[en] An anisotropic rock physics model based on effective medium theories is constructed for shales and applied to the Longmaxi Shale formation. In the rock physics model, intrinsic VTI (transverse isotropy with a vertical asymmetry axis) anisotropy due to preferred orientations of clay particles is quantified by introducing the clay lamination (CL) index in the Backus averaging method, and additional anisotropy enhanced by bed-parallel fractures is described by the Chapman model for multi-scale pore spaces. Rock physics templates are constructed based on the model in order to better understand the implicit relations between reservoir properties, elastic and mechanical properties and seismic parameters. A model-based method is proposed for inversion of the CL and aspect ratio (AR) of horizontal fractures, as well as VTI anisotropy parameters for the shale formation. Results indicate that the obtained CL index shows a negative correlation with clay content, which is further used in the prediction of S wave velocity (V _s) based on the constructed rock physics model. A more accurate prediction of V _s justifies the application of the constructed rock physics model and the proposed inversion method. Seismic modeling and inversion for the Longmaxi Shale are conducted. Seismic–well tie based on the propagator matrix method indicates that seismic signatures from the target gas shale can be identified by the waveform between a trough and a peak of a strong reflection. Examinations using Rüger’s theory of P-wave reflection coefficients indicate that the variations in vertical P-wave velocity V _P01 and the anisotropy parameter ε of the target shale provide interpretations for real seismic amplitudes versus offset (AVO) data. Seismic AVO inversion is then implemented based on Rüger’s theory. The inverted anisotropy parameter ε is further interpreted as the CL index, which reflects the microstructure of the solid matrix of the shale. Seismic inverted layer thickness and the P-wave anisotropy parameter ε of the target shale reservoir show good agreement with the values obtained at the location of involved well, which justifies the feasibility and applicability of the seismic inversion method. (paper)

[en] The rock physics model is an important tool for the characterization of shale reservoirs. We propose an improved anisotropic rock physics model of shale by introducing clay lamination (CL) index as a modeling parameter in effective medium theories. The parameter CL describes the degree of preferred orientation in distributions of clay particles, which depends on deposition and diagenesis history and determines intrinsic anisotropy of shales. Those complicated parameters of sophisticated methods that are difficult to quantify are substituted by CL. The applications of the proposed rock physics method include the inversion for anisotropy parameters using log data and the construction of a rock physics template for the evaluation of the Barnett Shale reservoir. Results show reasonable agreement between the P-wave anisotropy parameter ε inverted by the proposed method and those measured from core samples. The constructed rock physics templates are calibrated on well log data, and can be used for the evaluation of porosity, lithology, and brittleness index defined in terms of mineralogy and geomechanical properties of the Barnett Shale. The templates predict that the increase in clay content leads to the increase in Poisson’s ratio and the decrease in Young’s modulus on each line of constant porosity, which confirms the consistent and reveals quantitative relations of the two ways of defining the brittleness index. Different scenarios of mineralogy substitutions present the varied layout of constant lines on the templates. (paper)

[en] A set of parallel vertical fractures embedded in a vertically transverse isotropy (VTI) background leads to orthorhombic anisotropy and corresponding azimuthal seismic responses. We conducted seismic modeling of full waveform amplitude variations versus azimuth (AVAZ) responses of anisotropic shale by integrating a rock physics model and a reflectivity method. The results indicate that the azimuthal variation of P-wave velocity tends to be more complicated for orthorhombic medium compared to the horizontally transverse isotropy (HTI) case, especially at high polar angles. Correspondingly, for the HTI layer in the theoretical model, the short axis of the azimuthal PP amplitudes at the top interface is parallel to the fracture strike, while the long axis at the bottom reflection directs the fracture strike. In contrast, the orthorhombic layer in the theoretical model shows distinct AVAZ responses in terms of PP reflections. Nevertheless, the azimuthal signatures of the R- and T-components of the mode-converted PS reflections show similar AVAZ features for the HTI and orthorhombic layers, which may imply that the PS responses are dominated by fractures. For the application to real data, a seismic-well tie based on upscaled data and a reflectivity method illustrate good agreement between the reference layers and the corresponding reflected events. Finally, the full waveform seismic AVAZ responses of the Longmaxi shale formation are computed for the cases of HTI and orthorhombic anisotropy for comparison. For the two cases, the azimuthal features represent differences mainly in amplitudes, while slightly in the phases of the reflected waveforms. Azimuth variations in the PP reflections from the reference layers show distinct behaviors for the HTI and orthorhombic cases, while the mode-converted PS reflections in terms of the R- and T-components show little differences in azimuthal features. It may suggest that the behaviors of the PS waves are dominated by vertically aligned fractures. This work provides further insight into the azimuthal seismic response of orthorhombic shales. The proposed method may help to improve the seismic-well tie, seismic interpretation, and inversion results using an azimuth anisotropy dataset. (paper)

