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The dolomite gas reservoir of the Permian Chihsia Formation has recently become the main target of hydrocarbon exploration in the northwest Sichuan Basin. Owing to its substantially low porosity and permeability, the dolomite gas reservoir has become a great challenge for geophysicists in terms of distinguishing gas-filled dolomite thin layers. We have used limestone and dolomite core samples from the Permian in the northwest Sichuan Basin to investigate the physical characteristics of the dolomite gas reservoir in a laboratory. Dolomite has a comparatively higher permeability than limestone due to its higher fracture development. X-ray CT scanning indicates that a few large pore spaces dominate the pore space in limestone that cannot provide good pore connections, whereas small pores and fractures dominate the pore spaces in dolomite. The fracture development in the dolomites provides insignificant porosity that substantially increases the permeability. The [Formula: see text] ratio estimated from well-logging data in the northwest Sichuan Basin indicates significantly lower anomalies. These low [Formula: see text] anomalies validate the rock-physics analysis from the laboratory data and indicate that the [Formula: see text] ratio could be a good indicator of gas reservoirs in dolomite layers.

The emergence of big data is breaking the spatial and time limitations of urban waterlogging scenario description. The scenario data of different dimensions (e.g., administrative levels, sectors, granularities, and time) have become highly integrated. Accordingly, a structural and systematic model is needed to represent waterlogging scenarios for more efficient waterlogging response decision-making. In this article, a full-view urban waterlogging scenario is first defined and described from four dimensions. Next a structured representation of scenario element is given based on knowledge unit method. The full-view scenario model is then constructed by extracting the scenario correlation structures between different dimensions (called scenario nesting), i.e., inheritance nesting, feedback nesting, aggregation nesting, and selection nesting. Finally, a real-world case study in Wuhan East Lake High-tech Development Zone, China is evaluated to verify the reasonability of the full-view model. The results show that the proposed model effectively integrates scenario data from different dimensions, which helps generate the complete key scenario information for urban waterlogging decision-making. The full-view scenario model is expected to be applicable for other disasters under big data environment.

The seismic responses of the shale properties are critical for shale gas reservoir evaluation and production. It has been widely reported that the clay minerals have substantial influences on the seismic wave anisotropy and brittleness. Hence, knowing the seismic responses of the clay-rich shales and estimation of shale elastic properties are significant for the shale gas industry. A physical model containing two groups of shale blocks as the target formations is constructed in laboratory. The group S contains six shale blocks with different clay contents, and the group N contains six shale blocks with different porosity. The acquired 2D seismic data is used to analyze the seismic responses of two corresponding seismic lines. An anisotropic three-term inversion method is applied to one of the seismic inline to estimate the elastic properties the target shale blocks. The inversed attributes can be used to reveal the effects of shale clay contents. This study shows the substantial significance of using a physical model to observe the seismic responses of shale properties. The inversion results indicate that the anisotropic three-term inversion method could provide accurate results of elastic properties as well as the P- wave anisotropy parameter for shale formations.

Fractures usually spread over various scales and strongly influence velocity and anisotropy. We investigate elastic velocity and anisotropy in rocks with fractures of different sizes. Based on synthetic rocks with controlled fracture geometries, we create a set of rocks with fracture diameter of 2, 3 and 4 mm and the fracture thickness is 0.06 mm. P‐ and S‐wave velocities are measured at 0.1 MHz, while the rocks are saturated with water and air. For a fixed measurement frequency (0.1 MHz), velocities are higher in rocks with larger fractures, while anisotropy is higher in rocks with smaller fractures, even for the same fracture density. These phenomena are associated with the wave‐induced fluid flow process. Some novel effective medium theories are adopted to calibrate with the laboratory data and analyse the anisotropy affected by the fluid flow mechanism and fracture size. The results of our study demonstrate the significant effects of fracture scale on wave responses by effective medium theories in different ways. We suggest that these scale effects should be of considerable concern in some disciplines (e.g. shear wave splitting in earth's crust and hydraulic fracture monitoring with microseismic data). Considering the scale effects of fractures, the accuracy during these investigations would be improved.

