Theory of CBM AVO: I. Characteristics of anomaly and why it is so

Abstract

Based on the inverse correlations between methane content and elastic parameters of coalbed methane (CBM) reservoirs and on geomodulus approximation of Zoeppritz equations, we study characteristics of CBM amplitude versus offset (AVO) anomaly. Our research first suggests that CBM AVO cannot, and needs not, cling to Poisson’s Ratio as AVO for conventional gas (C-gas) does and that various approximations of Zoeppritz equations for C-gas AVO can be applied to CBM AVO though the elastic contrasts at boundaries of CBM reservoirs cannot be said to be small compared to unity. We then show that the sweet spots within a brine-saturated high- and/or medium-rank CBM reservoir can have only Class IV AVO anomaly. The reason is that all of the three elastic reflectivities, i.e., [Formula: see text] reflectivity, [Formula: see text] reflectivity, and [Formula: see text] reflectivity, at one and the same boundary of a CBM reservoir, are always of the same sign and that [Formula: see text] reflectivity or density reflectivity or both are significant in comparison with [Formula: see text] reflectivity. Finally, we propose that density reflectivity be an indicator of methane content of a CBM reservoir whereas [Formula: see text] reflectivity should be an indicator of permeability.