The Geophysical Identification?origin And Migration Patterns Of Shallow Gas In The Baiyun Sag,Northern South China Sea

In this paper,the geophysical characteristics,source,migration pattern and distribution of shallow gas in the Baiyun Sag of the Pearl River Mouth Basin in the northern South China Sea are studied.Based on the three-dimensional seismic and well logging data,the geophysical characteristic of the shallow gas in the study area are identified;the geochemical data are collected to discuss the gas source of the shallow gas;the distribution features of the shallow gas are identified according to the seismic reflector characteristic of the gas-bearing layer;and the migration model of the shallow gas in Baiyun Depression was analyzed by establishing geological model.Based on the three-dimensional seismic data,more than 30 lines were interpreted under the constraints of well logging LW3-1-1,and a large amount of shallow gas in the study area was identified.According to the seismic data,the gas-bearing strata has the characteristics of the weak coherence,strong amplitude reflector,low velocity,low instantaneous frequency,etc.According to the characteristics of seismic reflection of the gas-bearing strata,anomalous reflections such as strong reflection,acoustic dumb zone,gas chimney,gas channel,bright spot and polarity reversal are occurred in the seismic profile of the study area.The gas-bearing layer was recognized by the interpretation of the well logging data.Through the interpretation of the seismic profile,it can be seen that a large number of gas vertical migration channels are developed in the study area,and it can be concluded that the shallow gas origins from deep gas-bearing reservoir.And based on the collected geochemical data,the biogenic gas was discovered in the gas-bearing sand of the Pearl River Mouth Basin.Thus,the geophysical and geochemical analyzes indicate that the gas of the shallow sediment are regarded as a mixture of biogenic and thermogenic gas.In the study zone,we establish a migration model of the shallow gas based on the three-dimensional seismic data.The model shows the gas-bearing reservoir,the focused fluid migrating pathway and the accumulation of the shallow gas.Based on the faults,gas chimneys,mud diapirs mud volcano,pipeline,shallow superficial fault and migration canyon which are interpreted from seismic profile,can explain the distribution features of shallow gas.It can be seen that the distribution of the shallow gas in the study area is closely correlated with the deep gas reservoir,faults and the mud content of shallow rocks.