Search

We investigated velocity and anisotropic structure of the crust beneath the eastern margin of the Tibetan Plateau to better understand its deformation and evolution mechanism. We performed H‐κ and Pms anisotropy analyses to obtain crustal thickness, Vp/Vs ratio, fast polarization direction, and splitting time from 711 stations, and further conducted quality control using slowness, harmonic and statistical analyses. The Songpan‐Ganzi Block has a large splitting time and a fast polarization direction roughly parallel to the GPS motion and SKS fast direction. It also shows an overall high but complex distribution of Vp/Vs ratio, and large variations in crustal thickness, indicating that crustal deformation is likely caused by crustal shortening and lower crustal flow. The northern Sichuan‐Yunnan Rhombic Block (SYRB) is featured by a thick crust and high Vp/Vs ratio, suggesting that the crust is likely inflated by partial melting lower crustal rocks. The subblock also exhibits a strong azimuthal anisotropy with a splitting time greater than 0.6 s. The fast polarization direction aligns with the nearly N‐S extended direction and rotates clockwise in front of the Emeishan Large Igneous Province (ELIP). The observed anisotropy agrees with aligned amphibole minerals under a simple shear condition, supporting a southward lower crust flow being diverted by the ELIP. Anisotropy measurements on the southern SYRB are less robust and widely scattered, suggesting a deformation mechanism different from the northern SYRB. In addition, the southeastern margin of the Sichuan Basin shows a systematic pattern of crustal anisotropy consistent with a pure shear deformation mechanism. Plain Language Summary: We studied the crustal structure and deformation beneath the eastern edge of the Tibetan Plateau. By analyzing teleseismic data from 727 stations, we measured crust thickness, Vp/Vs ratio, and azimuthal anisotropy using receiver functions. Our findings show a complex crustal behavior pattern in the eastern Tibet margin. In the Songpan‐Ganzi Block, crustal shortening and lower crust flow seem to be causing deformation. The northern Sichuan‐Yunnan Rhombic Block, with its thick crust and high Vp/Vs ratio, appears to be influenced by partial melting of lower crust rocks. And a southward lower crust flow being diverted by the Emeishan Large Igneous Province. The eastern and southern part of this block shows a different pattern, indicating a unique deformation mechanism. Additionally, the southeastern margin of the Sichuan Basin appears to undergo a distinct type of crustal deformation. Key Points: A new crustal model with much more reliable and denser Pms anisotropic measurements is built in the eastern Tibetan MarginSongpan‐Ganzi Block is likely deformed by crustal shortening and lower crustal flowChannel flow may exist in the west low velocity zone (LVZ) while absent at the east LVZ [ABSTRACT FROM AUTHOR]

To obtain the influence of anisotropy and energy evolution characteristics on wellbore stability, the acoustic and mechanical anisotropy characteristics of shales are studied through various experiments, including scanning electron microscopy, ultrasonic pulse transmission, and uniaxial compression experiments, with the Longmaxi Formation shale in the southern area of the Sichuan Basin as the research object. The energy evolution characteristics of the Longmaxi Formation shale under different bedding angles are analyzed. The influence of anisotropy on the wellbore stability of shale formation is discussed on this basis. The results show that the acoustic and mechanical parameters, failure mode, and energy evolution characteristics of shale have significant anisotropy. Furthermore, the P-wave and S-wave time differences decrease with an increase in bedding angle. The compressive strength and Poisson's ratio decrease first and then increase with an increase in bedding angle. Meanwhile, the elastic modulus gradually increases with an increase in bedding angle. Rock samples with different bedding angles show diverse failure modes in mechanical tests, including splitting, shear, and shear-splitting failure. The total energy and elastic energy decrease first and then increase with an increase in bedding angle. Finally, the formation anisotropy affects the wellbore stability: the higher the formation anisotropy, the more vulnerable is the wellbore to instability. [ABSTRACT FROM AUTHOR]

Although many geophysical models have been proposed in the Longmenshan fault zone (LFZ) and its surrounding areas, the deep structure of the seismic gap and its constraint of the Wenchuan and Lushan earthquakes remain uncertain. Based on the compiled aeromagnetic data and Bouguer gravity data, we have tried to create a more detailed and reasonable magnetic and density model using 2D forward modeling and 3D inversion and made the deep structure of the LFZ visible. The research shows that structure is heterogenous across the LFZ. The earthquake epicenters are located in regions with high-magnetic anomalies and gravity gradients that are associated with rigid blocks that were likely to accumulate stress. However, the seismic gap shows low-magnetic anomalies and transition of gravity anomalies related to a weak zone. The Sichuan Basin has two NE-trending banded high-magnetic blocks extending beneath the LFZ that firmly support the idea that the crust of the Sichuan Basin has subducted downward the LFZ. More importantly, the basement subducts to approximately 33 km west of the Wenchuan–Maoxian fault, with a low dip angle beneath the middle segment of the LFZ, whereas the distance decreases to approximately 17 and 19 km under the southern segment. Thus, the crust of the Sichuan Basin beneath the middle segment extends farther than that beneath the southern segment, with the seismic gap as the transition zone. Therefore, we propose that the structural heterogeneity of the basement on the western margin of the Sichuan Basin may be the main reason for the different focal mechanisms and geodynamics of the Wenchuan and Lushan earthquakes. [ABSTRACT FROM AUTHOR]

