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... This indicates the formation of significant overpressure during the shale gas accumulation period and that the conditions for shale gas accumulation were beneficial. However, shale gas was subjected to destruction and leakage in the later stage [17,19,20]. The current shale gas reservoirs are, basically, at normal pressure. ...
... In addition, the Late Cretaceous-Cenozoic was the main period of hydrocarbon leakage and adjustment. In northern Guizhou, the Yanshan orogeny constitutes the fundamental controlling factor for the widespread normal pressure of regional gas reservoirs, and the heterogeneous modification of the Himalayan orogeny determines whether the current gas reservoirs could be effectively preserved [13,18,20]. Well YH1 is located in the eastern part of northern Guizhou, closer to the Jiangnan-Xuefeng orogenic belt. ...
... Well YH1 is located in the eastern part of northern Guizhou, closer to the Jiangnan-Xuefeng orogenic belt. Compared with the central region of northern Guizhou [17,18,20], the Yanhe region was impacted by the long-range effect of Pacific Plate subduction, which implies that the damage caused by the Yanshan orogeny was earlier and stronger. This caused severe leakage in the shale gas reservoir already formed, and the current gas content is poor in Well YH1. ...
Shale gas accumulation and leakage histories are significant for evaluating the prospects of hydrocarbon exploration. The Wufeng Formation in northern Guizhou is a potential target for shale gas exploration and development. This work selected a typical Well YH1 to conduct detailed fluid inclusion analyses in the Wufeng Formation. We integrated the inclusion results with systematic core description, thin-section microscopic observation, laser Raman spectroscopy, microscopic temperature measurement, and burial–thermal history to reveal shale gas accumulation and leakage histories in northern Guizhou. The results show that the source rocks of the Wufeng Formation occurred in oil generation since the Permian and entered the stage of natural gas generation at the end of the Late Triassic. During the Early Jurassic and Early Cretaceous, shale gas accumulated in the Wufeng Formation reservoirs, and the stratigraphical pressure coefficient ranged from 1.47 to 1.97. Under the influence of the Yanshan orogeny, northern Guizhou experienced a rapid uplift and exhumation since the end of the Early Cretaceous. Under the influence of this tectonic, the stratigraphical pressure plummeted to the normal level, adversely impacting the accumulated shale gas in reservoirs and resulting in leakage events since the end of the Early Cretaceous. The reservoirs were further destroyed and adjusted, and the gas continued to leak because of a long-term and large-scale uplift and exhumation during the Himalayan orogeny. This work clarified that shale gas accumulation and leakage histories are significant for evaluating petroleum exploration.
Incised marine valleys (IV) are hot topics in unconventional shale gas exploration. These depositional systems are developed during the rapid fall and subsequent slow rise in the sea level. The reservoir shale lenses are trapped in the naturally fractured shale facies of IV. Therefore, the natural fractures and fault systems inside this IV have an enormous impact on the perpendicular and horizontal movement of hydrocarbon-posture fluids. Every combination trap of a petroleum system has an exact fine-tuning frequency, which can remain usable for resolving the reservoir and seal. This study develops a cost-economic petroleum system using seismic attributes, broadband spectral decomposition-based inverted density structural simulation and inverted velocity structural simulation for imaging natural fractures and fault systems in the SW-Asian onshore Indus rift basin. A band-pass frequency filter of 6-22-48-62 Hz was applied on the IV with two lower-peak and higher-peak tuning frequencies of 22-Hz and 48-Hz. The 22-Hz tuning frequency resolves this IV architecture at its greatest plausible location of gas-posture reserves. The traditional mapping shows poor competence to image a fractured system. The 22-Hz frequency has resolved the dense fracture system in the westernmost region. 48-Hz frequency has resolved the top seal of fracture-less zones to provide high-density zones to stop vertical migrations of gas-bearing fluids inside the reservoir shale lenses of IV. The inverted velocity structural simulations have poorly imaged the E-W transforms fault. The 22-Hz frequency-based inverted density structural simulations have resolved the en-echelon geometry along a high-amplitude anomalous and E-W oriented transform fault from a point of inflexion of fractured reservoir shale lenses from source to reservoir beds of IV. The lateral extent of this en-echelon structure is ~1.35 km with a vertical depth of ~3281 to 3299 m. The inverted density structural simulation has resolved two transform faults, which were imaged at depths of 3282 and 3298 m. The zone between pseudo-densities of 2.1 to 2.4 g/c.c shows uniform sedimentations inside the IV. This implicates the deposition of laterally continuous and fractured fluvial-dominated reservoir shale reservoir lenses. The inverted density structural simulations have also resolved the least angle (<~2o) of the combination trap at pseudo-densities of 2.1 to 2.4 gm. /c.c. No vertical amplitude anomaly could be imaged for predicting the intensity of fractures and upward migration from source to reservoir beds, which implicates the regionally established traps inside the IV. The fractured reservoir has accumulated the shale gas-bearing fluids inside the IV, which implicates the tectonic subsidence. The combination trap experiences inclinations increase to 35o at a 2.6 gm. /c.c pseudo-density. Hence, inverted density structural simulation shows strong implications for the horizontal migrations of shale gas resources inside the IV. Consequently, conducted exploration strategies may serve as an analogue for the SW-Asian Indus Onshore rift basin and similar structural basins.
Great progress has been made in marine shale gas of Wufeng–Longmaxi formations in the Sichuan Basin. However, shale gas exploration in the complex structural belt around the Sichuan Basin still faces great challenges. In this study, shales of Wufeng–Longmaxi formations collected from the northern Guizhou were taken as the studied target, organic matter (OM) characteristics, mineral composition, pore structure, methane adsorption capacity and in situ desorption gas content were measured, and the controlling factors of shale gas content were further discussed. The results indicated that the sedimentary facies of Wufeng–Longmaxi formations in north Guizhou varies from shallow‐water shelf facies to deep‐water shelf facies from south to north, and organic‐rich shales are primarily distributed in Daozhen‐Xishui areas, with a maximum thickness of about 80–100 m. Organic‐rich shales are characterized by high total organic carbon (TOC) content, high thermal maturation and type I–II 1 kerogens, which can be comparable with those in commercially produced shale gas field in Sichuan Basin. High‐quality shale gas reservoirs generally have a high content of brittle minerals, making them easier to be fractured. OM pores are the dominanted pore type in the studied shales, followed by intergranular pores associated with brittle minerals, dissolution pores within carbonate grains and microcracks, while clay mineral‐related pores are poorly developed. The Wufeng–Longmaxi Formation shales generally have strong methane adsorption capacities, but these vary greatly across different areas. Shale gas adsorption capacity is primarily controlled by TOC content and thermal maturation level. Similarly, total gas content, including desorption gas and lost gas, varies greatly in different areas, and it is obviously lower than that in Fuling and Luzhou shale gas field, due to the loss of shale gas and low‐pressure coefficient in the complex structural zone. It is worth explaining that shale gas is not always low in northern Guizhou, which is determined by burial depth and the distance of great fractures. Shale gas content is relatively high in LY1 well and DY1 well in Xishui‐Daozhen area, and it is extremely low in TY1 well and AY1 well in Tongzi‐Zheng'an area. Shale gas content in the same structural unit is primarily influenced by TOC content, OM pore development degree and water saturation. However, different structural units have different shale gas contents, due to the differences in preservation conditions. Shale reservoirs with good preservation conditions, that is, wide and gentle structure, far from a large fault and great burial depth, generally have high shale gas contents.
