EARTH SCIENCE FRONTIERS
Volume 15, Issue 1, January 2008
Online English edition of the Chinese language journal
Cite this article as: Earth Science Frontiers, 2008, 15(1): 51–56.
RESEARCH PAPER
Gas Field of the Lithosphere and Possibilities of the Use of
Adsorbed Gases in Rocks in Prospecting for Mineral
Resources
V. I. STAROSTIN∗, L. S. KONDRATOV, D. M. VOINKOV
Moscow State University, Leninskie gory, Moscow 119992, Russia
Abstract: Methods for the extraction of adsorbed and absorbed gases from rocks (sediments) can be used in prospecting for
mineral deposits, because these gases are gradually accumulated and preserved in rocks. The thermodesorption flow is
supplemented with fluids from the earth's mantle. The fluids migrating to the surface produces the lithospheric gas field. The
background gas field formed in this way might be influenced by physical fields and fluids from different mineral deposits. Physical
fields of ore deposits may change the composition of the background gas field.
Key Words: lithospheric gas field; absorbed gas; mineral resource
The study of gases of the earth in prospecting for oil and
gas deposits was proposed by V.A. Sokolov in 1929 (USSR).
Free and dissolved gases were used for investigating the gas
field. Gases were used in prospecting for not only oil and gas
deposits but also ore deposits, practically in all countries. Free
gases are very mobile and depend on gradients of the
atmospheric pressure and temperature, thus reliability of the
information about the gas field was insufficient and
predictions of mineral resources were less effective. Attempts
to extract gases from rocks by mechanical and chemical
methods did not give reliable information about the
lithospheric gas field. The uniqueness of the lithospheric gas
field development thus remained unexplored. Practically,
gases of the underground atmosphere were determined on the
basis of residual gases, which were mainly measured in
summer. This information was unreliable because both
concentration and composition of gases in the gas field
significantly varied in between years in the same area.
Instability of the gas field in the underground atmosphere
based on the investigation of free gas could not satisfy the
requirement of geologists for the application of gas survey in
geological exploration[1].
Therefore, geochemists engaged in gas survey attempted to
solve this problem. In 1985, L.S. Kondratov (VNII geosystem,
State Scientific Center of the Russian Federation, Moscow)
started work related to the extraction of other forms of gas
from rocks[2]. They established that free and adsorbed forms of
gases were released from rocks when heated to 200–250 ºC
and 400–450 ºC, respectively. Existence of these forms of
gases in rocks was known long ago: as early as 1881, H.
Kaiser introduced the term “adsorption”, whereas the term
“sorption” (adsorption + absorption) was introduced by
McBen in 1909. The phenomenon of adsorption of gases by
charcoal was first described by K.V. Sheele in 1773. This
phenomenon was studied systematically for the first time by T.
Sosur in 1814[3]. For example, gas molecules enter a solid
body (absorption) and partly remain on the mineral surface
(adsorption), and the integral effect is known as sorption.
Capillary condensation implies that physical adsorption is
caused by capillary forces.
In most solid bodies, several types of constraint forces are
manifested simultaneously, although the predomination of
only one type is evident. For example, electrostatic forces
prevail in ion crystals, whereas homopolar forces prevail in
atomic lattices. An atom inside a solid body is subjected to the
impact of similar forces along all directions, whereas an atom
on the surface is subjected to disequilibrium forces, and the
resultant force is directed inside the body. Therefore,
Received date: 2007-09-20; Accepted date: 2007-12-30.
∗
Corresponding author. E-mail: [email protected].
Foundation item: Supported by the Project of the Russian Foundation for Basic Research (No.04-05-64045).
Copyright © 2007, China University of Geosciences (Beijing) and Peking University, Published by Elsevier B.V. All rights reserved.
V. I. STAROSTIN et al. / Earth Science Frontiers, 2008, 15(1): 51–56
processes of adsorption are spontaneous, which lead to a
decrease in the free energy of the system. Adsorbed particles
can be tightly connected to the adsorbent surface or can be
freely migrating along the surface along two dimensions.
Molecules could migrate in space along three dimensions
before adsorption. Therefore, adsorption is accompanied by a
decrease in the entropy, i.e., release of heat (exothermic
process). The amount of gas adsorbed by 1 g of adsorbent at
equilibrium is a function of temperature, pressure, and nature
of adsorbent and adsorbed matter. Adsorption of gas takes
place very quickly. For example, Freindlich measured the rate
of SO2 adsorption on charcoal by the photographic method
and found that the equilibrium was attained at less than 20 s.