Frequency-dependent anisotropy can be used for detection and evaluation of fractured reservoirs. In this study, three numerical models of fractured reservoirs are designed to study the frequency de...

The scale of the fractures can vary, making the seismic velocity and anisotropy substantially scale dependent. Two mechanisms of the scale-dependent phenomenon may be considered: scattering and wave-induced fluid flow. In this study, we measure the scale-dependent velocity and anisotropy effects through laboratory experiments on porous and non-porous artificial rocks containing aligned fractures. This allows us to isolate the effects of these two mechanisms for the first time, yielding some insights into the scale-dependent phenomenon. For short-wavelength waves, scattering dominates with less wave-induced fluid flow effects. For intermediate- and long-wavelength waves, the P-wave is strongly scale dependent mainly due to wave-induced fluid flow mechanism, and the slow shear-wave is also strongly scale dependent but due to both scattering and wave-induced fluid flow. However, the fast shear-wave is almost scale independent. Moreover, a multi-scale equivalent medium theory can model the P-wave propagation accurately.

SUMMARY Density is one of the most essential properties that determines the dynamic behavior of the Earth. Besides, density has been commonly used to investigate the mineral composition, porosity and fluid content of rock. Therefore, a reliable estimation of the density structure is one of the most important objectives in both global seismology and seismic exploration. However, seismic inversions of independent density estimates are ill-posed because density has a large trade-off with velocities. Shear wave propagation is sensitive to both density and the S-wave velocity. We show that the reflected SV-wave (SV-to-SV wave) at an incident angle of 22.5o depends only on density contrast, and at incident angle 30o it depends only on S-wave velocity contrast. Thus, density as well as S-wave velocity can be directly inverted from the reflected SV-wave as separate and independent parameters. The forward modelling has high accuracy, the inverse problem is well-posed and the misfit function can be easily regularized. Field data application demonstrates the proposed method can efficiently recover reliable and high-resolution density and S-wave velocity of fine sturctures. Thus, this method has great potential in geological interpretation including understanding regional Moho structure, crustal and mantle formation and evolution, and rock lithologic composition and fluid-filled porosity.

Shale is observed to have strong anisotropy due to its unique mineralogy and microstructure, and this anisotropy property has significant impact on seismic and well-log data. The organic-rich marine shale in the southern and eastern Sichuan Basin is one of the most important shale-gas reservoir formations in China. To investigate the elastic anisotropy of this shale and its influencing factors, we performed ultrasonic velocity measurements, X-ray diffraction analysis, rock-eval pyrolysis and vitrinite reflectance measurement on the samples from the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation. The experimental results show the that: (1) the velocity anisotropy of the Wufeng-Longmaxi (WL) shale varies from 10% to 50%, and most samples have strong anisotropy; (2) the P- and S-wave anisotropy parameters (Thomsen’s e and γ) increase with clay contents, but this relationship can be greatly affected by the clay orientation index; (3) organic matter content (OMC) is found to have little influence in seismic anisotropy for the over mature WL shale, whereas the OMC determines the magnitude of anisotropy of immature/mature shales (e.g. the Bakken shale or the Bazhenov shale) according to the published literatures, because organic matters in shales of different maturity have different morphologies and distributions; (4) the OMC of WL shale has positive correlation with quartz content, and this weakens the correlation between OMC and the magnitude of anisotropy to a certain extent. The results of this study provide an important rock-physics basis and data support for seismic anisotropy exploration, quantitative interpretation and resource evaluation of the organic-rich marine shales in southern China.

