Vp-Vs Relations of Organic-rich shales

Organic-rich shales can serve as both source rocks and reservoir rocks. They are becoming increasingly important exploration and exploitation targets; however our knowledge of organic-rich shale properties is poor. Mudrock line (Castagna et al., 1985) suggests that Vp/Vs ratios of shale are around 2, but Vp/Vs ratios of organic-rich shale vary from 1.5 to 1.7. What remains to be studied is what factors impact this difference. Understanding Vp-Vs relations for organic-rich source rock is vital for seismic characterization of shale reservoirs. This thesis explores several issues related to Vp-Vs relations of organic-rich shale, such as Vp/Vs ratio, P-wave and S-wave velocity anisotropies, and three Poisson’s ratios in a TI medium.

Parameters that affect the Vp/Vs ratio in organic-rich shale are studied in two parts: mineralogy and organic matter. First, we studied Vp/Vs ratios, P-impedance, and velocity anisotropies in three types of organic-rich shales: silica-rich, clay-rich, and calcareous shales. Vp/Vs ratios change with mineral composition. A simple two-layer model built with the Backus average is used to explain why silica-rich shale has the highest shear anisotropy and calcareous shale has the highest P-wave anisotropy among the three types of organic-rich shale. Well logging data are utilized to separate the effects of quartz, clay, and calcite on Vp/Vs, P-impedance, and density. Second, total organic carbon (TOC) and maturation of organic matter significantly affect shale properties, because of a large contrast between elastic properties of the organic and inorganic components. Basin and maturation models are built to simulate hydrocarbon generation during organic-rich shale evolution, which provides us the fundamentals for analyzing products and their concentrations in different maturation stages. The effect of organic matter with different maturity levels on immature, mature, and overmature shale properties are studied with rock physics models, which are based on effective medium theory. The immature and mature shale modeled results are plotted with the Bakken shale samples, which increase the credibility of the overmature model. The modeling results suggest that higher maturity level and TOC lead to lower Vp/Vs ratio and stronger anisotropy. When TOC is high, maturity level dominates; when TOC is low, TOC dominates.