The distribution of block boundary faults, both horizontally and vertically, has significant implications for crustal movements and structural variations. In the SE margin of the Tibetan Plateau, the south part of the Sichuan–Yunnan block is surrounded by the first-order block boundaries of the Honghe faults (HHF) trending NW–SE and the Xiaojiang faults (XJF) trending N–S. Crustal anisotropy provides valuable insights into the ongoing deformation and evolution. This study investigates the upper-crustal anisotropy at 26 stations, using local seismic waveforms from the temporary linear Honghe–Xiaojiang seismic broad-band Array (i.e. the HX Array) in 2017–2019 and regional permanent stations in 2012–2019. The PFS (i.e. polarization of fast S wave) shows an abrupt change in the Yimen region, which is the middle of the HX Array survey line between two large faults. The NW–SE dominant PFS orientations across the HHF agree well with the strike of faults and the direction of regional principal compressive stress, illustrating cracks or minerals aligned with the HHF, influenced by regional stress. The NE–SW dominant PFS orientations crossing the XJF differ from the faults strike or regional stress orientation, suggesting alignment with microcracks caused by the in situ principal compressive stress. The comparison between the upper-crustal and whole-crustal anisotropy demonstrates a noticeable PFS consistency across the two large faults, the HHF and the XJF. This observation implies that these two large faults may tectonically control a wide zone, with wide rigid widths of approximately 150 and 200 km, respectively. This study presents a new pattern indicating the control of deep physical properties on the boundary faults and uncovering the layered deformation in the upper and lower crust. [ABSTRACT FROM AUTHOR]

The economic exploitation of unconventional gas and oil in deep shale relies closely on effective hydraulic fracturing stimulations. However, the fracturing operations of deep shale reservoirs face challenges of insufficient fracture growth and a rapid decline in productivity due to the increasing in situ stress level. In addition, the shale strata on the margin of the Sichuan Basin are frequently folded and faulted, and the change in bedding inclinations significantly complicates the process of hydraulic fracturing. The investigation of the combined effect of the in situ stress level and bedding anisotropy on the hydraulic fracture configuration is vital for fracturing engineering design. To analyze this, we conducted hydraulic fracturing tests on shale cores to simulate the hydraulic fracture initiation and growth from a horizontally positioned perforation. By using acoustic emission detection and CT scans, the influence of natural stress levels and the angle of the shale's bedding on the process of hydraulic fracturing in shale and the resulting fracture geometry were analyzed. The results showed that the area of hydraulic fracture under a higher stress level (σ1 = 50 MPa, σ3 = 40 MPa) was about 13%~23% smaller than that created under the lower stress level (σ1 = 30 MPa, σ3 = 20 MPa) when the bedding angle was smaller than 60°. With the increase in bedding angle, the curves of the fracture area and fracture network index under two different stress levels presented similar decreasing trends. Also, the time from micro-crack generation to sample breakdown was significantly reduced when the bedding orientation changed from the horizontal to vertical position. The increasing stress level significantly increased the breakdown pressure. In particular, the fracturing of shale samples with bedding angles of 0° and 30° required a higher fluid pressure and released more energy than samples with larger bedding inclinations. Additionally, the measurement of the sample radial deformation indicated that the hydraulic fracture opening extent was reduced by about 46%~81% with the increasing stress level. [ABSTRACT FROM AUTHOR]

The 2008 Wenchuan earthquake, which occurred on the eastern edge of the Tibetan Plateau, produced significant time-dependent post-seismic deformation in the northeastern Tibetan Plateau. To explore the spatio-temporal evolution of crustal deformation and its impact on nearby active faults due to the Wenchuan earthquake, we first solve the velocity fields, strain rates and slip rates of the major faults at different stages of the earthquake cycle, based on GPS observations spanning approximately 10 yr before and after the Wenchuan earthquake. The results show that: (a) during the late interseismic phase, the GPS velocity fields relative to the Sichuan basin and strain rates near the Longmenshan fault (the seismogenic fault of the Wenchuan earthquake) are insignificant in magnitude, and the geodetic slip rates of the major faults are in good agreement with geological investigations; (b) After the Wenchuan earthquake, post-seismic deformation on both flanks of the causative Longmenshan fault is asymmetrically distributed, with significant deformation distributed between the Longmenshan fault and the Longriba fault. The post-seismic deformation decreases with increasing distance away from the Longmenshan fault. Furthermore, geodetic slip rates for the major active faults at the early post-seismic stage differ significantly from those at the late interseismic stage; (c) The crustal deformation in the northeastern Tibetan Plateau is generally small during the late interseismic phase, but it increases significantly during the early post-seismic stage and begins to decrease again as it progresses to the second post-seismic stage. We speculate that this kinematic evolution of crustal deformation is ascribed to that the Longmenshan fault was in a locking state during the late interseismic period, whereas the Longmenshan fault is no longer locked after the Wenchuan earthquake, and thus the marked post-seismic deformation occurred in the epicentral and its surrounding area. Then, the post-seismic deformation becomes weaker with time lapse, and generally the crustal deformation begins to slowly recover to the pre-seismic level. Using a forward model, we test whether post-seismic deformation caused by viscoelastic relaxation of the lower crust 4 yr after the Wenchuan earthquake can explain the observed spatio-temporal pattern of the crustal deformation. We find that this mechanism can account for the evolutionary pattern of crustal motion in the Songpan-Ganzi block east of the Longriba fault in central-east Tibet and central-eastern and southern regions of the West Qinling-Songpan Tectonic Syntaxis in east Tibet 4 yr after the Wenchuan earthquake. Moreover, the post-seismic viscoelastic relaxation of the Wenchuan earthquake is the main cause of loading rate variations of primary faults in the early post-seismic stage in the northeastern Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Resolving the detail of crustal deformation of the seismically and technically active Sichuan-Yunnan block located in the southeast margin of the Tibetan Plateau is crucial for understanding the lateral extrusion of the Tibetan Plateau. Using seismic data from two regional seismograph stations of the China Earthquake Networks Center and twenty recently deployed broad-band temporary stations from the MCD Array (Middle ChuanDian block seismic array), we constrained crustal thickness and seismic anisotropy at the west boundary of the Sichuan-Yunnan block, near the east Himalayan syntaxis by receiver function techniques including CCP stacking and sinusoidal moveout of PmS conversions. Crustal thickness varies significantly across major faults inside the block which increases by >20 km from west to east. East of the Chenghai Fault, a clear positive phase lies above the Moho and is interpreted to be caused by the magma intrusion related to a late Permian Emeishan large igneous province (ELIP). The crustal anisotropy shows strong zoning features and clockwise rotation with respect to due north. To the west of the Lijiang-Xiaojinhe and the Honghe Faults, anisotropic fast orientations are near NS, indicating strong crustal deformation and movement of southward crustal flow along the Nujiang-Lancangjiang Fault; to the east of this fault, anisotropic fast orientations are near NE. The variation of the anisotropic fast orientations indicates that the southeastward crustal flow from the Tibetan Plateau may be prevented by fast velocity anomalies beneath the ELIP. These observations provided new evidence for strong crustal deformation along the tectonic boundary in the southeastern Tibetan Plateau with the expansion of the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Although many geophysical models have been proposed in the Longmenshan fault zone (LFZ) and its surrounding areas, the deep structure of the seismic gap and its constraint of the Wenchuan and Lushan earthquakes remain uncertain. Based on the compiled aeromagnetic data and Bouguer gravity data, we have tried to create a more detailed and visible magnetic and density model beneath the LFZ using 2D forward modeling and 3D inversion. The research shows that structure heterogeneities are widely distributed beneath the LFZ. The earthquake epicenters show high magnetic anomalies and the edge of high Bouguer gravity anomalies that consist of rigid blocks where apt to accumulate stress. However, the seismic gap shows low magnetic anomalies and transition of Bouguer gravity anomalies related to a weak zone. The Sichuan Basin has two NE-trending banded high magnetic blocks extending beneath the LFZ that firmly support the crust of the Sichuan Basin was downward subduction toward the LFZ. More importantly, the basement subducts to approximately 33 km west of the Wenchuan-Maoxian fault with a low dip angle beneath the middle segment of the LFZ, whereas the distance decreases to approximately 17 and 19 km under the southern segment. Thus, the crust of the Sichuan Basin beneath the middle segment extends farther than that beneath the southern segment with the seismic gap as the transition zone. Therefore, we propose that the structural heterogeneity of the basement on the western margin of the Sichuan Basin may be the main reason for the different focal mechanisms and geodynamics of the Wenchuan and Lushan earthquakes. [ABSTRACT FROM AUTHOR]

