Page 170 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 147

Key
CH 4
ZHC

CH 4 Y'HC vol% CH 4 ]~HC 10 -4 voi %
30 I- o.3o I--
0.25 1000 Well
26 ).20 600
D.15 .-,--,- ,..-," 200 12 ".
22 ).10 Ir i _ J 4 it i i i i t t ! I I I I I
I'"I::-,-:"~ ,;:
60 CH 4 CH 4 Y'HCl0 "4 vol% I /
40 o, / K-T
i ~ f shY, ,..';.i j
20 0.5 /
, ,..., ,
o.4
I
c., Z.c~0 "4 vo. % $ ~ c. z.c~0 4vo, %
,,o w.,
10"40[ .. ~ 1. I "-~'I 140 / /I ~
30 ~ "" -" 100 10 8
/~ f - I ~o ~r- "~"'-
L- ! I I ! t t I I I ! I 60 It I ! I!ll II I ! I I
CH 4 y" HC 10 .4 vol % CH 4 T'HC 10 .4 vol %
1000 5 0.4 Well 79 I
Well 104 l /
800
600 4 3 0.3
400 2 0.2
200 5 ~J~ 1
31- t t -Fi i ! i t i J I i i i I I I I I I I ! I
10 20 30 5 15 27 31 3 16 25 5 15
September October August September October

Fig. 5-7. Methane and total hydrocarbon gases in subsoil before and after an earthquake
(reproduced with permission from Zorkin et al., 1977).

As to the second point, if selective adsorption is occurring, the volumes of material
escaping over geologic time should ultimately saturate (poison) the adsorber such that no
additional material can be adsorbed, or at best, material is exchanged in a steady-state.
The result will be a gradual return of the signal to the original composition. This is
clearly shown in a study by Zorkin (1977a).
There is, however, one important area where diffusion may be responsible for
compositional changes; near the soil-air interface. Methane should, due to its lightness
and zero net dipole moment, be preferentially lost (followed perhaps by ethane). This
would possibly result in an oilier gas signal at the surface. This could be countered by
the production ofbiogenic methane, which might partially compensate for this loss.