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Shale gas is a relatively clean-burning fossil fuel, produced by hydraulic fracturing. This technology may be harmful to the environment; therefore, environmentally friendly methods to extract shale gas have attracted considerable attention from researchers. Unlike previous studies, this study is a comprehensive investigation that uses systematic analyses and detailed field data. The environmental challenges associated with shale gas extraction, as well as measures to mitigate environmental impacts from the source to end point are detailed, using data and experience from China’s shale gas production sites. Environmental concerns are among the biggest challenges in practice, mainly including seasonal water shortages, requisition of primary farmland, leakage of drilling fluid and infiltration of flowback fluid, oil-based drill cuttings getting buried underground, and induced seismicity. China’s shale gas companies have attempted to improve methods, as well as invent new materials and devices to implement cleaner processes for the sake of protecting the environment. Through more than 10-year summary, China’s clean production model for shale gas focuses on source pollution prevention, process control, and end treatment, which yield significant results in terms of resource as well as environmental protection, and can have practical implications for shale gas production in other countries, that can be duplicated elsewhere.

Recently, shale gas research and industrial production in China have been significantly developed, which has enriched the theory of natural gas geology in China and increased the supply of domestic natural gas production. In the future, shale gas will obtain great achievements among the developments of the natural gas industry in China. The following four geological and geochemical issues about the exploration and research of shale gas have great significance: (1) China has to break through the single economic shale gas formation. The United States found at least thirty sets of economic shale gas formations in 29 basins. To date, only one basin in China (Sichuan Basin) has found one set of economic shale gas formation (Wufeng-Longmaxi formations). (2) In regards to the interval RO value of the economic shale gas formations and segment RO value of the gas fields, the interval RO values are 3.31% for the economic shale gas formation in China. However, it is 4.1% for that in the USA. The segment RO values are 2.34% for the gas fields in China and 3.5% for that in the USA. So the values are small in China, but big in the USA. Several shale gas fields derived from sapropelic sources in the United States were found in the oil generation window with RO = 1.0%–1.3%. Therefore, shale gas exploration in China also needs to pay attention to this stage. At the same time, attention should be paid to the exploration of shale gas derived from humic sources. (3) When it comes to the continuous high-yield period of the gas field, the world's first commercially developed shale gas field (Barnett) shows a high-yield forecast of 10 × 109 m3 per year for about twenty years. The development of the shale gas field in China is still in the construction stage. (4) In terms of the role of fracture in shale gas preservation and development, the fault has a dual effect on the enrichment of shale gas. In the United States, tectonic in the shale gas basin is stable, and faults are few, while structures in China in the shale gas basin are active, and faults are developed. Therefore, faults have more harm than good to the shale gas in China, which caused the fragmentation of the continuously distributed Wufeng-Longmaxi formations, limiting the area of the gas field.

Based on thin-section, argon-ion polished large-area imaging and nano-CT scanning data, the reservoir characteristics and genetic mechanisms of the Lower Silurian Longmaxi shale layers with different laminae and laminae combinations in the Sichuan Basin were examined. It is found that the shale has two kinds of laminae, clayey lamina and silty lamina, which are different in single lamina thickness, composition, pore type and structure, plane porosity and pore size distribution. The clayey laminae are about 100 μm thick each, over 15% in organic matter content, over 70% in quartz content, and higher in organic pore ratio and plane porosity. They have abundant bedding fractures and organic matter and organic pores connecting with each other to form a network. In contrast, the silty laminae are about 50 μm thick each, 5% to 15% in organic matter content, over 50% in carbonate content, higher in inorganic pore ratio, undeveloped in bedding fracture, and have organic matter and organic pores disconnected from each other. The formation of mud lamina and silt lamina may be related to the flourish of silicon-rich organisms. The mud lamina is formed during the intermittent period, and silt lamina is formed during the bloom period of silicon-rich organisms. The mud laminae and silt laminae can combine into three types of assemblages: strip-shaped silt, gradating sand-mud and sand-mud thin interlayers. The strip-shaped silt assemblage has the highest porosity and horizontal/vertical permeability ratio, followed by the gradating sand-mud assemblage and sand-mud thin interlayer assemblage. The difference in the content ratio of the mud laminae to silt laminae results in the difference in the horizontal/vertical permeability ratio.

The shales in the 2nd Member of Shanxi Formation in the east margin of the Ordos Basin were deposited in a marine-nonmarine transitional environment during the Permian. Based on the recent breakthroughs in the shale gas exploration and theoretical understandings on the shale gas of the study area, with a comparison to marine shale gas in the Sichuan Basin and marine-nonmarine transitional shale gas in the U.S., this study presents the geological characteristics and development potential of marine-nonmarine transitional gas in the study area. Four geological features are identified in the 2nd Member of the Shanxi Formation in the study area has: (1) stable sedimentary environment is conductive to deposition of widely distributed organic shale; (2) well-developed micro- and nano- scale pore and fracture systems, providing good storage capacity; (3) high content of brittle minerals such as quartz, leading to effectively reservoir fracturing; and (4) moderate reservoir pressure and relatively high gas content, allowing efficient development of shale gas. The 2nd Member of Shanxi Formation in the east margin of Ordos Basin is rich in shale gas resource. Three favorable zones, Yulin-Linxian, Shiloubei-Daning-Jixian, and Hancheng-Huangling are developed, with a total area of 1.28×104 km2 and resources between 1.8×1012 and 2.9×1012 m3, indicating a huge exploration potential. Tests of the 2nd Member of Shanxi Formation in vertical wells show that the favorable intervals have stable gas production and high reserves controlled by single well, good recoverability and fracability. This shale interval has sufficient energy, stable production capacity, and good development prospects, as evidenced by systematic well testing. The east margin of the Ordos Basin has several shale intervals in the Shanxi and Taiyuan formations, and several coal seams interbedded, so collaborative production of different types of natural gas in different intervals can be considered. The study results can provide reference for shale gas exploration and development and promote the rapid exploitation of shale gas in China.

At present, shale gas exploration and development in China is faced with some problems, such as the imperfect evaluation system of reservoir effectiveness and the limitations of reservoir evaluation system on efficient development of shale gas. In order to improve the content and the standard of reservoir evaluation, this paper analyzed the shortcomings and challenges in the static evaluation of shale gas reservoirs on the basis of existing reservoir evaluation, and established a method for evaluating shale gas reservoir effectiveness and a scheme for classifying pore systems. Then, the dynamic evaluation parameters after shale fracturing and their effects on drainage and production measures were discussed. In addition, the potential evaluation parameters of “automatic mitigating water blocking” were studied, and a comprehensive reservoir evaluation system of “static–dynamic” combination was established. And the following research results were obtained. First, new challenges to the shale gas reservoir evaluation are emerged as the lack of in-depth study on “reservoir effectiveness, dynamic evaluation parameter system after fracturing and drainage and production measures after fracturing”, which leads to the serious lag of existing shale gas reservoir evaluation system behind production. Second, the evaluation of reservoir effectiveness is mainly presented as the evaluation on the lower limit of effective porosity, and is embodied in the influence of clay bound water and unconnected pores on the development of shale gas. Third, the development of shale gas reservoir evaluation follows the trend of refining the static reservoir evaluation parameters, defining the potential evaluation indexes of “automatic mitigating water blocking” and establishing the reservoir comprehensive evaluation system of “static–dynamic” combination. Fourth, a post-frac dynamic evaluation system is determined for the potential evaluation indexes of “automatic mitigating water blocking” (e.g., wettability, water imbibition retention capacity, water imbibition expansion mode, expansion rate, and water imbibition cracking capacity). Fifth, a reservoir evaluation idea is put forward that “static evaluation of shale gas reservoir is the foundation and postfrac dynamic evaluation is the complement”, and a comprehensive reservoir evaluation system is established of “static–dynamic” combination suitable for the evaluation of marine shale gas reservoirs in China.

