Page 240 - Geochemical Remote Sensing of The Sub-Surface

Geochemical Remote Sensing of the Subsurface
Edited by M.Hale
Handbook of Exploration Geochemistry, Vol. 7 (G.J.S. Govett, Editor)
9 Elsevier Science B.V. All rights reserved 213
Chapter 6

GAS GEOCHEMISTRY SURVEYS FOR PETROLEUM

T. RUAN and Q. FEI

INTRODUCTION

Gas geochemical surveys appear inherently suitable for petroleum exploration
because oil and gas fields are natural accumulations of volatile hydrocarbons. They were
first attempted some sixty years ago by Laubmeyer (1933) and Sokolov (1935).
Nevertheless, there has since been only rather limited use of them. The reasons for this
are fourfold: (1) the fundamental concept of hydrocarbon gases escaping through
considerable thicknesses of "impermeable" cap rock has been slow to gain acceptance;
(2) gas geochemical sampling devices and detection methods have often been
insufficiently sophisticated and have therefore produced unsatisfactory data; (3) gas
anomalies are unstable and have often proved difficult to repeat; and (4) the petroleum
industry has achieved a high success rate with seismic exploration and hence has not felt
an acute need for an alternative exploration tool.
By the 1980's, however, the situation was beginning to change. A series of direct
observations was persuading petroleum geologists that volatile hydrocarbons related to
petroleum at depth are detectable at the surface (Jones and Drozd, 1983). Drilling proved
the significance of a few hydrocarbon-gas anomalies found some forty years earlier
(Horvitz, 1969; Horvitz, 1981). Technical advances in analytical instrumentation
(especially in gas chromatography and mass spectrometry) meant that concentrations and
isotopic compositions of hydrocarbons could be determined with the required
sophistication in terms of sensitivity and accuracy, and also comparatively quickly and at
reasonable cost. The importance of subtle non-structural oil and gas traps was recognised
by the petroleum industry, along with the difficulty of detecting them by conventional
seismic exploration.
The consequences of these developments in China were that petroleum companies,
geological institutions and the Sinica Academy carried out a series of gas geochemistry
research programmes on land in areas with different climate conditions and offshore.
This chapter outlines the background, some of the methods that were developed and
illustrates some of the results.