Page 175 - Geochemical Remote Sensing of The Sub-Surface

152 V.T. Jones, M.D. Matthews and D.M. Richers
TABLE 5-VI
Generation of C1-C4 hydrocarbons in vitro (average concentrations, ppm)

Rock type Depth (m) Hydrocarbon concentration, C (10 -4 cm3/kg) Relative loss,
Sealed kc sampler Open-hole sampler Ckr / Co~n
Sandstone 385 106243 119 893
Shale 575 2431 52 47
Shale 620 1610 35 46
Sandstone 640 36473 69 529

FACTORS INFLUENCING NEAR- SURFACE HYDROCARBON FLUX

The hydrocarbon flux near to the surface varies according to the supply of
hydroca~rbons and whether local chemical and biological conditions favour their
preservation or breakdown. In addition, hydrocarbon magnitudes at any given location
vary with time because of displacement by wind, rain and barometric pumping (Wyatt et
al., 1995).

Microbial activity

In a very extensive review, Price (1985) suggested that surface bacterial activity can
totally obliterate the gases in a microseep. That this is not typically the case has been
demonstrated by extensive research over both macroseeps and microseeps (Jones, 1984).
However, bacterial activity does probably contribute to the noisy appearance of soil-gas
seepage.

Barometric pumping

An example of gas flux related to barometric pumping has been demonstrated over
an underground propane-storage reservoir. This mined cavern is about 60 metres (200
feet) deep. In order to observe the gas flux related to atmospheric phenomena, plastic
ground sheets about 1.5 x 1.5 m (5 x 5 feet) were buried along their edges to contain any
gas flux. The variation with rainfall is shown as vertical bars in Fig. 5-10. A very large
seepage anomaly is shown by the dashed line at the right edge of the first bar. The rain
probably displaced the gas in the ground and caused it to come up underneath the ground
sheet. However, the same effect is not repeated every time it rains. Around the 19th,
20th, 21st and 22nd days of the month very small barometric changes were observed.
Nevertheless, small barometric lows have clearly-expressed gas-flux increases. Thus
falls in barometric pressure lead to a gas flux that escapes into the atmosphere. This