Page 241 - Geochemical Remote Sensing of The Sub-Surface

214 T. Ruan and Q. Fei

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SURFACE EOCHEMICAL NOMALIES ~
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- DESTRUCTION ~Ii!i
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DISPERSION

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Fig. 6-1. Conceptual model ot'surtace gas geochemical anomalies.

'!'t IF.ORETICAI. PRINCIPI~I']S

The basis for gas geochemical surveys for petroleum lies in modern concepts of
petroleum genesis. Petroleum originates mainly from degradation and thermal cracking
of disseminated organic matter in sedimentary rocks. Oil and gas fields are formed by
the migration of the resulting liquid and gaseous hydrocarbons and their accumulation in
favourable settings, termed traps. Accumulated petroleum is constantly dispersing
laterally and vertically, albeit slowly, and this can lead to disappearance or destruction of
the accumulation. Petroleum generation, migration, accumulation, dispersion and
destruction are, to varying degrees, taking place simultaneously in a continuous
evolutionary process. A surface geochemical anomaly can develop from any and all
stages in this process, and should be considered as an integral part of the oil or gas field
(Fig. 6-1). Conventional petroleum exploration tends to focus on the first three stages.
Exploration geochemistry, on the other hand, concentrates on the dispersion stage.
Surface macroseeps of oil and gas, which can be seen by the naked eye, led to the
discovery of some of the most famous oil fields in the world (Link, 1952). Today, almost
all of the visible macroseeps have been tested and drilled. Microseeps are the extension
of macroseeps into the non-visible range. Their detection therefore requires the use of
alternative methods, most obviously gas geochemical surveys.