The Sichuan Basin was a part of the Yangtze Carbonate Platform (YCP) during the Cambrian–Ordovician, and marine carbonates were deposited in the basin during this interval. Although previous studies have evaluated the paleogeography, paleoclimate and paleoecology of this basin, they have primarily focused on the paleoecology and biological evolution in the basin; however, analysis of paleogeography and paleoclimate is lacking. This study integrated outcrop sedimentological and magnetic fabric data to document sedimentary differentiation and anisotropy of magnetic susceptibility (AMS) within the YCP. The aims of this study were to infer paleowind directions during each epoch of the Cambrian–Ordovician and to constrain the paleogeographic location of the YCP. The northwestern, central and southeastern sides of the YCP were characterized by high-energy deposition (e.g. sub-angular to rounded intraclasts), medium-energy deposition (e.g. sub-angular to sub-rounded intraclasts) and low-energy deposition (e.g. angular to sub-angular intraclasts), respectively. The centroid D-Kmax values for the Early, Middle and Late Cambrian were 116° ± 52°, 145° ± 57° and 159° ± 62° from the present north, respectively; corresponding values for the Early, Middle and Late Ordovician were 169° ± 70°, 139° ± 73° and 91° ± 68° from the present north, respectively. Sedimentary differentiation and AMS results indicated that the prevailing wind directions during the Early Cambrian, Middle Cambrian, Late Cambrian, Early Ordovician, Middle Ordovician and Late Ordovician were 296° ± 52°, 325° ± 57°, 339° ± 62°, 349° ± 70°, 319° ± 73° and 271° ± 68° from the present north, respectively. The present study provides evidence for the location of the YCP during the Cambrian–Ordovician via the correspondence between the paleowind directions over the YCP and the trade winds in the Northern and Southern hemispheres. The novelty of this study lies in the following aspects: (1) it integrates microfacies and AMS analyses to establish paleowind patterns; (2) it constrains the paleo-hemispheric location of the YCP during the Cambrian–Ordovician; and (3) it provides a reference for further studies of the paleoclimate and paleogeography of the YCP during the Cambrian–Ordovician. [ABSTRACT FROM AUTHOR]

At present, hydrofracture technology is mainly used for shale gas exploration and exploitation, and the effect of volume fracturing is tightly linked to the mechanical properties (including anisotropy and brittleness) of shale. Meanwhile, Sichuan Basin is also an important area for unconventional natural gas exploration and development in China. This work analyzed mineral composition and microstructure of outcrop shale matrix in Lower Silurian Longmaxi Formation through XRD test, slice observation, and scanning electron microscope (SEM). How anisotropy affects shale deformation, strength and failure characteristics at different bedding angles (0°, 30°, 45°, 60°, and 90°) under diverse confining pressures (0 MPa, 25 MPa, 50 MPa, 75 MPa, and 100 MPa) in conventional uniaxial/triaxial experiments were discussed. The acoustic emission (AE) evolution was introduced to reveal the cracks closure and propagation of Longmaxi Formation shale and qualitatively characterized the brittleness of the shale. What's more, based on the existing brittleness index, this study proposed an indicator BInew that considers the shape of the stress–strain curve before and after the peak stress to evaluate the brittleness of the Longmaxi Formation shale. The index BInew can better describe the changes of brittleness with confining pressure and bedding angle. Finally, this work elaborately explained the relationship between AE characteristics, brittleness, and failure modes of shale. AE characteristics can not only qualitatively characterize shale brittleness but also show the failure process and modes of shale samples. Change of failure modes rests with the difference in brittleness and also reflects the level of brittleness. The change of brittleness with failure modes can be expressed as shear along bedding plane (Sh-Al) > splitting through bedding plane (Sp-Th) > conjugate shear failure (Co-Sh) > shear along bedding plane (Sh-Al) > shear through bedding plane (Sh-Th). Highlights: Uniaxial/triaxial compression and acoustic emission (AE) monitoring of Longmaxi formation shale were carried out. Influence of anisotropy of shale on mechanical properties under different confining pressures was analyzed. Different brittleness indexes were used to evaluate the brittleness of shale, and a new brittleness index was established. Relationship between AE characteristics, brittleness, and failure modes of shale was discussed in detail. [ABSTRACT FROM AUTHOR]