Poplar anthracnose caused by Colletotrichum gloeosporioides is a common disease affecting poplars globally that causes the destruction and alteration of poplar phyllosphere microbial communities; however, few studies have investigated these communities. Therefore, in this study, three species of poplar with different resistances were investigated to explore the effects of Colletotrichum gloeosporioides and poplar secondary metabolites on the composition of poplar phyllosphere microbial communities. Evaluation of the phyllosphere microbial communities before and after inoculation of the poplars with C. gloeosporioides revealed that both bacterial and fungal OTUs decreased after inoculation. Among bacteria, the most abundant genera were Bacillus, Plesiomonas, Pseudomonas, Rhizobium, Cetobacterium, Streptococcus, Massilia, and Shigella for all poplar species. Among fungi, the most abundant genera before inoculation were Cladosporium, Aspergillus, Fusarium, Mortierella, and Colletotrichum, while Colletotrichum was the main genus after inoculation. The inoculation of pathogens may regulate the phyllosphere microorganisms by affecting the secondary metabolites of plants. We investigated metabolite contents in the phyllosphere before and after the inoculation of the three poplar species, as well as the effects of flavonoids, organic acids, coumarins, and indoles on poplar phyllosphere microbial communities. We speculated that coumarin had the greatest recruitment effect on phyllosphere microorganisms, followed by organic acids through regression analysis. Overall, our results provide a foundation for subsequent screening of antagonistic bacteria and fungi against poplar anthracnose and investigations of the mechanism by which poplar phyllosphere microorganisms are recruited. IMPORTANCE Our findings revealed that the inoculation of Colletotrichum gloeosporioides has a greater effect on the fungal community than the bacterial community. In addition, coumarins, organic acids, and flavonoids may have recruitment effects on phyllosphere microorganisms, while indoles may have inhibitory effects on these organisms. These findings may provide the theoretical basis for the prevention and control of poplar anthracnose.
Riedel shear structures (RSS) are often observed in the embryonic stage of strike‐slip fault development, which can be depicted in the field through outcrops and coseismic surface ruptures. It is a critical concept linking the geomechanical behavior of individual earthquakes to structural geology at both local and regional scales. However, the influence of long‐term fluid injections on the developing process of RSS, as manifested by the common occurrences of injection‐induced earthquakes, has been rarely addressed. Here we document for the first‐time subsurface RSS expedited by long‐term wastewater disposal injections in western Canada. We study an earthquake sequence consisting of 187 events (ML ranging 1.3–3.9) between 1 January 2018 and 15 July 2021 in an area without any previous seismic history. According to 31 well‐constrained focal mechanism solutions, the injection‐related earthquake sequence exhibits various faulting types with the vast majority (87%) being compatible with the background stress regime (SHmax azimuth = N38°E). The orientation of derived nodal planes collectively indicates a model of RSS that consists of four primary strike‐slip structures striking 19° (R′), 79° (R), 94° (PDZ), and 109° (P), respectively. Moreover, six fault segments delineated from the relocated local seismicity are parallel to the substructures of RSS. Mohr‐Coulomb failure analysis further suggests a cumulative stress perturbation of up to 10.0 MPa. Our observations suggest that long‐term fluid injection can expedite the development of local fault systems. Therefore, it is probably important to consider the dimension of local/regional RSS in the assessment of the overall seismic hazard due to fluid injections.
Poplar anthracnose caused by Colletotrichum gloeosporioides is one of the most important diseases widely distributed in poplar growing areas in China, causing serious economic and ecological losses. In this study, three poplar species showed different resistance to poplar anthracnose: Populus × canadensis was resistant, P. tomentosa was susceptible, and P. × beijingensis showed intermediate resistance. However, it remains uncertain whether phenolic compounds in poplar are involved in this resistance. Therefore, we determined the concentrations of phenolic compounds and their antifungal activity. Before and after the C. gloeosporioides inoculation, 20 phenolic compounds were detected in P. × canadensis and the number increased from 12 to 14 in P. × beijingensis but decreased from seven to four in P. tomentosa. Thus, phenolic compounds may be positively correlated with the degree of disease resistance. We selected seven phenolic compounds for further analysis, which varied considerably in content after inoculation with C. gloeosporioides. These seven compounds were salicin, arbutin, benzoic acid, salicylic acid, chlorogenic acid, ferulic acid and naringenin, which helped poplars to limit the growth of C. gloeosporioides growth, differed in their antifungal effects, with phenolic acids having the strongest inhibitory effect. In addition, the optimal concentrations of different substances varied. We demonstrate that these phenolic compounds produced by poplar do play a certain role in the process of poplar resistance to anthracnose. These findings lay a foundation for future research into the antifungal mechanism of poplar trees and may be useful for enhancing the prevention and control of poplar anthracnose.
In southern Sichuan Basin, the main production layers are characterized by deeply buried, high stress difference, and complex structural conditions. The Luzhou area is far from large faults, and natural fractures are greatly important for shale gas storage and production. Multi-scale natural fractures control the migration, enrichment, and preservation conditions of shale gas, and facilitate the formation of complex fracture network under the action of hydraulic fracturing. In this study, based on the outcrops, drilling cores, geochemical tests, thin section, and other experiments, the development characteristics of the Wufeng-Longmaxi shale in the Luzhou block, Southern China, are statistically analyzed, and the controlling factors (e.g., tectonic factors, organic matter, mineral components, mechanical properties) are discussed in details. Fractures observed in the outcrops are mainly regional fractures with two groups of orthogonal joints. Similarly, fractures observed in the core are also mainly joints fractures perpendicular to the laminae with three typical features (high-density, high-angle, and unfilled). In detail, steeply dipping fractures (75-90°) account for 78.1% of all fractures, with 85.1% being unfilled, 78.1% having a longitudinal extension less than 4 cm, and 65.1% having a spacing less than 2 cm. In brief, there exist cross-scale similarity among outcrops, cores, and microscopic thin sections, which is critical to the shale gas preservation conditions. Based on this understanding, further research is conducted on the relationship between the fracture density and gas content, which shows that (i) when the fracture density is less than 122 number/m, TOC content and fracture density together positively dominate gas content; (ii) when fracture density exceeds 122 number/m, gas content appears negative with fracture density, and TOC content is not the critical factor anymore. The above study establishes quantitative limits for shale gas preservation in the study block. It may assist in providing references for determining the sweet spot area and further deep shale gas exploration and development in the southern Sichuan Basin.
The brittleness of shale reservoirs is of great significance for shale gas development. The brittleness index is the most intuitive parameter reflecting reservoir brittleness. The key exploration and development strata of shale gas in Pengshui area of southeastern Chongqing are Wufeng Formation of Upper Ordovician and Longmaxi Formation of Lower Silurian, and the high-quality shale reservoir is Wufeng–Long 1 section. The results show that the high brittleness grade of shale corresponds to low Poisson's ratio and high Young's modulus, but the relative distribution index of brittle minerals also affects brittleness. Through the analysis and calculation of brittle minerals and clay minerals in reservoir, the brittle index is calculated using rock physical parameters and mineral composition, and the advantages and disadvantages of the two are compared. Then a new formula for calculating brittleness index is obtained by combining rock physical parameters and mineral composition. Finally, the brittleness of high-quality shale reservoir in southeast of Chongqing area is evaluated comprehensively. Compared with the measured brittleness, the new formula has good effect. The average brittleness index of the Wufeng–Long 1 is more than 70% in Nanchuan area, China. The lateral distribution is stable and the fracturing conditions are good.
Marine shale gas in the southern Sichuan Basin is the most successful area of shale gas exploration and development in China. In order to open up new shale gas fields and search for new shale gas reserves and production replacement blocks, it is necessary to continuously establish and complete the standards on shale gas reservoir evaluation and area selection under different structural settings. The early exploration practice shows that shale in the mountainous complex structural area along the southwestern margin of the Sichuan Basin varies greatly in the gas bearing property, so systematical analysis and study on the shale gas enrichment mechanisms in this area is of great significance to searching for new shale gas exploration areas. Based on drilling data of 5 wells in the mountainous complex structural area along the southwestern margin of the Sichuan Basin, the main factors controlling the shale gas bearing property and the shale gas enrichment model were discussed based on the systematical experiments and analysis of the outcrops, cores and cuttings of Longmaxi Formation shale from the aspects of organic geochemistry, physical property, gas content, hydrocarbon generation history and pore evolution history, so as to lay a solid geological basis for the birth of a new shale gas area. And the following research results were obtained. First, the shale of the first submember of first Member of Longmaxi Formation (S1l1¹) in the mountainous complex structural area along the southwestern margin of the Sichuan Basin has an average TOC of 3.02%–4.97% and Ro of 2.38%–3.37%, and the average total gas content in local enrichment zones is up to 4.62 m³/t, so it is classified as quality shale. Second, the detailed studies on hydrocarbon generation history indicate that the shale has the characteristics of “low thermal evolution rate and low maturity”. It is characterized by late hydrocarbon generation, low thermal evolution rate, low current maturity and short late diffusion time, which are favorable for shale gas enrichment. Third, the pore evolution history reveals that shale pore evolution can be divided into six stages, i.e., sharp pore reduction, pore reduction, organic pore formation, pore preservation, organic pore dissipation and karstification, and organic pore and TOC are the most direct control factors of shale gas content. Fourth, favorable shelf facies belt is conducive to the formation of large-scale reservoir space and effective pores, and diversities of preservation conditions under different structural styles and at different structural positions control different pore evolution stages. The shale gas in the mountainous complex structural area is characterized by horizontal zoning and differential enrichment. “Low thermal evolution rate and low maturity” and “slow structural uplifting” are beneficial to the long-term enrichment and preservation of shale gas. In conclusion, the mountainous complex structural area along the southwestern margin of the Sichuan Basin has a shale gas enrichment model of “sedimentation controlling source rocks, diagenesis controlling reservoirs and structure controlling preservation”. This research result provides idea and reference for searching for new shale gas areas and fields.
