Abstract
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.
Highlights
Shales represent intrinsic elastic anisotropy that is much stronger than other sedimentary rocks
This paper focuses on the shale-gas reservoir characterization by using an amplitude variation with offset (AVO) inversion method for vertical axis of symmetry (VTI) media and a statistical-rock-physics-based quantitative interpretation method
The AVO inversion for VTI media is based on a modified anisotropic PP-wave reflection coefficient consisting of three attributes
Summary
Shales represent intrinsic elastic anisotropy (transverse isotropy with vertical axis of symmetry [VTI]) that is much stronger than other sedimentary rocks. Vernik and Nur (1992) find that shale anisotropy is attributed to the presence of organic matter. Hornby et al (1994), Lonardelli et al (2007), and Wenk et al (2007) suggest that such intrinsic anisotropy is mainly caused by preferentially orientated clay platelets and cracks. Porosities of clay-bound water, free water, and free gas, and organic matter content are used to model all five effective elastic constants of the VTI shale by using a combination of varied anisotropic effective-medium theories, e.g., Backus averaging, self-consistent approximation theory, and the differential effective medium theory. Zhang (2019) analyzes major factors of intrinsic anisotropy of the Lower Silurian shale: (1) The magnitude of anisotropy is mainly determined by clay contents and increases with the clay-bound water porosity, and (2) the presence of organic matter and pores of free water/free gas has a negative contribution to anisotropy of this overmature shale because the former are observed to distribute randomly within the rock matrix, and the latter have varied geometry and a wide range of orientations.
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