A Bayesian Network Model for Yellow Rust Forecasting in Winter Wheat

International audienceYellow rust (YR) is one of the most destructive diseases of wheat. We introduced the Bayesian network analysis as a core method and develop a large-scale YR forecasting model based on several important meteorological variables that associate with disease occurrence. To guarantee an effective model calibration and validation, we used multiple years (2010–2012) of meteorological data and the ground survey data in Gansu Province where the YR intimidated most severely in China. The validation results showed that the disease forecasting model is able to produce a reasonable risk map to indicate the disease pressure across the region. In addition, the temporal dispersal of YR can also be delineated by the model. Through a comparison with some classic methods, the Bayesian network outperformed BP neutral network and FLDA in accuracy, which thereby suggested a great potential of Bayesian network in disease forecasting at a regional scale

Characterization of shale gas enrichment in the Wufeng Formation–Longmaxi Formation in the Sichuan Basin of China and evaluation of its geological construction–transformation evolution sequence

Shale gas in Upper Ordovician Wufeng Formation–Lower Silurian Longmaxi Formation in the Sichuan Basin is one of the key strata being explored and developed in China, where shale gas reservoirs have been found in Fuling, Weiyuan, Changning and Zhaotong. Characteristics of shale gas enrichment in the formation shown by detailed profiling and analysis are summarized as “high, handsome and rich”. “High” mainly refers to the high quality of original materials for the formation of shale with excellent key parameters, including the good type and high abundance of organic matters, high content of brittle minerals and moderate thermal evolution. “Handsome” means late and weak deformation, favorable deformation mode and structure, and appropriate uplift and current burial depth. “Rich” includes high gas content, high formation pressure coefficient, good reservoir property, favorable reservoir scale transformation and high initial and final output, with relative ease of development and obvious economic benefit. For shale gas enrichment and high yield, it is important that the combination of shale was deposited and formed in excellent conditions (geological construction), and then underwent appropriate tectonic deformation, uplift, and erosion (geological transformation). Evaluation based on geological construction (evolution sequence from formation to the reservoir) includes sequence stratigraphy and sediment, hydrocarbon generation and formation of reservoir pores. Based on geological transformation (evolution sequence from the reservoir to preservation), the strata should be evaluated for structural deformation, the formation of reservoir fracture and preservation of shale gas. The evaluation of the “construction - transformation” sequence is to cover the whole process of shale gas formation and preservation. This way, both positive and negative effects of the formation-transformation sequence on shale gas are assessed. The evaluation models based on this strategy would be more accurate, reliable and would avoid bias derived from indiscriminate and simplistic use of all parameters in the models

Genetic mechanism of high-quality shale gas reservoirs in the Wufeng–LongmaxiFms in the Sichuan Basin

The Upper Ordovician WufengFm and the Lower Silurian LongmaxiFm are important strata for shale gas exploration and development in the Sichuan Basin, but the genetic mechanism, evolutionary history and the controlling effect of mineral diagenetic evolution on the formation of shale gas reservoirs are not clear. In this paper, the evolution history of organic matter pores and the diagenetic evolution of minerals were analyzed based on the analysis of petrology, mineralogy and organic geochemistry, combined with basin simulation and practical shale gas exploration and development. Then, the types and genetic mechanisms of high-quality shale gas reservoirs were discussed, and the development intervals of high-quality shale gas reservoirs were determined. And the following research results are obtained. First, the shale gas development intervals of Wufeng–LongmaxiFms in the Sichuan Basin are mainly dominated by siliceous shale, limy siliceous shale and clayey shale. Rock type has an important controlling effect on the types and characteristics of shale reservoir space. Siliceous shale and limy siliceous shale have the highest reservoir capacity with the most developed organic pores. Second, the diagenetic evolution of minerals controls the formation of shale gas reservoirs. Biogenic silica, formed in the early diagenetic stage, together with terrestrial detrital silica and pyrite, constitutes particle support lattices in the form of microcrystalline aggregates, so as to resist the compaction effectively and preserve a great number of residual intergranular pores, which is beneficial to the formation of high-quality shale gas reservoirs. Third, siliceous shale in the WF2–LM4 graptolite zone (from WufengFm to the bottom of LongmaxiFm) presents a high-quality reservoir genetic mechanism of “multicellular algae controlling hydrocarbon source, biogenic silica controlling framework, and co-evolution controlling a high-quality reservoir”. In conclusion, the siliceous shale and limy siliceous shale in the WF2–LM4 graptolite zones are the main development intervals of high-quality shale gas reservoirs in the Sichuan Basin. It is also a valuable reference for the Upper Ordovician-Lower Silurianshale gas exploration and development in other countries and regions worldwide

