Estimating Organic Enrichment in Shale Gas Reservoirs Using Elastic Impedance Inversion Based on an Organic Matter−Matrix Decoupling Method

Abstract

The accumulation of organic matter is the basis for gas generation and significantly affects the ultimate gas production in shale reservoirs. Estimation of organic enrichment using seismic data is essential for shale gas characterization. The commonly used correlations between elastic properties and organic matter content for a particular area are locally applicable but may not be workable for other zones. Herein, a general physics-based approach is proposed to predict organic enrichment in shales. An organic matter-matrix decoupling amplitude variation versus offset (AVO) formula is constructed to straightforwardly quantify seismic signatures of organic matter via an introduced organic matter-related factor (M_c). Then, the elastic impedance (EI) function is established from the decoupling AVO formula to compute M_c. The proposed EI inversion method is suitable for capturing organic enrichment, particularly in the case of inadequate petrophysics information for reliable evaluation of M_c using log data as a constraint in the inversion. The developed AVO formula and EI function regard the organic matter as solid pore-fillings, presenting a more reasonable model for organic shales. Numerical tests show that M_c exhibits enhanced sensitivity to organic matter content with respect to the regularly used elastic properties. The real data applications indicate that the estimated M_c agrees well with the gas production in horizontal development wells, suggesting that M_c is a good indicator of favorable gas areas. The proposed approach may have broader potential applications and can be extended to detect other fluids and solid-saturated hydrocarbon reservoirs such as shale oil, heavy oil, and gas hydrates.