Jianhua Geng

3D Kirchhoff prestack time migration in average illumination-azimuth and incident-angle domain for isotropic and vertical transversely isotropic media

Conventional offset domain prestack migration tends to bring ambiguity and migration artifacts because it smears energy from different angles at the image point. To avoid this, prestack depth migration implementations in angle domain have been investigated in the past decades. As an efficient imaging tool, angle domain Kirchhoff prestack time migration is still useful and was proposed recently. However, existing algorithms cannot handle ray bending and anisotropy correctly. Practically, azimuth analysis for fractured reservoirs should be carried out after migration for most geological settings. Unfortunately, the existing migration algorithm implicitly involves some kind of binning to source-receiver azimuth, which may not be the real wave-path azimuth, especially for side-scattering or out-of-plane waves. In this paper, we present an algorithm for 3D Kirchhoff prestack time migration in average illumination azimuth and incident angle domain, which matches true wave path naturally and more accurately. To handle ray bending and vertical transversely isotropy, we propose several approaches to estimate two-way traveltime and the corresponding angular attributes through extended offset-to-angle mapping. Based upon these approaches, our 3D prestack time migration can provide high-quality common-image gathers for amplitude variation with incident angle and/or amplitude variation with offset and azimuth analyses, even in media with slight to moderate lateral heterogeneity. The 2D and 3D synthetic examples prove the validity of our methods.

Frequency‐ and angle‐dependent poroelastic seismic analysis for highly attenuating reservoirs

We extend the frequency- and angle-dependent poroelastic reflectivity to systematically analyse the characteristic of seismic waveforms for highly attenuating reservoir rocks. It is found that the mesoscopic fluid pressure diffusion can significantly affect the root-mean-square amplitude, frequency content, and phase signatures of seismic waveforms. We loosely group the seismic amplitude-versus-angle and -frequency characteristics into three classes under different geological circumstances: (i) for Class-I amplitude-versus-angle and -frequency, which corresponds to well-compacted reservoirs having Class-I amplitude-versus-offset characteristic, the root-mean-square amplitude at near offset is boosted at high frequency, whereas seismic energy at far offset is concentrated at low frequency; (ii) for Class-II amplitude-versus-angle and -frequency, which corresponds to moderately compacted reservoirs having Class-II amplitude-versus-offset characteristic, the weak seismic amplitude might exhibit a phase-reversal trend, hence distorting both the seismic waveform and energy distribution; (iii) for Class-III amplitude-versus-angle and -frequency, which corresponds to unconsolidated reservoir having Class-III amplitude-versus-offset characteristic, the mesoscopic fluid flow does not exercise an appreciable effect on the seismic waveforms, but there exists a non-negligible amplitude decay compared with the elastic seismic responses based on the Zoeppritz equation.

Azimuth-preserved Local Angle Domain Imaging and Its Application to Prestack Time Migration

Employing 3D prestack migration in azimuthal analysis allows for some structural dip and provides for higher signal to noise ratio in the measurements. Standard common-offset migration does not preserve azimuthal information. Recent attempts to move this analysis into migrated space concern source-receiver azimuth and offset on the surface, and generally involve some form of data sectoring. In fact, surface-azimuth and offset may be poor representations of the direction of wave path in the subsurface. Sectored migration may produce noisy prestack images owing to limited fold within the sectors and often requires large sectors leading to poor statistics for analysis of anisotropic properties. To tackle these problems, we present an azimuth-preserved local angle domain imaging approach that honors the local directional characteristics of the subsurface wave path in isotropic and vertical transversely isotropic media. The example has shown the advantage of our azimuth-preserved local angle domain Kirchhoff prestack time migration algorithm for azimuthal analysis.

Combined Bayesian AVO inversion with rock physics to predict gas carbonate reservoir

Summary A better understanding of fluid effects on carbonate rock properties is considered to be key issue in applying AVO inversion to carbonate reservoir characterization. Analysis of well log data in a gas carbonate reservoir of Southwestern China indicates that gas does influence carbonate’s elastic rock properties, and low VP/VS ratio and low acoustic impedance can be treated as calibration for the interpretation of seismic inversion result. We invert prestack seismic data using Bayesian linearized AVO inversion to estimate elastic properties and assess their uncertianty. We also show how to combine a credible seismic inversion result with rock physics analysis to identify gas carbonate reservoir.

An Effective Reservoir Parameter for Seismic Characterization of Organic Shale Reservoir

Sweet spots identification for unconventional shale reservoirs involves detection of organic-rich zones with abundant porosity. However, commonly used elastic attributes, such as P- and S-impedances, often show poor correlations with porosity and organic matter content separately and thus make the seismic characterization of sweet spots challenging. Based on an extensive analysis of worldwide laboratory database of core measurements, we find that P- and S-impedances exhibit much improved linear correlations with the sum of volume fraction of organic matter and porosity than the single parameter of organic matter volume fraction or porosity. Importantly, from the geological perspective, porosity in conjunction with organic matter content is also directly indicative of the total hydrocarbon content of shale resources plays. Consequently, we propose an effective reservoir parameter (ERP), the sum of volume fraction of organic matter and porosity, to bridge the gap between hydrocarbon accumulation and seismic measurements in organic shale reservoirs. ERP acts as the first-order factor in controlling the elastic properties as well as characterizing the hydrocarbon storage capacity of organic shale reservoirs. We also use rock physics modeling to demonstrate why there exists an improved linear correlation between elastic impedances and ERP. A case study in a shale gas reservoir illustrates that seismic-derived ERP can be effectively used to characterize the total gas content in place, which is also confirmed by the production well.

Petrophysical Characterization of Pore Type in Tight Gas Carbonates of Southwestern China

Exploration and production in tight gas carbonates reservoir requires a comprehensive petrophysical characterization of rock microstructure in low-porosity and low-permeability tight carbonates. The complex pore system in tight gas carbonate makes pore shape must be taken into account for estimating elastic properties in rock physics modeling, differential effective medium(DEM) theory can be used to study the relationship between changing pore geometries and elastic properties. In this article, detailed rock physics modeling steps was put proposed, and on this basis pore type inversion algorithm was developed according to the concluded velocity-porosity relationship. Finally, Geological knowledge, core data, and well log data are integrated to discuss the pore shape effect on elastic properties of carbonate rocks in tight gas carbonates reservoir of Southwestern China, and the given pore type inversion result fits well with features of pore geometry which was revealed by thin section of cores and FMI image of local area, indicating this method can be can be effective to characterize complexity of pore system in tight gas carbonates.

DSR equation prestack tau migration in heterogeneous media

Prestack migration methods based on the double-squareroot (DSR) equation are modified to operate in the two-way vertical traveltime (tau) domain in this paper. Unlike the traditional time imaging algorithms in which at most mild lateral variations of velocity are treated, the DSR equation tau migration approach includes reasonable treatment for media with strong lateral inhomogeneity. To address the problems that the full 3-D DSR equation prestack tau migration could meet in practical applications, we present a method for downward continuing common-azimuth data in the theoretical frame of cross-line common-offset migration. Migration of the Marmousi model proves that it works well in strong contrast media. The real data example illustrates the validity of common-azimuth prestack tau migration in production applications.