Thin interbeds are typical reservoirs in eastern China. Thin layers and fractures bring huge challenge to fluid identification in anisotropic reservoir. This study focuses on thin-fractured reservoirs and amplitude attributes of shear-wave splitting, and consequently predicted fluid type in fractured reservoir based on the response of fast and slow S-waves to fluids. 3D HTI viscoelastic equation was employed to analyze amplitudes of split S-waves through fluid-filled and fractured media, including oil- and water-saturated synthetic models. Similar to velocity anisotropy, amplitude anisotropy was proposed to avoid the calculation of S-wave quality factor. Amplitude ratio and substation derived from amplitude anisotropy were used to identify fluid type. Example from the Luojia area of Shengli oilfield was used to demonstrate the effectiveness of the inversion method. Results show that amplitude ratio and amplitude subtraction are useful to distinguish fluids, while the former works better than the latter.

Fractures are widely distributed within the Earth’s crust and have a substantial influence on wave propagation. Although many studies have been conducted on fracture characterization based on seismic anisotropic attributes, natural horizontal fractures and hydraulic fracture networks have yet to be characterized. Here, we investigate the influence of the fracture orientation and saturation on the Vp/Vs ratio via laboratory observations and theoretical modeling. Then, we discuss the feasibility of using the Vp/Vs ratio to predict natural horizontal fractures and evaluate hydraulic fracture networks. Our study demonstrates that an abnormally low Vp/Vs ratio, together with a lack of shear wave splitting, could be used to predict horizontal fractures. Fracture network development that results from hydraulic fracturing stimulation is associated with a much higher Vp/Vs ratio and no shear wave splitting; these two characteristics could be used to estimate fracture development and evaluate hydraulic fracturing stimulation.

Robert Stuart Middleton, 1944–2021

Study of the Krauklis wave in fractured media reveals the potential for characterizing fracture geometry. In this paper, a finite element scheme is proposed to simulate the Krauklis wave in...

The Lower Silurian shale-gas formation in the south of the Sichuan Basin represents a strong transverse isotropy with vertical axis of symmetry (VTI) feature. Successful characterization of shale-gas formation requires handling the great influence of anisotropy in the seismic wave propagation. Seismic amplitude variation with offset (AVO) inversion for VTI media using PP-waves only is a difficult issue because more than three parameters need to be estimated and such an inverse problem is highly ill posed. We have applied an AVO inversion method for VTI media based on a modified approximation of the PP-wave reflection coefficient. This approximation consists of only three model parameters: the acoustic impedance (attribute [Formula: see text]), shear modulus proportional to the anellipticity parameter (attribute [Formula: see text]), and the approximated horizontal P-wave velocity (attribute [Formula: see text]), which can be well-inverted and have great interpretation capability in shale-gas reservoir characterization. A statistical-rock-physics method was then applied to the inverted attributes for quantitative interpretation of the shale-gas reservoir. A Markov random field is combined with Bayesian rule to improve the continuity and accuracy of the interpretation results. Shales can be successfully discriminated from surrounding formations by using the attribute pair [Formula: see text]-[Formula: see text], and the organic-rich gas-bearing shale can be successfully identified by using the attribute pair [Formula: see text]-[Formula: see text]. Comparison between the prediction results and well logs demonstrates the feasibility of the inversion and quantitative interpretation approaches.

Hengchang Dai, 1957–2021

Carbonate fractured gas reservoir detection is very significant for the process of oil-gas exploration. It is difficult to characterize the reservoirs properly by traditional post-stack seismic attributes, because of the complexity of mineralogical composition and fluid type. Based on rock physics, it becomes possible to effectively solve this problem. In this paper, we combine seismic azimuthal anisotropy analysis and P-wave (primary wave) dispersion inversion based on the appropriate rock physics model in order to provide a method for carbonate fractured gas reservoir prediction. Firstly, referring to the geology and logging data of a carbonate fractured reservoir in the S area of Tarim basin in western China, we introduce the Voigt–Reuss–Hill theory into the Chapman model and set up an appropriate model which includes the influences of lithology and physical and fluid properties. Then, through seismic forward modeling and inversion based on this model, we find that attenuation azimuthal anisotropy is very sensitive to fracture density, and P-wave dispersion is closely linked to fluid type. By comprehensive analysis of these two attributes, we can characterize the reservoirs well. Finally, both attributes were applied to analysis of seismic field data for carbonate gas reservoir discrimination in the S area of the Tarim basin. The results show that zones with strong attenuation anisotropy and intense P-wave dispersion are likely to be favorable gas reservoirs. This is consistent with trial production data, hence demonstrating the great advantages of our method in carbonate gas exploration.