Lower Paleozoic dark shale is developed in the western Middle Yangtze Block, which lays a material foundation for the enrichment and accumulation of marine shale gas. In order to ascertain the control action of geological structures on the differential preservation of shale gas and reveal the key factors in shale gas preservation, this paper firstly analyzed the structure characteristics of this area, carried out structure pattern recognition and structural belt division, and studied structural deformation mode and intensity. Based on this, the relationships between different structure styles and shale gas preservation conditions were analyzed. Finally, combined with the structural deformation and the lithofacies paleogeographic characteristics of marine shale, the favorable exploration zones of shale gas were proposed. And the following research results were obtained. First, the western Middle Yangtze Block can be divided into four structural deformation belts, and three types of piggyback structural patterns have been identified, including restricted type, weakly reformed type and strongly reformed type. Second, the restricted type is located in the northwestern part of Hunan and Hubei Provinces. In this pattern, piggyback structure is incomplete and thrust belt and Compressive fold belt are developed. Third, the weakly and strongly reformed types are located in the western parts of Hunan and Hubei, and Wulingshan area, respectively. They both have complete piggyback structures, but the former has lower deformation intensity and has never undergone the late superimposed reformation. Fourth, there are three structural transfer belts in the western Middle Yangtze Block, i.e. the structural transfer belt between the East Sichuan fault–fold belt and West Hunan–Hubei fault–fold belt, the structural transfer belt between West Hunan–Hubei fault–fold belt and Wulingshan fault–fold belt, and the structural transfer belt between the outcrop and the hinterland of Middle Yangtze Block. The first one is structurally transformed at the Qiyueshan fault. The East Sichuan fault–fold belt on the west is an ejective fold with low fault density and formation denudation intensity, where shale gas is enriched in anticlines and slopes; while the West Hunan–Hubei fault–fold belt on the east is a trough-like fold with strong faulting and high formation denudation intensity, where shale gas is enriched in residual synclines. In conclusion, shale gas preservation conditions of Upper Ordovician Wufeng Formation–Lower Silurian Longmaxi Formation in this area are the best in Zigui syncline, thrust–detachment zone and western margin of Qiyueshan fault. The favorable exploration areas of shale gas of Lower Cambrian Niutitang Formation are distributed in the western flank of Yichang Slope, Kaixian thrust zone, compressive fold zone and thrust–detachment zone.
Correlation coefficients are abundantly used in the life sciences. Their use can be limited to simple exploratory analysis or to construct association networks for visualization but they are also basic ingredients for sophisticated multivariate data analysis methods. It is therefore important to have reliable estimates for correlation coefficients. In modern life sciences, comprehensive measurement techniques are used to measure metabolites, proteins, gene-expressions and other types of data. All these measurement techniques have errors. Whereas in the old days, with simple measurements, the errors were also simple, that is not the case anymore. Errors are heterogeneous, non-constant and not independent. This hampers the quality of the estimated correlation coefficients seriously. We will discuss the different types of errors as present in modern comprehensive life science data and show with theory, simulations and real-life data how these affect the correlation coefficients. We will briefly discuss ways to improve the estimation of such coefficients.
The Nojima Fault Zone, on which the 1995 Mw 6.9 Kobe (Japan) earthquake occurred, is characterized by pulverized cataclastic rocks with numerous fractures and subsidiary faults. The fractures and subsidiary faults were observed in trench walls and using borehole televiewer (BHTV) images from boreholes NFD‐1 (~1,000‐m depth) and AFD‐1 (~700‐m depth), which were drilled through the Nojima Fault and its branch, the Asano Fault, respectively. Measurements show that the orientations of fractures and subsidiary faults in the two boreholes are concentrated at N10–70°E, averaging N40°E. In contrast, the orientations of fractures and subsidiary faults in the walls of a trench across the main surface trace of the Asano Fault are constrained to N0–40°E, averaging N20°E, parallel to the general trend of the Asano Fault but different with that observed in the deep holes at the Ogura site. Structural analysis shows that the fractures and subsidiary faults are mainly concentrated in Riedel Y and R shears of the right‐lateral strike‐slip Nojima Fault and its branch Asano Fault. Our findings demonstrate that the fractures and subsidiary faults within the Nojima Fault Zone were formed by dextral strike‐slip shearing under the current ENE‐WSW to E‐W compressive regional tectonic stress field, related to ongoing subduction of the Pacific and Philippine Sea plates beneath the Japanese Islands.
Cytospora is a genus including important phytopathogens causing severe dieback and canker diseases distributed worldwide with a wide host range. However, identification of Cytospora species is difficult since the currently available DNA sequence data are insufficient. Aside the limited availability of ex-type sequence data, most of the genetic work is only based on the ITS region DNA marker which lacks the resolution to delineate to the species level in Cytospora. In this study, three fresh strains were isolated from the symptomatic branches of Elaeagnus angustifolia in Xinjiang Uygur Autonomous Region, China. Morphological observation and multi-locus phylogenetic analyses (ITS, LSU, ACT and RPB2) support these specimens are best accommodated as a distinct novel species of Cytospora. Cytospora elaeagnicola sp. nov. is introduced, having discoid, nearly flat, pycnidial conidiomata with hyaline, allantoid conidia, and differs from its relatives genetically and by host association.
Quantitative analysis of free gas and adsorbed gas contents in shale reservoirs are great significance for efficient exploration and development of shale gas. Based on the isothermal adsorption experiment of shale samples from Wufeng-Longmaxi Formation of JYA well in Jiaoshiba area and Langmuir volume model, the relationship between shale adsorption capacity and temperature, pressure, organic carbon content, quartz and clay mineral content is analyzed. Besides, the key parameters such as Langmuir volume and Langmuir pressure are dynamically calibrated by combining grey correlation method. A new model for calculating adsorbed gas and free gas is established, which takes fully into account the formation temperature, pressure, total organic carbon and shale mineral components. The results showed that the gas content of shale calculated by the new dynamic modified model is in good agreement with the actual gas content characteristics of shale reservoirs. The new model fully takes into account the vertical and horizontal heterogeneity of mineral components and its influence on shale adsorption capacity. That is not only suitable for the tectonic stability area but also for the gas content analysis in the area with strong tectonic movement. It is concluded that the modified calculation model can effectively predict the adsorbed gas, free gas and total gas content of shale reservoirs under formation conditions, which can be used as an indicator for the analysis and prediction of the exploration and development potential of shale gas wells.
Among many clustering algorithms, the K-means clustering algorithm is widely used because of its simple algorithm and fast convergence. However, the K-value of clustering needs to be given in advance and the choice of K-value directly affect the convergence result. To solve this problem, we mainly analyze four K-value selection algorithms, namely Elbow Method, Gap Statistic, Silhouette Coefficient, and Canopy; give the pseudo code of the algorithm; and use the standard data set Iris for experimental verification. Finally, the verification results are evaluated, the advantages and disadvantages of the above four algorithms in a K-value selection are given, and the clustering range of the data set is pointed out.