The fast initial adsorption was followed by the slow sorption.
He compared the volumes of different gases adsorbed by 1 g
of adsorbent at different pressures and temperatures and
showed that adsorption (gas volume) increased with increase
in the boiling temperature of gas: H2 (5 cm3, -252 ºC), N2 (8
cm3, -195 ºC), O2 (8 cm3, -182 ºC), CH4 (16 cm3, -164 ºC),
and CO2 (48 cm3, -78 ºC). Arrhenius showed the existence of
an analogous relation between volumes of different gases
adsorbed by charcoal and values of constant a, in the van der
Waals equation. O. Schmidt showed the relation between the
temperatures of evaporation of gases and their values of
adsorption. Links between some properties of gases, which
characterize the process of condensation, and their van der
Waals adsorption should be taken into consideration in the
investigation of the lithospheric gas field based on the
adsorbed form of gas in rocks.
Amount of the adsorbed gas is usually expressed as the
volume of gas under normal conditions (0 ºC and 760 mm Hg
at 1 g of adsorbent). In this study, adsorbed gases are given in
cm3 per 1000 g of rock (cm3/kg).
For the method of extraction of adsorbed gas from rocks,
the calcination of different types of rock at 600 ºC (as silicagel
for chromatography) was carried out, and hydrocarbon gas
(HCG) of a certain composition (C1-C3-C4) was introduced
into the heated rock. An attempt was made to extract the
adsorbed gases from the rock at different temperatures (from
150 to 350 ºC). Gas of the C1-C3-C4 type was released from
the rock when heated to 200–250 ºC (depending on the rock
type). For working with any rock type, the accepted
desorption temperature is 225 ºC for the adsorbed gas.
Heating of different rock types (from limestones to dunites
and kimberlites) at an increasing rate of 50 ºC (from 100 to
900 ºC) showed three peaks for the release of high gas
concentrations at 200–250 ºC, 400–500 ºC, and 700–800 ºC.
Each peak showed significant differences in the concentration
level and composition. The first peak corresponded to the
release (thermodesorption) of the adsorbed gas, which is
characterized by a high share of unsaturated components of
the HCG and a low share of methane. The second peak
(desorption of the absorbed gas) was characterized by a high
share of methane and its light saturated homologues. The third
peak was characterized by the release of gases with a high
share of the lightest unsaturated homologues of methane, i.e.,
HCG components that are most actively transferred into the
condensate state in microfractures and subjected to capillary
forces (Fig. 1).
Thus, rocks (not the underground atmosphere) contain gases
in three forms: adsorbed, absorbed, and capillary condensate.
Gases in closed pores and vacuoles should be considered free
gases that are temporarily closed for their free migration in the
underground atmosphere.
To investigate gases in rocks (lithosphere), it is
technologically most convenient to determine the presence of
adsorbed gases. Therefore, the lithospheric gas field was studied
by measuring the adsorbed gases in rocks of different regions
(from the North and South poles, and from the Atlantic to the
Pacific oceans) in borehole sections (including the ones at great
depths) and in subsoil sediments. The uniqueness of the
lithospheric gas field development at various stages of the
section and in structures of different orders (from mega
anticlines to local anticlines 2–3 km across) was determined. It
might be suggested that structures of different orders include
ring anomalies in the gas field development, and the section
shows wave structure of the gas field. The sedimentary part of
the section includes three anomaly zones: (1) at the contact
between sedimentary rocks and basement rocks, (2) in the
midsection of the sedimentary cover, and (3) in subsoil deposits.
The basement rocks are characterized by wave distribution of
gas concentrations, with separate narrow zones of high gas
concentrations related to the fracturing of rocks in the section.
The level of gas concentration in the section is higher than the
ads
Clarke level for H 2 . Zones of high concentrations correspond
to adsorbed HCGs (HCGads) of the lightest composition (high
shares of methane). Distribution of adsorbed H2, HCG, and CO2
in the section is rather similar, indicating their common genesis
(Fig. 2). The redox potential (Eh) decreases with depth in
accordance with the wavy character of its distribution.