Types and distribution of the shale sedimentary facies of the Lower Cambrian in Upper Yangtze area, South China

Based on comprehensive analysis of outcrops, cores, rock thin sections, mineral X-ray diffraction and Argon ion-milling – scanning electron microscopy, nine lithofacies types and five facies marks in the Lower Cambrian Meishucun Formation and Qiongzhusi Formation in the Upper Yangtze are identified, a classification scheme of the shale sedimentary facies is proposed, and the deposition and evolution model of the shale in the Lower Cambrian is figured out. This research shows there are difference in sedimentation, types and distribution of the sedimentary facies in the Meishucun Formation and Qiongzhusi Formation. The main sedimentation modes in the Meishucun Stage were mechanical-chemical and biological sedimentation, the sedimentary facies (from west to east) were carbonate ramp, shelf, and slope and bathyal basin. The main sedimentation of the Qiongzhusi Stage was clastic mechanical, argillaceous flocculation and biological deposition, and the sedimentary facies were shore, shelf, and ramp and bathyal basin. There are two depositional centers of organic-rich shale in the Upper Yangtze which are the prospective areas for shale gas exploration. The first one is in the Ziyang-Changning area in nearly north-south strike, which is characterized by multiple thin layers. The other one is in the Western Hubei-Eastern Chongqing-Middle Guizhou and Yichang-Jianshi-Fangxian, which is characterized by the thick-layer shale. The shale gas exploration in these areas should take pertinent strategies in line with their differences in the future. Key words: shale, lithofacies, sedimentary facies, Lower Cambrian Qiongzhusi Formation, Lower Cambrian Meishucun Formation, Upper Yangtze area, Sichuan Basi

An overview of the characteristic of typical Wufeng–Longmaxi shale gas fields in the Sichuan Basin, China

The Wufeng Formation–Longmaxi Formation in the Sichuan Basin in South China is the key stratum for shale gas exploration and production. To date, three national shale gas demonstration zones have been developed. Nevertheless, there are still some test wells that have not yet been commercialized. In this study, the geological characteristics of commercial and non-commercial zones are analyzed, as are the main controlling factors of high-producing wells (high estimated ultimate recovery; EUR), and the reasons for low-production wells (low EUR) by dissecting the three national shale gas demonstration zones and the main shale gas exploration wells. The results of this study indicate the following: (1) The black shale in the WF2–LM4 graptolite zone is deposited in the Craton depression on the Upper Yangtze plate, which provides a relatively stable tectonic environment for tectonic deformation and uplift destruction. The large shale thickness and weak tectonic activity jointly result in shale gas being enriched mainly in the deep-water shelf. (2)The regional fault has a destructive effect on shale gas preservation, and the shale gas reservoir is likely to be destroyed. In the areas close to the regional fault, multiple fracture-fluid migration activities caused by multistage tectonic movements are also detrimental to shale gas preservation. Conversely, shale gas is generally well preserved in areas far from regional faults. (3) The black shale thickness in the WF2–LM4 graptolite zone in the deep-water shelf area controls the shale gas field distribution. Furthermore, the horizontal well trajectory in the WF2–LM4 graptolite zone determines the shale gas well test production and EUR. The results of this work will provide a reference for shale gas exploration and development of the Wufeng Formation–Longmaxi Formation in the Sichuan Basin, as well as the Silurian strata in other parts of the world

Differences in Pore Type and Pore Structure between Silurian Longmaxi Marine Shale and Jurassic Dongyuemiao Lacustrine Shale and Their Influence on Shale-Gas Enrichment