The studies on the paleo-fluid in the Ordovician–Silurian detachment zone in the Middle–Upper Yangtze area focus on the origin of high-density methane inclusions and the evolution process of formation pressure, but rarely deal with the significance of paleo-fluid to shale gas preservation. In this paper, the relationship between fracture formation and detachment zone was analyzed by observing the Ordovician–Silurian outcrops in western Hunan–Hubei area and by investigating the geological characteristics of fracture veins in the drilling cores. Then, the significance of paleo-fluid forming environment and detachment zone to shale gas preservation was studied by using inclusion compositions of fracture veins and homogenization temperature test data. Finally, accumulation–dispersion modes of shale gas in the detachment zone were established. The following results were obtained. First, the detachment zone is lithologically composed of silicite with intercalated shale at the Ordovician–Silurian interface. In the detachment zone, rocks are broken and small crumples are developed. The conjugate-vertical joints are relatively developed in the silicite above and below the detachment zone. Second, multi-stage and multi-type inclusions, especially the aqueous inclusions, are developed in the veins of the detachment zone. Third, the infiltration depth of ancient meteoric water along the detachment zone in the study area is over 4000 m. High-density overpressure methane inclusions were captured in the detachment zone during early stage, while normal-pressure methane and nitrogen inclusions were captured during late stage. Fourth, the ionic constituents of inclusions recording the fluid activity during the late stage was characterized by high sodium-chloride coefficient, high desulfurization coefficient and low metamorphic coefficient, and it is indicated that the sealing capacity of the shale in the detachment zone gets worse. Fifth, the accumulation and dispersion of shale gas in the detachment zone within the study area is divided into three modes, i.e., syncline, broad anticline and closed anticline. In conclusion, the detachment zone in the study area is permeable. Synclines and closed anticlines developed in detachment structures are unfavorable for the accumulation of shale gas, while broad anticlines are favorable. Keywords: Western Hunan–Hubei area, Ordovician, Silurian, Detachment zone, Paleo-fluid, Inclusions, Homogenization temperature, Ionic constituent, Shale gas, Preservation condition
Through detailed analyses of the distribution characteristics of organic-rich shale, appearance features of high-quality shale, microscopic characteristics of shale reservoir rocks, fracability, and the relationship between preservation conditions and shale gas enrichment in Upper Ordovician Wufeng Formation−Lower Silurian Longmaxi Formation in Sichuan Basin, theoretical understandings and specific suggestions with respect to the exploration and development of shale gas in China are summarized and proposed respectively. Important geological understandings in the exploration and development of shale gas of the Wufeng Formation–Longmaxi Formation in the Sichuan Basin can be summarized into the following aspects: depositional environment and depositional process control the distribution of organic-rich shale; high quality shale in “sweet spot segments” are commonly characterized by high content of organic carbon, high brittleness, high porosity and gas content; organic pores are important storage space for the enrichment of shale gas; preservation conditions are the key factor for the geological evaluation of shale gas in structurally complex regions; shale gas can be considered as “artificial gas reservoirs” and the fracability assessment is essential for high-production; nanoscale storage space and the mode of occurrence control the special seepage characteristics of shale gas. The following suggestions are proposed for the development of China's shale gas industry: (1) focus more on fundamental research to achieve new breakthrough in the geological theory of shale gas; (2) emphasize exploration practices to have all-round discoveries in multiple strata; (3) study the regularities of development and production to establish new models of shale gas development; (4) think creatively to invent new technologies to tackle key problems; (5) explore the management innovation to create new mechanisms in shale gas development.
In order to investigate the potential of the Lower Cambrian shale gas in South China, a study on the accumulation condition of shale gas from the Niutitang Formation was conducted. In addition, the favorable accumulation area of shale gas in northern Guizhou was evaluated. The study and evaluation were both analyzed according to the geochemical analysis data of the core and outcrop samples in this paper. The result shows that the Niutitang Formation shale in northern Guizhou is formed in the shallow sea sedimentary environment, which features moderate buried depth (<3000 m), high organic-rich shale thickness (30–110 m), high organic abundance (average TOC>3.0%), type I kerogens, over-mature thermal evolution level (Ro>2.0%), low porosity, abundant micro- and nano-meter pores and fractures, rich brittle mineral (average content >40%), and relatively high gas content (average 1.5 m³/t). The aforementioned feature indicates that the Niutitang Formation in northern Guizhou has good accumulation condition for shale gas. Through the studies above, it is proposed that the southeastern Shiqian Yuqing Shibing area is the most favorable area for shale gas accumulation in northern Guizhou.
For the sake of figuring out the influential mechanisms of structural characteristics on the productivity of shale gas wells, the structural characteristics of the Jiaoshiba Block in the Fuling shale gasfield, Sichuan Basin, were analyzed. Then, based on well test data of more than 190 horizontal wells, the effects of structures on shale gas well productivity were discussed systematically, and the main structural factors of different structural units in the Jiaoshiba Block that influence the productivity of shale gas wells were clarified. The following results were obtained. First, the structural units in the Jiaoshiba Block were obviously different in structural characteristics and their deformation strength is different. Second, the influence of structural characteristics on shale gas well productivity is directly manifested in gas-bearing property and fracturing effect. The stronger the structural deformation and the more developed the large faults and natural fractures, the more easily shale gas escapes and the poorer the gas bearing property will be, and vice versa. Third, The stronger the structural deformation, the more developed the fractures, the greater the burial depth and the higher the compressive stress of negative structures, the worse the fracturing effect will be, and vice versa. And fourth, Tectonics is the key factor controlling the difference of shale gas productivity between different structural units in the Jiaoshiba Block, but the main structural factors influencing the productivity are different in different structural units.
Flower structures are typical features of wrench fault zones. In conventional studies, two distinct kinds of flower structures have been identified based on differences in their internal structural architecture: (1) negative flower structures characterized by synforms and normal separations and (2) positive flower structures characterized by antiforms and reverse separations. In addition to negative and positive flower structures, in this study, a third kind of flower structure was identified in a divergent-wrench fault zone, a hybrid characterized by both antiforms and normal separations. Negative flower structures widely occur in divergent-wrench fault zones, and their presence indicates the combined effects of extensional and strike-slip motion. In contrast, positive and hybrid flower structures occur only in fault restraining bends and step overs. A hybrid flower structure can be considered as product of a kind of structural deformation typical of divergent-wrench zones; it is the result of the combined effects of extensional, compressional, and strike-slip strains under a locally appropriate compressional environment. The strain situation in it represents the transition stage that in between positive and negative flower structures. Kinematic and dynamic characteristics of the hybrid flower structures indicate the salient features of structural deformation in restraining bends and step overs along divergent-wrench faults, including the coexistence of three kinds of strains (i.e., compression, extension, and strike-slip) and synchronous presence of compressional (i.e., typical fault-bend fold) and extensional (normal faults) deformation in the same place. Hybrid flower structures are also favorable for the accumulation of hydrocarbons because of their special structural configuration in divergent-wrench fault zones.
Three evidences show that the Riedel shear pattern is similar to the faulted basin pattern in a strike-slip fault zone. (a) The pattern of Riedel shears is not dependent on the scale observed. The extensional (transtensional) zones in the small strike-slip faults are similar to the basin distributions along large fault zone; (b) The depocenter along a fault is the same as the location of maximum fault displacement; The basins commonly show elongated form as the faults. (b) The basins related to Riedel shears can be reproduced by physical and numerical experiments. Based on above factors, we proposed six patterns of sedimentary basin related to Riedel shears: en echelon basin, horsetail basin, beaded basin, S-type (Z-type) basin, §-type basin, and complicated lattice basin. (1) The width of en echelon basins increases with the increase of shear displacement in the principal shear zone. (2) Geometry of horsetail basins and brush basins are similar to each other. The horsetail basins are generally found in the extensional tip zones of a strike-slip fault, whereas the brush basins develop in any place in a shear zone. (3) Beaded basins are formed due to extension of releasing zones in the braided strike-slip fault. Individual basin is in the form of lenses. (4) Z-type basins are formed due to sinistral shear and S-type basins are formed due to dextral shear. (5) §-type basins are typical pattern of pull-apart basins in the case of Ls/Ln <1, whereas for Ls/Ln >> 1, the beaded pattern is expected, where Ls is the transfer length of a pull-apart basin and Ln is the spacing between two en echelon fault segments. (6) Lattice basins are developed in the distributed shear zones with anastomosing faults.
