Page 26 - 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)
@2000 Elsevier Science B.V. All rights reserved.
Chapter 1

GENESIS, BEHAVIOUR AND DETECTION OF GASES IN THE CRUST

M. HALE

INTRODUCTION

The search for minerals can be traced back for millennia and the search for petroleum
for more than a century. Despite this difference, the early days of both relied on clear
surface manifestations of the commodities sought in the near-subsurface. Over the
course of time, countless gossans and oil seeps have acted as the spur for the discovery
of resources at depth. Ultimately all such surface indications are exhausted, and new
clues are needed if further subsurface resources are to be discovered. Enter remote
sensing, the science of gathering data describing distant objects. Geophysical techniques
have contributed a wealth of data for suggesting the presence of mineral deposits and
petroleum (including natural gas) accumulations in the subsurface, and are set to
continue to be vital exploration tools. There has been considerable scientific interest in
extending geochemical methods to the acquisition of data that aid the search for deep
mineral deposits and petroleum reservoirs. This involves studying the genesis and
geochemical behaviour of elements and compounds that are naturally associated with
these resources at depth and are able to migrate to the surface. This chapter considers
those elements and compounds that are gases at ambient temperatures. Models of the
dispersion of less volatile species are put forward in Chapters 2 and 3.
Gases exhibit a high degree of geochemical mobility and their dispersion is
unconstrained by gravity. These dispersion characteristics represent a potentially powerful
combination of attributes in exploration. If mineral deposits or petroleum accumulations are
judged or can be shown to liberate a gas into a porous medium such as overlying rock,
overburden or soil then, in the simplest case, the gas will form a broad spherical halo.
According to Oakes (1984) the parameters of such a halo are described by the formula:

p - 2m / 37tr3p

where a mass of gas m, released into a medium of porosity p, produces a dispersion halo of
radius r with a mean partial pressure O 9 As the value of r increases, that of p decreases,
and vice-versa. The resulting hemisphere of gas above the source is a particularly appealing