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II. Carbon Cycle in Deep Karst Systems

1999-07-10KDL 3206

II. Carbon Cycle in Deep Karst Systems


Study on the Release of Deep Source CO2?in Western China

Yang Lizheng et al.(Chengdu University of Technology)

Base on the data of numerous hot springs in western China, the CO2?contents of hot springs are calculated, the sources of CO2?in hot water are discussed, the release process of CO2?in hotwater is analyzed, and the release quantities are estimated.

1.The CO2?content of hotsprings, which all occurred in carbonate rock strata, is relatively high. The maximum reaches 2575mg/L.The hot spring numbers, which CO2?content exceeds 500mg/L, make up 35% of the totals, mean value of CO2?content is 512.12mg/L in Tibet, 494.1mg/L in western Sichuan Province and 332.6mg/L in western Yunnan Province respectively. The statistical data indicate clearly that the CO2?content in hotspring is abundant, and it is aproved that the hydrothermal activity zone in Tibet contains the richest CO2content in China.

2.The carbon stable isotope data explain that the CO2?in hotspring mainly derived from inorganic CO2?in the deep earth. The deep source CO2?has three different causes: (1)mantle-derived CO2, features a lower d13C value (-3%o~-6%o, Tengchong geothermal area in Yunnan Province is a typical CO2?release area of this kind (Fig.1); (2) metamorphic CO2, features a higher d13C value (around 0%oor >0%o, typical region is located in Erdaoqiao, Kangding city, Sichuan Province (Fig.1); and (3) mixed origin CO2, features a middle d13C value (0%o~?-3%o, the percentage of metamorphic CO2?in Kangding hotsprings is very high, average value account for 49.3~86.9%?of the whole, and mantle-derived CO2?account for 13.1~50.7%.

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Fig.1 Scatter diagram of d18O, d13C
1.Yangbajin 2.Tengchong 3.Kangding

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3.In order to calculate the emission quantity of hotspring CO2, the paper put forward the concept of discharge modulus, it means that the emitted CO2?quantity of hotspring in unit time:

Mf?= (Ch?- Co)Q

Where Mf?- Modulus of discharge(mg/L)

Q - Discharge of hotspring water (L/s)

Ch- CO2?content of hot water (mg/L)

Co- CO2?content of shallow seepage water (mg/L)

Based on the statistic results from about 700 hot springs, the emission quantity of deep source CO2?is 2.68x 105t/a according to the formula above, in which 1.681x 105t/a for Tibet, 0.616x105t/a for West Yunnan, and 0.375x105t/a for West Sichuan. The emission is uneven spatially, so it can be divided into 3 sections: First is from Yangbajin to Naqiu; Second is from Changdu to Kangding; Third is from Tengchong to Kunming(Fig.2).

4.According to the field measurement of Kangding hotspring, the release quantity of CO2?decrease gradually with the increase of water flow distance, it presents an attenuated curve. The release quantity is about 38.3% of the total in a flow distance of 100m , that is , about 40% of deep source CO2?transforms into gas, and immediately release into atmosphere. The other 60% enters into surface water with hot water.

The hydrothermal activities of hotspring in western China, especially in geothermal area of Himalayan Mountain , are an important way for deep source CO2?to release, these deep source CO2?is a major component part of the global carbon cycle.

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Fig. 2 Release intensity distribution of deep source CO2?in Tibet and its neighbouring area

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Conceptual model of karstified oil-gas bearing reservoirs' formation according to new data on reduction sulphates by hydrocarbons and?
carbon cycle

A.V.Petukhov (Industrial Institute, Ukhta, Komi Republic, Russia)

Large amount of oil and gas deposits containing hydrogen sulphide of different quantities has been discovered in the World. For example : Lacq field in France; Tengiz field in Kazakhstan; Astrakhanskoe and Orenburgskoe gas fields in Russia and many others. All these fields are karstified carbonate reservoirs with sulphate beds. Such oil and gas deposits have been found in the Timan-Pechora basin too. These karstified deposits are known to be in Vuktyl, Rassokhin, West Tebook, Vozey, Kharyaga, Kochmes, Lemva and other fields. Oil and gas deposits are located in Low and Upper Paleozoic karst carbonate reservoir from the Ordovician to the Low Perm.