The Qaidam Basin, especially its western part, is characterized by numerous northwest-trending folds and faults. Understanding the style and formation mechanism of these structures is crucial for unravelling the deformation of the Qaidam Basin and the broader tectonics of the northern part of the Tibetan Plateau. Based on seismic data, we reconstruct the structural framework of the central part of the western Qaidam Basin, and find that: (1) the structures in this area display positive flower geometries in two-dimensional (2D) seismic profiles and helicoidal shapes in 3D space; (2) these positive flower structures began to develop during the middle to late Miocene (15–8 Ma); and (3) these positive flower structures are controlled by left-lateral basement faults and formed in a random temporal sequence. The left-lateral features represent the strike-slip component of distributed deformation, consistent with published global positioning system and seismicity data for oblique convergence across the north of the Tibetan Plateau. Collectively, they perform the same role as the discrete Haiyuan, Kunlun, and Altyn Tagh faults in adjacent areas.
Dolomite genesis is a century-old mystery in sedimentology. To reveal the mechanism of dolomite genesis, two core problems need to be addressed. The first is the origin and migration mechanism of Mg2+-rich fluids during the dolomitization process. The second is the kinetic barrier caused by Mg2+ hydration during dolomite precipitation at low temperatures. To address these problems, our study, based on detailed petrological, sedimentological, geochemical (major and trace elements), and isotopic (C-O-Mg) analysis, clarified the source and migration of Mg2+-rich fluids and the kinetic barrier mechanism of low-temperature dolomite precipitation in the Upper Sinian Qigebulake Formation and the Lower Cambrian Xiaoerbulake Formation in the Tarim Basin. First, we found that the Mg2+-rich fluids required for the dolomitization of dolomite in the Xiaoerbulake Formation were primarily derived from the Early Cambrian marine fluid. At the interface of the sedimentary cycle, δ26Mg values fluctuated considerably, indicating that the sequence interface was the starting point and channel for the migration of dolomitized fluids. Sea level variation plays a major role in controlling the dolomitization process of the Xiaoerbulake Formation. Second, the Qigebulake Formation contains low-temperature dolomite with Mg2+-rich fluids supplied by seawater, microorganisms, and sedimentary organic matter. Comprehensive analysis shows that the dolomite of the Qigebulake Formation was formed by microbial induction by anaerobic methane bacteria. Finally, the properties and sources of dolomitization fluids and the formation process of dolomite were the reasons for the difference in the Mg isotope composition of dolomite during the Sinian-Cambrian transition. This study reveals the genetic mechanism of the Sinian-Cambrian dolomite in the Tarim Basin and establishes a new method to explain the genesis of microbial dolomite by C-O-Mg isotopes, providing a reference for the reconstruction of the formation and evolution of dolomites.
Based on the analysis of the deformation styles in different tectonic belts of the Middle-Upper Yangtze region, as well as the dissection of typical hydrocarbon reservoirs, this study determined the controlling effects of deformations on the hydrocarbon accumulations, obtaining the following results. The Middle-Upper Yangtze region experienced significant deformations during the Late Indosinian (T2–T3), the Middle Yanshanian (J3–K,), and the Himalayan, and five styles of tectonic deformations mainly occurred, namely superimposed deep burial, uplift, compressional thrusting, multi-layer decollement, and secondary deep burial. The distribution of hydrocarbon reservoirs in the piedmont thrust belts is controlled by the concealed structures on the footwall of the deep nappe. The gentle deformation area in central Sichuan experienced differential uplift, structural-lithologic hydrocarbon reservoirs were formed over a wide area. The eastern Sichuan-western Hunan and Hubei deformation area experienced Jura Mountains-type multi-layer detachment, compressional thrusting, and uplift. In relatively weakly folded and uplifted areas, conventional structural-lithologic hydrocarbon reservoirs have undergone adjustment and re-accumulation, and the shale gas resources are well preserved. In the strongly deformed areas, conventional hydrocarbon reservoirs were destroyed, while unconventional hydrocarbon reservoirs have been partially preserved. The marine strata in the Jianghan Basin experienced compression, thrusting, and denudation in the early stage and secondary deep burial in the late stage. Consequently, the unconventional gas resources have been partially preserved in these strata. Secondary hydrocarbon generation become favorable for conventional hydrocarbon accumulations in the marine strata.
The organic-rich shale of the Wufeng-Longmaxi Formation in the Northeast Yunnan area where faults develop has a considerable basis and potential for hydrocarbon generation. A total 16 core samples from well ZD2, which is affected by faults, and well ZY1, which is located in a residual syncline, were collected. The effect of faults on the shale pore fracture and shale gas preservation was investigated primarily through core fracture statistics, field emission scanning electron microscopy, nuclear magnetic resonance (NMR), and triaxial mechanical tests. The results show that high-angle shear and bedding fractures are more developed in well ZD2 than well ZY1, indicating that faulting promotes the development of both bedding and structural fractures. The pore types and morphology of samples affected by faults are significantly different in terms of the compaction of organic matter (OM) pores, reduction of interparticle (interP) pores, increase in dissolution pores and structural microfractures. NMR pore size distributions of the samples primarily show a bimodal distribution, and mesopores (2-50 nm) are the dominate pores. However, faulting can weaken the correlation of pore structure with total organic carbon and minerals content. Comparative studies reveal that the effects of faults on pore structure are manifested as an increase in porosity, a decrease in average pore size, and a leftward shift of the PSD curve. Strongly heterogeneous dissolution substantially increases shale porosity. Tectonic extrusion compacts the OM and interP pores, which reduces the average pore size. The triaxial compressive strength (σTCS) of the samples from well ZY1 is higher than the normal stress on the fault plane. However, considering the correlation between σTCSand mineral content, the Upper Longmaxi Formation has a better fault sealing ability compared to the Wufeng-Lower Longmaxi Formation. Together with the gas-bearing properties of the wells, the Himalayan faults have a good sealing ability in Mugan syncline. Hence, the Himalayan faults with satisfactory sealing can effectively block the escape channels of shale gas along the bedding in a syncline. The wings of gentle syncline covered by overthrusting Himalayan faults are a favorable structural style for shale gas enrichment.
Remarkable breakthroughs have been achieved in exploration of marine shale gas, acting as important cleaner energy resources in complex tectonic regions, encouraging us to accurately estimate different reservoir capacity of tectonically deformed gas shales. In particular, organic matter (OM) pore system, acting as the most important storage space for high-over-mature marine gas-shale reservoirs in South China has not yet been specifically targeted and comparatively studied in a regional and differential tectonic deformation regime. A set of shale drilling core samples from the Upper Silurian to Lower Ordovician Wufeng-Longmaxi Formations in the Sichuan Basin and its periphery were targeted by a multi-methodical approach utilizing organic geochemistry and mineralogical investigations, FE-SEM observation and digital image processing and extraction, and combined fluid intrusion (N2 and CO2 adsorption analysis). From undeformed shales (UDS) through slightly deformed shales (SDS) to intensely deformed shales (IDS), an obviously and progressively “Triple Jump” reduction in multi-scale pore volume and specific surface areas, and characteristic parameters for OM pores including the plane porosity (Phi) (the average dropped from 23.14% to 10.33%), the equivalent circle diameter (ECD) (the average dropped from 28.38 nm to 10.09 nm), the perimeter over area (PoA) (the average rose from 0.1126 to 0.2718), and the dominant pore diameter (DOM size) (the average dropped from 113.80 nm to 20.78 nm). The reactivation of seepage channels and intrusion of brittle minerals are proposed as two main microscopic forcing mechanisms, updated and innovative conceptual models are proposed to reconstruct an OM pore response processes under a differential tectonic deformation regime in panoramic view for Lower Paleozoic marine gas shale reservoirs of China. Thus, an evolution of spatial resolution spanning a total of three different scales respectively as structural styles, shale reservoir architectures, and microscopic petrological compositions are together portrayed.