Anisotropic characteristics are mainly attributed to fractures in fractured reservoirs. The Dogger formation in the Paris basin is a kind of fractured carbonate reservoir, and the nine-component surface seismic and vertical seismic profiling (VSP) data were acquisited in this area. To characterize the fracture attributes, we analyzed the differential variation of each attribute of the vector wavefield, such as velocity ratio, polarizations, amplitudes and differential travel times. The results described using these parameters had a good match with the overall shear wave anisotropy in the Dogger formation. Two VSP data verified our results. There was about four percent anisotropy in the horizontal direction, which could be interpreted as the porosity and permeability changing along the Dogger layer. The differential shear-wave amplitude from the top of the Dogger shows an overall dimming. There were about six intervals in the shear wave section, which showed that shear wave processes and interpretation provided more information than the P-wave section. The interpretation with shear wave anisotropy showed advantages in revealing detailed lateral variations in fractured reservoirs.

Spectral decomposition based on sparse constrained inversion, as proposed in recent years, is a high-resolution time-frequency domain analysis method. However, the traditional inverse spectral decomposition (ISD) algorithm is based on an inverse wavelet transform (IWT) whose mother wavelet is a Ricker wavelet function instead of an adaptive wavelet, which causes poor continuity and instability of the ISD results. In this paper, we have extended ISD into other linear transforms and have developed an ISD based on inverse S transform (ISD-IST), which has higher continuity and stability due to the more adjustable and adaptable mother wavelet of the S transform. Comparison of conventional methods and the ISD-IST shows that the ISD (ISD-IWT and ISD-IST) methods have higher resolution and accuracy and can provide more reliable time-frequency phase spectra, and it further verifies the greater continuity and stability of ISD-IST over ISD-IWT. Frequency-dependent AVO (FAVO) inversion is regarded as a potential hydrocarbon identification technique (Wilson et al 2009 SEG Technical Program Expanded Abstracts pp 341–5). However, it is frequency-decomposition based, which means that inversion results can be greatly affected by the effect of time-frequency analysis (Wu et al 2010 SEG Technical Program Expanded Abstracts pp 425–9; Wu et al 2014 Geophys. Prospect. 62 1224–37). Combining the FAVO inversion and high-resolution time-frequency decomposition ISD allows better dispersion attributes to be obtained. Results from application in Southwest China demonstrate that the high frequency-resolution of ISD is helpful in the extraction of gas-induced frequency anomalies from post-stack data, and its high time-resolution helps to obtain high time-resolution gas-induced dispersion results from FAVO inversion. The results of FAVO-ISD not only agree better with well information, but also supply more credible boundary information for the reservoirs. ISD-IST can provide more continuous results and smoother boundary information for the gas reservoir than ISD-IWT.

Aiming at the prediction of carbonate fractured-vuggy reservoirs, we put forward an integrated approach based on seismic and well data. We divide a carbonate fracture-cave system into four scales for study: micro-scale fracture, meso-scale fracture, macro-scale fracture and cave. Firstly, we analyze anisotropic attributes of prestack azimuth gathers based on multi-scale rock physics forward modeling. We select the frequency attenuation gradient attribute to calculate azimuth anisotropy intensity, and we constrain the result with Formation MicroScanner image data and trial production data to predict the distribution of both micro-scale and meso-scale fracture sets. Then, poststack seismic attributes, variance, curvature and ant algorithms are used to predict the distribution of macro-scale fractures. We also constrain the results with trial production data for accuracy. Next, the distribution of caves is predicted by the amplitude corresponding to the instantaneous peak frequency of the seismic imaging data. Finally, the meso-scale fracture sets, macro-scale fractures and caves are combined to obtain an integrated result. This integrated approach is applied to a real field in Tarim Basin in western China for the prediction of fracture-cave reservoirs. The results indicate that this approach can well explain the spatial distribution of carbonate reservoirs. It can solve the problem of non-uniqueness and improve fracture prediction accuracy.