The Sichuan Basin is a significant region for exploration and development of shale gas in China, and it is essential to clarify the impact of deep shale gas reservoir parameters on cost-effective development at scale to ensure national energy security. Rock physics modeling is a significant means of communicating the physical and elastic parameters of rocks. A rock physics modeling method applicable to fractured shale gas reservoirs is proposed for the current situation of complex fluid relationships in shale gas reservoirs and unclear characteristics of gas identification seismic response. In this paper, based on the Self Consistent Approximation (SCA) model and the differential effective medium (DEM) model, the anisotropic source of shale is used as a starting point to add bound water, kerogen, clay, and brittle minerals, the Schoenberg linear slip theory is used to add fracture disturbance effects, and then the Brown–Korringa model is used to perform fluid replacement under anisotropic conditions. Finally a rock physics model applicable to fractured shale gas reservoirs is obtained, and the established rock physics model is used for analysis of elastic parameters, Thomsen parameters, and fracture weakness parameters. Rock physics tests were performed on shale in southern Sichuan as an example. The experimental results show that the model established by the process can accurately invert the longitudinal and transverse wave velocities of the shale, which can provide a conceptual basis for the study of fractured shale gas reservoirs. [ABSTRACT FROM AUTHOR]

Electrical exploration has become an important means of shale gas reservoir exploration and evaluation, and is expected to play a key role in the later stages of reservoir fracturing and development. At present, the research on the electrical response characteristics of shale gas reservoirs and their relationship with reservoir parameters is extensive and in-depth, but there is little research on their complex resistivity anisotropy characteristics and influencing factors, which restricts petrophysical modeling and reservoir parameter prediction, and reduces the reliability of shale gas exploration and reservoir evaluation by electromagnetic methods. In this paper, shale samples from the Longmaxi Formation and the Wufeng Formation of shale gas wells in southern Sichuan were collected, the complex resistivity of 34 shales in bedding direction and vertical bedding direction were measured, and the induced polarization (IP) parameters of shales were extracted by inversion. The electrical anisotropy response characteristics under different temperature and pressure conditions were analyzed, and the influencing factors and laws of complex resistivity anisotropy of shales were revealed. Combined with the test results of shale porosity and permeability, the evaluation model of resistivity, polarizability and porosity and permeability parameters was established. The research results have formed a set of testing methods and analysis techniques for electrical anisotropy of shale reservoirs, which are mainly based on complex resistivity parameter testing. It is helpful to understand the electrical anisotropy characteristics of shale gas reservoirs in southern Sichuan; this will provide the theoretical and physical basis for shale gas reservoir evaluation and fracturing monitoring by electrical exploration. [ABSTRACT FROM AUTHOR]

No shale-rock physical model has been established in the observation coordinate system. To this end, this paper carried out anisotropic wave velocity tests on shale rock and compared the Thomsen, Daley, and Berryman solutions to characterize anisotropic acoustic wave velocity. Finally, the Daley solution was selected. Based on basic rock physical models, such as SCA and DEM methods, and combined with the Daley solution, an anisotropic shalerock physical model was established in the observation coordinate system and applied in Well B1 in the Luzhou area, Sichuan Basin. Our research conclusions were as follows: 1. for the samples from the same core, the P-wave velocities in three directions were in the order VP11 >VP45 >VP33, shear-wave velocity VS11 was the largest, but VS33 and VS45 did not follow the law of Vs33 >Vs45 for some samples; 2. the Daley solution, which not only considers the accuracy requirements but also has a complete expression of P-, SV-, and SH-waves, is most suitable for characterization of anisotropic wave velocity in this study area; 3. the rock physical model constructed in the observation coordinate system has high accuracy, in which the absolute value of the relative error of the P-wave slowness was between 0% and 5.05% (0.55% on average), and that of shear-wave slowness was between 0% and 6.05% (0.59% on average); 4. the acoustic waves recorded in Well B1 in the observation coordinate system were very different from those in the constitutive coordinate system. The relative difference of the P-wave was between 6.76% and 30.84% (14.68% on average), and that of the S-wave was between 7.00% and 23.44% (13.99% on average). The acoustic slowness measured in the observation coordinate system, such as in a deviated well or a horizontal well section, must be converted to the constitutive coordinate system before it can be used in subsequent engineering applications; 5. the anisotropic shale-rock physical model built in the observation coordinate system proposed in this paper can provide basic data and guidance for subsequent pore pressure prediction, geomechanical modeling, and fracturing stimulation design for deviated and horizontal wells. [ABSTRACT FROM AUTHOR]

