Inference of Organic Carbon Content of Shale From Zoeppritz PP-AVO Inversion: Case Study of Avalon Shale, Delaware Basin

Abstract

Accurate source rock characterization is increasingly important since unconventional shale serves as a significant energy resource. In particular, estimation of total organic carbon (TOC) content of shale has direct influence on the well placement, analyzing quality of unconventional reservoirs. We estimate organic carbon content of Avalon shale in Delaware Basin from density values obtained by an amplitude variation with offset (AVO) inversion. The estimation is based on an empirical relationship between kerogen volume fraction and density of the shale. An analysis of measured well logs and laboratory data for the formation shows that density is the geophysical parameter most strongly correlated with organic carbon content compared to other invertible geophysical parameters from seismic data. For an accurate density inversion, we apply a nonlinear AVO inversion using full Zoeppritz equations for P-wave reflection amplitudes instead of using its linearized approximations. With a near angle range, the inversion can reliably predict correct density unlike conventional linearized AVO inversion. With a far angle range, the inversion underestimates density, with an error that depends on the degree of anisotropy of P-wave velocity. However, the underestimated density can be corrected, once the degree of anisotropy of P-wave velocity is determined. Since the inversion accurately determines horizontal and vertical P-wave velocities when a far angle range and a small angle range are applied respectively as input data, the anisotropy degree can also be determined. Consequently, organic carbon content of the shale can successfully be estimated from both inverted and corrected densities.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019