Page 256 - Geochemical Remote Sensing of The Sub-Surface

Gas geochemistry surveys for petroleum 229

Q Cl>400ul/kg @ Cl>600ul/k9 0 1Okra
I ,I
Fig. 6-10. Contour map of C~ in soils, Pinqiao oil field, Ordos Basin.

making the anomaly-to-background contrast very low, so the contour intervals used to
map the results have to be selected carefully. Using contours at 480, 500 and 520 ~tl/kg
of C~ the known oil field is clearly outlined by the discontinuous patches constituting a
low relief annular anomaly (Fig 6-10). A small area in the southeast comer of the survey
area was also interpreted as an annular anomaly, and the presence of oil there has since
been confirmed by drilling.

Lixian Depression

The Lixian Depression, some 500 km south of Xi'an, is filled by about 10,000 m of
Palaeozoic and Mesozoic sediments. Mature source rocks occur throughout the
stratigraphic sequence, but the complicated geological structure has impeded exploration
and there are as yet no commercial discoveries of oil or gas. Located in southern China,
the region has a very wet climate and is largely covered by rice fields, so that soils in the
region are invariably moist.