The strength of radial anisotropy can effectively reveal vertical or horizontal material flow, which may provide significant dynamic implications. The South China Block (SCB) is a classic laboratory to study strong deformation under compression from the surroundings. We construct a radially anisotropic 3‐D shear‐wave model of the entire SCB using multimode surface wave tomography with a lateral resolution of 2.0°. Distinctive structural contrasts are revealed in the S‐wave velocity anomalies in the Cathaysia Block and the Yangtze Craton. In the Cathaysia Block, a low‐velocity anomaly becomes evident with increasing depth, particularly in the VSH model. The Yangtze Craton is, however, characterized by a high‐velocity anomaly, especially in the Sichuan Basin, where the high‐velocity feature can extend down to a depth of 300 km. Interestingly, an abrupt change in radial anisotropy is observed beneath the Sichuan Basin, with positive radial anisotropy (faster VSH) in the uppermost mantle and negative radial anisotropy (faster VSV) at depths from 80 to 250 km. The lower negative anisotropic layer may reflect the directional arrangement of constituent materials caused by eastward compression and erosion of mantle flow under the Tibetan Plateau. In addition, remarkable negative radial anisotropy is found in the asthenosphere beneath Hainan Island and its surroundings. The features beneath Hainan Island and its surroundings may be related to the Hainan mantle plume in the Cenozoic. Plain Language Summary: Seismic radial anisotropy represents that the vertically (VSV) and horizontally (VSH) polarized shear waves propagate at different wave velocities in the Earth's medium, which is useful to reflect the deformation characteristics and the deformation pattern of the medium, so as to constrain the mantle dynamics and tectonic processes of the crust and mantle. In this study, we measure multimode Rayleigh wave and Love wave phase velocity dispersion to construct a 3‐D shear‐wave model with radial anisotropy in the entire South China Block. Our results show that the shear‐wave velocity under the Yangtze Craton is anomalously fast and beneath the Cathaysia Block is anomalously slow. Moreover, we find different manifestations of radial anisotropy in the lithosphere of the Sichuan Basin, in which the negative radial anisotropy of 80–250 km is inferred to be affected by eastward compression and erosion of mantle flow under the Tibetan Plateau. A remarkable negative radial anisotropy beneath the Hainan Island and its surroundings in the asthenosphere is interpreted to reflect the Hainan mantle plume in the Cenozoic. Key Points: An anisotropic shear wave model beneath South China is constructed by multimode surface wave tomography with improved vertical resolutionSichuan basin shows negative radial anisotropy, reflecting the influence of eastward compression and mantle flow under the Tibetan PlateauThe low‐velocity anomaly and strong negative radial anisotropy of the Hainan Island may be related to the Hainan mantle plume [ABSTRACT FROM AUTHOR]

Crustal thickness and Poisson's ratio have played significant roles in determining the crustal structure, compositions and deformation. We improved the algorithm for joint estimation by receiver function and gravity data on the optimization of initial model, sediment-correction, as well as estimation of associated density parameter, and utilized it to estimate the crustal thickness and Poisson's ratios in eastern China. The uncertainty analyses verified the reliability of our model. Our results demonstrate that the crustal thickness in eastern China increases gradually from east (about 26.6 km) to west (about 62.4 km) with an NNE-trending gradient zone along the line of Great Khingan-Taihang-Wuling Orogenic belt. The crustal Poisson's ratios in eastern China, ranging from 0.19 to 0.31, reflect the characteristics of different tectonic blocks. Low crustal Poisson's ratios are mainly presented along the sutural and orogenic belts between tectonic blocks. Low Poisson's ratio associated with the Central Asian Orogenic Belt accretionary wedge suggests that it extends further east than previously thought, beneath the Songliao Basin (between the city line of Sonid Right Banner-Linxi-Tongliao-Jilin-Wangqing and Bayan-Huade-Chifeng-Liaoyuan-Helong). After reducing the influence of the sediment, banded-like high-Poisson's ratios show up in Sichuan Basin and Ordos Basin. Combined with the magnetic data and petrophysical properties, we speculate it to be the intrusions of intermediate-basic rocks in the basement. High Poisson's ratios presented in the northern Songliao Basin may be related to multistage magmatic intrusions and eruptions since Meso-Cenozoic. High Poisson's ratios in Panxi rift are related to magmatic activities caused by mantle plumes. [ABSTRACT FROM AUTHOR]

For the purpose of clarifying the seismic response characteristics of fractured-cavity reservoirs of Dengying Formation in the central Sichuan Basin, the paper first intends to establish three geological models of fracture cave reservoirs based on drilling, logging, and core data of the Dengying Formation in the central Sichuan Basin. Then, the formation reflection is calculated with reference to anisotropic Horizontal Transverse Isotropy (HTI) medium. Finally, further research on Amplitude Variation with Azimuth (AVAZ) seismic forward modeling has been conducted to clarify the seismic response characteristics of different reservoir types in the study area. The results suggest that: Seismic response characteristics of fractured-cavity reservoirs are controlled by incident angle and azimuth angle of seismic waves in different types of reservoirs. The incident angle of the seismic wave controls the difference in amplitude caused by different micro-fracture densities, and the azimuth angle controls the identification ability of the micro-fracture direction. The increase in incident angle brings about a gradual decline in amplitude. The magnitude reaches the highest when the azimuth is parallel to the normal direction of the fracture surface; however, it'll come down to the lowest as the azimuth is perpendicular to the normal direction of the fracture surface. The fracture density fails to affect the amplitude as long as the azimuth angle is parallel to the direction of the fracture. However, the decreased amplitude reflects the increasing fracture density as the azimuth angle is identical to the normal direction of the fracture surface. The comparison between the theoretical model of three different types of fractured-cavity reservoirs and the actual uphole trace shows that the model has high accuracy. The prospect of seismic identification of fractured-cavern reservoirs, based on the results, can provide us with feasible and applicable evidence for future research on seismic identification of reservoirs and prediction of fracture distribution in the Dengying Formation of central Sichuan. [ABSTRACT FROM AUTHOR]