The Silurian Longmaxi (S1l) marine shale and Jurassic Dongyuemiao (J1d) lacustrine shale in the Sichuan Basin, West China have attracted considerable attention from the oil-and-gas industry in China. Currently, the differences in pore types and pore structures between them are poorly understood, which has limited shale-resource exploration in the Sichuan Basin. This paper systemically compares the pore characteristics of Longmaxi shale and Dongyuemiao shale and investigates their impact on shale-gas enrichment by integrating field-emission scanning electron microscopy (FE–SEM), X-ray diffraction (XRD), low-pressure gas (CO2 and N2) adsorption and mercury-intrusion porosimetry, high-pressure sorption isotherms, gas-saturation measurement, molecular-dynamics simulation, etc. The results show that the S1l organic-rich marine shale and the J1d lacustrine shale have different pore types and pore structures. The S1l shale is dominated by organic pores, mainly micropores and mesopores with ink-bottle-like pore shapes, while the J1d shale is primarily composed of clay-mineral pores, mainly mesopores and macropores with slit- or plate-like pore shapes. Organic pores can provide considerable storage space for shale-gas enrichment in S1l marine shale, which also determines the adsorption capacity of shale reservoirs. Although organic pores are not the most prevalent in the Dongyuemiao lacustrine shale, they also play an important role in enhancing reservoir quality and absorbed-gas enrichment. Clay-mineral pores contribute weakly to the storage space of J1d-lacustrine-shale reservoirs. Mesopores are the most important form of storage space in both S1l shale and J1d shale, contributing significantly to shale-gas enrichment. Micropores are secondary in importance in S1l marine shale, while macropores are secondary contributors to pore volume in J1d lacustrine shale

Coevolutionary Dynamics of Organic-Inorganic Interactions, Hydrocarbon Generation, and Shale Gas Reservoir Preservation: A Case Study from the Upper Ordovician Wufeng and Lower Silurian Longmaxi Formations, Fuling Shale Gas Field, Eastern Sichuan Basin

Shale gas deposits are self-sourced, self-accumulating, and self-preserving in the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation of the Fuling Shale Gas Field in the eastern Sichuan Basin. They were both seemingly mixed by secondary oil cracking and kerogen cracking gases during the high maturation window. The reservoir space primarily consists of mineral pores and organic matter (OM) pores, and the shale gas was mainly trapped by a high-pressure system. In this study, the Fuling O3w-S1l Shale Gas Field in the eastern Sichuan Basin was used as a case study to discuss the coevolutionary process and organic-inorganic interactions of hydrocarbon generation, accumulation, and preservation. The results indicate that the processes and mechanisms of organic-inorganic interactions and coevolution of hydrocarbon generation and reservoir preservation are quite different among the shale graptolite zones (GZ) with respect to hydrocarbon generation, types and characteristics of shale gas reservoirs, seal characteristics, and their spatiotemporal relations. In the WF2-LM4 GZ, the favorable OM, biogenic authigenic quartz and organic-inorganic interactions are highly coupled, leading to the high level of coevolution demonstrated within the field, as well as to the favorable conditions for shale gas accumulation. Conversely, the overlying LM5-LM8 GZ seemingly exhibits early densification and late charge and has a reverse mode of reservoir development (i.e., low degree of coevolution). These two coevolutionary processes were conducive to the development of a high degree of spatiotemporal matching between the reservoir (i.e., WF2-LM4 GZ) and the seal (i.e., LM5-LM8 GZ). This is due to underlying differences in their coevolutionary histories. The synthetic work presented here on the coevolutionary processes and mechanisms of formation for organic-inorganic interactions and hydrocarbon generation and reservoir preservation reveals insights into the driving mechanisms of shale gas enrichment, providing a basis for effectively predicting favorable enrichment intervals for shale gas worldwide

Genesis of Bedding Fractures in Ordovician to Silurian Marine Shale in Sichuan Basin

