AVO Based Prediction of Hydrocarbon Occurrence in Sand Reservoirs and Assessment of Uncertainty. The Abo field Case History

Maurizio Cardamone
ENI- E&P Division, San Donato Milanese, Italy

The presented case history concerns an advanced AVO Fluid Inversion study carried out in the OML 125 block of the Nigeria Deep Offshore. Here the need of a reliable and robust tool to analyze the possible distribution of different hydrocarbons in the possible reservoirs was definitely an issue. The request for increased reliability and more advanced implementation of the AVO method, capable of predicting the occurrence of different fluids, but also to estimate the uncertainty of the prediction was set as a top objective of the technology development. A quite innovative and integrated methodology, called "AVO Fluid Inversion" (AFI) has been jointly developed by ENI and Hampson-Russell. Its goal is that of reliably estimating the probability that an assigned AVO response, measured from real prestack seismic data (Intercept, Gradient or Narrow and Far Stack amplitudes), can be ascribed to the presence of either brine, gas, oil in a sand reservoir, given the specific geological parameterization. The developed software compares the real AVO responses at the several horizons/targets/objects in the study area with a generalized model, which takes into account the expected (or guessed) variability of all the petrophysical parameters involved into the AVO phenomenon. This model is developed through the statistical analysis of all available wireline logs and borehole processed data in a large area of interest. The Abo field case history represents one of the first industrial applications of the method, blind tested with the first two exploration wells, and then successfully validated by the many following production wells.