The northeastern Tibetan Plateau serves as the frontier for the northeastward expansion of the plateau. In this area, the Tibetan Plateau interacts with the surrounding blocks, such as the Alxa Block, the Ordos Block, the Kunlun-West Qinling belt and the Sichuan Basin. Because of this expansion and interaction, this area suffers from intense deformation. At present, the evolution and deformation mechanisms of the northeastern Tibetan Plateau remain controversial. To provide new insights into these mechanisms, in this study, we conduct tomography of the P-wave velocity and radial anisotropy structures beneath the northeastern Tibetan Plateau. We choose a total of 667 teleseismic earthquakes from August 2006 to October 2020. Waveforms of these earthquakes were recorded by 921 broadband seismic stations in the northeastern Tibetan Plateau and surrounding areas. We first perform cross-correlation on waveforms of each station pair and obtain 770,749 P-wave traveltime differences. Then, we invert the differential traveltime data by applying eikonal equation-based teleseismic tomography. Finally, the P-wave velocity and radial anisotropy structures at depths from 30 to 800 km below the northeastern Tibetan Plateau are obtained. Our tomographic model shows clear low-velocity anomalies and positive radial anisotropy in the lower crust under the northeastern Qilian orogen, the northeastern Songpan-Ganzi belt and the western Qinling fold zone. These features are integrated to demonstrate the existence of lower crustal flow in the study area. Prominent low-velocity anomalies and positive radial anisotropy are found in the uppermost mantle beneath the Qilian orogen, the northeastern Songpan-Ganzi belt and western Qinling fold zone. These characteristics are combined to infer a weak lithosphere and horizontal asthenospheric flow under these tectonic units. Both the Ordos Block and the Sichuan Basin exhibit clear high-velocity anomalies and negative radial anisotropy in the uppermost mantle, thereby reflecting the high mechanical strength of the lithosphere beneath these blocks. High-velocity anomalies are also present in the upper mantle under the northern Chuandian block, potentially implying the northward subduction of the Indian plate. Furthermore, the front of the subducted Indian plate is imaged close to the Xianshuihe fault rather than the Kunlun fault. [ABSTRACT FROM AUTHOR]

As a result of the combined influence of multi-stage and multi-directional tectonic activities, the subsurface medium in complex structural areas can easily develop intricate fault systems with varying scales and directions. Post-stack seismic and azimuth-stack attributes have limited precision in recognizing such complex fault systems. In this paper, we propose an adaptive fault enhancement workflow in offset vector tile (OVT) domain based on anisotropy theory to improve the recognition ability of seismic data for multi-directional faults and minor faults. The vertical directions of fault strikes and tectonic strikes estimated by the gradient structure tensor (GST) can be used as sensitive observation azimuths of faults. At the same time, according to the anisotropic forward modeling and real data amplitude variation with offset (AVO) analysis of minor faults in marine shale of Wufeng-Longmaxi formations in Sichuan Basin, China, the incident angles with the largest difference between isotropic and anisotropic reflection coefficients can be used as sensitive incident angles of faults. A new fault enhancement seismic data can be reconstructed from the OVT gathers by traversing the sensitive azimuths and incident angles adaptively. The 3D model and real data indicate that the faults enhanced by the workflow we proposed are clearer in shape, smaller in scale, and richer in details, which is consistent with geology and well. • We propose an adaptive fault enhancement workflow in OVT domain. • The workflow considers the azimuth and incidence contained in OVT gathers. • We discussed the recognition ability of angle stack for minor faults. • The result of shale reservoir in Sichuan Basin is consistent with geology and well. [ABSTRACT FROM AUTHOR]

The vast majority of teleseismic XKS (including SKS, SKKS and PKS) shear wave splitting studies interpret the observed splitting parameters (fast orientation and splitting time) based on the assumption of a spatially invariant anisotropy structure in the vicinity of a recording station. For such anisotropy structures the observed splitting parameters are either independent of the arriving azimuth of the seismic ray paths if the medium traversed by the ray paths can be represented by a single layer of anisotropy with a horizontal axis of symmetry (i.e. simple anisotropy), or demonstrate a periodic variation with respect to the arriving azimuth for a more complicated structure of anisotropy (e.g. multiple layers with a horizontal axis of symmetry, or a single layer with a dipping axis). When a recording station is located near the boundary of two or more regions with different anisotropy characteristics, the observed splitting parameters are dependent on the location of the ray piercing points. Such a piercing-point dependence is clearly observed using a total of 360 pairs of XKS splitting parameters at three stations situated near the northeastern edge of the Sichuan Basin in central China. For a given station, the fast orientations differ as much as 90°, and the azimuthal variation of the fast orientations lacks a 90° or 180° periodicity which is expected for double-layered or dipping axis anisotropy. The observed splitting parameters from the three stations are spatially most consistent when they are projected at a depth of ∼250 km, and can be explained by shear strain associated with the absolute plate motion and mantle flow deflected by the cone-shaped lithospheric root of the Sichuan Basin. [ABSTRACT FROM AUTHOR]

The safe and efficient development of ultra-deep shale gas deeper than 4500 m is closely related to the comprehensive evaluation of the mechanical behavior, brittleness and anisotropy characteristics of reservoir rocks under in-situ stress conditions. However, the above-mentioned information about ultra-deep shale is rarely reported. This study aims to fill this gap by systematically investigating the mechanical and anisotropic characteristics of cores, with 0° and 90° bedding orientations, from different layers in the vertical depth of 4900–4930 m, under confining pressure of 110 MPa. Furthermore, a new brittleness index with three parts related to brittle failure, i.e., the energy storage performance in the pre-peak stage, the performance of elastic energy used to maintain the post-peak failure, and the severity of the energy released during the post-peak stage, all of which are physically meaningful, was proposed. Subsequently, the index was validated and applied to the brittleness evaluation of ultra-deep reservoir rocks. The brittleness was evaluated in the following descending order: black shale of Longmaxi Formation ＞ gray-black shale of Wufeng Formation ＞ limestone of Baota Formation. Overall, the Wufeng- Longmaxi Formation shale still exhibits brittle failure under such high confining pressure, while the Baota Formation limestone exhibits quasi-ductile failure, which is consistent with the phenomenological behavior of cores. Further, various experimental techniques have been employed to study the brittle and quasi-ductile failure mechanisms of reservoir rocks. The anisotropy evaluation results show that the anisotropy degree of the mechanical and brittleness parameters corresponding to the deep formation is significantly lower than that of the shallow formation. The anisotropy degree of strength and deformation parameters (Young's modulus, Poisson's ratio) is closely related to the content of quartz and clay minerals, respectively. The above results are expected to provide data support and theoretical guidance for the development of deep and ultra-deep shale gas in the southern Sichuan region of China. • The mechanical characteristics of ultra-deep reservoir rocks in the vertical depth range of 4900–4933 m are reported. • The influence of the bedding effect on the mechanical behavior of reservoir rocks under high confining pressures is studied. • A new three-part brittleness index in which each part has a clear physical meaning related to brittle failure is proposed. • The brittle and quasi-ductile failure mechanisms of reservoir rocks under high confining pressures are revealed. [ABSTRACT FROM AUTHOR]

