Quantifying AVO uncertainties related to calcite-cemented beds using Monte Carlo simulation

Abstract

Calcite cement often occurs locally forming thin layers of calcite-cemented sandstone characterized by high seismic velocities and densities. Because of their strong impedance contrast with the surrounding rock, calcite-cemented intervals produce detectable seismic reflection signals that may interfere with target reflections at the top of a reservoir. In this case, the amplitude variation with offset (AVO) of the effective seismic signature will be altered and may even create a false hydrocarbon indication. From Monte Carlo simulation, we find that the presence of thin calcite-cemented beds increases the uncertainty of Bayesian pore-fluid classification based on the AVO attributes intercept and gradient. In the case example of a North Sea turbiditic oil and gas field, the probability of a false positive hydrocarbon indication increases from 3–5% to 18–21% assuming an equal probability of the occurrence of brine, oil, and gas. The results confirm that calcite-cemented beds can create a pitfall in AVO analysis. Realistic estimates of AVO uncertainty are crucial for the risk assessment of well placement decisions.