The types of organic-matter (OM) pores in shale gas reservoirs were comprehensively investigated from the Wufeng-Longmaxi (W-L) mudstones in the Changning gas field, southern Sichuan Basin, China. The aim of this research is to provide the origin and significance of OM pores development in the nanometer-scaled pore structures within shale rocks. To understand the contribution of OM pores development to the W-L mudstones, the thin-section, OM extract from bulk cores, mineralogy, pore size distribution, and image analyses by scanning electron microscopy (SEM) and atomic force microscopy (AFM) were comprehensively carried out. Combining SEM photographs and AFM observations, two types of OM pores were identified, including structural OM pores and non-structural spongy OM pores. Through organic petrology analyses, the structural OM pores evolve from telalginites mainly sourced from phytoplankton and acritarch, while the non-structural spongy OM pores are derived from bacteria, benthic algae and certain graptolites. Three basic lithofacies were observed in the first member of the W-L shales, based on the results of the low-pressure N2 adsorption. We did not find any complex interaction between mineralogy and OM pores evolution. However, siliceous shales generate the most amount of bitumen which is the main OM pores producer, and the rigid siliceous particles can make the shelter of bigger OM pores. OM pores connectivity therefore increases within the siliceous shales. OM pores development, especially structural OM pores, has a crucial effect on shale gas exploitation.

Recent significant breakthroughs have been made of the Lower Silurian shale gas in the Changning-Zhaotong blocks recently, the key exploration area of China. Through comparative research of the gas geochemistry, Longmaxi Formation shale gas is mainly composed of methane (98.8%), with extremely low wetness coefficient (0.49%), and possesses negligible quantities of ethane and propane. The Non-hydrocarbon gases are mainly composed of small-scale quantities of N2 and CO2 of mixed origin. Enriched δ13C1 (from −29.4‰ to −26.7‰) and δ13C2 (from −34.5‰ to −31.6‰) values, together with sapropelic organic matter, indicate a typical overmature oil-type gas. Abnormal geochemical characteristics of shale gas (δ13C1 > δ13C2 > δ13C3) result from merged secondary impacts in the high-maturity and well-confined shale reservoirs, like the mixing of secondary cracking gases, Rayleigh fractionation of C2 when transition metals and water undergo redox reactions. Notable geographic variation can also be observed, as the δ13C1 and δ13C2 values of the Longmaxi Formation become more positive toward the thrust-faulted folds developed in the southern CZ blocks.

Organic-matter-rich shales are the main target rocks for unconventional oil and gas exploration and development across the world. In China, shale-gas geological resources are estimated as approximately 110 × 1012 m3, with recoverable gas reserves of ca. 20 × 1012 m3. Recoverable shale-oil reserves are estimated as ca. 5 × 109 t. A total 35 important organic-matter-rich shale units have been recognized from Mesoproterozoic to Cenozoic strata across the entire China. These shales are categorized according to their origin under marine, marine–nonmarine transitional and lacustrine conditions. Shales of marine origin, with ca. 9 × 1012 m3 recoverable resources, dominate China's potential in terms of total volume of organic-carbon. Currently, the most favorable marine shales for oil and gas exploration are found in the Sichuan Basin within the lower Cambrian Qiongzhusi Formation and in the Wufeng-Longmaxi formations of uppermost Ordovician through lower Silurian. A fortuitous combination of of sea-level variations, of paleo-productivity, of tectonic activity causing development and migration of partially closed deep basin depocenters, and of sediment accumulation rates controlled the extensive deposition and distribution of organic-matter-rich shales in these Wufeng and Longmaxi formations. Organic-matter-rich shales in marine-nonmarine transitional facies associated with coal measures occur in North China within the Carboniferous and Permian, and in South China within the Permian. These Carboniferous-Permian organic-matter-rich shales are important source rocks for the gas fields in the Ordos and Sichuan Basins. Abundant organic-rich shales are also widely distributed within coal-bearing clastics and coal-measure shales of fluvial, lacustrine, and swamp facies in Upper Triassic to Middle Jurassic successions of many basins. Lacustrine organic-rich shales were deposited during the Permian through Neogene in various freshwater to saline lake settings. Lacustrine organic-matter-rich shales are the main oil source rocks in the Songliao, Bohai Bay, Ordos and Junggar basins. Lacustrine algae contributed to the rain of organic matter; and the preservation of organic matter and distribution of organic-rich shale was controlled by lake currents, water depth and oxygen-poor conditions, with enhanced preservation when buried by turbidity currents. Algal blooms were partly induced by trace nutrients from volcanic ash falls in all of these lacustrine basins. Seawater intrusion into the freshwater lake of the Songliao Basin promoted some episodes of black shales. Saline lacustrine basins, such as middle Permian Junggar Basin, contain organic-rich dolomite mudstone that mainly formed during hot climate conditions when the lakes had high salinity and stratified water columns that deprived the bottom waters of oxygen, thereby preserving massive amounts of organic matter. Laminated calcite-rich mudstone in the saline lacustrine settings formed in more brackish waters under stable warm conditions and weak biological activity. The modeling of the factors controlling the distribution of organic-matter-rich shales within China's basins is important for the exploration and development of unconventional oil and gas resources.

The pore structure characterization and its controlling factors in overmature shales are keys to understand the shale gas accumulation mechanism. Organic matter in source rocks is a mixture of various macerals that have their own specific evolutionary pathways during thermal maturation. Pores within macerals also evolve following their own path. This study focused on petrographic characterization and maceral controls on porosity in overmature marine shales in China and the United States. Shale from Ordos Basin in China was also selected as an example of overmature transitional shale for maceral comparison. Organic petrology techniques were used to identify maceral types and describe morphological features in detail; scanning electron microscopy techniques were then used to document the abundance and development of pores within macerals. Helium measurement, mercury intrusion capillary pressure, and CO2 adsorption were especially applied to quantify the pore structure of Wufeng-Longmaxi shale from Sichuan Basin in China. The vitrinite reflectance equivalent of the studied overmature samples is ~2.4%. The macerals within the studied marine shales are composed mainly of pyrobitumen and zooclasts. At this maturity, pyrobitumen develops abundant gas-related pores, and their volume positively correlates to gas content. Three types of pyrobitumen and its related pore structure are characterized in Wufeng-Longmaxi shales. Zooclasts contribute to total organic carbon (TOC) content but little to porosity. When the TOC content is above 1.51% in Wufeng-Longmaxi samples, the TOC content positively correlates to quartz content. Organic matter strongly controls micropore development. Pores of diameter ~ 0.5 nm provide a significant amount of micropore volume. Clay mineral and quartz contents control micro- and macropore increments in organic-lean shales. MICP results indicate that pores within 3-12 nm and 900-2500 nm account for a major contribution to pore volume obtained. Determining the proportions of pyrobitumen to zooclasts within the total organic matter in pre-Devonian organic-rich marine shales is important in predicting porosity and gas storage capacity in high-maturity shales.

