Rock physics analysis: a tool for lithology and fluid prediction in the Gulf of Thailand

Abstract

SUMMARY The Tertiary rift basins of the Gulf of Thailand are major hydrocarbon producing areas. The reservoirs in these basins are mostly fluvial sands of Miocene and Oligocene age. Gas is found mostly in central basins whereas there are more oil discoveries in western marginal basins. The main objective of this study is to understand the vertical and spatial trends of different rock physics parameters which can be used to differentiate lithology and fluids in these basins. Cross-plot and fluid substitution analysis were performed to determine lithology and/or fluid sensitive rock properties. Cross-plot analysis shows that sands have low P-velocity and density at shallow depths as compared to shale but the contrast of P-velocity between sand and shale decreases significantly at deeper levels. However, density shows significant contrast between sand and shale throughout the zone of interest and is therefore a more useful lithology discriminator. Density can also distinguish highly gas-saturated sands (80%) from water-wet sands throughout the zone of interest. On the other hand, oil-bearing sands cannot be so easily discriminated from water-wet sands. In comparison Vp/Vs can only successfully resolve high porosity sands (>16%) and gas sands when used in combination with P impedance. This regional rock physics study indicates that appropriate inversion techniques for lithology and fluid prediction studies need to be considered carefully. Post stack P-impedance volumes generated by inversion are very useful at shallower levels down to 1900 to 2000 metres but at deeper levels, density volumes generated by pre-stack simultaneous inversion are more appropriate.