Page 254 - Geochemical Remote Sensing of The Sub-Surface

Gas geochemistry surveys for petroleum 227

concentrations than those found in the anomalies, so hydrocarbons must have been
added. Both of these observations can be explained by ascending hydrocarbons being
trapped during the formation of secondary carbonates, and released and detected by the
acid treatment method used to analyse the samples. The river valley, filled with loose
sediments, might follow a good conduit for ascending hydrocarbon gases.
Still other factors point to the significance of the anomalies. The hydrocarbon
anomalies in the river bank sediments include the complete range from C1 to C5, which
excludes the possibility of the hydrocarbon anomaly being of surface biological origin.
The aeolian sand is a comparatively poor medium for the development and detection of
hydrocarbon anomalies in soil, but is highly suitable for the detection of Hg in soil air,
for which there is no obvious source other than natural gas at depth.
Subsequently the scientific research well was drilled as planned. When it penetrated
Lower Ordovician carbonates it produced 16.3 x 104 m 3 per day of natural gas. The
scientific research well thus became the discovery well in this region. Since the well was
drilled directly on the surface hydrocarbon gas anomaly, the soil gas survey traverse was
thought to cross an apical or linear anomaly. Further work, however, was to reveal that
the gas field has an annular anomaly.
Encouraged by the gas discovery, the Changqing Oil Company decided to explore
the full extent of the field. In 1990, a 5035 km 2 area was covered by a gas geochemical
survey with a target sample density of 1-2 per km 2 which yielded a total of 6162 sample
sites. At each site C~ and He were measured in the interstitial soil air, and soil samples
were analysed in the laboratory by the acid treatment method for Cj-C5 and by thermal
desorption at 250~ for Hg. The random data points were gridded by inverse distance
squared weighting, smoothed by means of a 3 x 3 moving average and contoured.
The resulting hydrocarbon gas patterns and Hg pattems are presented in Figs. 6-8 and
6-9, respectively (which also show the location of the traverse discussed above and
shown in Fig. 6-7). The regional pattems comprise an arc of anomalous values extending
from the southwest to the northeast. This arc is interpreted as part of an annular anomaly
that is still open to the north and west. The exploration implication is that a gas field
underlies the low values in the northwest of the survey region. In fact the earlier gas
survey traverse and the scientific research well that became the discovery well were
already located in this part of the region.
Subsequently the Changqing Oil Company drilled more than twenty wells, most of
them commercial, in the prospective area (Figs. 6-8 and 6-9). The highest production
from a well in this area is more than 120 x 104 m 3 per day. The gas reserves in the
Jingbian field are likely to exceed 10 ~ m 3 as the field is not yet closed off to the north
and west by surface exploration.
The gas production layer is a Lower Palaeozoic karstic plateau at a depth of 3000 m,
surrounded by palaeo-valleys filled with a Permo-Carboniferous coal series (which could
be the source of the natural gas). The annular surface geochemical anomaly may be
related to the edge of the buried plateau. In the production layer, the gas is under a
pressure of 120 atmospheres. Therefore the average pressure gradient between the gas