The M S 6.4 earthquake occurred in Yangbi County, Yunnan Province, China, on 21 May, 2021. The earthquake is located on the west side of the Red River fault, the western boundary of the Sichuan-Yunnan block in the SE margin of the Tibetan Plateau. The Yangbi M S 6.4 earthquake has aroused great concern due to its unique geographical location and frequent strong earthquakes in Yunnan. In this study, a total of 4695 local earthquakes of the Yangbi M S 6.4 earthquake sequence are relocated using the double-difference location method, including the mainshock source location of 99.879°E, 25.687°N, and depth of 9.67 km. The mainshock has a moment magnitude of 6.1, and one of the fault-plane solutions is 137°, 83° and − 163° in strike, dip, rake respectively. Combined with the relocation and focal mechanism solutions results, it is suggested that the seismogenic fault of the Yangbi M S 6.4 earthquake is a NW-SE trending fault on the southwest side of the Weixi-Qiaohou fault (WQF), with high dip of SW, which is a previously-unknown blind fault. The regional 2D stress field shows that the principal compressive stress around the Yangbi earthquake is NNW-SSE and nearly N-S direction, which appears to be deflected counterclockwise by a small angle on its east side and changed to N-S and NNE-SSW direction on its west side, while both the north and south sides of the Yangbi earthquake show directional coherence. Strong differential deformation distorts in the upper crust to the northeast of the seismic source as reflected by tectonic stress field and the upper crust seismic anisotropy, indicating strong influence of Lijiang-Xiaojinhe fault (LXF). The stress field in the source area is jointly controlled by southeastward movement of southeastern margin of Tibetan Plateau, the northeastward movement of Indian plate and the differential movement between the different tectonic blocks. • The Yangbi M s6.4 earthquake sequences are relocated to reveal two burst areas of small earthquakes. The seismogenic fault is a blind fault. • The new detailed 2D stress field is released, and a model of stress field is reported around the source area. • Strong differential deformation distorts in the upper crust to the northeast of the source area. [ABSTRACT FROM AUTHOR]

The Sichuan basin which embodies salt and shale gas resources, is characterized by low seismicity. Seismicity in the southeastern Sichuan basin increased dramatically in recent years, including a series of moderate to strong earthquakes. We investigate seismic anisotropy in the upper crust by performing shear wave splitting analyses on local earthquakes recorded at seventeen stations around the Changning-Xingwen area, southeastern Sichuan basin between January 2016–February 2021. We perform an automatic shear wave splitting analysis and obtain 2926 high-quality splitting measurements by examining 5174 local earthquakes. The delay times range between 0.08 and 0.20 s, and the anisotropy magnitude ranges between 1.9% and 5.5% with a mean of 4.2%, indicating that the study area has significant strength of anisotropy. The fast polarization directions are highly variable reflecting a combined effect of complex local structure and stress field in this area. In the southeastern part of the study area, fast directions are aligned parallel to NW- and NE-oriented surface structures, suggesting structural anisotropy. In the northwestern part of the study area, fast-polarization directions align parallel to the strike of folds and the direction of maximum horizontal compressive stress, probably reflecting the combination of both stress- and structural-controlled anisotropy. Whereas at these stations of central part, fast directions tend to align parallel to the direction of maximum horizontal compressive stress, suggesting stress-induced anisotropy. We examined the temporal variation of seismic anisotropy associated with the 2019 M S 6.0 Changning earthquake. The significant changes in splitting parameters during the Changning earthquake period may only reflect the spatial changes in seismic anisotropy. • A more detailed spatial distribution of crustal anisotropy is revealed in the Changning-Xingwen area. • Crustal anisotropy is attributed to stress in the middle area and surface structures in the southeastern area. • Crustal anisotropy in the northwestern area is probably caused by a combination of folds and stress. • No temporal changes in anisotropy parameters were observed. [ABSTRACT FROM AUTHOR]

The resistivity anisotropy of shale has a non-negligible effect on resistivity logs. Many studies have shown that the macroscale factors of shale anisotropy are depositional environment, compaction process, and diagenesis process, and the microscale factors mainly include lamination, orientation of clay minerals, and microfractures. However, the mechanism of laminations, pores, organic matters, pyrite and clay minerals driving resistivity anisotropy remains unclear. In this research, experiments and simulations were performed to investigate the characteristics and mechanism of the resistivity anisotropy of shale. Two sets of shale samples were collected at different inclination angles from the Lower Silurian Longmaxi Formation in the Pengshui area, southeastern Sichuan Basin, China. Core sample resistivities were measured in several water saturation states established using imbibition, pressurized saturation and centrifuge methods. Scanning electron microscopy, thin sections, X-ray diffraction, the mercury injection capillary method, and N 2 adsorption were also performed. Resistivity simulations were applied to three-dimensional models derived from the microimages. The results of the experiments and simulations show that Longmaxi shale has strong resistivity anisotropy. The resistivity anisotropy coefficient (λ) of Longmaxi shale ranged from 1.3699 to 2.1623. Longmaxi shale has a strong lamination structure. In the submillimeter scale, strawberry-like pyrite (3.8%–10.2%, v /v) aggregates in elongated strips with an extension direction consistent with the horizontal direction. Due to the differential compaction, the clay minerals (21.1%–26.4%, v /v) also tend to be parallel to the formation bedding. The most obvious directional distribution of pores occured in the organic pores. Most interparticle pores between detrital minerals did not exhibit anisotropic characteristics. Because of the horizontal distribution of lamination, pyrite, clay minerals and organic pores, a higher resistivity is measured in the vertical direction than in the horizontal direction. • Lamination, distribution of minerals and pores affect shale resistivity anisotropy. • Large quartz and feldspar grains in laminae block vertical flow of electricity. • Accumulation of strawberry-like pyrite decreases horizontal resistivity of shale. • Organic pore exhibits more horizontal distribution characteristics than other pores. [ABSTRACT FROM AUTHOR]

