Phenomenological models for estimating and constraining c13 for transversely isotropic hydrocarbon source rocks

Abstract

Hydrocarbon source rocks can be adequately approximated as transversely isotropic (TI) media. The elastic properties of a TI medium are defined by five stiffness parameters: [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text]. The laboratory estimation of [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] is straightforward, with each of the stiffness parameters determined by a single velocity measurement in an orthogonal direction. For [Formula: see text], we need the information of [Formula: see text], [Formula: see text], [Formula: see text], and at least one oblique velocity. Consequently, it is usually more difficult to estimate [Formula: see text] than the other stiffness parameters in the laboratory, and it is even more challenging to acquire it from the field. Therefore, it is important to find the relations between [Formula: see text] and the other stiffness parameters so that [Formula: see text] can be estimated from the orthogonal elastic tensor elements [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] that can be much more economically and reliably acquired. There are phenomenological models for estimating [Formula: see text] from the other stiffness parameters, but their accuracy is not always satisfactory. We find that [Formula: see text] has strong correlations with [Formula: see text] and [Formula: see text], and that [Formula: see text] generally underestimates [Formula: see text] and [Formula: see text] generally overestimates [Formula: see text]. The average of [Formula: see text] and [Formula: see text] can be a much more precise phenomenological model to approximate [Formula: see text]. We also find that [Formula: see text] is generally greater than [Formula: see text] for hydrocarbon source rocks. Therefore, we determine that [Formula: see text] should lie between [Formula: see text] and [Formula: see text] for hydrocarbon source rocks.