For seismic data interpretation, horizon picking based on seismic events in stacked or migrated seismic sections is essential for obtaining information on subsurface structures. This conventional work is time-consuming via manual implementation. In this paper, we develop an automatic seismic event tracking method of horizon interpretation using the dynamic time warping (DTW) algorithm. The proposed method consists of two steps: calculating local time shifts between adjacent traces through a pilot trace and then event tracking. In the method, the DTW algorithm is applied to calculate time shifts between two adjacent traces, and an improved multitrace DTW strategy is proposed to improve the robustness. One synthetic seismic trace is used to demonstrate the DTW algorithm, and a synthetic seismic section is used to verify the feasibility of the proposed method handling contaminated seismic data with noise. Finally, we apply the method to a 3D physical model dataset. The result indicates that the proposed method is quantitatively feasible for seismic event automatic tracking and is reasonably stable for noisy seismic section flattening, which also has the potential to extract seismic horizon slices effectively.

Oil–water discrimination is of great significance in the design and adjustment of development projects in oil fields. For fractured reservoirs, based on anisotropic S-wave splitting information, it becomes possible to effectively solve such problems which are difficult to deal with in traditional longitudinal wave exploration, due to the similar bulk modulus and density of these two fluids. In this paper, by analyzing the anisotropic character of the Chapman model (2009 Geophysics 74 97–103), the velocity and reflection coefficient differences between the fast and slow S-wave caused by fluid substitution have been verified. Then, through a wave field response analysis of the theoretical model, we found that water saturation causes a longer time delay, a larger time delay gradient and a lower amplitude difference between the fast and slow S-wave, while the oil case corresponds to a lower time delay, a lower gradient and a higher amplitude difference. Therefore, a new class attribute has been proposed regarding the amplitude energy of the fast and slow shear wave, used for oil–water distinction. This new attribute, as well as that of the time delay gradient, were both applied to the 3D3C seismic data of carbonate fractured reservoirs in the Luojia area of the Shengli oil field in China. The results show that the predictions of the energy attributes are more consistent with the well information than the time delay gradient attribute, hence demonstrating the great advantages and potential of this new attribute in oil–water recognition.

Seismic anisotropy can help to extract azimuthal information for predicting crack alignment, but the accurate evaluation of cracked reservoir requires knowledge of degree of crack development, which is achieved through determining the crack density from seismic or VSP data. In this research we study the dependence of seismic anisotropy on crack density, using synthetic rocks with controlled crack geometries. A set of four synthetic rocks containing different crack densities is used in laboratory measurements. The crack thickness is 0.06 mm and the crack diameter is 3 mm in all the cracked rocks, while the crack densities are 0.00, 0.0243, 0.0486, and 0.0729. P and S wave velocities are measured by an ultrasonic investigation system at 0.5 MHz while the rocks are saturated with water. The measurements show the impact of crack density on the P and S wave velocities. Our results are compared to the theoretical prediction of Chapman (J App Geophys 54:191–202, 2003) and Hudson (Geophys J R Astron Soc 64:133–150, 1981). The comparison shows that measured velocities and theoretical results are in good quantitative agreement in all three cracked rocks, although Chapman’s model fits the experimental results better. The measured anisotropy of the P and S wave in the four synthetic rocks shows that seismic anisotropy is directly proportional to increasing crack density, as predicted by several theoretical models. The laboratory measurements indicate that it would be effective to use seismic anisotropy to determine the crack density and estimate the intensity of crack density in seismology and seismic exploration.