Nature fractures developed in tight gas-bearing sand reservoirs significantly increase reservoir porosity and permeability and vary in scale, angle, aperture, density. The detection of effective hydrocarbon fractures are requirements for commercial production from these reservoirs. Based on the equivalent medium theory, we propose an approach to eliminate the influence of low-productivity horizontal bedding fractures and directly extract the orientation and intensity information of effective high-angle fractures in the reservoir where horizontal bedding fractures are mainly developed. First, we assume that the complex tight gas-bearing sand reservoir is considered as a weakly orthorhombic medium with a single suite of parallel vertical fractures embedded in a vertical transverse isotropy (VTI) medium. Then, we derive a linearized relationship between the reflection coefficient and the fracture orientation and intensity in an orthorhombic medium. Furthermore, we implement a Bayesian elastic impedance (EI) inversion workflow, in which the seismic data are transformed to EI datasets, and subsequently EI datasets are used to estimate the fracture orientation and intensity. Finally, the proposed approach is demonstrated by tests on synthetic examples and field data sets. The predicted results illustrate the approach is feasible and robust, and agree well with the log data and geological interpretation. Compared with the conventional method, our approach has the greater advantages to accurately predict the fracture characteristics. We conclude that the proposed approach can reliably produce the distributions and properties of vertical fracture systems, which may provide crucial fracture information for tight gas-bearing sand reservoirs characterization. • A novel method is used to predict fracture signatures of tight sand reservoirs. • The influence of low-productivity horizontal bedding fractures is eliminated. • The orientation and intensity of high-angle fractures are directly estimated. • The new method has a great correlation with geological law and log interpretation. [ABSTRACT FROM AUTHOR]

Continued northeastward compression owing to the colliding Indian subcontinent has led to pronounced surface uplift and significant crustal and mantle deformation in the Sichuan–Yunnan region along the southeastern margin of the Tibetan Plateau. Seismic anisotropy is an effective physical property for characterising crustal and upper-mantle deformation, as these deformation mechanisms can be investigated by analysing the splitting of compressional-to-shear-wave (P-to-S) converted phases (XKS, which includes the SKS, SKKS, PKS and PKKS phases) at the core–mantle boundary. However, we cannot distinguish any crustal and/or mantle anisotropy variations owing to the relatively low vertical resolution of XKS splitting. Here we compute the P-wave receiver functions from teleseismic events recorded at 90 permanent and 350 mobile broadband seismic stations that were established by the ChinArray project in the Sichuan–Yunnan region and analyse the P-to-S converted phases at the Moho interface (Pms) to measure the crustal anisotropic parameters. The Pms splitting results are then regarded as the crustal anisotropic parameters, and the anisotropic parameters in the upper mantle beneath 347 stations are extracted from the XKS splitting parameters by removing this crustal anisotropy. The results indicate that an approximately E–W anisotropic fast-wave polarisation direction (FPD) exists in the upper mantle beneath the southeastern margin of the Tibetan Plateau, and that the splitting times range from 0.06 to 1.39 s, with an average splitting time of 0.93 s. The approximately E–W anisotropic FPD in the upper mantle is mainly due to asthenospheric flow induced by the eastward subduction of the Indian Plate beneath Myanmar. The small splitting times within the southern Sichuan–Yunnan diamond-shaped block (SYDSB) may be attributed to vertical growth of the lithosphere induced by mantle-plume-related magmatic uplift. [Display omitted] • P-to-S converted phases at the Moho are used to estimate crustal anisotropy. • Anisotropy in the upper mantle is extracted from the XKS splitting. • This upper-mantle anisotropy is due mainly to asthenospheric flow. • There is no an obvious deformation transition zone beneath the SE Tibet near ~26°N. [ABSTRACT FROM AUTHOR]

Crustal anisotropy beneath a dense broadband seismic array across the Longmenshan fault belt is measured based on P-to-S converted phases from the Moho and an intra-crustal interface. The crustal thickness and the average Vp/Vs values are also estimated by a new H-κ-c method. The crustal anisotropy in Aba sub-block mainly comes from the middle-lower crust and the NW-SE fast polarization directions (FPDs) may reflect the direction of the crustal flow. A sharp variation of FPDs is observed in the vicinity of the Longriba faults, combined with the Vp/Vs values and other geological and geophysical observations, we suggest the Longriba faults may block the eastward flow of crustal materials. East of the Longriba faults, the consistence between the FPDs and the direction of the maximum horizontal compression, and low Vp/Vs values indicated that the crustal thickening may be the dominant deformation mechanism in the Longmenshan sub-block. The crustal anisotropy in the Longmenshan fault belt is mainly manifested as fault-parallel FPDs, probably related to fluid-filled fractures in this area. The Sichuan basin is weakly anisotropic and the NE-SW FPDs may reflect the tectonic strike. Combined with previous observations, our results suggest that the crustal flow and crustal thickening are co-exist in the crustal deformation in eastern Tibet. [Display omitted] • We measure small-scale variations of crustal anisotropy beneath eastern Tibet. • Crustal flow beneath Aba sub-block and crustal thickening beneath Longmenshan sub-block. • The Longriba faults block the crustal flow of the plateau. [ABSTRACT FROM AUTHOR]