The Late Ordovician mass extinction (LOME) was a severe biocrisis during the Phanerozoic and in this extinction, trilobites, brachiopods, mollusks, graptolites, conodonts and other types suffered severe mortality, and 85% of marine invertebrate species were eliminated. The potential cause of this mass extinction remains unclear, although glaciation and marine anoxia have received the greatest attention. Herein, we present findings on pyrite morphology, pyritic sulfur isotopic composition and total organic carbon content (TOC) to analyze the evolution of redox conditions across the Ordovician-Silurian boundary in the Yangtze area of South China. Our results show that long-term euxinic conditions interrupted by episodic dysoxic/oxic events occurred from the upper Katian stage to the lower Rhuddanian stage. The two extinction pulses of LOME were coincident with euxinic conditions, suggesting that marine euxinia may have been the main cause of this mass extinction event.

The Sichuan Basin is a major target for shale gas exploration in present China because of its rich gas stored in abundant black shales with multiple bed series. For further guidance or reference, field exploration and development practices in the shale reservoirs Upper Ordovician Wufeng–Lower Silurian Longmaxi shale reservoirs were studied in terms of development stages and progress, favorable conditions for shale gas accumulation, bottlenecking issues on theories and technologies related to shale gas development, and so on. The following findings were obtained. (1) Shale with rich organic matters originated from the deep shelf has a good quality and great thickness in the continuous beds. The relatively stable wide buffer zones in synclines (anticlines) provides favorable conditions for shale gas accumulation and preservation with well-developed micro-fractures and overpressure as necessary factors for a great potential of high shale gas productivity. (2) The bottlenecking technical issues restricting the shale gas industrial development in this study area include the following aspects: understandings of rich-organic matter shale sedimentary facies and modes, shale reservoir diagenetic process and evaluation systems, shale gas generation and accumulation mechanism, geophysical logging identification and prediction of shale gas layers, low resource utilization rate, great uncertainty of shale gas development, no technological breakthrough in the exploration of shale gas reservoirs buried deeper than 3500 m. In conclusion, this study area will be the major target for the shale gas exploration and development in China in a rather long period in the future. Keywords: Sichuan Basin, Upper Ordovician Wufeng–Lower Silurian Longmaxi, Shale gas, Exploration and development, Progress, Challenge, Prospect

The dominant factors controlling development of microfractures in the black shale and the origin of microfractures in the sweet spot intervals were discussed of the Ordovician Wufeng Formation−Silurian Longmaxi Formation in Shuanghe outcrop profile, Changning, Sichuan Basin. For the target interval, holographic photograph statistics of microscopic composition of 203 big thin sections and 203 small thin sections, TOC content of 110 samples, 110 whole rock X-ray composition, and main trace elements of 103 samples were tested and analyzed. The results show that the microfractures include bedding microfractures and non-bedding microfractures. The bedding microfractures are mostly plane slip microfractures, lamellation microfractures and echelon microfractures. The non-bedding microfractures are largely shear microfractures and tension microfractures. Vertically, the density of microfractures is the highest in SLM1 Member of Longmaxi Formation, decreases from SLM2 Member to SLM5 Member gradually, and drops to the lowest in Wufeng Formation. The microfracture density is positively correlated with siliceous content and negatively correlated with the carbonate content. The finer the grain size of the black shale, the higher the density of the microfractures is. The microfracture density is controlled by biogenic silicon: the higher the content of biogenic silicon, the higher the microfracture density is. Under the effect of ground stress, microfractures appear first in the lamellar interfaces. Regional tectonic movements are the key factor causing the formation of microfractures in the sweet spot interval, diagenetic contraction is the main driving force for lamellation fractures, and the pressurization due to hydrocarbon generation is the major reason for the large-scale development of microcracks. Key words: microfracture, TOC content, lamina, lithology, black shale, Silurian Longmaxi Formation, Ordovician Wufeng Formation, Sichuan Basin

Recently, shale-gas exploration in the Lower Silurian has been conducted in the Weiyuan area, southwestern Sichuan Basin, which has a special symbolic significance for the development of shale gas in China. According to the first systematic analysis of its geochemical characteristics, methane accounts for the vast majority (95.7%) of the Longmaxi Formation shale gas in the Weiyuan area, with extremely low wetness (C2–3/C1–3%) of 0.57%. Non-hydrocarbon gases consist of small amounts of N2 and inorganic-induced CO2. The δ13C-CH4 values of shale gas range from −36.9‰ to −34.0‰, and the δ13C-C2H6 values range from −42.8‰ to −37.5‰, indicating that the gases originate from sapropelic kerogen. A reversal in carbon-isotopic composition, i.e., δ13C-CH4 > δ13C-C2H6 > δ13C-C3H8, results from combined secondary effects such as secondary cracking and diffusion under extremely high thermal maturity. Geologic features of organic-rich shale and secondary effects at the overmature stage together lead to differences in geochemical characteristics of shale gas in the Weiyuan and Changning and Fuling (CF) areas, which can be used to indicate the distribution of geochemical parameters and predict shale-gas production.

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

Based on the drilling data of the Silurian Longmaxi Formation in the Sichuan Basin and periphery, SW China, the Ro lower limits and essential features of the carbonization of organic matter in over-high maturity marine shale were examined using laser Raman, electrical and physical property characterization techniques. Three preliminary conclusions are drawn: (1) The lower limit of Ro for the carbonization of Type I-II1 organic matter in marine shale is 3.5%; when the Ro is less than 3.4%, carbonization of organic matter won't happen in general; when the Ro ranges from 3.4% to 3.5%, non-carbonization and weak carbonization of organic matter may coexist; when the Ro is higher than 3.5%, the carbonization of organic matter is highly likely to take place. (2) Organic-rich shale entering carbonization phase have three basic characteristics: log resistivity curve showing a general “slender neck” with low-ultralow resistance response, Raman spectra showing a higher graphite peak, and poor physical property (with matrix porosity of only less than 1/2 of the normal level). (3) The quality damage of shale reservoir caused by the carbonization of organic matter is almost fatal, which primarily manifests in depletion of hydrocarbon generation capacity, reduction or disappearance of organic pores and intercrystalline pores of clay minerals, and drop of adsorption capacity to natural gas. Therefore, the lower limit of Ro for the carbonization of Type I-II1 organic matter should be regarded as the theoretically impassable red line of shale gas exploration in the ancient marine shale formations. The organic-rich shale with low-ultralow resistance should be evaluated effectively in area selection to exclude the high risk areas caused by the carbonization of organic matter. The target organic-rich shale layers with low-ultralow resistance drilled during exploration and development should be evaluated on carbonization level of organic matter, and the deployment plan should be adjusted according to the evaluation results in time. Key words: Lower Silurian, Longmaxi Formation, marine shale, thermal maturity, organic matter carbonization, resistivity logging, exploration risk