Azimuthal P-wave amplitude variation with offset (AVO) offers a method for the characterisation of a naturally fractured system in a reservoir. This information is important for the analysis of fluid flow during production of, for example, oil, petroleum and natural gas. This paper provides a modelling scheme by incorporating the squirt-flow model for the prediction of velocity dispersion and attenuation with azimuthal reflectivity method for the calculation of frequency-dependent seismic responses. Azimuthal AVO responses from a fractured poroelastic sandstone layer encased within shale are investigated based on the proposed method. Azimuthal reflections are a combination of the dynamic information including the contrast in anisotropic properties, anisotropic propagation and attenuation within the layer, as well as tuning and interferences. Modelling results indicate that seismic responses from the top of the sandstone layer are dominated by reflection coefficients, and show azimuthal variations at far offset which is consistent with conventional azimuthal AVO theory. Reflections from the base, however, demonstrate complex azimuthal variations due to anisotropic propagation and attenuation of transmission waves within the layer. Tuning and interferences further complicate the azimuthal AVO responses for thinner layer thickness. The AVO responses of top reflections show no azimuthal variations for lower fluid mobility, while those of base reflections show visible and stable azimuthal variations even at near and moderate offsets for different fluid mobility. Results also reveal that it would be practical to investigate wavetrains reflected from the fractured layers that are regarded as integrated units, especially for thinner layers where reflections from the top and base are indistinguishable. In addition, near-offset stacked amplitudes of the reflected wavetrains show detectable azimuthal variations, which may offer an initial look at fracture orientations before AVO analysis.

Shale is observed to have strong transverse isotropy due to its complex intrinsic properties on a small scale. An improved rock physics model has been developed to effectively model this intrinsic anisotropy. Several effective medium theories (Backus averaging, differential effective medium theory and self-consistent approximation) are validated and used in different steps of the workflow to simulate the effects of clay minerals, crack-like pores, kerogen and their preferred orientation on the elastic anisotropy. Anisotropic solid clay is constructed by using different clay mineral constituents instead of assuming it to be an equivalent isotropic or transversely isotropic medium. We differentiate between the voids associated with clay and the voids associated with other minerals based on their varied geometries and their different contributions to the anisotropy. The degree of alignment of clay particles, interconnected pore fluid and kerogen has a great influence on the elastic properties of shale. Therefore, in addition to the pore aspect ratio (asp), a new parameter called the lamination index (LI) related to the distribution of clay particle orientation is proposed and needs to be estimated during the modeling. We then present a practical inversion scheme to enable the prediction of anisotropy parameters for both vertical and horizontal well logs by estimating the lamination index and the pore aspect ratio simultaneously. The predicted elastic constants are demonstrated by using the published laboratory measurements of some Greenhorn shale, and they show better accuracy than the estimations in the existing literature. This model takes different rock properties into consideration and is thus generalized for shale formations from different areas. The application of this model to the well logs of some Upper Triassic shale in the Sichuan basin, and the analyzed results, are presented in part 2 of this paper.

The presence of CO 2 in hydrocarbon reservoirs can cause significant changes in seismic wave properties. In turn these properties can be used to map CO 2 saturation in hydrocarbon reservoirs or aquifers — either from natural sources or by injection from the surface. We present the results of a synthetic modelling study of the effects of supercritical CO 2 saturation on P-wave attenuation in a medium consisting of four horizontal layers, including a target aquifer. The target aquifer is modelled fully by an effective medium containing pores saturated with brine and/or CO 2 and randomly-aligned microcracks at different densities. The other layers are modelled solely by their bulk seismic velocities and densities. We first compute synthetic seismograms for a reference case where the third layer is completely isotropic with no cracks, no pores and no fluid saturation. We then calculate synthetic seismograms for finite crack densities of 0.01, 0.02 and 0.03 at varying degrees of CO 2 saturation in the third layer. The results of our analysis indicate that attenuation is sensitive both to CO 2 saturation and the crack density. For a given crack density, attenuation increases gradually with decreasing percentage of CO 2 saturation and reaches a maximum at around 10% saturation. The induced attenuation increases with crack density and with offset. These observations hold out the potential of using seismic attenuation as an additional diagnostic in the characterisation of rock formations for a variety of applications, including hydrocarbon exploration and production, subsurface storage of CO 2 or geothermal energy extraction.

