Porosity identification using amplitude variations with offset: Examples from South Sumatra

Abstract

Widespread deposition of platform and reefal carbonates of the Baturaja limestone formation occurred during the Miocene epoch in the South Sumatra basin. Although significant oil and gas deposits have been discovered in the porous facies, porosity within the Baturaja limestone has been observed to vary widely between tight platform facies and highly porous reefal facies, making predrill prediction of porosity an important exploration objective. I use amplitude‐versus‐offset seismic modeling to distinguish between porous and tight Baturaja limestone facies. Amplitude variations with offset for reflections from two Baturaja reefs in the South Sumatra basin were studied: one, a proven gas reservoir, the other, an interpreted reef that had not yet been drilled at the time of study. The seismic data were processed judiciously to preserve and enhance amplitude effects, which were then modeled using the Bortfeld approximation for reflection coefficients. A key assumption was that the [Formula: see text] ratio of limestone depends primarily on minerology rather than on porosity or pore‐fluid content. The modeling showed that porous and tight limestone facies have unique and different reflection-coefficient variation patterns with angle of incidence. Good agreement was found between observed data and the modeling results, indicating that the modeling of amplitude variations with offset can be used as a lithology discriminant. In the second case, a predrill prediction of porosity was confirmed by subsequent drilling.