The Late Ordovician mass extinction (LOME, ca. 445 Ma) was the first of the "Big Five" Phanerozoic extinction events and comprised two extinction pulses. Proposed kill mechanisms include glacially induced global cooling and the expansion of water-column anoxia and/or euxinia (sulfidic conditions), but no general consensus has been reached with regard to the precise role of these mechanisms. A more definitive understanding is hampered by poorly constrained temporal links between the extinction pulses and climate change, and by uncertainty over the spatial distribution and intensity of euxinia. Here, we utilize Fe speciation and Mo concentrations, in addition to the chemical index of alteration weathering proxy, to reconstruct ocean redox conditions and climate change across a Late Ordovician to Early Silurian shelf-to-slope transect on the Yangtze Shelf Sea (a siliciclastic-dominated shelf basin in South China). These data show two cycles of expanded euxinia corresponding to the two pulses of the LOME, suggesting a strong causal relationship. Significantly, we show that intermittent or weak euxinia developed during the first extinction pulse, which likely accounts for the loss of benthic fauna and some planktonic organisms and nektonic groups. By contrast, the development of more intense euxinia throughout the water column during the second pulse likely drove survival fauna to extinction. Superimposed upon this, significant global cooling occurred across the first extinction phase, reflecting a secondary role in driving the extinction of certain low-latitude taxa.

Based on various test data, the composition, texture, structure and lamina types of gas-bearing shale were determined based on Well Wuxi 2 of the Silurian Longmaxi Formation in the Sichuan Basin. Four types of lamina, namely organic-rich lamina, organic-bearing lamina, clay lamina and silty lamina, are developed in the Longmaxi Formation of Well Wuxi 2, and they form 2 kinds of lamina set and 5 kinds of beds. Because of increasing supply of terrigenous clasts and enhancing hydrodynamics and associated oxygen levels, the contents of TOC and brittle mineral reduce and content of clay mineral increases gradually as the depth becomes shallow. Organic-rich lamina, organic-rich + organic-bearing lamina set and organic-rich bed dominate the small layers 1-3 of Member 1 of the Longmaxi Formation, suggesting anoxic and weak hydraulic depositional setting. Organic-rich lamina, along with organic-bearing lamina and silty lamina, appear in small layer 4, suggesting increased oxygenated and hydraulic level. Small layers 1-3 are the best interval and drilling target of shale gas exploration and development. Key words: Sichuan Basin, Longmaxi Formation, fine-grained sediment, shale gas, lamina, Well Wuxi 2

The development of shale gas in the United States has made a breakthrough, profoundly changed the pattern of oil and gas supply. It created the “shale gas revolution” on a global scale. Based on the practice of shale gas development in China and abroad, this paper aims to: (1) Summarize 5 basic characteristics, which are shale gas resource distribution, reservoir space, sweet spot area (section), hydrocarbon type and development mode. (2) Divide China shale development into three stages: scientific exploration, technological breakthrough and mature development. The United States is divided into three stages: scientific exploration, technological breakthrough and mature development. The United States is divided into three stages: scientific exploration, technological breakthrough and mature development. (3) Identify 4 challenges in the future development of China shale gas industry. It includes non-marine shale gas potential, core technology and equipment for resource deep than 3500 m, complex surface “factory mode” production, human geography and other non-technical factors. (4) Process economic evaluation under the conditions of government financial subsidies. China's shale gas project FIRR is about 8.0%–9.0%. Considering the global shale gas resources, consumer demand and other factors, it global shale gas production is expected to reach 1.1 × 1012m3 by 2040. Keywords: Shale gas, Black shale, Shale gas revolution, Nano pore, Sweet spot area, Sweet spot section, Artificial gas reservoir

In this paper, the shale of Upper Ordovician Wufeng FmâLower Silurian Longmaxi Fm was taken as an example to reveal the distribution patterns and the main sedimentation controlling factors of high-quality shale in the Upper Yangtze region. This study was made from the aspects of plate movement, fluctuation of sea level, palaeo-productivity, deposition rate and paleogeographic environment, based on the field outcrop sections and drilling data of the southern Sichuan Basin, together with geochemical element testing and biostratigraphical analysis results. The following findings were obtained. First, the collision and joint of Yangtze Plate and its periphery plates and the intra-plate deformation were gentle in the early stage, strong in the late stage, gentle in northwest and strong in southeast, so the sedimentation center in the Upper Yangtze region migrated to the northwest and the closure of sea area in the southern Sichuan Basin changed from weak in early stages to strong in late stages. Second, at the turning period from Ordovician to Silurian, sea levels presented the cycle change of deepâshallowâdeepâshallow. Due to the combination of high sea levels and a stable ocean basin in early stages, an extensive anoxic tectonic sedimentary space favorable for organic matters preservation was formed in the sea floor. Third, due to the effect of tectonic movements and sea closure, palaeo-productivity of sea areas in the southern Sichuan Basin presented a trend of being high in the early stage and low in the late stage, and the deposition rate was also low in the early stage and high in the late stage. And fourth, extensive deposition and distribution of shale rich in organic matters and silica was mainly controlled by a stable sea basin with a low subsidence rate, a relatively high sea level, semi-closed waters and low deposition rates. To sum up, the high-quality shale in the Upper Yangtze region is characterized by multiphase superimposition, lateral extension and northwestward renewal of sedimentary ages, and it is mainly deposited in WufengâRhuddanian age and secondarily in Aeronian age. The main exploration layers are WufengâRhuddanian in southernâeastern Sichuan Depression, RhuddanianâAeronian in Weiyuan area, and WufengâTelychian in centralânorthern Sichuan Basin. Keywords: Upper Yangtze region, Late OrdovicianâEarly Silurian, High-quality shale, Bentonite, Sea level, Palaeo-productivity, Deposition rate, Main exploration layers, Southern Sichuan Basin