The effective utilization of shale bedding fractures is of great significance to improve shale gas recovery efficiency. Taking the Wufeng–Longmaxi Formation shale in Sichuan Basin as the research object, the formation process and mechanism of bedding fractures in marine shale are discussed, based on field observation and description, high-resolution electron microscope scanning, fluid inclusion detection, and structural subsidence history analysis. The results show that the formation of bedding fractures is jointly controlled by sedimentary characteristics, hydrocarbon generation, and tectonic movement: the development degree of bedding (fractures) is controlled by the content of shale organic matter and brittle minerals, and bedding fractures formed in the layers with high organic matter; tectonic movement created stress environment and space for bedding fractures and promoted the opening of bedding fractures; the time for calcite vein to capture fluid is consistent with the time of oil-gas secondary pyrolysis stage. The formation of the calcite vein is accompanied by the opening of fractures. The acid and oil-gas generated in the hydrocarbon generation process occupied the opening space and maintained the bedding fractures open. The study of the formation process of bedding fractures is helpful to select a suitable method to identify bedding fractures, and then effectively use it to form complex fracture networks in the fracturing process to improve shale oil and gas recovery

A potential linkage between excess silicate-bound nitrogen and N₂-rich natural gas in sedimentary reservoirs

Funding: This work was supported by the National Natural Science Foundation of China (42102171, 41927801, 41972132), the Fundamental Research Funds for the Central Universities (2652019098), and the China Scholarship Council (202006405019). EES acknowledges funding from a NERC grant (NE/V010824/1).Molecular nitrogen (N2) released from sedimentary rocks during metamorphism is an important component of the biogeochemical nitrogen cycles. However, the importance and variability of this metamorphic N2 flux from rock nitrogen to Earth's surface environment remains largely unexplored. Here we present a comprehensive bulk rock C-N and N2 concentration dataset from the lower Cambrian shale across the Yangtze Block. The results reveal a spatial trend of excess silicate-bound nitrogen in the lower Cambrian shale throughout the Yangtze Block, which is interpreted as partial assimilation of ammonium (NH4+) with high concentrations of NH4+ accumulating in the euxinic water column and in sediment pore waters at shelf and slope environments during sedimentation. The remarkable spatial coupling between silicate-bound nitrogen in bulk rock shale and N2 concentration in modern shale reservoirs indicates that the high proportion of silicate-bound nitrogen may act as an important control on the formation of N2-rich gas in shale reservoirs during metamorphism. These N2-rich reservoir rocks may have affected the surface environment through tectonic movement over Earth's history. Our results therefore identify a novel linkage in the nitrogen cycle and provide evidence for the importance of metamorphism on the return of rock nitrogen back to the surface environment. We further reveal that the metamorphic N2 gas flux from the geosphere to the atmosphere is dependent on environmental conditions during sediment deposition.PostprintPeer reviewe

Shale gas content evaluation for deep strata and its variation: a case study of Weirong, Yongchuan gas fields in Sichuan Basin

The exploration and development of shale gas in China are recently focusing on the deep layers. However, the occurring characteristics of shale gas under high temperature and high pressure are not clear, which imposes great restrictions to deep shale gas development on a massive scale. Taking the deep shale in Weirong and Yongchuan regions of Sichuan Basin as research objects, for core samples with different organic carbon content and porosity, the isothermal adsorption experiments and porosity testing at high temperature and high pressure (135 ℃, 80 MPa) were carried out, and the theoretical values of shale adsorption gas, free gas and total gas content and compared with actual values. Results show that: (1) The content of adsorbed gas in deep shale increases gradually with pressure growing up. When the pressure surpasses 40 MPa, the increment flattens out, making 4.46 cm3/g the maximum of adsorbed gas content. (2) The theoretical gas content of shale increases with the growing formation pressure. The total gas content reaches its maximum 11.3 cm3/g under 80 MPa, which is made up with calculated free gas with an average of 6.8 cm3/g and adsorbed gas with an average of 4.5 cm3/g, accounting for about 60% and 40% of the total gas content, [JP]respectively. The ratio of free gas versus adsorbed gas gets larger with a greater buried depth. (3) Based on field desorption experiment, the actual measurement maximum gas content of well Weiye 11-1 is 5.95cm3/g, the minimum value is 3.29cm3/g, and the average content is 4.52 cm3/g. Compared with the theoretical value of 10.3 cm3/g, the result indicates that nearly 50% of gas leaked in the process of formation uplift, displaying the complexity of the deep shale gas preservation conditions. It is recommended to strengthen research on preservation conditions