Wave velocity in shale in the Wufeng-Longmaxi formation in Sichuan Basin

Analyzing the pattern of wave velocity is important for velocity prediction and amplitude-versus-offset analysis during unconventional seismic exploration. Several parameters, including the maturity and microstructure of organic matter (OM), the wave velocity, the mineral composition and the density of shale from Upper Ordovician Wufeng and Lower Silurian Longmaxi, formations were experimentally evaluated in this study. To investigate the factors influencing the wave velocity, the obtained data were analyzed together with numerical calculations and previous experimental results. A theoretical model of an anisotropic differential equivalent medium (DEM) was then established and the following conclusions were drawn: (i) different from Bakken shale, OM in the Wufeng-Longmaxi (WL) shale is over-mature and patchy, with higher elastic parameters for the kerogen; (ii) different from other areas, the amount of kerogen (being over-mature) in WL shale is not correlated with the wave velocity; (iii) bogenic quartz minerals were abundant in the tested samples, and their amount was positively correlated with OM, whereas OM was negatively correlated with the amount of clay; (iv) clay, quartz and kerogen contents had different association with the rock skeleton, resulting in a V-shaped relationship between the quartz content and wave impedance, with the slope of the growing branch being higher than that of the decreasing branch and (v) the anisotropic DEM model effectively proves the influence of OM maturity and patch shape on shale velocity. Also, the V-shaped relationship between quartz and wave impedance caused by skeleton change are verified. This study provides valuable data as well as a theoretical basis for seismic interpretations in the study area.