The reservoir characteristics of the Upper Ordovician Wufeng-Lower Silurian Longmaxi Formations in southern Sichuan Basin were preliminarily revealed in this study by identifying and quantitatively evaluating the fracture pores of five appraisal wells in the central and northern parts of the southern Sichuan Depression by several methods. Four conclusions were reached as follows: (1) In the central zone of the Depression, the deep reservoir space of the Wufeng-Longmaxi producing pay is composed mainly of matrix pores and the microcracks are not common, whether on the local structural highs, flanks or lows. The physical properties are similar to that of the matrix pores in Changning, Weiyuan and Fuling gas fields. (2) In the northern zone of the Depression, the deep reservoir space of the Wufeng-Longmaxi black shale is composed mainly of matrix pores, and fracture pores mainly occur in local discrete intervals, with a total porosity range from 3.5% to 6.7%, on average 5.3%, and fracture porosity of 0-2.1%, on average 0.3%. (3) In the central and northern parts of the southern Sichuan Depression, the Wufeng-Longmaxi producing pays have undeveloped fracture pores and chiefly extensively distributed matrix pores, indirectly indicating relatively stable tectonic activities and corresponding weaker reservoir reworking there than Fuling field located in eastern Sichuan Basin. (4) The size and distribution of the gypsum-salt layer in the Cambrian are the key controlling factors of fracture pore development in the Wufeng-Longmaxi Formations. Therefore, the areas including Wellblocks L7, GS1, eastern Sichuan Basin and western Hubei province, where gypsum-salt layer in the Cambrian is thick and stable, and fracture intervals are likely to occur in the Wufeng-Longmaxi producing pay controlled by decollement structure above salt structure since the Yanshan Movement, are the potential favorable areas for fracture pore development. Key words: Sichuan Basin, southern Sichuan Depression, shale gas, Upper Ordovician Wufeng Formation, Lower Silurian Longmaxi Formation, matrix pore, fracture pore, gypsum-salt layer, decollement

A continuously cored well covering the Carboniferous-Permian Benxi, Taiyuan, and Shanxi formations in the eastern Ordos Basin provides a rare window into environmental conditions of the marine-continental transitional strata. Integration of petrographic, mineralogical, and high-resolution geochemical data allows marine-continental investigating the depositional environment, organic abundance, detrital influx, paleoclimate, paleo-water salinity, and paleoredox conditions of transitional facies. The marine-continental transitional strata in the Upper Carboniferous to Lower Permian consist of complex interbedding of sandstone, shale, coal and limestone. TOC contents display a considerable change in the vertical direction, with high TOC content in coal, moderate TOC in shale, low TOC content in limestone and no noticeable TOC enrichment in sandstone. The Upper Carboniferous Benxi Formation is dominated by marine facies then changing into transitional facies. The Lower Permian Taiyuan Formation is characterized by multiple alternation of marine and transitional facies. Marine limestones are characterized by low detrital influx proxies, whereas transitional shales show high detrital influx input and oxic environment. The Lower Permian Shanxi Formation is a typical marine-continental transitional depositional system, which is characterized by complex interbedding of shales, sandstones and coals. Sandy intervals were deposited in deltaic and tidal flat environment and characterized by high Ti and Al content, low TOC (0.09–0.97%, avg. 0.31%), relatively low U/Th (0.11–0.32, avg. 0.17), showing limited influence of seawater. Shale intervals were deposited in a lagoonal environment with high TOC (0.94–40.64%, avg. 5.43%), Sr/Ba and trace elemental proxies values (U/Th, 0.18–3.27%, avg. 0.44%; MoEF, 0.95–32.3, avg. 8.56; UEF, 0.67–15.17, avg. 2.53). This study shows that high-resolution geochemical characteristics of elements could be reliable indicators for sedimentary facies analysis, and hence could provide broader implications for utilizing trace element geochemistry from marine-continental transitional periods in which sedimentary environment changes frequently and organic-rich sediment accumulated under varied conditions.

As a new expansion area for natural gas exploration, the Yichuan-Huanglong area improves the complete evolutionary sequence of the Paleozoic natural gas in the eastern part of the Ordos Basin, but the geochemical characteristics and the late gas generation processes involved have not been discussed. By collecting and analyzing gas samples from existing wells, the following insights were obtained. The alkane components had a dichotic characteristic, the wetness of the O1m5–C2b natural gas (average: 3.85%) was relatively higher than that of the Permian natural gas (average: 0.88%). The carbon isotopic compositions exhibited partial reversal (δ13C-CH4 >δ13C-C2H6), and the Permian natural gas from the coal-measure strata had abnormally low δ13C-C2H6 values (

In recent years, petroleum assessment incorporates the petroleum system concept method instead of the play concept method to estimate petroleum resources, because conventional-unconventional petroleum resources are more reliably estimated by the genetic similarities of petroleum fluids in the petroleum system. Conventional-unconventional oil resources co-exist in the Lucaogou-Permian petroleum system in the Jimusar sag of the Junggar basin, Northwest China. The mean in-place resources (IPR) of tight oil of the Lucaogou Formation (LF) were estimated at 2.0 billion tons by an integration method in the Jimusar sag. Based on the assessment of the technically recoverable coefficient by drill productivity, it is estimated that the mean technically recoverable resources (TTR) of tight oil are about 0.11 billion tons. The mean IPR and TTR of conventional oil of the Wutonggou Formation (WF) were evaluated at 0.36 billion tons and 0.06 billion tons by analogy methods, respectively. The ratio of mean IPR of unconventional oil to conventional oil in the Jimusar is about 5:1, but the ratio of mean TTR is only about 2:1. In addition, tight oil development is higher cost than conventional oil so far. Therefore, the development of tight oil in the Jimusar sag should be a careful consideration. Anyway, the integrated resource assessment of unconventional and conventional oil in the Jimusar sag could provide as a classic example for other lacustrine petroleum systems.

From the aspects of shale gas resource condition, main exploration and development progress, important breakthrough in key technologies and equipment, this paper systematically summarized and analyzed current situation of shale gas development in China and pointed out five big challenges such as misunderstandings, lower implementation degree and higher economic uncertainty of shale gas resource, and still no breakthrough in exploration and development core technologies and equipment for shale gas buried depth more than 3500 m, higher cost and other non-technical factors that restrict the development pace. Aiming at the above challenges, we put forward five suggestions to promote the shale gas development in China: (1) Make strategies and set goals according to our national conditions and exploration and development stages. That is, make sure to realize shale gas annual production of 20 × 10 9 m 3 , and strives to reach 30 × 10 9 m 3 . (2) Attach importance to the research of accumulation and enrichment geological theory and exploration & development key engineering technologies for lower production and lower pressure marine shale gas reservoir, and at the same time orderly promote the construction of non-marine shale gas exploration & development demonstration areas. (3) The government should introduce further policies and set special innovation funds to support the companies to carry out research and development of related technologies and equipment, especially to strengthen the research and development of technology, equipment and process for shale gas bellow 3500 m in order to achieve breakthrough in deep shale gas. (4) Continue to promote the geological theory, innovation in technology and management, and strengthen cost control on drilling, fracturing and the whole process in order to realize efficient, economic and scale development of China's shale gas. (5) Reform the mining rights management system, establish information platform of shale gas exploration and development data, and correctly guide the non-oil and gas companies to participate in shale gas exploration and development.

