Elastic anisotropy and its influencing factors in organic-rich marine shale of southern China

Abstract

Shale is observed to have strong anisotropy due to its unique mineralogy and microstructure, and this anisotropy property has significant impact on seismic and well-log data. The organic-rich marine shale in the southern and eastern Sichuan Basin is one of the most important shale-gas reservoir formations in China. To investigate the elastic anisotropy of this shale and its influencing factors, we performed ultrasonic velocity measurements, X-ray diffraction analysis, rock-eval pyrolysis and vitrinite reflectance measurement on the samples from the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation. The experimental results show the that: (1) the velocity anisotropy of the Wufeng-Longmaxi (WL) shale varies from 10% to 50%, and most samples have strong anisotropy; (2) the P- and S-wave anisotropy parameters (Thomsen’s e and γ) increase with clay contents, but this relationship can be greatly affected by the clay orientation index; (3) organic matter content (OMC) is found to have little influence in seismic anisotropy for the over mature WL shale, whereas the OMC determines the magnitude of anisotropy of immature/mature shales (e.g. the Bakken shale or the Bazhenov shale) according to the published literatures, because organic matters in shales of different maturity have different morphologies and distributions; (4) the OMC of WL shale has positive correlation with quartz content, and this weakens the correlation between OMC and the magnitude of anisotropy to a certain extent. The results of this study provide an important rock-physics basis and data support for seismic anisotropy exploration, quantitative interpretation and resource evaluation of the organic-rich marine shales in southern China.