diagrams of TOC, argillaceous minerals, and siliceous minerals: high organic-rich siliceous shale (HOSS), high organic-rich argillaceous shale (HOAS), medium organic-rich siliceous shale (MOSS), medium organic-rich argillaceous shale (MOAS), and low organic-rich shale (LOS). The type of organic matter in the studied samples is mainly Type I kerogen and Type II kerogen, predominantly Type II1 kerogen. The kerogen mostly lie within the mature zone in the study area. Various types of pores have been identified in the studied shale: intergranular pores, intragranular pores, intercrystalline pores, organic matter pores, and seams around organic matter. The pores are commonly nanoscale to micrometer in scale, with diameters ranging from 10 nm to several microns. The S1 content in shale is positively correlated with the macropore content, indicating that macropores in shale are the main effective oil storage spaces and are important for oil-bearing reservoirs. There is a good positive relationship between the macropore volume of shale and the content of organic matter. Organic matter in the shale can be beneficial for generating organic matter pores, dissolution pores, and seams at organic matter edge, resulting in better physical properties of shale reservoirs. There is a negative relationship between the quartz/feldspar content and macropores content, indicating that quartz and feldspar are detrimental for the macropore volume development. The lithofacies type is one of the important factors controlling the macropore volume. MOAS and HOAS are favorable lithofacies for the development of macropores. The findings of this study can be utilized for hydrocarbon exploration and development in the lacustrine shale formation of the Ordos Basin and other similar basins.

Sedimentary differentiation caused by glaciation and orogeny during the Ordovician–Silurian transition in the southern Sichuan Basin, China

The Wufeng–Longmaxi shales, which were deposited during the Ordovician–Silurian transition in the southern Sichuan Basin, exhibit distinct lithofacies transitions. The shift from organic-rich siliceous shale to organic-poor mixed shale and argillaceous shale is attributed to the influences of the Hirnantian glaciation and the Kwangsian Orogeny. Nevertheless, debates persist regarding the mechanisms underlying these significant lithofacies transformations. This study employs sequence stratigraphy, petrology, and geochemical analysis of key wells to unveil crucial insights. The findings indicate: 1) the Wufeng Formation to the Long-1 Member could be divided into two third-order sequences; 2) the waterbody underwent a transition from anoxic and strongly restricted conditions to oxic and moderately restricted conditions; and 3) the trend in formation thickness changed from an E–W orientation to a NE–SW orientation. The results suggest that considerable sediment condensation and anoxia resulting from glacier melting in the early Rhuddanian played a pivotal role in the formation of organic-rich siliceous shale. During the Middle Rhuddanian–Aeronian, the accelerated uplift caused by the Kwangsian Orogeny offset the transgression induced by glacier melting, leading to shallow water conditions and increased terrigenous influx. This diluted the organic matter content in the sediment, and subsequent turbidity currents altered sediment composition, driving the lithofacies transformation into organic-poor mixed shale and argillaceous shale. The study posits that the sedimentary differentiation in the southern Sichuan Basin during the Ordovician–Silurian transition is linked to the initiation and progression of the far-field effects of the Kwangsian Orogeny.

Reservoir Characteristics and Influencing Factors of Organic-Rich Siliceous Shale of the Upper Permian Dalong Formation in Western Hubei

