Log Quality Assessment and Data Correction for AVO

Abstract

AVO modelling requires the relationships of porosity, shale, water saturation, mineral and fluid properties plus P and S-wave sonic and density logs, which in turn provide useful information for rock physics study. Despite modern logging developments, the bad quality of log data caused by borehole environment or mud-filtrate invasion is still problematic and can affect the AVO analysis so that synthetic gathers cannot be obtained accurately. The bad log data must be recognized and corrected and could be divided into three classes: (a) relatively easy routine log curve editing, (b) environmental and (c) mud-filtrate invasion. The detailed methods with a case study are given for the application of synthetic AVO modelling. These include using Biot-Gassmann fluid substitution and an improved time-average method to outline the petrophysical process in an invaded hydrocarbon zone, using the Zoeppritz approximation, the input P-wave, S-wave sonic and density logs are firstly converted from depth to two-way travel time and the reflectivity at each given angle is then calculated. The result shows a satisfactory agreement between a synthetic seismic gather (generated using the corrected logs) and a gather substituted by Gassmann fluid substitution (80% oil saturated).