Simultaneously, fractured zone (horizon II, borehole SG-3) is
reflected by a sharp increase in Eh and a decrease in pH.
Histograms of the HCGads composition of rocks in the SG-3
section show sharp variation in the HCGads composition at both
upper and lower stages of the section (horizons II, III, and VII)
because of the fracturing of rocks. Other horizons (I, IV, V, and
VI) of the section, in which the gas field is mainly related to the
diffusional penetration of deep gases, show a similar HCGads
composition (Fig. 2). The curves showing the contrast of
HCGads components at these horizons, relative to the
background (horizon V), indicate similarity of the genesis of
gases, which is characteristic of deep gases dissolved in water.
Gases of different forms are accumulated simultaneously in
rocks during the geological time. The highest concentrations
are characteristic of absorbed gases, whose concentrations are
V. I. STAROSTIN et al. / Earth Science Frontiers, 2008, 15(1): 51–56
Fig. 1
Principle scheme of various gases from rocks (during their heating)
V. I. STAROSTIN et al. / Earth Science Frontiers, 2008, 15(1): 51–56
Fig. 2
Distribution of gas-geochemical parameters of absorbed gas fields in rocks along the Kola Superdeep borehole SG-3 section
V. I. STAROSTIN et al. / Earth Science Frontiers, 2008, 15(1): 51–56
nearly one order of magnitude higher than that of the adsorbed
or capillary gases; i.e., if the HCGads concentration is about 1
cm3/kg (for all rock types), the concentration of the absorbed
gas will reach 10cm3/kg. During the thermal decomposition
(pyrolysis) of organic matter in rocks, not more than 0.1–0.3
cm3/kg of HCGads is formed, which is almost two orders of
magnitude lower than the concentration of absorbed HCGs
accumulated in rocks. The earth’s surface, in the past,
contained rocks, which accumulated gases at the
above-indicated level; thus, because of critical pressures and
temperature in deep zones, these gases are released. As a
result, fluid flows of thermodesorbed gases to the surface are
formed. These flows interact with HCGs formed during
catagenesis of organic matter of rocks. The HCGs dissolved in
the flow were transported to the surface and were accumulated
in traps (hydrocarbon pools). The thermodesorption flow is
supplemented with fluids from the earth’s mantle. The fluids
migrating to the surface produces the lithospheric gas field.
The background gas field formed in this way might be
influenced by physical fields and fluids from different mineral
deposits. If oil and gas deposits introduce gases into the field,
the HCG composition of the background gas field becomes
more concentrated and heavier. Physical fields of ore deposits
can change the composition of the background gas field.
Deep gases of kimberlite bodies (pipes) affect the
background gas field of sedimentary rocks. This is reflected in
anomalous concentrations and compositions of gases even in
the overlying rocks[4,5]. On the basis of the distribution of
concentrations and compositional variations of the adsorbed
gas in subsurface rocks, zones with possible mineral deposits
can be seen, which affects the gas concentration and
composition of the background gas field.
The above methods for the extraction of adsorbed and
absorbed gases from rocks (sediments) can be used in
prospecting for mineral deposits and assessment of the state of
ecology because these gases are gradually accumulated and
preserved in rocks. They retain the “memory” about the
migration of fluids and the possible influence of various
technological and physical fields. The relation between the
concentrations of adsorbed and absorbed gases might indicate
zones with the most active input of deep fluids at present. This
might be used for forecasting earthquakes.
References
[1]
Kondratov L S. Gas field of the lithosphere. Geoinformatika,
2006(3): 87–90.
[2]
Kondratov L S, Ershova M V. Hydrocarbon gases in rocks in
connection with their application in the prospecting for mineral
resources. Geologiya Razvedka, 1986 (7): 83–88.
[3]
Stephan
Brunauer.
Absorption
of
gases
from
rocks.
Inostrannaya Literatura, Moscow, 1948:147.
[4]
Kondratov L S, Kudryavtseva G P, Starostin V I, et al.
Peculiarities of the gas filed of kimberlite pipes. Doklady
Akademii Nauk, 2005, 404(6): 817–820.
[5]
Kondratov L S, Starostin V I, Voinkov D M, et al. Gases in
rocks possibilities of their utilization in the prospecting for
kimberlite pipes. Vestnik MGU, Ser. Geology, 2006, 24–31.