Marine shale gas in the southern Sichuan Basin is the most successful area of shale gas exploration and development in China. In order to open up new shale gas fields and search for new shale gas reserves and production replacement blocks, it is necessary to continuously establish and complete the standards on shale gas reservoir evaluation and area selection under different structural settings. The early exploration practice shows that shale in the mountainous complex structural area along the southwestern margin of the Sichuan Basin varies greatly in the gas bearing property, so systematical analysis and study on the shale gas enrichment mechanisms in this area is of great significance to searching for new shale gas exploration areas. Based on drilling data of 5 wells in the mountainous complex structural area along the southwestern margin of the Sichuan Basin, the main factors controlling the shale gas bearing property and the shale gas enrichment model were discussed based on the systematical experiments and analysis of the outcrops, cores and cuttings of Longmaxi Formation shale from the aspects of organic geochemistry, physical property, gas content, hydrocarbon generation history and pore evolution history, so as to lay a solid geological basis for the birth of a new shale gas area. And the following research results were obtained. First, the shale of the first submember of first Member of Longmaxi Formation (S1l1¹) in the mountainous complex structural area along the southwestern margin of the Sichuan Basin has an average TOC of 3.02%-4.97% and Ro of 2.38%-3.37%, and the average total gas content in local enrichment zones is up to 4.62 m³/t, so it is classified as quality shale. Second, the detailed studies on hydrocarbon generation history indicate that the shale has the characteristics of "low thermal evolution rate and low maturity". It is characterized by late hydrocarbon generation, low thermal evolution rate, low current maturity and short late diffusion time, which are favorable for shale gas enrichment. Third, the pore evolution history reveals that shale pore evolution can be divided into six stages, i.e., sharp pore reduction, pore reduction, organic pore formation, pore preservation, organic pore dissipation and karstification, and organic pore and TOC are the most direct control factors of shale gas content. Fourth, favorable shelf facies belt is conducive to the formation of large-scale reservoir space and effective pores, and diversities of preservation conditions under different structural styles and at different structural positions control different pore evolution stages. The shale gas in the mountainous complex structural area is characterized by horizontal zoning and differential enrichment. "Low thermal evolution rate and low maturity" and "slow structural uplifting" are beneficial to the long-term enrichment and preservation of shale gas. In conclusion, the mountainous complex structural area along the southwestern margin of the Sichuan Basin has a shale gas enrichment model of "sedimentation controlling source rocks, diagenesis controlling reservoirs and structure controlling preservation". This research result provides idea and reference for searching for new shale gas areas and fields. © 2021, Natural Gas Industry Journal Agency. All right reserved.
Natural fractures are essential in the formation of shale gas reservoirs and have been the focus of study for shale gas development. Tectoclases provide most storage space for gas and are largely controlled by the paleo-tectonic stress field in shale reservoirs of the Niutitang Formation, northern Guizhou area. An accurate prediction of the development and distribution of tectoclases in the reservoirs is of great significance to exploring and developing shale gas sweet spots in the area. Based on geological structure evolution and fracture characterization, this study is focused on factors that control the fracture development in the Niutitang Formation shale reservoirs in northern Guizhou through characterization and modeling of geomechanisms and tectonic movements. A geomechanical model is formulated for the shale reservoirs against the geological background of the area. On this basis, the fractures are predicted by using the acoustic emission data. Numerical simulation results show that the development and distribution of tectoclase is controlled by fault zones, some of which have no obvious turning points with tectoclase in the middle sections being more developed and fragmented than those at the two ends. andSome of these have obvious S-shaped turning points where tectoclases are the most developed and fragmented.
Complex and ever-changing tectonism is one of the main factors affecting the shale gas preservation conditions of Lower Silurian Longmaxi Formation in the southern Sichuan Basin. In order to clarify the characteristics of various structural deformation and its relationships with the preservation conditions of Longmaxi Formation shale gas, structural belt division based on seismic, outcrop and drilling data were investigated. Structural deformation, evolution, and main control factors were analyzed and shale gas preservation areas were considered. Results revealed that the southern Sichuan Basin could be divided into three structural belts, namely basement thrust structural belt (piedmont zone of Qiyue Mountain and Dalou Mountain), caprock detachment structural belt (the area between Qiyue Mountain, Dalou Mountain and Huaying Mountain), and basement uplift structural belt (the area to the west of southern Huaying Mountain), which have the section structural characteristics of "double detachment layers and basement thrust fault", "three detachment layers and ramp-flat fault" and "deep extension and shallow compression", respectively. At the end of the Silurian, unconformity between the Permian and the Silurian was formed. From the Late Jurassic to the Early Cretaceous, the NE-SW trending structures began to develop in the Qiyue Mountain through eastern Sichuan Basin. In the Late Cretaceous, near E-W trending structure began to develop in the Dalou Mountain and its north area. In the Cenozoic, near S-N trending structure was formed in Chishui area and its north area. The formation of structures in the southern Sichuan Basin is related to the distribution of detachment layers and the stress mode of deformed series of strata. The detachment layers are mainly the Middle-Lower Cambrian gypsum-salt rock, the Longmaxi Formation shale and the Lower Triassic Jialingjiang Formation gypsum-salt rock, and the stress mode of deformed series of strata is dependent on the basement and its activity mode. In the basement thrust structural belt, some basement faults break through gypsum-salt rock, due to strong vertical uplift and smaller thickness of Middle-Lower Cambrian gypsum-salt rock, besides horizontal compression at the basin margin.
Achieving a thorough understanding of how primary sedimentary granularity drives considerable heterogeneity in internal reservoir attributes of terrigenous fine-grained deposits is of great significance. We investigated the quantitative differentiation and its corresponding driving forces of physical reservoir properties and pore-structure characteristics of silty-mud sediments in the Upper Triassic Xujiahe Formation (SW China) using a multi-method approach. The results show that the micro-mesopore volume and surface area of mudstones / shales are apparently higher than those of silty mudstones and a remarkable threefold rise in average permeability also presents. Extensively distributed bitumen pores occurring mostly along brittle mineral grains or forming clay-organic complexes make considerable contributions to shrinking microcracks. Furthermore, an evidently higher concentration of clay minerals in mudstone / shale reservoirs is primarily responsible for development of the two types of clay inter-crystalline pores distributed along grain aggregates and between well-oriented platelets. These two major causes facilitate the formation of micro-bedding fractures / non-bedding microfractures and connected fracture and pore-fracture networks, and also high-quality argillaceous reservoirs by strongly enhancing storage spaces and seepage capacities. Finally, a conceptual model is established for interpreting a differential reservoir-forming mechanism and corresponding two-sided effects on petrophysical and reservoir quality properties for continental silty-mud sediments.
Organic rich Wufeng-Longmaxi shales (WLS) occur widely in the Upper Ordovician–Lower Silurian strata in southeastern Sichuan Basin, SW China. However, complex structural deformation and faulting activity in this area since the Mesozoic time has had significant impact on shale gas preservation. Thus, difference in the structural deformation strength is an important constraint for differential shale enrichment in a shale reservoir. Fracture analysis and bedding surface normal stress data obtained from the outcrop, core and fluid inclusion samples were used to determine the vertical and lateral constraints on shale gas preservation conditions. Using the finite element method, 2D mechanical models were established in the Songkan area, based on seismic inversion, rock mechanics and acoustic emission tests. Paleotectonic stress fields were estimated for the Middle Yanshanian (K1¹/K1²), Late Yanshanian (K2²/K2³) and Himalayan （E-N）times. Fracture type distribution was determined from the rock failure criterion, and the shale bedding surface normal stress calculated by dip angle and burial depth. The results indicate that optimal conditions for shale gas preservation occur where the maximum principal stress values reach the fracture development stage (around 63.96–73.8 MPa). When the value increases to over 65 MPa, shale bedding surface closure favors the shale gas preservation. Based the vertical and lateral constraints, the best preservation area occurs where fracture development coincides with the shale bedding closure.
Significant breakthroughs of shale gas exploration have been made in Lower Cambrian and Sinian shale in the north margin of the Yangtze Block, South China. The drill wells with industrial gas flow located in the southern margin of the Huangling dome. Base on the geological survey, 2D seismic, geochronological and drill wells data, the tectonic evolution history of Huangling dome was studied, and its control effect on the preservation condition of shale gas was discussed. The result shows that the Huangling dome might undergo four tectonic stages: (1) About 800 Ma, granite intrusion in the Huangling dome basement, primarily of granites replaced metamorphism rocks; (2) 800–200 Ma, no significant tectonic movement with slowly buried history; (3) From 200 Ma, multi-phase uplift and the sedimentary rocks was eroded in the core of the Huangling dome. Shale gas in the Cambrian and Sinian strata was well preserved in the margin of the Huangling dome as the following reasons: (1) The Sinian shale was buried about 7.8 km in-depth during Middle Jurassic, source rocks have a suitable thermal maturity for shale gas; (2) The rigid basement of the Huangling dome was mainly composed by homogeneity granite, without intensive deformation. As the main challenges of the widely distributed Lower Cambrian and Sinian shale are high-maturity and intensive deformation, a geological unit with a dome probably is a favorable zone for the old age shale gas. Therefore, it indicates that the adjacent zone of the Xuefengshan, Shennongjia and Hannan are the geological units with a dome and probably have potentials for the exploration of shale in the Lower Cambrian and Sinian.