Summary A well log penetrating the Lower Silurain shale (S1l) in the south of the Sichuan Basin shows that this shale formation represents strong VTI (vertically transverse isotropy) properties. Anisotropy has significant influence on the seismic amplitude variation with offset (AVO) response, so AVO analysis and inversion ignoring the influence of anisotropy can be inaccurate. In this parper, an anisotropic AVO inversion is introduced based on a modified approximation of the PP-wave reflection coefficient for VTI media. Three attributes including the acoustic impedance (attribute A), the shear modulus weighted by an anelliptical anisotropy parameter (attribute B), and the approximated horizontal P-wave velocity (attribute C) can be simultaneously inverted by using a Bayesian-framework-based inversion scheme. A statistical rock-physics workflow is then applied to the inverted attributes for quantitative seismic interpretation. The field-data application on a lower Silurian-age shale-gas reservoir shows the feasibility of both approaches. The shale formation can be discriminated from surrounding formations by using the combination of the attributes A and C, and the gas-bearing shale can be identified through the combination of the attributes A and B.

A novel model combined with fractal theory and the discrete fracture network (DFN) model is proposed to simulate radon migration in fractured media. In the novel model, the fracture center and length distribution are determined by a first-order model, and the fracture orientations are modeled with the von Mises-Fisher distribution. The model is further developed into a computer software that can calculate the radon diffusion coefficient and exhalation rate of fractured media. An outcrop map is used to validate the ability of the model to describe complex fractures, and radon migration in natural fractures at an outcrop site is predicted using the model. Results show that the model has good reliability and robustness. Evaluation of the representative elementary volume (REV) indicates the existence of REV during the modeling of radon migration in fracture networks. Following an exponential law, REV size decreases with the increase in the length exponent.

Natural shale samples, particularly well-preserved, drilled core samples, are extremely difficult to obtain for laboratory research. Multiple tests must be carried out on one sample, and some samples are disposed after destructive tests. Therefore, rarity and non-reusability of samples strongly restrict shale studies. In this study, based on statistical data from the world’s major shale block, a new type of synthetic shale was physically constructed via a process of interfusion, stuffing, and compaction using quartz, clay, carbonate, and kerogen as the primary materials, according to statistical data from the world’s major shale blocks. Further evaluation of the synthetic shale involved the use of scanning electron microscopy imagery and analysis of its anisotropic characteristics in comparison with natural shale. The synthetic shale had a laminated microstructure similar to natural shale, and its velocity anisotropy corresponded to Thomsen’s anisotropy of a transversely isotropic medium. The results of tests for homogeneity and repeatability indicated that the construction process was stable and that several identical synthetic samples, which were satisfactorily similar to natural shale, could be produced for both iterative and destructive tests. The composition of each mineral, as well as the density, porosity, permeability, and anisotropy of the samples, were all variable. Therefore, a series of synthetic samples could be obtained with properties set to meet the requirements of petrophysical experimentation. Moreover, gas or oil saturation was also considered in the construction of the synthetic shale, meaning that the characteristics of gas or oil saturation (or the complete range of data from dry to saturated samples) could be tested using the synthetic shale.