The source rocks and reservoirs in the Middle Permian Lucaogou Formation of Jimusar sag, Junggar Basin, NW China and its source-reservoir assemblages were studied to reveal accumulation mechanism of the tight oil, on the basis of core observation, thin slice identification, X-ray diffraction analysis, TOC test and rock pyrolysis test of source rock samples, and mercury injection and nuclear magnetic resonance of reservoir samples, combined with burial history and hydrocarbon-generating history of source rocks. It is concluded that the Lucaogou Formation source rocks mainly have type I and II organic matter, with an average TOC of 4.6%, indicating they are high-quality source rocks in low mature – mature stage; and they can be divided into three types, shale, mudstone and silty mudstone. The reservoir rock in Lucaogou Formation features low porosity and low permeability; mercury injection and NMR (nuclear magnetic resonance) tests show that nano-scale pores and throats (with radius of 50−500 nm) constitute the main reservoir space, and the reservoirs can be classified into three types, muddy siltstone, dolomitic siltstone, and dolarenite according to lithology. The source rocks and reservoirs alternate with each other in vertical direction and are distributed continuously in lateral direction, and the source rocks have been generating oil and gas constantly. It is concluded through comprehensive analysis that since the Late Jurassic, the source rocks in Lucaogou Formation have been generating oil continuously and massively, causing high pressurization, and providing constant driving force and oil supply, as a result, the oil has been charged into the tight reservoirs effectively and continuously, forming abundant tight oil resources. Key words: tight oil, accumulation mechanism, source rock, reservoir, Permian Lucaogou Formation, Jimusar sag, Junggar Basin

A comprehensive analysis was carried out on the geochemical characteristics of 15 shale gas samples from the Changning area to study the carbon isotopic composition features of shale gas and the reversal cause of carbon isotopic composition in post-mature shale gas in the Silurian Longmaxi Formation of the Changning area, Sichuan Basin. Through the analysis of alkane gas component and carbon isotopic composition, combining with the research on carbon isotopic composition from the Longmaxi Formation of the Fuling and Weiyuan areas in Sichuan Basin, the methane from the Longmaxi Formation shale gas accounts for 97.11% to 99.45%, the average gas wetness is 0.49% representing typical dry gas, abnormal average δ13C1 value as −28.2‰ and the average of δ13C2 values is −33.2‰, in view of sapropel-type kerogen, the Longmaxi Formation shale gas belongs to the typical oil-associated gas. With the increasing degree of thermal evolution, the wetness of shale gas decreases gradually, and carbon isotopic composition of methane becomes heavier, and the carbon isotopic composition of ethane and propane will reverse, but the carbon isotopic composition of ethane and propane in the post-mature shale gas of the Changning area stays in the post-stage of reverse and will not get continuously heavier. The abnormal heavy carbon isotopic composition of methane and the reversal phenomenon of carbon isotopic (δ13C1>δ13C2>δ13C3) mainly generate from the secondary cracking effect in the post-mature stage and reactions between ethane with ferrous metals and water under Reileigh fractionation situation. Furthermore, the high temperature is also one of the important influence factors. Key words: Sichuan Basin, Changning area, Silurian Longmaxi Formation, shale gas, carbon isotope

Over-high maturity is one of the most vital characteristics of marine organic-rich shale reservoirs from the Lower Paleozoic in the south part of China. The organic matter (OM) in shale gas reservoirs almost went through the entire thermal evolution. During this wide span, a great amount of hydrocarbon was available and numerous pores were observed within the OM including kerogen and solid bitumen/pyrobitumen. These nanopores in solid bitumen/pyrobitumen can be identified using SEM. The imaging can be dissected and understood better based on the sequence of diagenesis and hydrocarbon charge with the shape of OM and pores. In terms of the maturity process showed by the various typical cases, the main effects of the relationship between the reservoir porosity and organic carbon abundance are interpreted as follows: the change and mechanism of reservoirs properties due to thermal evolution are explored, such as gas carbon isotope from partial to complete rollover zone, wettability alteration from water-wet to oil-wet and then water-wet pore surface again, electrical resistivity reversal from the increasing to decreasing stage, and nonlinearity fluctuation of rock elasticity anisotropy. These indicate a possible evolution pathway for shale gas reservoirs from the Lower Paleozoic in the southern China, as well as the general transformation processes between different shale reservoirs in thermal stages.

Although the annual production of shale gas in China was 13 × 10 8 m 3 in 2014, a systematic study on geochemical and isotopic characteristics of these unconventional gases has not been well addressed. In the present study, almost all shale gas samples available in China, including marine shale gas from the Wufeng-Longmaxi Formation in the Sichuan Basin (O 3 w–S 1 l) and terrestrial shale gas from Chang 7 Member (T 3 y 7 ) in the Ordos Basin, were collected and analyzed for their geochemical and isotopic compositions. The shale gas from the Wufeng-Longmaxi Shale is dry gas with an average methane content of 98.38% and records a highest content of CH 4 in the world (99.59%), which is consistent with the very high thermal maturity levels of the gas shales that have equivalent vitrinite reflectance (EqVRo) values between 2.4 and 3.6%. The δ 13 C 1 values are correspondingly heavy and record a heaviest δ 13 C 1 values (−26.7‰) for the shale gases found in the world as well. The average values of δ 13 C 1 , δ 13 C 2 and δ 13 C 3 for the Wufeng-Longmaxi shale gas are −31.3‰, −35.6‰, and −47.2‰, respectively, displaying a complete carbon isotopic reversal (i.e., δ 13 C 1 > δ 13 C 2 > δ 13 C 3 ). δ 2 H CH4 and δ 2 H C2H6 has an average value of −148‰ and −173‰, respectively, also yielding a hydrogen isotopic reversal (i.e., δ 2 H CH4 > δ 2 H C2H6 ). The Chang 7 shale has an average TOC content of 13.81% with vitrinite reflectance (VRo) values between 0.7 and 1.2%. The Chang 7 shale gas is wet gas with an average methane content of 84.90% and is rich in heavy gaseous hydrocarbons (C 2 –C 5 ). The respective values of δ 13 C 1 , δ 13 C 2 and δ 13 C 3 are −48.7‰, −36.4‰ and −31.3‰, displaying a positive carbon isotopic distribution pattern (i.e., δ 13 C 1 13 C 2 13 C 3 ). The average δ 2 H CH4 , δ 2 H C2H6 and δ 2 H C3H8 values are −256‰, −244‰ and −188‰, respectively, and are characterized by a positive distribution pattern (i.e., δ 2 H CH4 2 H C2H6 2 H C3H8 ). The differences in gas wetness and carbon and hydrogen isotopic distribution patterns between the shale gases from the Wufeng-Longmaxi and Chang 7 shale are attributed to variations in thermal maturity levels. CO 2 is present in low content in both the Wufeng-Longmaxi and Chang 7 shale gases, mostly less than 1%. δ 13 C values for the CO 2 in the Wufeng-Longmaxi Formation are between 8.9 and −9.2‰, indicating an inorganic origin from the cracking of carbonate mineral in the shales under high temperatures. In contrast, δ 13 C values of the CO 2 in the Chang 7 shale gas range from −8.2 to −22.7‰, indicating an organic origin from the thermal degradation of organic matter. Helium in both the Wufeng-Longmaxi and Chang 7 shale gases is dominantly of curst origin in terms of their R/Ra ratios that vary from 0.01 to 0.08. Positive carbon isotopic distribution pattern is typical of primitive thermogenic gas. However, it can be converted into complete or partial carbon isotopic reversal patterns due to secondary alteration. The causes that yield carbon isotopic reversal include (1) mixing of gases with the same source but different thermal maturity levels; (2) secondary cracking of oil or wet gas; (3) formation water-involved reactions; (4) gas diffusion; and (5) carbon isotope exchange between alkane gases at high temperature. Among them, carbon isotopic exchange between alkane gases at high temperature is a key factor. Nine plots have been drawn based on the shale gases from China, USA and Canada. Among them, the plot of δ 13 C 2 versus wetness demonstrates a “lying-S” shape with two inflection points on the gas wetness axis. The wetness value of 1.4% represents a critical point from pyrolytic gas (primary cracking gas) to cracking gas (secondary cracking gas) and whereas the wetness value of 6% marks the end of oil generation. On the diagram of wetness versus δ 13 C, shale gases with wetness values greater than 1.6% are characterized by positive carbon isotopic distribution pattern, whereas a complete or partial carbon isotopic reversals are observed for shale gases with wetness values less than 1.6%.