[en] Conventional AVO techniques are based on the linear approximation of Zoeppritz equations for the inversion of elastic properties across a subsurface interface. However, for a layered model consisting of internal multiple layers beyond seismic resolution, it can be difficult to use the traditional AVO inversion methods due to the practicalities of picking amplitudes, resolution problems, and thin-layer effects. We propose an improved AVO inversion method applied to a layered model for inversion of layer thickness and properties by incorporating the frequency content of the wavelet. The proposed method uses the propagator matrix method as a theoretical description of the frequency-dependent reflection coefficients for the layered model in an inversion scheme. We test the proposed method on a single layered model with synthetic data to produce an inversion for layer thickness and porosity, and then investigate the feasibility of the method for the characterization of the fracture zone in the Bakken formation. For the inversion of the reference crack density and layer thickness of the fracture zone in the Bakken formation, the minima of the objective functions generate inversion results that indicate a reasonable fit with the true model parameters. The inversion error may result from the intrinsic complexity of reflections from a layered model, in which several different combinations of layer thicknesses and associated properties may produce similar frequency-dependent coefficients. By contrast, in the multicomponent model data example, the inversion of converted PS-wave data seems to be less stable compared to PP-wave data. The potential of the proposed AVO inversion method may include applications to complex models, such as a sandstone/shale interbedded system, or a formation that presents internal heterogeneity. (paper)

[en] Irradiation breeding is an important technique in the effort to solve food shortages and improve the quality of agricultural products. In this study, a field test was implemented on the M3 generation of two mutant pea plants gained from previous neutron radiation of pea seeds. The relationship between agronomic characteristics and yields of the mutants was investigated. Moreover, differences in physiological and biochemical properties and seed nutrients were analyzed. The results demonstrated that the plant height, effective pods per plant, and yield per plant of mutant Leaf-M1 were 45.0%, 43.2%, and 50.9% higher than those of the control group, respectively. Further analysis attributed the increase in yield per plant to the increased branching number. The yield per plant of mutant Leaf-M2 was 7.8% higher than that of the control group, which could be related to the increased chlorophyll content in the leaves. There was a significant difference between the two mutants in the increase in yield per plant owing to morphological variation between the two mutants. There were significant differences in SOD activity and MDA content between the two mutants and the control, indicating that the physiological regulation of the two mutants also changed. In addition, the iron element content of seeds of the two mutants was about 10.9% lower than in the seeds of the control group, a significant difference. These findings indicate that the mutants Leaf-M1 and Leaf-M2 have breeding value and material value for molecular biological studies. (authors)

[en] We construct a rock physics workflow to link the elastic properties of shales to complex constituents and specific microstructure attributes. The key feature in our rock physics model is the degrees of preferred orientation of clay and kerogen particles defined by the proportions of such particles in their total content. The self-consistent approximation method and Backus averaging method are used to consider the isotropic distribution and preferred orientation of compositions and pores in shales. Using the core and well log data from the Barnett Shale, we demonstrate the application of the constructed templates for the evaluation of porosity, lithology and brittleness index. Then, we investigate the brittleness index defined in terms of mineralogy and geomechanical properties. The results show that as clay content increases, Poisson's ratio tends to increase and Young's modulus tends to decrease. Moreover, we find that Poisson's ratio is more sensitive to the variation in the texture of shales resulting from the preferred orientation of clay particles. Finally, based on the constructed rock physics model, we calculate AVO responses from the top and bottom of the Barnett Shale, and the results indicate predictable trends for the variations in porosity, lithology and brittleness index in shales. (paper)