Derisking low-saturation gas in Tertiary turbidite reservoirs

Abstract

Occurrences of low-saturation gas (LSG) in tertiary turbidite reservoirs are common and often characterized by anomalous amplitude/amplitude variation with offset (AVO) response, which can be classified as false positives. LSG cases are difficult to derisk with geophysical methods because the anomalous responses may be very similar to success-case (oil or gas) anomalies. LSG failures may represent true residual gas in cases of seal integrity failure (“blown” trap) or variable saturation of gas in low net-to-gross laminated sandstone/siltstone or siltstone/mudstone intervals (“generic LSG”), which represent a type of reservoir failure. Derisking exploration opportunities burdened with a high possibility of LSG failure require an integrated evaluation of the geophysical evidence in the proper context of charge setting, reservoir/seal stratigraphy, structural setting, and trap geometry. Although geophysical methods may not provide fully conclusive predictions, the integration of geophysical observations with a geologic framework and realistic geologic chance factors can result in effective derisking of potential low-saturation failure cases through the estimation of geophysical scenario likelihood values and Bayesian probability updates.