To elucidate the reservoir characteristics of organic-rich siliceous shale of the Upper Permian Dalong Formation in western Hubei, this study focused on the drilling cores of Well ED-2. Various techniques, including a mineral composition analysis, an organic carbon content analysis, a vitrinite reflectance measurement, a total porosity determination, field emission scanning electron microscopy (FE-SEM), and low-pressure CO2 and N2 physical adsorption tests, were employed to analyze the mineralogy, organic geochemistry, total porosity, and pore structure characteristics. Additionally, the factors influencing the reservoir performance of the Dalong Formation shale were investigated. The results indicated that the Dalong Formation’s shale was characterized as an organic-rich siliceous shale. Organic matter was mainly of sapropelic type, with a relatively high thermal evolution degree and Ro ranging from 2.59% to 2.76%. The total porosity of the Dalong Formation’s siliceous shale was low, indicating poor reservoir properties. Organic matter pores were highly developed, mainly the ones formed after the hydrocarbon generation of solid asphalt. Micropores and mesopores were the dominant pore types in the shale, with macropores being significantly less abundant. The study further revealed that the pore volume and specific surface area exhibited a significantly positive correlation with total organic carbon (TOC) content and clay minerals, while demonstrating a weak negative correlation with quartz content. The comprehensive analysis revealed that there were two factors contributing to the poor physical properties of organic-rich siliceous shale in the Dalong Formation. Firstly, in siliceous shale with a high quartz content, the siliceous component was partly derived from the siliceous cementation of hydrothermal fluids. This process led to the formation of secondary quartz that filled intergranular pores, resulting in a decrease in macropore volume, total porosity, and a weak negative correlation with quartz content. Secondly, in siliceous shale with a relatively high clay mineral content, the organic matter was subjected to stronger compaction due to the relatively low content of brittle minerals. This compaction caused the destruction of most macropores, leaving behind primarily micropores and mesopores. Consequently, the average pore size decreased, leading to poor physical properties.

Effect of Sedimentary Facies Characteristics on Deep Shale Gas Desserts: A Case from the Longmaxi Formation, South Sichuan Basin, China

Shale gas is one of the hot spots of energy development. Due to the strong heterogeneity and low physical properties of shale gas reservoirs, and the complex influencing factors of pore development, it is necessary to conduct a comprehensive analysis of the control factors of high-quality reservoirs. The sedimentary characteristics, mineral composition, pore structure and controlling factors of high quality reservoir development are studied on the basis of thin section, scanning electron microscopy (SEM) and QEMSCAN analysis, X-ray diffraction (XRD), total organic carbon (TOC), Nano-CT scanning and Low-pressure N2 adsorption (N2GA) analysis on shale samples from the Longmaxi Formation in South Sichuan Basin. The results show the following: (1) According to lithology, sedimentary structure, organic carbon content and mineral composition, six sedimentary microfacies can be divided. (2) Organic matter pores are developed in organic-rich siliceous shale and organic-rich silty shale at the bottom of the first member of the Longmaxi formation, with average porosity of more than 5% and permeability of more than 2 × 10−3 μm2, which is conducive to the formation of high-quality shale gas reservoirs. (3) The contents of siliceous and TOC are positively correlated with porosity and specific surface area, while the contents of carbonate and clay minerals are negatively correlated with reservoir quality. (4) In the first member of the Longmaxi formation, the sedimentary water depth becomes shallower from bottom to top, and the sedimentary environment changes from a reduction to an oxidation environment. The contents of siliceous and organic matter decrease, while the contents of clay minerals and carbonate minerals show the opposite trend. The difference in sedimentary microfacies affects the distribution of mineral and organic matter and controls the heterogeneity of the shale reservoir.

Characteristics of Lithofacies in Deep Shale Gas Reservoirs in the Southeast Sichuan Basin and Their Influence on Pore Structure

The characteristics of lithofacies in the shale reservoir of the Wufeng and Longmaxi formations located in the Luzhou gas field were studied using the three-end-member method (mineralogical components). In addition, the microscopic characteristics and pore genesis of different lithofacies were studied and compared using TOC, FESEM, and digital core images. The results of the present study showed the development of five lithofacies: 1) quartz-rich argillaceous shale; 2) quartz/clay-mixed shale; 3) calcareous/clay-mixed shale; 4) calcium-rich argillaceous shale; and 5) clay-rich siliceous shale. The degree of pore development between lithofacies was highly heterogeneous. In addition, data indicated that total shale porosity increased with the increase in TOC. Thus, the pores were mainly related to organic matter. The enrichment of framboidal pyrite in quartz-rich shale (S1, S-3) and mixed shale facies (M-2, M-3) is essential during the formation of high-quality reservoirs. On the other hand, the content of organic matter in clayey shale was low, which does not favor the development of high-quality reservoirs. The differences in lithofacies pore structures are controlled by the sedimentary environment. The strong retention and reduction environment are the most favorable features for the formation of organic-rich siliceous shale lithofacies, which promote the preservation of organic matter and the development of reservoirs.

