IN this First Break article form November 2024, TGS experts David Went*, Jon Rogers and Felicia Winter demonstrate that lithology has a very strong impact on the amplitude variation with offset or angle (AVO/AVA) response and that shale and brine sand responses need to be identified with confidence before any prognosis of a hydrocarbon signature is made.
Abstract
Well and seismic data from the Leonian and Liberian basins confirm the presence of a working petroleum system, with syn and post-rift Cretaceous intervals containing excellent source rocks, reservoir sands and sealing shales. Rock property studies conducted on well log data indicate that lithology (sand versus shale) has a strong impact on amplitude variations with offset or angle (AVO/AVA).
Elastic seismic inversions confirm the observations made from well logs. The key learning is that shale and brine sand responses need to be clearly identified before any prognosis of an AVO anomaly resulting from the presence of hydrocarbons is made.
Introduction
The deep waters of the Atlantic margin have proven to be a good place to explore for hydrocarbons in recent years with large commercial discoveries having been made, for example, in Brazil, Guyana, Ghana and Namibia (Figure 1a). Most discoveries have been made in siliciclastic plays in stratigraphic traps (e.g. Daily et al 2013, Hedley et al 2022). In these plays, there is typically a heavy reliance on seismic interpretation to define traps and on amplitude analysis, including amplitude variation with offset or angle (AVO/ AVA), to suggest hydrocarbon presence. Well data typically plays an important part in calibrating seismic responses. However, in newly emerging plays it is commonly scarce, so it is important to maximize the learnings from the existing well data to improve subsurface interpretation and increase the chances of commercial success.
The purpose of this article is to show the results of a rock property study and seismic inversion workflow conducted on the offshore West African Leonian and Liberian Basins (Figure 1b). Seismic rock properties determined from well logs provide an important link to interpretation of seismic responses in the subsurface (Castagna et al 1993). Rock properties determined from well logs in the Leonian and Liberian basins highlight that lithology has a very strong impact on the amplitude variation with offset or angle (AVO/AVA) response and a key learning is that shale and brine sand responses need to be identified with confidence before any prognosis of a hydrocarbon signature is made. Armed with this understanding, material exploration opportunities may be identified, each of which has the potential to transform this region from a frontier system to a commercial hydrocarbon province.
Read the full article HERE.