Organic-rich shale formations have unique properties that significantly differ from conventional formations because of low permeability, anisotropy and multiscale heterogeneity. These properties are the results of lithology variation, depositional process, diagenesis, fabric and thin lamination, pore structure as well as the distribution and maturity of the organic matter which influence the rock properties and wave velocities. In this paper, the influence of rock composition, organic matter (TOC) and orientation of formation lamination on acoustic velocity in organic-rich shales was experimentally evaluated. A correlation was developed describing the dependence of the compressional wave velocity on stress, rock composition, organic matter, rock lamination orientation as well as the fluid composition.
After its development using core-scale measurements of the preserved shale core samples in the laboratory, the correlation was modified to include the fluid saturation using well log data in order to obtain more accurate in situ estimation of the compressional velocity. The correlation presented here can be utilized to predict velocity, TOC, and orientation of the formation lamination or potentially the in-situ pore pressure when one of the variables are unknown. The results of this study can be utilized in economic planning and optimization of the field development for organic-rich shale source rocks. Further study is in progress to verify the validity of the correlation in other shale formation field applications as the correlation presented here is specifically developed and customized for Eagle Ford shale formation.
Acoustic velocity is in general controlled by stress (Wyllie et al. 1958), lithology (Kenter et al. 1997), rock texture (Wyllie et al. 1956; Jizba 1991; Tutuncu et al. 1993; Guadagno and Nunziata 1993; Eberli et al. 2003), rock fabric (Podio et al. 1968; Nur and Simmons 1969; Vernik and Nur 1992), burial diagenesis (Schoonmaker et al. 1985; Kenter et al. 1997), pore fluid(s) type and saturation (Wang et al. 1990; Liu et al. 1994), temperature (Timur 1977; Vernik and Nur 1992), as well as signal frequency and amplitude (Tutuncu et al. 1994; 1997 a, b). In this study, several of these factors (mainly stress, rock compositions, TOC, fluid compositions and rock lamination orientation) have been evaluated with an objective of developing a correlation to understand the influence of these factors on compressional velocity in organic-rich shales.