Exploration potential of different lithofacies of deep marine shale gas systems: Insight into organic matter accumulation and pore formation mechanisms

With the development of exploration and exploitation technologies, deep shale gas systems are gradually becoming an important target for the future petroleum industry. However, there is still a lack of systematic geological studies for deep shale gas systems, which restricts further exploration. In this paper, the deeply buried shales (more than 4000 m in depth) of the first member of Silurian Longmaxi (S1l1) at the southeastern margin of the Sichuan Basin are selected as the research object, and thin sections, scanning electron microscope (SEM) observations, total organic carbon (TOC) testing, nitrogen adsorption (NA), and major and trace element analyses are used to systemically identify the different lithofacies and delineate geological and geochemical features. Moreover, we reconstruct the paleoenvironment, summarize the controlling factors and mechanisms of organic matter accumulation and pore formation, and finally reveal the favorable exploration lithofacies. Our results show that there are four lithofacies that can be identified in deep S1l1 shale, namely, laminated organic-rich siliceous shale (S), laminated organic-rich mixed shale (RM), banded organic-lean mixed shale (LM), and banded organic-lean argillaceous shale (CM). Organic matter-hosted pores are the primary type in their pore system. The S and RM shales were mainly deposited under dysoxic to anoxic conditions with moderate watermass restriction, low terrestrial detrital influx, and high paleoproductivity, and the silica mainly originated from the diagenesis process (biogenic quartz); the LM and CM shales were sedimented under oxic conditions with high terrestrial detrital influx and low paleoproductivity, and the silica mainly originated from terrestrial detrital influx. Redox conditions, paleoproductivity, and terrestrial detrital influx together controlled the organic matter enrichment in the deep S1l1 shale, and the TOC content further influences the physical properties of the shale, thus determining the exploration potential of different lithofacies. Moreover, the significant impacts of lithofacies on the geological and geochemical features of the deep shale gas system are demonstrated. Compared with the middle-shallow shale gas system in the study area, due to less tectonically uplifted and tectonically active shale, the deep shale has more extraordinary preservation conditions, overpressure inheritance development, and higher gas amounts. The S and RM in Sll1 in the study area have better resource potential and should be given more attention in the future.

Variation in the brittle-ductile transition of Longmaxi shale in the Sichuan Basin, China: The significance for shale gas exploration

The Longmaxi Formation has abundant shale gas resources. High-yield wells have been obtained in medium-shallow and deep areas in the Sichuan Basin. Quantitative research on the brittle-ductile change in rocks is a basic geological component for caprock evaluation, and it can also be applied to shale gas exploration. This paper fully considers the difference between the lower shale (N1) and upper shale (N2) of the Longmaxi gas-bearing shale. Through total organic carbon (TOC), mineral components, field emission scanning electron microscopy (FE-SEM), and triaxial mechanics experiments, the two groups of shale samples, N1 and N2, were compared and analyzed, and the brittle-ductile transition was quantitatively studied. Based on previous work, a method for the fast calculation of the key over consolidation ratio (OCR) was proposed. The results show that N1 is mainly organic-rich siliceous shale, N2 is primarily organic-poor clay shale and mixed shale, and N1 is generally more brittle than N2. The key OCRs of N1 and N2 are 2.16 and 2.60, respectively. The key confining pressures at which N1 and N2 become fully ductile are 87.87 MPa and 70.28 MPa, respectively. Therefore, the key depths of the brittle zones of N1 and N2 (H b1 and H b2 ) and the key depths of the ductile zones of N1 and N2 (H d1 and H d2 ) are obtained. On this basis, a comprehensive model for the Longmaxi shale gas' brittle-ductile transition exploration zone in the Sichuan Basin and H b1 and H b2 maps of the Longmaxi gas-bearing shale have been established. Through comprehensive vertical and horizontal analysis, Longmaxi shale gas exploration risks in different blocks of the Sichuan Basin have been clarified. Therefore, this study of the variation in the brittle-ductile transition has an important guiding role in refining the exploration of Longmaxi shale gas in the Sichuan Basin. • There are obvious differences between the lower shale and the upper shale. • Difference of brittle-ductile transition makes shale gas exploration more complicated. • The new method can quickly calculate the threshold value of over consolidation ratio. • The new model provides more comprehensive guidance for shale gas exploration.

