Reservoir characterization via amplitude versus offset analysis and impedance inversion in the Thrace Basin of northwest Turkey



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The fundamental parameters in seismic reservoir characterization are P-wave, S-wave velocities, and density. Understanding the relation of these parameters with the pore-fluid content and lithology is the key for this research. The purpose of the study is to detect and outline a hydrocarbon-saturated reservoir in the Thrace Basin of NW Turkey, which is primarily gas-prone, using four 2-D long-offset reflection lines and conventional well logs data that were acquired by Turkish Petroleum (TPAO). The major means are amplitude versus offset analysis and impedance inversion. In addition, petrophysical analysis is performed to understand and determine the rock properties including porosity, lithology and pore fluid content. Well-2 is interpreted, and reservoir interval is discovered in the depth range of 878-899 m. According to the available petrophysical data, the reservoir consists of high porosity shaly-sandstone with 38% water saturation. Seismic data is processed through Kirchhoff Time Migration to image subsurface geology with an optimum resolution. The processing flow is designed to preserve true amplitude which is required for satisfactory amplitude versus offset (AVO) analysis. AVO modeling is performed to understand the AVO response differences between brine and gas saturated cases. In the absence of measured shear wave velocity, it is predicted using primary wave velocity from sonic log based on Gassmann fluid substitution and Castagna equations. Furthermore, AVO analysis is carried out using NMO-corrected CDP gathers at Well-2 location to discriminate gas reservoir from background lithology. Class III type of AVO Anomaly which is known as “classic bright spot” is observed demonstrating amplitude is increasing as offset increases for both the top and base of the reservoir. Acoustic impedance inversion has been applied to characterize the rock properties of the reservoir zone. The reservoir interval is distinguished as a zone of relatively low-impedance and information regarding lateral extent of the reservoir sand is obtained, as well. Elastic impedance inversion using range-limited angle stacks demonstrates an increase in amplitude with offset/angle indicating that although the anomalous amplitude can be seen at the near offset stack, the reservoir is brightening and is more visible at far offset.



Reservoir characterization, Amplitude versus Offset (AVO) Analysis, Impedance inversion, Thrace Basin