The Lower Silurian Longmaxi Formation marine shales are the main shale gas reservoirs in the Sichuan Basin, SW China. The present study analyzes the characteristics of the Longmaxi shales using samples selected from wells N1 and N2 in the Changning block, southern Sichuan Basin. Several methods, including X-ray diffraction (XRD), scanning electron microscopy (SEM), helium gas expansion and low pressure nitrogen (N2) adsorption/desorption, were used to investigate the mineralogical composition, geochemical characteristics, pore structure and other characteristics of the shales. The total organic carbon (TOC) contents of the samples range from 0.8 to 6.429%, and the thermal maturity is at the overmature stage. Two wells are rich in quartz and clay, with the average contents of 39.4% and 29.6%, respectively. The Longmaxi shales have the specific surface area and total pore volume around 16.6 m2/g and 0.0165 mL/g, respectively. It is suggested that the TOC content contributes significantly to the average pore size, specific surface area and total pore volume, and the thermal maturity strongly influences the development of porosity, as does the mineralogical composition. Based on imagine analysis, we classify shale gas microreservoirs into three types: organic matter with microfractures, organic matter alone and organic matter with mineral pores (interparticle pores (InterP pores) and intraparticle pores (IntraP pores)). The first type is the best for shale gas exploitation. The second type and the type of organic matter with InterP pores are better, while the type of organic matter with IntraP pores is almost useless. The analysis results show that the microreservoirs of well N1 mainly contain the type of organic matter with InterP pores, while the microreservoirs of well N2 are dominated by the type of organic matter with InterP pores and the type of organic matter with microfractures. Combing all the analysis results and their influencing factors, we consider that well N2 is more favorable for gas production than well N1.

The stratigraphic distribution and lithofacies features of the Guanyinqiao Member of the Upper Ordovician Wufeng Formation around the Sichuan Basin were studied based on the data of drilling, outcrops, rock minerals, geochemistry and logging. The stratigraphic distribution of the Guanyinqiao Member is generally dominated by sea-level variation. This formation is in conformable contact with the overlying and underlying formations inside and locally outside the basin and is locally lost outside the basin. The Guanyinqiao Member is usually composed of deep-water siliceous shale and calcareous and siliceous shale facies, similar to those overlying and underlying lithologies inside the basin; and composed of shallow marl facies or lost, greatly different from the surrounding lithologies in the conformable area outside the basin. The study confirmed that the lithofacies of the Guanyinqiao Member is of great significance to the identification of the reservoir properties of the Wufeng and Longmaxi formations shale gas and to the selection of shale gas. The exploration and development of shale gas is promising in the depressions in the south, east and northeast of the Sichuan Basin, where good reservoirs are developed. The zone where the Guanyinqiao Member is lost has no “sweet spot” formation and is risky in exploring shale gas. The reservoir quality and exploration prospect of the Guanyinqiao Member marl zones are between the two zones mentioned above. Key words: Sichuan Basin, shale gas reservoir, exploration risk, Guanyinqiao Member, stratigraphic distribution, lithofacies feature, sedimentary environment

Based on element geochemical and total organic carbon (TOC) analyses of core samples from the Silurian Longmaxi Formation black shales in the Sichuan Basin, this study tried to figure out the temporal changes of palaoenvironmental settings for the shale deposition. The redox-sensitive element ratios, such as U/Th, V/Sr, V/Cr, Ni/Co and V/(V + Ni) are useful indicators to define the redox condition of marine black shale deposition. Element geochemical analysis of Longmaxi Shale samples taken from well W201 and Z106 in the southern Sichuan Basin shows that the organic-rich shales at the bottom of the Longmaxi Formation were deposited in an anoxic environment, while the upper part organic-poor shales were deposited in an oxic environment. Meanwhile, the sedimentary and paleontological characteristics show that pyrite and microfossils are concentrated mainly in the lower organic-rich black shales, indicating also an anoxic environment. All these geochemical, sedimentary and paleontological criteria indicate a paleoenvironmental change from bottom anoxic to middle and upper dysoxic/oxic conditions for the Longmaxi Formation shales. Furthermore, geochemical indicators should be combined with sedimentary and paleontological features in the study of palaeoenvironment conditions for shale deposition.

Silica diagenesis is one of the most important aspects of black shale diagenesis and critically controls the reservoirs properties of black shales. Three stratigraphic sections of the Wufeng and Longmaxi Formations in the Sichuan Basin along a proximal to distal transect were investigated to study quartz types and their silica sources, as well as their influences on reservoirs properties including rock mechanical properties and porosity preservation. Six types of quartz including detrital quartz, recrystallized radiolaria, siliceous fossil fragments, microcrystalline quartz, quartz overgrowth, and hydrothermal quartz veins were identified based on their occurrences and morphologies under the petrographic microscope and scanning electron microscope and cathodoluminescence color and intensity. Original biogenic silica now exists as recrystallized radiolaria and microcrystalline silica in the mudstone matrix and follows systematic distribution patterns both laterally and stratigraphically. The biological origin of this silica component is supported by the presence of recrystallized radiolaria with preserved spines, preservation of “ghosts” of the central capsule, serrated edges reflecting the spherical lattices of the outer shell, and in addition preserved sponge spicules. A further source of silica for microcrystalline quartz formation was silica released in the course of clay mineral (smectite to illite) transformation. The different types of quartz make different contributions to the reservoir properties of shales, with microcrystalline quartz having the most important impact. Microcrystalline quartz enhances the rock mechanical properties of shales and protects initial porosity from mechanical compaction through a rigid framework of interconnected diagenetic silica grains. Pores between microcrystalline quartz grains are shielded from complete mechanical compaction and provide space for accumulation of bitumen and oil later in burial history. Subsequently, these pore-filling organic matter develop secondary organic nanopores at higher thermal maturity. Although biogenic silica content has been used as a paleoproductivity proxy, the method used to calculate it is based on questionable assumptions, and model parameters need to be fine-tuned for a given shale succession on the basis of petrographic observations. In addition, the commonly observed positive correlation between total organic carbon (TOC) and biogenic silica is invalid for very low sedimentation rates and extremely high concentrations of biogenic silica. From low to extremely high biogenic silica content in organic-rich sediments, a parabolic relationship between TOC and biogenic silica should be common.