On one hand, H2S is an aggressive component of natural hydrocarbons causing hydrosulphuric corrosion in the pipes and equipment. That is why even little hydrogen sulphide in hydrocarbons is considered to be harmful mixture. Its presence makes prospecting, exploration, development, well drilling, oil and gas transportation difficult and requires special expensive equipment resistant to sulphur affect. Such equipment is produced abroad.

On the other hand, gas-chemical plants can be built in the regions of large gas field containing hydrogen sulphide (10 per cent and more). Such plants have been built on Lacq gas field in France, on Astrakhanskoe and Orenburgskoe fields in Russia. These plants can produce cleared hydrocarbon gas and natural sulphur. In this connection the problem of sulphureous gas origin and its deposit formation has become a very important economic one as well. Conducting of gas deposits' exploration requires separate searches of sulphureous gas and gas without sulphur. Besides that, success of such deposits' exploration will depend on the discovery of relationship between sulphureous gas and karst processes.

Hydrogen sulphide can be the product of many geochemical reactions which take place in different geochemical conditions. There are some hypotheses concerning to H2S origin in natural gases. The main of them are as follows :

1.Hydrogen sulphide origin during reduction of sulphates by hydrocarbons with participation of hydrocarbon-consuming bacteria and related organisms;

2.Hydrogen sulphide formation during decomposition of organic substance in the rock;

3.Origin of hydrogen sulphide as the result of natural hydrocarbons' interaction with natural sulphur;

4.Formation of hydrogen sulphide as result?of chemical reaction between natural hydrocarbons and sulphate rocks.

Only the 4th hypothesis can explain high concentration of hydrogen sulphide in natural hydrocarbon gases. This hypothesis is more successful. It explicates all the laws which have been discovered during investigations of sulphureous gas deposits. But, unfortunately, all previous laboratory experiments on chemical reduction of sulphates by hydrocarbons were unsuccessful. I think it was because all these experiments didn't take into account of geodynamical and seismic-tectonic conditions in earth's crust. It worth paying attention to relationship of sulphureous oil-gas fields with oil-gas bearing basins of tectonic active marginal platforms located on the areas which have intensive lateral stress.

Formation of disjunctive and plicative dislocations in tectonic active areas of oilbearing basins leads to apparition of high pressures, elastic wave impulses and displacement stresses which stir up so called "mechanic-chemical" reactions. It lead to chemical interaction of sulphate rocks with hydrocarbons even under conditions of low temperature.

We carried out the experiments which conditions close to natural seismic-tectonic conditions. We used special apparatus which is called Bridgman's anvil. During experiments compression deformation has been done under high pressures. Specimens CaSO4+CnH?2n+2?were pressed tablets which were subjected to fast deformations' displacement under elastic wave force, pressure 1-20 kb. During the experiment the sample was subjected to one axis compression in the steel cylinder. The elastic waves were made with explosion under pressure of synthetic polymer plates which were located over steel piston. Mechanical force of elastic waves to specimens has been produced through the piston. Subjected to elastic waves the sat-nples were investigated with the help of radiospectrometres "Varian 12A" and "SE/X2547".

The experiments confirmed the suggested hypothesis. We fixed chemical reduction of sulphates by hydrocarbons according to the following scheme:

~(CH2-CH2)n-CH3?+ CaSO4?-(CH2-CH2)-CH2?+ (HSO42-)-
-(CH2-CH2)-CH2O2?+ (HSO42-) =RO2?+ (HSO42-) (1)

We suppose that more deep reduction of sulphates by hydrocarbons is realized in the earth's crust. The reaction between hydrocarbons and sulphates proceeds in two stages according to Angler-Gofer scheme:

CaSO4?+ CH4?- CaS + CO2?+2H2O

CaS + CO2?+2H2O - CaCO3?+ H2S (2)

During this reaction both H2S and CO2?are formed. Ascending fluids carrying H2S and CO2?reach the surface in the vicinity of the karstic carbonate rocks. The CO2?and carbonic acid dissolution is balanced by subsequent calcite cementation. The H2S arrives at the oxidation zone, sulphuric acid is formed which subsequently causes major dissolution and karstification. Significant pyrite mineralization during cementation filling the fractures in carbonate reservoirs is strong indication of H2S formation after generation fractures. The chemical reduction of sulphates by hydrocarbons according to this scheme is confirmed by isotope determination of the secondary calcite and sulphur in karstified carbonate oil-gas bearing reservoirs.