Biostratigraphy and reservoir characteristics of the Ordovician Wufeng Formation—Silurian Longmaxi Formation shale in the Sichuan Basin and its surrounding areas, China

Through graptolite identification in profiles, graptolite zone division, contour map compilation, and analysis of mineral composition, TOC content, lamina distribution features of shale samples, the biostratigraphic and reservoir characteristics of Ordovician Wufeng Formation—Silurian Longmaxi Formation in the Sichuan Basin and its peripheral are sorted out. There are 4 graptolite zones (WF1 to WF4) in Wufeng Formation and 9 (LM1 to LM9) in Longmaxi Formation, and the different graptolite zones can be calibrated by lithology and electrical property. The shale layers of these graptolite zones have two depocenters in the southwest and northeast, and differ in mineral composition, TOC, and lamina types. Among them, the graptolite zones of lower WF2 and WF4 are organic matter-poor massive hybrid shale, the upper part of WF1–WF2 and WF3 have horizontal bedding hybrid shale with organic matter, the LM1–LM4 mainly consist of organic-rich siliceous shale with horizontal bedding, and the LM5–LM9 graptolite zones consist of organic-lean hybrid shale with horizontal bedding. The mineral composition, TOC and lamina types of shale depend on the paleo-climate, paleo-water oxidation-reduction conditions, and paleo-sedimentation rate during its deposition. Deposited in oxygen-rich warm water, the lower parts of WF1 and WF2 graptolite zones have massive bedding, low TOC and silicon content. Deposited in cooler and oxygen-rich water, the WF4 has massive bedding, high calcium content and low TOC. Deposited in anoxic water with low rate, the upper part of WF2, WF3, and LM1–LM4 are composed of organic rich siliceous shale with horizontal bedding and high proportion of silt laminae. Deposited in oxygen rich water at a high rate, the graptolite zones LM5–LM9 have low contents of organic matter and siliceous content and high proportions of silt lamina.

Rock physical and chemical alterations during the in-situ interaction between fracturing fluid and Silurian organic-rich shales in China

Hydraulic fracturing has been proved to be an effective way to unlock shale gas resources. During this process, fracturing fluid comes into contact with shale reservoirs and interact with the rocks. This could cause alteration of the pore systems in shales, thereby affecting gas transport and production. To illustrate this geochemical process and identify the physical and chemical alterations of shales, fluid-shale interaction at in-situ conditions is studied for typical marine organic-rich shales from the Silurian Longmaxi Formation in the Sichuan Basin, using actual fracturing fluid and deionized water over a 5-day period. The minerals and pore structures before and after the reaction were examined by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), QEMSCAN, computed tomography (CT) scanning, nitrogen adsorption testing and organic geochemical analysis. Results showed that, the fracturing fluid and deionized water caused indiscernible changes to the organic matter, but a variety of alterations on carbonate minerals. Calcites and dolomites were markedly dissolved, but quartzs, albites and illites remained relatively unaltered. More dolomites crystals were precipitated in the deionized water-shale interaction than in the fracturing fluid-shale interaction, and some of the fractures seemed to be filled by flocculation of friction reducers or deposited mineral particles. The fracturing fluid could enlarge the pore volumes and improve pore connectivity, and the invasion depth of fracturing fluid after 120 h’ was around 60 μm. The dissolved pores and loosening/shedding of particles detected by CT increased through time, and the calculated 3D pore volume increased by 67% at 24 h, 117% at 72 h, and 430% at 120 h compared to that of the initial sample. The XRD and nitrogen adsorption data indicated that new pore volumes increased with higher contents of carbonate minerals in the sample and longer treatment by fracturing fluid. The mineral and pore-scale alterations of shale reservoirs indicated that fracturing fluid-shale interaction (FFSI) enlarged the reservoir reconstruction volume, increased well performance during shut-in period and should be considered as part of the well treatment optimization process.

