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.