The exhumation of the deep crustal rocks through brittle crust by extensional tectonics is recognized in orogens of all ages. Paleostress analysis are generally used to understand the brittle exhumation process. In this study, we reconstructed the paleostress of faults of the Neoproterozoic Ambaji Granulite, South Delhi Terrane of the Aravalli- Delhi Mobile Belt, NW India by analyzing the fault slip data in Win Tensor program. Several NE-SW and WNW-ESE faults have been mapped in the area and found to be normal faults with a few strike-slip faults. The strike- slip faults are pre-kinematic to normal faults. Tensor solutions for 237 fault slip data points estimate WNW- ESE extension for strike slip faults and NW-SE direction extension for normal faults. From these results, we interpret that the NE-SW striking, orogen-parallel normal faults were produced from a NW-SE directed extensional stress and are primarily responsible for brittle exhumation of the granulite through crustal extension and thinning at 764 - 650 Ma. This is comparable to earlier studies on brittle exhumation along the Southern Tibet detachment in the Higher Himalayas. On a more regional scale, our results are in agreement with the extensional tectonics that affected the entire Aravalli-Delhi Mobile Belt and adjoining continents of the Neoproterozoic- Cambrian Gondwanaland Supercontinent.
There have been few systematic studies on the potential of the shallow Longmaxi shale in Lower Silurian strata although it develops widely outside the Sichuan Basin in southern China. The Longmaxi shale samples with a burial depth of 362-394 m were taken from the XK2 well in the Xishui area, Guizhou Province and their gas-in-place (GIP) and connate water contents were reported. The relationships of the GIP and connate water contents with the TOC content, mineral composition and porosity were investigated, and the controlling factors of the GIP content were discussed. The results show that the GIP content of the shales varies from 0.83 m³/t to 3.04 m³/t, with an average of 1.67 m³/t, and the connate water content is in the range of 5.01-12.38 mg/g, with an average of 7.46 mg/g. The GIP content of the shales is positively correlated with the TOC,but is not obviously related to the mineral composition, while the connate water content shows a definite positive linear correlation with the clay mineral but shows no correlation with the TOC. Both the organic matter (OM)-hosted and inorganic matter (IM)-hosted pores of the shales are extensively developed, with a total porosity of 3.03-5.50 %. Most of the shale gas occurs in the OM pores, while the connate water is mainly distributed in the IM pores. The TOC content controls the porosity and pore structure of the shales, and therefore, TOC content is considered to be the primary controlling factor of the GIP content. The clay-hosted pores are abundant with the connate water, which notably decreases the sorption capacities of clay minerals, making minor contribution to the GIP content.
In this work, we report the results of a multidisciplinary study describing the structural architecture and diagenetic evolution of the Rocca di Neto extensional fault zone developed in poorly lithified sandstones of the Crotone Basin, Southern Italy. The studied fault zone has an estimated displacement of ∼90 m and consists of: (1) a low-deformation zone with subsidiary faults and widely spaced deformation bands; (2) an ∼10-m-wide damage zone, characterized by a dense network of conjugate deformation bands; (3) an ∼3-m-wide mixed zone produced by tectonic mixing of sediments with different grain size; (4) an ∼1-m-wide fault core with bedding transposed into foliation and ultra-comminute black gouge layers. Microstructural investigations indicate that particulate flow was the dominant early-stage deformation mechanism, while cataclasis became predominant after porosity loss, shallow burial, and selective calcite cementation. The combination of tectonic compaction and preferential cementation led to a strain-hardening behavior inducing the formation of “inclined conjugate deformation band sets” inside the damage zone, caused by the kinematic stress field associated with fault activity. Conversely, conjugate deformation band sets with a vertical bisector formed outside the damage zone in response to the regional extensional stress field. Stable isotope analysis helped in constraining the diagenetic environment of deformation, which is characterized by mixed marine-meteoric signature for cements hosted inside the damage zone, while it progressively becomes more meteoric moving outside the fault zone. This evidence supports the outward propagation of fault-related deformation structures in the footwall damage zone.
The studies on the paleo-fluid in the Ordovician–Silurian detachment zone in the Middle–Upper Yangtze area focus on the origin of high-density methane inclusions and the evolution process of formation pressure, but rarely deal with the significance of paleo-fluid to shale gas preservation. In this paper, the relationship between fracture formation and detachment zone was analyzed by observing the Ordovician–Silurian outcrops in western Hunan–Hubei area and by investigating the geological characteristics of fracture veins in the drilling cores. Then, the significance of paleo-fluid forming environment and detachment zone to shale gas preservation was studied by using inclusion compositions of fracture veins and homogenization temperature test data. Finally, accumulation-dispersion modes of shale gas in the detachment zone were established. The following results were obtained. First, the detachment zone is lithologically composed of silicate with intercalated shale at the Ordovician–Silurian interface. In the detachment zone, rocks are broken and small crumples are developed. The conjugate-vertical joints are relatively developed in the silicate above and below the detachment zone. Second, multistage and multi-type inclusions, especially the aqueous inclusions, are developed in the veins of the detachment zone. Third, the infiltration depth of ancient meteoric water along the detachment zone in the study area is over 4 000 m. High-density overpressure methane inclusions were captured in the detachment zone in the early stage, while normal-pressure methane and nitrogen inclusions were captured in the late stage. Fourth, the ionic constituents of inclusions recording the fluid activity in the late stage was characterized by high sodium chloride coefficient, high desulfurization coefficient and low metamorphic coefficient, and it is indicated that the sealing capacity of the shale in the detachment zone gets worse. And fifth, the accumulation and dispersion of shale gas in the detachment zone within the study area is divided into three modes, i.e., syncline, broad anticline and closed anticline. In conclusion, the detachment zone in the study area is permeable. Synclines and closed anticlines developed in detachment structures are unfavorable for the accumulation of shale gas, while broad anticlines are favorable. © 2018, Natural Gas Industry Journal Agency. All right reserved.
The primary quality of shale in the Fuling Shale Gas Field, Sichuan Basin, is areally less different, but the gas production rates vary greatly from different development wells. This is resulted from the areal difference of shale gas preservation conditions. For a quantitative evaluation of shale preservation conditions in this area, the structural characteristics of this field (e.g. the force applied on the structure, and structural form and fracture development degree) were analyzed comprehensively and the structural types were classified based on the detailed structure interpretation results, together with geological, well logging and gas well testing data. Then, based on the single-well testing production of shale gas, the main indexes influencing the preservation conditions of marine shale gas of the Upper Ordovician Wufeng-Lower Silurian Longmaxi Fms in this area were studied from the macroscopic and microscopic aspects. And finally, the preservation types were classified according to the determined evaluation indexes. The following research results were obtained. First, the structures in the Fuling Shale Gas Field can be divided into a stable type, a weakly-deformed type and a complex type. Second, the following 6 parameters are taken as the evaluation indexes of shale gas preservation conditions in this area, including structural form, fracture characteristic, roof and floor condition, formation pressure, porosity and pore size. Third, gentle morphological structures and major structures far from the boundary are the base of shale gas preservation. The effective barrier and roof & floor sealing can significantly reduce the damage of structural reworking to gas reservoirs. Overpressure environment, porosity and pore size are the comprehensive representations of preservation conditions. And fourth, three types of structures correspond to three typical types of shale gas preservation respectively, among which a stable structure is the most favorable one. © 2018, Natural Gas Industry Journal Agency. All right reserved.
In studies with two continuous variables usually named the x-values and y-values a linear relation between the two variables can be assessed with the help of the Pearson correlation coefficient R. R is a measure of strength of association, and varies from −1 to +1. Instead of a traditional Pearson correlation analysis a Bayesian analysis of linear correlation is possible.
