Velocity reversal as a direct hydrocarbon indicator in the anomalous high temperatured and over pressured DF1-1 Gas Field below 2100 m in the Yinggehai Basin, South China Sea

Abstract

In the DF1-1 Gas Field in the Yinggehai Basin, South China Sea, the velocity-depth plot and velocity spectra show significant variations from the classic linear trend, exhibiting a universal reversal phenomenon. Velocity parameters derived from velocity spectral analyses of the seismic data and sonic logs indicate that the ``Interval Velocity' universally reverses below 2100 m, corresponding to the starting depth of overpressure in the field. There is a distinct difference between the gas-bearing sandstones and the surrounding rocks in the shallow strata with depths <2100 m, however, such a difference becomes less apparent beyond 2100 m in the middle-deep strata.