Seismic AVO Analyses and the Impact of Anelastic Attenuation and Seismic Tuning - A Case Study from North Sea, close to Well 2/4-14
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There are verified thin sandstone layers saturated with oil and gas condensate at shallow depths in Block 2/4, Central North Sea. Gas-saturated sandstone layers are known to be good targets for AVO- analyses (Amplitude versus Offset). AVO is a (semi)-quantitative method used to distinguish seismic reflectors representing gas-saturated sandstone from other lithologies and fluid contents. However, noise including other wave propagation effects can bias the AVO-responses, there are thus many pitfalls related to AVO. This study presents and compares real seismic AVO-responses and a synthetic AVO-response of a thin gas-saturated sandstone layer. The main focus of the study is the impact of seismic tuning and anelastic attenuation on AVO-analysis. We believe these two wave propagation effects have a significant impact on the seismic AVO-response. Considering the synthetic model, we used Zoeppritz equation in order to estimate the reflection coefficient as a function of P-wave angle of incidence, where the acoustic properties were decided based on well logs. Anelastic attenuation was estimated trough a constant-Q model, and seismic tuning trough a wedge model. For the seismic AVO-response, we present four AVO-analyses of the same reflector in Block 2/4 close to well 2/4-14 using seismic data acquired from four different sail lines. The amplitudes were fitted to an approximation of Zoeppritz equation described by Swan , Aki and Richard  and further stated in Castagna et al. . The equation is written in terms of intercept(A), gradient(B) and curvature (C). The results of the AVO-analyses show a negative gradient of different magnitude, and a pronounced positive curvature-term. The results from the synthetic AVO-response show that both seismic tuning and anelastic attenuation decrease the magnitude of the amplitude from near to far offsets. In order to quantify the effects, the ratio of the tuning factor near to far offset was calculated. The tuning ratio is in the range [1.02, 1.07]. Multiples are excluded from the analysis. Similarly, the ratio of the anelastic attenuation term at near to far offset is in the range [1.15, 1.22], for a Q-factor equal to 100 and 70, respectively. Both estimations assumed a dominant frequency of 50Hz. Considering a lower frequency of 40Hz, the seismic tuning had an increased impact (ratio of 1.13), whereas the anelastic attenuation effect decreased (ratio of [1.12, 1.17]). The anelastic attenuation and seismic tuning do contribute to the decrease in the magnitude of amplitude observed at greater offsets. The synthetic amplitudes increased slightly from P-wave angle of incidence 10 - 30degrees when both anelastic attenuation and seismic tuning were accounted for. There are also suggested other effects that impact the response of the reflection coefficient with offset; NMO-stretch, velocity errors, anisotropy, multiples or most likely, a combination of them all.