Online Journal for E&P Geoscientists

Al-Sokhiber, Mohanad S.^*1; Bakhorji, Aiman ²; Al Otaibi, Mohammed ³
(1) Saudi aramco, Dhahran, Saudi Arabia. (2) Saudi Aramco, Dhahran, Saudi Arabia. (3) Saudi Aramco, Dhahran, Saudi Arabia.

Rock physics guided amplitude vs. offset (AVO) provides reliable predictions of reservoir properties, such as porosity, lithology, and pore-fluid type. Quantitative interpretation of these properties is required to help reduce the drilling risk for exploration and reservoir development. This case study incorporates two wells that were drilled for gas exploration targeting clastic reservoirs in Eastern Saudi Arabia. Rock physics modeling was conducted for the two wells including fluid substitution and porosity perturbation after testing of the reservoirs showed they were tight. AVO analysis was also carried out for all the models to help interpret observed seismic response

Fluid substitution modeling using Gassmann’s equation (1951) was conducted for the gas and wet cases. The first well exhibited a class IV AVO response with a negative reflection coefficient that dimmed with offset. The gas case showed a stronger amplitude response relative to that of the wet. The second well showed a class I AVO response for the wet case and changed to a class III AVO response when gas was present. Based on these modeled AVO seismic responses, intercept-gradient analysis was carried out to interpret the change in seismic response due to the presence of gas.

Porosity perturbation was also performed using Raymer’s model (1980) with increases of 25%, 50% and 75% of the original porosity. AVO synthetics were generated for all these models and compared to that of the in-situ case. Changes in porosity show a more pronounced impact on the seismic response as compared to changes in pore fluid. Intercept-gradient analysis was also carried out to draw some interpretation rules of thumb that can help the geoscientists better understand the seismic response variation with changes in porosity.