The effect of aqueous solution containing H2S, CO2?and products of their oxidation (H2SO4?and H2CO3) on carbonate rocks promote their karstification and formation of complicated carbonate reservoirs as shown in figure. Such complicated karstified reservoirs have been found in the Timan-Pechora basin both on the surface and in the depth of 3,000 m or more.

As a result of our experiments we made conclusions as follows:

1.The experiments show that chemical reduction of sulphates by hydrocarbons is possible in conditions of earth's crust under low temperature.

2.In the conditions of impulsive high pressure with displacement's influence on CaSO4?+ CnH2n+2?paramagnetic particles (HSO4?2-?) and RO2?are formed. This confirms that dehydration of hydrocarbons takes place and a hydrogen atom moves to sulphate.

3.It is confirmed that in the presence of changeable valency metal like Co in admixture (CaSO4?+ CoSO4?+ CnH2n+2) the particles of Con?are formed. This shows that deep reduction processes take place. The changeable valency metal presenting in these reactions takes part in transportation of electrons. They are likely to include further reduction of (HSO4?2-) . Rather more the deep reduction of sulfates take place and sulphur are reduced to the form S2-?which as H2S is volatilized.

4. The experiments explained the reason of sulphureous gas and oil's location into carbonate sulphate rock association. The results of the experiments can be used for prediction of sulphureous gas and oil, if there is some information about neotectonic and seismic active local areas and thickness of sulphate rocks in the section.

5. During chemical reduction of sulphates by hydrocarbons the acidic gases H2S and CO2?are formed. As result of dissolution of these gases in the water the acids formed, which promote active karstifing of carbonate rocks. New sulphate and carbonate minerals are formed during such karst development processes. Deposition of these minerals into karstified carbonate reservoirs completes carbon cycle in the geochemical processes.

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Fig.1 Conceptual model of fracture-karst zones' formation over petroleum deposits

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Tufa deposits in the Taroko Gorge limestone area, Taiwan,China

Kazuhisa YOSHIMURA?1), Youji INOKURA?2), Kota TAKAHASHI?3),
Yuan-Yang Chen?2)?, Ching-Nan Liu?4)?and Morgan Chen?4)

1)Department of Chemistry, Faculty of Science, Kyushu University,
Ropponmatsu, Chuo-ku, Fukuoka, 810 Japan
2) Research Institute of Kyushu University Forests, Sasaguri,
Fukuoka, 811-24 Japan
3)Department of Agricultural Engineering, Faculty of Agriculture,
Kyushu University, Hakozaki, Higashi-ku, Fukuoka, 812 Japan
4)Taroko National Park Headquarters, Fu-su, Hualien,

Taiwan , China

(From J.speleol. Soc. Japan, 21: 59-64, November 15, 1996)

Abstract

During a speleological survey conducted in the Taroko Gorge limestone area by the Kyushu University Exploration Club in cooperation with the Taroko National Park Headquarters, tufa deposits were found for the first time in Taiwan. A tufa cascade about 20 m high has formed from a karst spring near the Liwu Hsi River in the garden of Changchun Temple. The cliffs upstream of Changchun and in Meiyuan, a tributary area of the Liwu Hsi River, are covered with tufa deposits. The tufa-forming water in these areas was supersaturated with calcite, but otherwise contained no typical chemical components. Annual layer structures, characteristic of tufa deposits in Japan, could not be observed; because of the subtropical climate in Taiwan, there are no significant variations in biological activity. Bryophytes and/or higher plants were enclosed in the tufa deposits due to high deposition rate.

Fig.1 Tufa deposits in the Taroko National Park and the Taroko Gorge limestone area, Taiwan. (a) Changchun Temple; (b) cascade tufa of Changchun Temple; (c) cross-section of tufa deposit of Changchun Temple; (d) cliff covered with tufa deposit upstream of Changchun Temple; (e) cave entrance covered with tufa cascade in Meiyuan, a tributary area of the Liwu Hai River.

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