Exploration Geophysics - Volume 51, Issue 3, 2020

In the fields of oil and gas exploration, various quantitative seismic interpretation techniques have been developed to identify lithology and pore fluids based on the rock physics. However, discrimination between oil and brine in pores is often difficult because the seismic response of oil and brine are similar. Poisson impedance (PI) is an effective indicator for distinguishing oil from brine in pores. Target correlation coefficient analysis (TCCA) is widely used to calculate the “c” factor of PI. However, PI analysis using the TCCA method can lead to misinterpretation when the resistivity log is affected by lithology and porosity, as well as the pore fluid type. To overcome this problem, we developed a more effective PI workflow composed of three steps that focus on lithology impedance, porosity impedance, and fluid impedance. In each step, we exclude the unnecessary area for hydrocarbon exploration such as shale, tight, and brine-saturated areas. We applied our method to marine field data that contained shale and tight intervals and demonstrated that the proposed approach can be used to identify potential oil reservoirs more effectively from seismic data.

Full-waveform inversion (FWI) is one of the most promising inversion methods in geophysics due to its theoretical completeness and high resolution. However, the inversion ability of FWI strongly relies on the accuracy of the initial model and the quality of the low-frequency data. For FWI, it is important to accurately recover low and middle wavenumber components (the background model). In order to invert for the background model, we use a new strategy to compose the plane-wave using reference source points. With the new algorithm, more plane-waves can be composed for a single ray parameter to obtain the background model by plane-wave multi-scale full-waveform inversion (PMFWI) method. By controlling the ray parameter in this method, the low to middle wavenumber components can be recovered in sequence. Composing several plane-waves for a single ray parameter enables the illumination to be significantly improved, even for incomplete data. As only a small number of plane-waves are needed in this method, the computation burden greatly decreases. Analysis of numerical tests also verifies that the proposed inversion strategy is robust, to a certain extent, for high-frequency or noisy data. Application of this method on a modified portion of a SigsBee 2A model illustrates that, combined with conventional FWI, PMFWI has suitable model accuracy, even for coarse initial model data and high-frequency data.

Reservoir characterisation using a crossplot of elastic properties can be used to determine fluid and lithology in a seismic survey area. P-impedance and P-wave/S-wave ratios are commonly used as axis parameter for a 2D crossplot. To achieve this goal, the fluid and lithology must first be identified using well log data. However, when the well log data are too sparse, they cannot encompass the full suite of reservoir properties. Stochastic forward modelling (SFM) methods have been developed to overcome the sparseness problem. However, when the results of SFM are plotted on the 2D crossplot, the augmented data for different facies frequently overlap. To overcome this problem, we propose a probabilistic reservoir characterisation using 3D crossplotting of the SFM results. Axis-parameters of the 3D crossplot consist of the seismic attributes by which the facies are distinguished. The acoustic impedance (Ip), pseudo gamma ray (GR) log, and pseudo water saturation (Sw) log were used as the axis parameters of the 3D crossplot. To perform SFM, pseudo GR and pseudo Sw log data must be expressed mathematically with well log data. Linear multi-regression analysis was used to derive the mathematical relationships of the different parameters. The probability distributions of the pseudo GR and pseudo Sw logs were extracted using these relationships. Using the probability distributions of the Ip, pseudo GR log, and pseudo Sw log, the data were augmented by Monte Carlo simulation. The trivariate probability density function (3D PDF) of each facies was determined by the mean and covariance of the augmented data. The pseudo GR log and pseudo Sw log volumes were extracted using a probabilistic neural network. Finally, a Bayesian inference was applied to calculate the facies probabilities using the 3D PDFs. We confirmed that the proposed method is more effective than the conventional reservoir characterisation method using 2D crossplot of SFM results.

To improve the efficiency of seismic waveform inversion, we construct a novel high-order (6th-order) frequency-domain nearly analytic discrete method for forward modelling in inversion processes. Compared with some existing numerical schemes, this method is more powerful in suppressing numerical dispersion and enhancing the accuracy of computational results based on coarse discrete grids. We describe the discretisation of the frequency-domain wave equation, investigate the efficiency of wave-field simulation and perform numerical dispersion analysis. The inversion efficiency can also be improved when the inexact rotated block triangular preconditioned Krylov iteration solvers are incorporated into the corresponding linear system. Moreover, the inversion behaviour of frequency-domain NAD methods is also systematically discussed as one of its important applications. We take three representative media models as examples and the reliable inversion results are obtained reflecting the effectiveness of the proposed methods.

Because a vertical seismic profile (VSP) survey includes the installation of geophones into a borehole, the results of data processing are influenced by the deviation error of the borehole trajectory. In particular, when full waveform inversion (FWI) is applied to VSP data, receiver location errors distort the results of velocity inversion. Under this circumstance, we attempt to estimate the exact locations of receivers and resolve the distortion of the inverted velocity model by adding receiver locations to the inversion parameters. This makes the FWI process a multi-parametric inverse problem. In this case, it is desirable to invert each parameter group alternatively. Because receiver coordinates depend on borehole trajectory, the inverse problem is regularised by adding a term measuring the curvature of the borehole trajectory gradient to obtain a physically meaningful solution. In addition, we propose a grid search method to improve the estimation of receiver coordinates. In the case of VSP data, the inversion of receiver coordinates using the grid search method is more efficient and robust than using a local descent method. Through numerical examples, we verify that the proposed inversion algorithm can improve FWI results and mitigate velocity distortion. We also demonstrate that the convergence of FWI can be improved by applying the grid search method to the estimation of receiver coordinates.

We evaluated imaging alteration associated with porphyry copper style mineralisation using aeromagnetic data in the Highland Valley Copper (HVC) district as part of the Natural Sciences and Engineering Research Council of Canada (NSERC) and Canada Mining Innovation Council (CMIC) Footprints project. The first step of the investigation involved construction of a 3D geological model based on surface and regional information, and petrophysical data of the host rocks. From the petrophysics, it was observed that hydrothermal alteration affected the magnetic susceptibility of some batholithic host rocks at HVC, and low susceptibility values were associated with the altered rocks. From this observation, an alteration index was devised, differencing, based on the magnetic susceptibility, between unaltered and altered batholith host rocks. Three-dimensional inversions were conducted using geological constraints obtained from surface and borehole geology as well as physical property data. Then the alteration index methodology was tested using 3D inversions and residual models within five overlapping blocks covering portions of the batholith. The indices inferred from inverted susceptibility were compared to surface mapped alteration. For some felsic facies of the batholith, the alteration index derived from inverted magnetic susceptibility corresponds to the mapped alteration and delineates the known porphyry Cu system district. However, it is not true for others where alteration was less magnetite-destructive and more focused in structures.