Page 209 - Geochemical Remote Sensing of The Sub-Surface

186 V.T. Jones, M.D. Matthews and D.M. Richers

4
E
Q.
O.
3
LU
Z
<C
2~
rr
Q.
I
, 0
SOUTH WHIP COVE WEST LIMB
40~176 EAST ANTICLINE NW--" HANGING ROCK
.~ No. 9254 ANTICLINE
LOST
2000, ," ~ ~ ~ RIVER
z
g 0
>-2000
W
W
-4000'
-6000 Oriskany Sandstone g
VERTICAL EXAGERAT)ON 2:1 Metws
Fig. 5-30. Cross-section through the Lost River oil field, West Virginia, and profile of propane in
soil-gas (reproduced with permission of Veridian-ERIM International from Matthews et al.,
1984).

pattern continues to be the most common type found in conjunction with important oil
and gas accumulations.
Numerous explanations have been put forth as to why halos form around
hydrocarbon accumulations. Most of these link the phenomena to the impedance effect
of a diagenic mineralisation zone overlying the main part of the petroleum accumulation.
Such a zone would tend to reduce the ability of gases to seep vertically, except along
well-pronounced fracture systems. Hence, most transport would be deflected around the
edges of the occluded zone. The occluded zone could form by any number of diagenic
processes. Rosaire (1940) suggested that the greater solubility of carbon dioxide in
petroleum, as compared to water, results in the conversion of bicarbonates to less soluble
carbonates over an accumulation. An initial chimney effect would result in a greater
supply of bicarbonate being present above an accumulation resulting in the cementation.
Rosaire (1940) also proposed the reduction of sulphates to sulphides over an
accumulation. Fenn (1940) reintroduced another process that was first introduced by
Mills and Wells (1919). This model is based on the evaporation of ground moisture as
the result of gas expansion which results in the subsequent precipitation of minerals at
shallow depths. The origin of the blocked central portion over an accumulation implies
that gas-induced evaporation occurs more effectively over an accumulation than along
its margins. This model is consistent with results on the variations in unusual chemical