In this paper, we implement three scales of fracture integrated prediction study by classifying it to macro- (> 1/4λ), meso- (> 1/100λ and < 1/4λ) and micro- (< 1/100λ) scales. Based on the multi-scales rock physics modelling technique, the seismic azimuthal anisotropy characteristic is analyzed for distinguishing the fractures of meso-scale. Furthermore, by integrating geological core fracture description, image well-logging fracture interpretation, seismic attributes macro-scale fracture prediction and core slice micro-scale fracture characterization, an comprehensive multi-scale fracture prediction methodology and technique workflow are proposed by using geology, well-logging and seismic multi-attributes. Firstly, utilizing the geology core slice observation (Fractures description) and image well-logging data interpretation results, the main governing factors of fracture development are obtained, and then the control factors of the development of regional macro-scale fractures are carried out via modelling of the tectonic stress field. For the meso-scale fracture description, the poststack geometric attributes are used to describe the macro-scale fracture as well, the prestack attenuation seismic attribute is used to predict the meso-scale fracture. Finally, by combining lithological statistic inversion with superposed results of faults, the relationship of the meso-scale fractures, lithology and faults can be reasonably interpreted and the cause of meso-scale fractures can be verified. The micro-scale fracture description is mainly implemented by using the electron microscope scanning of cores. Therefore, the development of fractures in reservoirs is assessed by valuating three classes of fracture prediction results. An integrated fracture prediction application to a real field in Sichuan basin, where limestone reservoir fractures developed, is implemented. The application results in the study area indicates that the proposed multi-scales integrated fracture prediction method and the technique procedureare able to deal with the strong heterogeneity and multi-scales problems in fracture prediction. Moreover, the multi-scale fracture prediction technique integrated with geology, well-logging and seismic multi-information can help improve the reservoir characterization and sweet-spots prediction for the fractured hydrocarbon reservoirs.

Conventional elastic impedances are derived as scalars by means of the integration of reflectivity. In this sense, they are attributes of the seismic reflection but not the intrinsic physical property of the subsurface media. The derivation of these expressions shares the same assumptions as the reflectivity approximations, such as weak impedance contrast, small angle of incidence, or weak anisotropic media, and thus it limits the accuracy and interpretation capability. The exact P/SV impedance matrices relating the stress and strain represent the mechanical property of the subsurface media and yield reflection coefficients at an arbitrary angle of incidence. We have extended the impedance matrices to a transversely isotropic medium. The resulting elastic impedances (stress/velocity ratios) can be used to characterize those unconventional reservoir formations with strong seismic anisotropy, such as shale-gas and coal-bed methane. Our numerical analyses determined their variations with the phase angle and anisotropy parameters. The exact expressions of the P- and S-wave elastic impedances are used to model the seismic reflection coefficients, and thus they can be inverted simultaneously if all of the types of reflection waves are available. We then derive approximations of quasi-P-wave elastic impedances for seismic inversion of anisotropy parameters and further interpretation. Our applications on real logs and seismic data for a turbidite fan reservoir and a shale-gas reservoir determined the robust interpretation capability of derived elastic impedances in lithology characterizations.

A rock physics model is a very effective tool to describe the anisotropy and mechanical properties of rock from a seismology perspective. Compared to a conventional reservoir, modelling a shale reservoir requires us to face two main challenges in modelling: the existence of organic matter and strong anisotropy. We construct an anisotropic rock physics workflow for a typical shale reservoir in Southwest China, in which the organic matter is treated separately from other minerals by using a combination of anisotropic self-consistent approximation and the differential effective medium method. The standard deviation of the distribution function is used to model the degree of lamination of clay and kerogen. A double scan workflow is introduced to invert the probability of pore aspect ratio and lamination simultaneously, which can give us a better understanding of the shale formation. The anisotropic properties of target formation have been analysed based on the proposed model. Inverted Thomsen parameters, especially the sign of delta, are analysed in terms of the physical properties of rock physics modelling.

The anisotropic?viscoelastic wave equation is a generalized expression of a transversely isotropic?viscoelastic medium. The viscoelastic horizontal?tranverse isotropic (HTI) medium is a special case. Considering the horizontal symmetry axis of viscoelastic HTI media, we develop elasticities and wave equations for preserving its transverse isotropy. To model anisotropic?viscoelastic wave propagation, we apply the fourth-order Runge?Kutta and Fourier pseudospectral method to discretize the wave equation. The convolutional perfectly matched layer (CPML) absorbed boundary condition is applied to the 3D modeling algorithm and the result shows that it absorbs reflected energy efficiently. We present three models to investigate anisotropic?viscoelastic waves. Two half-space models demonstrate the azimuthal attenuation, which appears both on the PP and PSv wave. The quality factor is greater along the fracture direction for both the PP and PSv waves. Analysis shows that frequency-dependent amplitude attenuation behaves differently along the fracture azimuth. This demonstates that we can use this property for reservoir detection.