Mineralogy and geochemistry of Lower Silurian black shales from the Yangtze platform, South China

Early Silurian black shales are widely deposited over the entire Yangtze platform and their origin are highly controversial. In order to identify the controlling factors of these black shales formation, the mineralogy and geochemistry of the Early Silurian organic-rich shales from the Yangtze Platform, South China, were investigated by using an optical microscopy, X-ray diffraction analysis, X-ray fluorescence spectroscopy, inductively coupled plasma mass spectrometry and field emission scanning electron microscopy. These black shales are characterized by high total organic carbon (0.37%–10.47%, avg. 3.86%) and total sulphur (0.02%–1.68%, avg. 0.67%) content as well as enrichment of some trace elements, including V (70.66 ppm–1740.52 ppm, avg. 312.75 ppm), Cr (68.72 ppm–301.94 ppm, avg. 117.97 ppm), U (3.90 ppm–52.49 ppm, avg. 16.58 ppm), Ba (316.69 ppm–2985.82 ppm, avg. 1523.38 ppm) and rare earth elements (94.26 ppm–340.50 ppm, avg. 202.46 ppm). The minerals mainly consist of quartz and clay mineral with average content of 54.8% and 29.3%, respectively. K-feldspar, dolomite, calcite and pyrite are next in abundance, followed by trace amounts of apatite, barite and Cr-bearing minerals. The dissolution of biogenic silica and recrystallization during burial is an important source of quartz cement. In addition to biological sources, terrestrial, volcanic and authigenic sources are responsible for these mineralogical and geochemical compositions. The Al2O3/TiO2, Zr/Sc, Th/Sc, LREE/HREE and Eu anomalies indicate that the sediment-source region is derived from a mixture of felsic and intermediate rocks of the old upper continental crust. The Th/U and V/(V + Ni) ratios, Ce anomalies and authigenic U suggest that these black shales are deposited under an anoxic bottom water environment. The elevated concentrations of U-V-Ni-Cu-Zn-Cr-Ba and REE in these shales are predominated by the marine environment during deposition and influenced by biologic production in water columns and volcanic ash fall. High marine productivity resulting from volcanic nutrients promote the anoxic/sulfidic water column conditions of the restricted Yangtze basin, further resulting in organic matter enrichment. Elemental enrichment processes usually result in a valence change under anoxic environment, which is associated either with the clay minerals or direct precipitation from sea water. Understanding the coenrichments of organic carbon and trace elements in these black shales is important for studying the formation mechanism and resource potential of hydrocarbon and critical elements.

Pore Size Distribution and Fractal Characteristics of the Nanopore Structure in Organic-Rich Shales Using the Neimark-Kiselev Fractal Approach.

Shale gas has been playing an increasingly important role in meeting global energy demands. The heterogeneity of the pore structure in organic-rich shales greatly affects the adsorption, desorption, diffusion and flow of gas. The pore size distribution (PSD) is a key parameter of the heterogeneity of the shale pore structure. In this study, the Neimark-Kiselev (N-K) fractal approach was applied to investigate the heterogeneity in the PSD of the lower Silurian organic-rich shales in South China using low-pressure N₂ adsorption, total organic carbon (TOC) content, maturity analysis, X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) measurements. The results show that (1) the fractal dimension DN-K obtained by N-K theory better represents the heterogeneity of the PSD in shale at an approximately 1-100 nm scale. The DN-K values range from 2.3801 to 2.9915, with a mean of 2.753. The stronger the PSD heterogeneity is, the higher the DN-K value in shale is. (2) The clay-rich samples display multimodal patterns at pore sizes greater than 20 nm, which strongly effect the PSD heterogeneity. Quartz-rich samples display major peaks at less than or equal to a 10 nm pore size, with a smaller effect on the PSD heterogeneity in most cases. In other brittle mineral-rich samples, there are no obvious major peaks, and a weak heterogeneity of the PSDs is displayed. (3) A greater TOC content, maturity, clay content and pore size can cause stronger heterogeneity of the PSD and higher fractal dimensions in the shale samples. This study helps to understand and compare the PSD and fractal characteristics from different samples and provides important theoretical guidance and a scientific basis for the exploration and development of shale gas resources.

