Direct hydrocarbon indicator (DHI) pitfall assessment in prospecting pliocene globigerina biogenic gas play in “X structure”, Madura Strait, East Java Basin

Abstract

Direct Hydrocarbon Indicator (DHI) has a major role in the exploration of Pliocene globigerina biogenic gas play in Madura Strait, part of East Java Basin. This study aimed to assess the DHI pitfall within the X Structure which triggered by the drilling result of Well X3 (dry hole) that contradict with Well X1 that successfully discovered gas even though both wells targeted the same reservoir body and DHI features. Low gas saturation that decreases the compressional velocity (Vp) is predicted to be the main reason behind the DHI pitfall. Considering the limitation of qualitative interpretation to investigate the DHI pitfall, this research uses seismic quantitative interpretations, such as fluid substitution modeling, amplitude variations with offset (AVO), seismic inversion, and rock physics analysis to get a full understanding behind the DHI features in X Structure. The 2D marine seismic survey consists of 49 seismic lines and wells data were used in this research. Fluid substitution and Vp-Vs-ρ modelling confirm that the DHI pitfall is related to the non-economic gas saturation (Sg < 10 %) within the reservoir. As for the AVO and inversion analysis indicated that the gas water contact (GWC) level is around 1,389 ms (± 4,023 ft SSTVD). Rock physics analysis shows that the poor reservoir quality is predicted to be the reason behind the low gas accumulation within the reservoir. Several properties cut off for gas zone were derived such as, P-impedance < 5,700 gr/cc*m/s, Vp/Vs < 2.0, porosity > 40 %, dan VCL < 22 %. This research concluded that the seismic quantitative interpretations were successfully used to assess the DHI pitfall and to delineate the economic gas accumulation zone.