To verify recent single-well data from the Sichuan Basin, we analyzed the molecular and isotopic composition of late-mature Longmaxi shale gases from the southwestern portion of the Fuling block. The gases are highly enriched in methane (>96.1%) and have low C2+ hydrocarbon contents (

Taking the Late Ordovician Wufeng Formation (WFF) shale in the Upper Yangtze region as an example, we conducted a lithofacies distribution, thickness quantification, and paleo-topographic reconstruction of the Late Ordovician graptolite zones. Specifically, we focused on the Late Katian Dicellograptus complexus and the Early Hirnantian Metabolograptus extraordinarius within a chronostratigraphic framework, using the Geographic Information System (GIS) and 310 stratigraphic sections (incl. drilling) obtained from the Geobiodiversity Database (GBDB). Reconstruction of the geographic distribution indicates that the WFF and the synchronous sediments in the Upper Yangtze region contain 8 litho-stratigraphic units, which are geographically distributed across 7 provinces/municipalities and do not exhibit significant variations in lithofacies. The black graptolite shale extends in a broad swath from east to west within the basin, while the other lithofacies deposited during the same period are present on the periphery of the basin. All these strata were deposited in a normal neritic epicontinental sea environment, except for the flysch sediments in the southern Hunan area. The thickness reconstruction involves a comparison of three spatial interpolation methods, including Inverse Distance Weighting (IDW), Kriging, and the Radial Basis Function (RBF). Based on a general verification, IDW is considered to be the optimal method since it has the minimum standard deviation and variance. Based on the contours obtained from the IDW model, the WFF black shale is estimated to have an overall area of 0.67 × 106 km2, an average thickness of 6.2 m, and a total volume of 3902 km3. This shale was deposited over a 2.83 Ma period. Therefore, the volume of shale deposited per million years is estimated to be 1379 km3/my and the average thickness of shale deposited per million years is 2.37 m/my. The Hirnantian paleo-water-depth values obtained using 275 sections were used to infer the Late Katian paleo-topography. These results suggest that the Yangtze platform was surrounded by ancient highlands to the west, south, and north, exhibiting a paleo-geographic framework characterized by one uplift and four depressions. This setting blocked water circulation, causing the water to be contained and forming a closed and restricted marine environment, which was one of the major factors controlling the deposition of the organic-rich WFF shale. With the advent of the big data era of geology, the methodology of GIS-based technology is readily exportable to any resource play having spatial distribution pattern. Results can be provided rapidly and efficiently generated from geological data.

Shale gas extraction through hydraulic fracturing and horizontal drilling is increasing in China, particularly in Sichuan Basin. Production of unconventional shale gas with minimal environmental effects requires adequate management of wastewater from flowback and produced water (FP water) that is coextracted with natural gas. Here we present, for the first time, inorganic chemistry and multiple isotope (oxygen, hydrogen, boron, strontium, radium) data for FP water from 13 shale gas wells from the Lower Silurian Longmaxi Formation in the Weiyuan gas field, as well as produced waters from 35 conventional gas wells from underlying (Sinian, Cambrian) and overlying (Permian, Triassic) formations in Sichuan Basin. The chemical and isotope data indicate that the formation waters in Sichuan Basin originated from relics of different stages of evaporated seawater modified by water-rock interactions. The FP water from shale gas wells derives from blending of injected hydraulic fracturing water and entrapped saline (Cl ∼ 50,000 mg/L) formation water. Variations in the chemistry, δ

Rock brittleness is now one of the significant parameters for geological evaluation of shale gas reservoirs due to the wide application of hydraulic fracturing in shale gas development. Currently, the evaluation of shale brittleness is generally based on mineral composition or elastic parameters, which both derive from the exploration experiences in North America. Due to the easier access of mineral composition data through x-ray diffraction method than rock mechanical data through array sonic logging method, the mineral-based brittleness indices have stronger practicability. However, there is not a unified definition of brittle minerals yet, which now usually refers to quartz or quartz and carbonate minerals. Considering the specific conditions of Wufeng–Longmaxi Formation in Southern Sichuan Basin; depositional environment of calcareous shelf; and higher content of calcite, dolomite, and pyrite, we redefined brittle minerals according to the elastic parameters of each mineral, taking quartz, dolomite, and pyrite which best satisfy the standard of “Young’s modulus>30 GPa and Poisson’s ratio

Research on major controlling factors of the high-quality shale in Upper Ordovician Wufeng–Lower Silurian Longmaxi Formation is a major subject. Based on the information of outcrops and drilling data acquired from Southern Sichuan combined with geochemical elemental analysis and biostratigraphy, this paper analyzed the sedimentary characteristics of Wufeng–Longmaxi organic-rich shale in Sichuan Basin, including plate movements, fluctuation of sea level, paleoproductivity, deposition rates, and paleogeographic environment, and revealed the distribution and sedimentary controlling factors of high-quality shale. Four preliminary conclusions were drawn: (1) The collision and joint of Yangtze Plate and its periphery plates, as well as the intraplate deformation, were gentle in early stage, strong in late stage, gentle in northwest, while strong in southeast, thus forming the northwestward migration of sedimentation center in Sichuan Basin, and the evolution of southern Sichuan sea closeness from weak to strong. (2) Sea level changed following the cycle of deep→shallow→deep→shallow at the turn of Ordovician–Silurian. High sea level and stable ocean basin in early stage created extensive anoxic tectonic sedimentary space which was in favor of organic matter preservation. (3) Influenced by tectonic movements and sea closeness, paleoproductivity of Southern Sichuan marine presented the trend of high in early stage and low in late stage, while the deposition rate also changed from slow in the early stage to fast in the late stage. (4) Extensive deposition and distribution of organic-rich silicic shale was mainly controlled by stable sea basin of low deposition rate, relatively high sea level, semienclosed water, and low deposition rate. The research showed that the high-quality shale in Sichuan Basin is characterized by multiphase superimposition, horizontal extension, and northwest renewal of sedimentary age; Wufeng–Rhuddanian is the main depositional period of high-quality shale, Aeronian comes next; the main exploration layers are Wufeng–Rhuddanian in southern–eastern Sichuan depression, Rhuddanian–Aeronian in Weiyuan, and Wufeng–Telychian in central-northern Sichuan.