Characterization and evaluation of shale lithofacies within the lowermost Longmaxi-Wufeng Formation in the Southeast Sichuan Basin

Based on drilling cores, thin sections, FIB-SEM and geochemistry data, shale lithofacies in the lowermost part of the Longmaxi-Wufeng Formation were studied in the Southeast Sichuan Basin. This formation is the main shale gas producing layer in Jiaoshiba and its surrounding areas. In this article, with the combination of the three-end-member method (using mineral components) and TOC, six lithofacies, namely, organic-rich siliceous shale, organic-rich mixed shale, organic-rich argillaceous shale, organic-lean siliceous shale, organic-lean mixed shale, and organic-lean argillaceous shale, were identified. Relevant parameters that could represent reservoir quality of the different lithofacies, including porosity, permeability, pore spaces, TOC and gas content, were analyzed and compared. In the end, some of those parameters (e.g., porosity, TOC and gas content), together with special mineral contents (e.g., QMF and clay), were used to establish shale lithofacies evaluation criteria. The results show that organic-rich siliceous shale could be considered as an excellent lithofacies for shale gas and is mainly concentrated in the first section; organic-rich mixed shale, organic-lean siliceous shale and organic-lean mixed shale are assumed to be moderately promising for shale gas, while organic-rich argillaceous shale and organic-lean argillaceous shale are ranked as the least-promising lithofacies in the research area. Consequently, the first section contains the most favorable lithofacies for shale gas exploration.

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.

Relationship between the origin of organic-rich shale and geological events of the Upper Ordovician-Lower Silurian in the Upper Yangtze area

Abstract Based on detailed sedimentological observations and geochemical analyses, four geological events have been identified, and a genetic depositional model has been established within a sequence stratigraphic framework. The organic-rich siliceous shale interbedded with dense bentonite layers in the transgressive systems tract 1 (TST 1) is attributed to the first stage of high-frequency volcanism (which started at ∼447.62 Ma) associated with the collision of the Cathaysia and Yangtze Blocks. The presence of the organic-fair lime mudstone/shelly limestone indicates that Hirnantian Glaciation in the highstand systems tract (HST). The occurrence of organic-rich siliceous shale interbedded with scarce bentonite layers in the transgressive systems tract 2 (TST 2) is attributable to the second stage of low-frequency volcanism (which started at ∼443.83 Ma) and to postglacial effects. The formation of organic-fair mixed shale with silty laminae is attributed to bottom-current activity in the early highstand systems tract (EHST). The occurrence of organic-poor clay-rich shale is due to terrigenous input in the late highstand systems tract (LHST). Our research also shows that the high marine productivity caused by volcanic ash and a high burial rate in an anoxic depositional setting led to organic matter and siliceous mineral enrichment in the TST 1 and TST 2. The favorable organic matter preservation conditions and the postglacial biotic recovery played significant roles in the formation of the organic-rich shale in the TST 2.

Application of a lithofacies model to characterise the gas shale in the Wufeng and Lungmachi formations, NE Sichuan Basin, China

ABSTRACTA practical model that uses a combination of the variations in total organic carbon (TOC) content and mineralogy is developed to determine the shale facies. By using a ternary plot, four main lithofacies and 14 subfacies were identified. This method is applied to describe the shale lithofacies and to identify the favourable lithofacies in the Wufeng and Lungmachi formations based on drill cores from well WX2, NE Sichuan Basin, China. The results show five classes of lithofacies are present and the organic-rich siliceous shale at the base of the Lungmachi Formation (LMF) is the most promising lithofacies, as it has the highest in-situ gas content, abundant TOC, and a high brittleness index, which are all favourable for the generation and development of shale gas. No significant changes in lateral continuity exist in the study area. The core based model can be used to quantify shale lithofacies in other areas.

