Page 18 - Geochemical Remote Sensing of The Sub-Surface

Contents XVII

Physical basis for migration of hydrocarbons to the surface ..................................... 143
Basic assumptions ................................................................................................. 143
Physical transportation by effusion ....................................................................... 144
Physical transportation by diffusion ...................................................................... 144
Hydrocarbon residence sites at surface ...................................................................... 148
Free gas ................................................................................................................. 149
Bound gas ............................................................................................................. 150
Choice of free gas or bound gas ........................................................................... 150
Factors influencing near- surface hydrocarbon flux .................................................. 152
Microbial activity .................................................................................................. 152
Barometric pumping ............................................................................................. 152
Earthquakes ........................................................................................................... 153
Sampling and measurement methods ........................................................................ 155
Atmospheric techniques ........................................................................................ 155
Soil gas ................................................................................................................. 159
Dissolved gas ........................................................................................................ 170
Headspace gas ....................................................................................................... 172
Disaggregation ...................................................................................................... 173
Acid extraction ...................................................................................................... 176
Fluorescence ......................................................................................................... 179
Sampling strategy ...................................................................................................... 180
Data interpretation ..................................................................................................... 182
Preferential pathway model .................................................................................. 182
Geochemical populations ...................................................................................... 187
Case histories ............................................................................................................ 192
Neuquen Basin, Argentina .................................................................................... 192
High Island area, Gulf of Mexico ......................................................................... 195
Great Basin, Railroad Valley, Nevada .................................................................. 197
Overthrust Belt, Wyoming-Utah ........................................................................... 208
Conclusions ............................................................................................................... 211

Chapter 6. Gas geochemistry surveys for petroleum .................................................................... 213
Y. Ruan and Q. Fei
Introduction ............................................................................................................... 213
Theoretical principles ................................................................................................ 214
Indicator gases ...................................................................................................... 216
Gas migration ........................................................................................................ 217
Surface expressions of hydrocarbon migration ......................................................... 218
Gas anomalies ....................................................................................................... 218
Alteration .............................................................................................................. 218
Modes of occurrence of gases in microseeps ............................................................. 219
Free molecules ...................................................................................................... 220
Adsorbed molecules .............................................................................................. 220
Microbubbles ........................................................................................................ 221
Mineral constituents .............................................................................................. 221
Practical methods ...................................................................................................... 222
Soil air .................................................................................................................. 222