Seismic characterization of source rocks: Why should it be assessed on a site-by-site basis?

Abstract

Seismic characterization of source rocks (SRs) became widely used in exploration risk assessment, partly driven by the evaluation of petroleum systems conditioned by the presence of organic-rich SRs. Generally, rock physics combined with seismic amplitude variation with offset (AVO) analysis and inversion of seismic data are used to detect SR presence and to assess SR lateral and vertical variations measured in total organic carbon (TOC) content (in weight percent [wt%]). Despite its great potential, this method suffers from a range of pitfalls and uncertainties. In this study, based on several data sets, we highlight the variability of seismic responses of SRs. From well data, rock property studies of SRs show that the relation between acoustic impedance, which is the product of density and P-wave velocity, and TOC turns out not to be representative in SRs with TOC contents less than approximately 4–5 wt%. In the screening phase of rock-physics data, SR also reveals a large range of Poisson's ratio values, which relates to P-wave and S-wave velocities and has a direct impact on AVO. Moreover, from real seismic data, AVO analysis gave support for this complex behavior, highlighting AVO class I, III, and IV anomalies. Therefore, the expectation that the top of SR intervals would feature a “clear dimming with offset” (AVO class IV) should not be generalized for SR identification, especially in frontier areas lacking nearby well calibration, as suggested by the results of this study.