Page 258 - Geochemical Remote Sensing of The Sub-Surface

Gas geochemistry surveys for petroleum 231

additionally reveal an annular anomaly in the west of the area and a linear anomaly
running southwest-northeast across the area.
Seismic data have demonstrated three sub-surface southwest-northeast trending faults
in the area. The annular gas anomaly partially overlies an uplifted block, a structurally-
favourable setting for an oil or gas trap. The linear anomaly follows the trend of another
of the subsurface faults.

CONCLUSIONS

Gas geochemical surveys of light molecular-weight hydrocarbons (C~-C5) and Hg
have proved to be useful methods of detecting oil and gas fields in the Ordos Basin of
northern China and of indicating potential oil or gas fields in the Lixian Depression of
southern China. The annular anomaly is the type most usually found in these regions,
although apical and linear anomalies occur under particular conditions.
If gas geochemical surveys are to be applied effectively, proper attention should be
paid to the mode of occurrence of gases in the soil. Using available geological and
geophysical data, a microseepage migration model should be developed for each survey
area, and the sampling and analytical techniques should then be chosen to suit the model.
Lithology, soil type and surface conditions should also be considered. These can play
important roles in the distribution and intensity of anomalies, as the pre-discovery
traverse over the Jingbian gas field demonstrates. Finally, if the entire field is not
covered by the gas geochemical survey, anomalies may be missed or their shape and
significance misinterpreted.