Lithofacies classification and its effect on pore structure of the Cambrian marine shale in the Upper Yangtze Platform, South China: Evidence from FE-SEM and gas adsorption analysis

Lithofacies classification and storage capacity are of great significance for shale gas. In this study, shale lithofacies classification and its effect on pore structure from the Niutitang shale in the Upper Yangtze Platform were analyzed by experiments of field emission scanning electron microscope (FE-SEM) and gas adsorption analysis. Results show that Niutitang shale can be divided into 12 kinds of shale lithofacies on the basis of TOC content and mineral composition. Pore types and size distribution are significantly distinct of different shale lithofacies since TOC content and mineral compositions do affect shale pore structure differently in their own ways. Specifically, TOC content has the most significant positive effect on shale pore structure, indicating that the higher the TOC content is, the larger the pore volume and surface area are. Negative correlation has been found between calcareous mineral and pore volume and surface area, indicating the decrease of pore volume and surface area with the increase of calcareous mineral content. Compared to argillaceous mineral, siliceous mineral has stronger effect on pore structure. Pore types are also distinctly different in different shale lithofacies which also significantly affect shale gas storage capacity. Organic matter pore is the most abundant pore type in the organic-rich siliceous shale while no OM pore has been found in the organic-poor calcareous shale. Above all, the organic-rich siliceous shale has the largest pore surface area as well as gas storage capacity. While the organic-poor calcareous shale has the smallest gas storage capacity with the poor pore volume and surface area. It would be an effective method to optimize the favorable shale lithofacies in guiding the exploration and development of shale gas.

Lithofacies and sedimentary sequence of the lower Cambrian Niutitang shale in the upper Yangtze platform, South China

The lower Cambrian Niutitang shale in the Upper Yangtze Platform is a hot target for shale gas exploration in China. In this paper, the lithofacies and sedimentary evolution characteristics about this shale formation are studied on the basis of outcrop investigation and experimental measurements. The variations on geochemistry, mineralogy, lithofacies and stratigraphic architecture of this shale are analyzed. This shale is rich in organic matter, with an average TOC content of 5.5%. Quartz and clay mineral dominate the mineral composition, with an average content of 57.3% and 29.8%, respectively. The TOC content correlates positively to quartz content, but negatively to clay mineral content. Trace element ratios of V/Cr, V/(V + Ni) and Ni/Co indicate anoxic to dysoxic conditions prevailed during the Niutitang shale deposition period in Early Cambrian, and moderate to weak degree of restriction are inferred by the covariation of Mo and TOC content. Five lithofacies are identified based on TOC content, mineral composition, texture and fabric: organic-rich siliceous shale, silty siliceous shale, calcareous siliceous shale, argillaceous shale, and silty mixed shale. Four lithofacies associations are interpreted to represent four different depositional facies from shelf margin to abyssal environments. The Niutitang Formation generally shows a shallowing-upward prograding depositional sequence and can be divided into two large shale cycles. From bottom to top, the organic matter content and anoxic degree decreases largely with the sea level fall. The average content of quartz deceases, but the average content of clay mineral increases. Stratigraphic correlation for this shale reveals an obviously decreasing trend of quartz content, but an increasing trend of organic matter and clay mineral contents from slope to basinal facies.

Lithofacies characteristics and its effect on gas storage of the Silurian Longmaxi marine shale in the southeast Sichuan Basin, China

Based on a detailed description and analysis of outcrops and drilling cores, onsite gas desorption, and laboratory testing data, shale lithofacies characteristics and its effect on gas storage of the Silurian Longmaxi Formation marine shale in the southeast Sichuan Basin have been studied. Twelve types of shale lithofacies are classified based on the organic matter content and mineral composition, of which 9 types were identified in the study area based on their dramatically differences in color, grain size, lamination, organic matter content, mineralogy, density, and other physical properties. The bottom of the Longmaxi shale Formation is primarily organic-rich siliceous shale with stable distribution of thickness and good continuity. The upper section has characteristics of rapid vertical and lateral change in lithofacies, exhibiting a strong spatial heterogeneity. Organic-rich siliceous shale in the lower section, showing the highest content of gas desorption in situ, has a high content of organic matter, a high brittleness index and good permeability, which is conducive to shale gas storage, hydraulic fracturing, and exploitation. Organic-rich argillaceous shale was also observed to have the highest methane adsorption capacity. The interval with lithofacies association of organic-rich siliceous shale with organic-rich argillaceous shale interlayer is the pay zone for shale gas generation and accumulation.