Journal of Petroleum Geology, Vol.5,
No.2, pp. 173-190, 1982
©Copyright 2000 Scientific Press,
Ltd.
THE NATURE OF THE UPPER
400 KM OF THE EARTH AND ITS POTENTIAL AS THE SOURCE FOR
NON-BIOGENIC PETROLEUM
A. A. Giardini*, Charles E.
Melton** and Richard S. Mitchell***
*Department of Geology, University
of Georgia, Athens, Ga. 30602, USA.
**Department of Chemistry,
University of Georgia, Athens, Ga. 30602, USA.
***Department of Environmental
Sciences, University of Virginia, Charlottesville, Va. 22903,
USA.
Abstract
It has been reported that the upper 400 km
of the Earth outgassed 1015 tons of petroleum-related fluid over the past 3
billion years. By assuming Fischer-Tropsch reactions and crustal
entrapment, 0.1% of this quantity can account for the entire
amount of petroleum estimated to be in the Earth's crust. In this
paper, the authors attempt to define in some detail the past and
present nature of this deep-source for non-biogenic petroleum.
The principal data used are seismic, and the nature of the matter
that is found totally enclosed in natural diamonds from
world-wide continental sources. New data are presented on matter
included in diamond cubes from Zaire, Africa, noteworthy among
which are the identification of plagioclase, silicate glass, and
solid hydrocarbon inclusions. The general conclusions on the
upper 400 km of the Earth are: (1) Diamonds, the sample carriers
from the inner Earth, formed over the depth range <= 70 <=
370 km in a partially molten silicate environment that contained
C and H components. (2) From top to bottom, the growth
environment varied from aqueous, carbonate-rich and felsic to
aqueous, hydrogen-rich and ultramafic. (3) Petroleum and
petroleum-forming constituents were present throughout. (4) The
depth range of diamond formation is found to correlate with the
present depth profile of the suboceanic asthenosphere. Because of
this, it is concluded that the general nature of this region of
the Earth has not changed much since the earliest known time of
diamond formation (~3 b.y. ago) in spite of material transport to
the surface and isostatic adjustment of the
lithosphere-asthenosphere boundary. The discovery of significant
solid hydrocarbon in a diamond cube suggests that the hydrocarbon
content of the upper region of the asthenosphere may be twice
that indicated by occluded fluid alone. This tends to support a
suggestion based on the quantity of occluded fluid in kimberlite
that 1016 rather that 1015 tons of petroleum and petroleum-forming material
have outgassed from the Earth during the past 3 billion years. An
outgassing of 1016 tons would have permitted crustal accumulation of
up to 105 tons per km in a 10-million year span of Cenozoic
time, consistent with the existence of Cenozoic petroleum fields.
By analogy with N2 outgassing, it is estimated that about 1015 tons of
non-biogenic petroleum remain to be outgassed from the Earth.
