Class IV AVA Anomalies in High TOC Shale, a Case Study from Namibia, West Africa

This abstract, written by Spectrum’s David Eastwell, was originally submitted for the EAGE 2019 Convention held in June 2019.

Amplitude vs Angle (AVA) analysis has been practically utilized in oil exploration since the 1980s after Richards and Frasier 1976, and Aki and Richards 1980 approximated the Zoeppritz equations which calculate the partitioning of seismic wave energy at a physical boundary as a function of incidence angle. AVA testing now forms a key component of the identification of potential hydrocarbon drilling prospects and risk assessments thereof. AVA has become a staple component of the exploration process and many rock physics models and workflows exist within the public domain and within Oil companies.

In 2011 a study was published by Loseth et al which began to explore the possibility that shales or “Thick <20m Claystones” exhibit a number of seismic characteristics which could be used to assist interpreters in the identification and characterization of marine source rocks on seismic data in the absence of wells. This study was broadly based on the observation that kerogens have approximately half of the density of otherwise non organic shales for an equivalent volume, and therefore increases in TOC (Total Organic Carbon) reduces the bulk rock density and therefore Acoustic Impedance of a rock non-linearly as shown in Figure 1.

Figure 1: The relationship between Total Organic Carbon % and Acoustic Impedance. From Loseth et al 2011.

One characteristic of shale source rocks proposed by Loseth et al 2011 was that the top and base of a thick high total organic content (TOC) claystone should show a class 4 AVA response. Class 4 AVA anomalies show a decrease in amplitude with increasing reflection angle. This is distinctly different to the conventional use of AVA which looks for increases in amplitude with increasing offset as an indicator of hydrocarbons within reservoir rocks. Class 4 AVA responses are relatively uncommon in clastic sequences and the confirmation of the presence of one can be a quantitative indicator of source presence.

Subsequent publications including Avseth and Carcione 2015 have expanded upon the initial work by Loseth et al and demonstrate the relationship between rock properties such as Acoustic Impedance (AI), Shear Impedance (SI) and Vp/Vs ratios amongst others, with TOC and other shale rock properties. Similarly, Yengu and De-hua Han 2013 calculate the effect of maturity within the Bakken Shale onshore USA and the predicted p wave reflectivity (R) (Figure2) as a function of angle. These studies give credit to the statement that High TOC claystones will exhibit a class 4 AVA response, and that AVA attributes and derivative attributes such as Gradient, Intercept and Vp/Vs ratio should vary as a function of both TOC and maturity due to the changes in rock properties as a result of variations in kerogen content, fluid content and porosity.

 

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Class IV AVA Anomalies in High TOC Shale, a Case Study from Namibia, West Africa