The structural characteristics of the Jiaoshiba Block in the Fuling Shale gasfield, Sichuan Basin, were analyzed. Based on well test data of over 190 horizontal wells, the effects of structures on shale gas well productivity were discussed systematically, and the main structural factors of different structural units in the Jiaoshiba Block that influence the productivity of shale gas wells were clarified. Results showed that the structural units in the Jiaoshiba Block were obviously different in structural characteristics and their deformation strength is different. Also, the influence of structural characteristics on shale gas well productivity is directly manifested in gas-bearing property and fracturing effect. The stronger the structural deformation and the more developed the large faults and natural fractures, the more easily shale gas escapes and the poorer the gas bearing property will be, and vice versa. The stronger the structural deformation, the more developed the fractures, the greater the burial depth and the higher the compressive stress of negative structures, the worse the fracturing effect will be, and vice versa. And fourth, Tectonics is the key factor controlling the difference of shale gas productivity between different structural units in the Jiaoshiba Block, but the main structural factors influencing the productivity are different in different structural units.
Triangle zones are important structures found in foreland fold-and-thrust belts all over the world and are commonly associated with tectonic wedging. However, tectonic wedging and consequently the formation of passive hinterland verging roof thrusts requires particular mechanic conditions such as pre-fractured rocks, syntectonic sedimentation, or a specific stratigraphic layering with variable mechanically weak and rigid formations. In this contribution models of triangle zones and hypotheses regarding their formation are reviewed. Our results show that the term “triangle zone” is often used in a contradictory sense and interpretations are affected by large uncertainties. Passive roof thrusts are not necessarily required in all cases and suggested models of triangle zones, even if they are geometrically and kinematically viable, are hampered by their mechanical implications. With respect to a large number of published triangle zones and associated kinematic as well as mechanic models we present a new definition and classification scheme, which is discussed and applied to natural examples worldwide. We show that natural examples of triangle zones can be separated into two types: (1) detachment dominated and (2) ramp dominated triangle zones. Both types imply particular mechanic conditions of involved detachments and regional dynamics. Regarding the large uncertainties associated with triangle zone interpretation we suggest to only use the term “triangle zone” in a very stringent manner and only if irrefutable evidences for stratigraphic repetitions within a duplex are given. The results should then be validated with additional kinematic and mechanic considerations. Geometric and kinematic uncertainties as well as implications of the proposed model explaining observed triangular structures should be reflected in the nomenclature used.
Over the past five years, great progress has been made in shale gas exploitation, which has become the most driving force for global gas output growth. Hydrocarbon extraction from shale helps drive the USA on the road to energy independence. Besides, shale oil & gas production has been kept in a sustained growth by continuous improvement in drilling efficiency and well productivity in the case of tumbling oil prices and rig counts. Shale gas reserves and production have been in a rapid growth in China owing to the Lower Palaeozoic Wufeng and Longmaxi shale gas exploitation in the Sichuan Basin, which has become an important sector for the future increment of gas reserves and output in China. However, substantial progress has been made neither in non-marine shale gas exploitation as previously expected nor in the broad complicated tectonic areas in South China in for which a considerable investment was made. Analysis of the basic situation and issues in domestic shale gas development shows that shale gas exploitation prospects are constrained by many problems in terms of resources endowment, horizontal fracking technology, etc. especially in non-marine shale deposits and complicated tectonic areas in South China where hot shales are widely distributed but geological structures are found severely deformed and over matured. Discussion on the prospects shows that the sustained and steady growth in shale gas reserves and production capacity in the coming years lies in the discovery and supersession of new shale plays in addition to Wufeng and Longmaxi shale plays, and that a technological breakthrough in ultra high-pressure and ultra deep (over 3 500 m buried in the Sichua n Basin) marine shale gas exploitation is the key and hope. © 2017, Natural Gas Industry Journal Agency. All right reserved.
Fenggang area in North Guizhou is one of the typical blocks where the Lower Cambrian Niutitang Fm-an important shale gas payzone is located. In this paper, samples from Lower Cambrian Niutitang Fm were experimentally tested by means of X-diffraction, scanning electron microscope (SEM), liquid-nitrogen cryogenic adsorption and nuclear magnetic resonance (NMR) to analyze its accumulation and recovery conditions of shale gas. The X-diffraction results show that the Lower Cambrian Niutitang Fm shale in Fenggang area contains quartz as the main mineral. Its brittle mineral content is high and clay mineral (mainly illite) content is low. The SEM results show that shale pores are mainly composed of nanometer organic matter pores, mineral dissolution pores, mineral intercrystal pores and moldic pores, as well as a large number of closed and semi-closed microcracks. The liquid nitrogen absorption experiments show that the specific surface area and pore volume of shale are large, and micropores are the main contributor to the specific surface area and pore volume of shale. H2 type hysteresis loop is presented in adsorption and desorption curves and there are a lot of "ink bottle" type pores in the shale. The NMR experiments show that shale pores are poorly connected, and that the pore size distribution measured by the NMR is consistent with that in liquid nitrogen absorption experiments, mainly ranging between 3 and 10 nm. It is finally concluded that the reservoir conditions of the Niutitang Fm in North Guizhou are favorable for shale gas accumulation and fracturing, suggesting a good potential of shale gas recovery. © 2016, Natural Gas Industry Journal Agency. All right reserved.
The main controlling factors of organic matter accumulation in the Upper Ordovician Wufeng–Lower Silurian Longmaxi Formations are complex and remain highly controversial. This study investigates the vertical variation of total organic carbon (TOC) content as well as major and trace element concentrations of four Ordovician–Silurian transition sections from the Upper Yangtze Platform of South China to reconstruct the paleoenvironment of these deposits and to improve our understanding of those factors that have influenced organic matter accumulation in these deposits.
The Huangshi structure, as one of the NWW-trending S-shaped structures in the southwestern Qaidam Basin, holds important implications for unraveling the regional structural pattern. There are four dominant sets of surface strike-slip fractures at the core of the Huangshi structure. The fractures with orientations of N28°E, N47°E and N65°E correlate well with conjugate Riedel shears (R′), tension fractures (T) and Riedel shears (R) in the Riedel shear model, respectively. Two conjugate strike-slip fracture sets occur at the surface of the Hongpan structure (secondary to the Huangshi structure) and the southwestern part of the Huangshi structure. In seismic sections, the Huangshi structure is present as a positive flower or Y-shaped structure governed by steeply dipping faults, whereas Hongpan and Xiaoshaping structures, located symmetrically to the Huangshi structure, are thrust-controlled anticlines. The Riedel shear pattern of surface strike-slip fractures, the positive flower or Y-shaped structure in seismic sections and the NW-trending secondary compressional anticlines consistently demonstrate that the Huangshi structure is dominated by left-lateral strike-slip faults which comprise a strike-slip fault network. Considering the similar S-shaped configuration and NWW trend of structures across the southwestern Qaidam Basin, it can be further speculated that these structures are also predominantly of left-lateral strike-slip types.
Based on a comparison of geological and surface conditions for shale gas in China and America, we conclude that marine strata in South China having similar properties to those of America are targets for shale gas exploration, and the Sichuan Basin and its surroundings are favorable exploration areas. After six years of research and exploration, the Fuling shale gas field, which is the first major commercial shale gas play in China, was finally found in the Sichuan Basin in 2012 and then successfully developed. The shale gas enrichment and high productivity in the Wufeng-Longmaxi Formations is mainly controlled by the original depositional environment which determines organic matter content and post-depositional conditions that promote shale preservation and gas retention within the shale after thermal maturation. Deposition and distribution of the shales are controlled by the platform depression environment. Thickness and TOC content are the evaluation parameters for play. The late preservation conditions control the enrichment of shale gas. There are favorable preservation conditions and high degree of shale gas enrichment in the internal of Sichuan Basin, which are characterized by widely distributed Triassic gypsolyte layers and high pressure coefficient. The areas where both original depositional environment and preservation conditions are favorable should be given priority in play evaluation. Comprehensive analysis of the shale gas reservoir indicates that organic matter content is the principal control on shale gas enrichment, the organic micro-pores are the main reservoir space, horizontal bedding fractures ensure horizontal seepage, and that the high siliceous mineral content ensures good fracturing effect, high pressure coefficient indicates high degree of shale gas enrichment. These five properties have genetic relationships and statistical correlation and are characterized by “five properties in one position” distribution features in vertical. The high quality shales with “five properties in one position” feature is conducive to the enrichment of shale gas and benefit for its development, which is the main development of shale gas layer and key objectives of horizontal well trajectory. © 2016, Editorial Office of Earth Science Frontiers. All right reserved.