Heat flow distribution and some aspects of formation of thermal field in the Caspian region

Размещено на http://www.allbest.ru/

heat flow distribution and some aspects of formation of thermal field in the Caspian region

heat flow thermal field

Summary

The paper deals with distribution of heat flow in the water area of the Caspian Sea. Geothermal data from this region are given as results of measurements by sea thermal probes and as data from wells from the shelf zone. Moreover, there were used data from wells from the coastal zones. There was constructed a map of heat flow of the Caspian Sea water area. There were discovered high anomalies of the heat flow that were not reflected in the previously constructed maps. The areas of the earth crust where non-conductive heat-transportation prevails link these anomalies. It may be active zones of faults, mud volcanos and other types of dislocations of the earth crust.

Preface. The Caspian region with its energetic resources has been in the centre of attention for a long time. Investigation of the heat field is very important for the assessment of oil and gas potential of the region. Different companies conduct vast geothermal investigations in this region: in deep zones of the water area - by marine and in the shelf - by well technique.

Geological background. The Caspian Sea is the world's largest lake which was formed in site of Meso-Cenozoic sea basins of Tethys and Paratethis existing there before. One can identify five large geostructural elements in the Caspian Sea: the South Caspian basin (SCB), the Absheron Sill, the Middle Caspian basin (MCB), the Mangyshlak Sill and the North Caspian basin (NCB).

The SCB for a short geologic period subsided deeply. According to A.V. Mamedov's data [1998] the maximum thickness of Jurassic deposits is up to 4000 m (south of the Absheron Sill including its south flank). The Middle Caspian turned into land and in the South-Mangyshlak depression thickness of Jurassic deposits is up to 1800-2000 m. The Cretaceous in the Caspian Sea and its framings continued tendencies of the Jurassic. In the SCB the downwarping became more intensive (the maximum thickness of Cretaceous deposits is more than 4000 m). In the MCB the maximum thickness of Cretaceous deposits is more than 2400 m and the NCB it is up to 1400 m. The maximum downwarping in Paleogene occurred in the SCB. It envolved a part of the Absheron Sill (2500 m and more). In the Middle Caspian the maximum thickness of Paleogene deposits is up to 1400 m and in the NCB it is 2000 m. The intensive downwarping started in the Early Pliocene and in Quaternary it went on intensively. Total thickness of Neogene-Quaternary deposits in the SCB is up to 10 km and in the NCB it is only 4 km [Mamedov, 1998].

Factors influencing the heat flow value. According to our calculations the sedimentation rate in Jurassic in the SCB was 120-180 m/My, if the maximum thickness of the sedimentary cover for about 30 km. In Cretaceous and in Paleogene it became lower and in Pliocene it reached avalanche values - 1,8 km/My. Results of modeling of thermal evolution of the basin with account of non-stability of the heat field demonstrated that temperature in the base of the sedimentary layer changed 400-500^oC [Mukhtarov and Adigezalov, 1999; Mukhtarov et al., 2003]. Opinions have appeared lately on that the total thickness of the sedimentary cover in the SCB is up to 30 km. This may result that the rate of sedimentation at early stages of sedimentation will be more than it was calculated for the 20 km thickness of sediments. As a result share of deep heat flow in the sedimentary thickness will be much more lower.

Investigators Levin and Viskovski [2000] think that in Jurassic the rate of sedimentation in the South Caspian basin varied from 10-25 to 50 m/My. Temperatures in the base of the system were 150-200^oC. They increased in deeper blocks up to 300-450^oC. In Cretaceous the rate of sedimentation was 2,5-10 m/My. Temperatures in the base were from 50-150 to 250-300^oC. In Oligocene-Miocene system the rate of sedimentation was 0,025-0,4 km/My. Temperatures in the base of Miocene deposits were 50-100^oC and in some blocks they only were up to 200^oC. In Pliocene-Quaternary system the rate of sedimentation changed from 0,75 to 1,75 m/My. Temperatures were from 100-150^oC to 200-300^oC.

One should take into account that in the SCB there were drilled a lot of wells. While considering temperature data from these wells one can be sure than the temperature regime of sedimentary layers uncovered by the wells is moderate enough [Table 1].

Sedimentation rate is one of the factors affecting the heat regime in sedimentary basins. With avalanche rates of sedimentation there occurs intensive decay of deep heat flow. For this reason one can observe relatively low values of the heat flows density throughout the Caspian region.

Table 1. Borehole maximum temperature data in SCB

It should be noted that values of the heat flows determined by a well method (20-40 mW/m² in wells of the Baku archipelago and in the Absheron Sill) were lower than values of the heat flows determined by sea sondes (30-50 mW/m² and more). Without special investigations one can hardly judge about the reason of this difference.

1. Heat flow distribution map in the water area of the Caspian Sea

Crosses - points of determination of density of the heat flow by a well method.

Rhombuses - points of determination of density of the heat flow by see thermal soundings.

Heat flow distribution map in the water area of the Caspian Sea. To construct the map of heat flows in the water area of the Caspian Sea (Fig. 1) there were used results of the determination of the heat flow by marine thermal probes (Catalogue …, 1973; Lubimova et al., 1976; Tomara, 1979; Aliyev et al., 1979; Lebedev and Tomara, 1981). Moreover, there were used data obtained by the well method in the shelf zone and in the on-shore territory (Catalogue …, 1973; Kashkai and Aliyev, 1974; Ashirov, 1984; Aliyev, 1988).

The main features of distribution of the heat flow in the water area of the Caspian Sea are very well correlated with tectonic peculiarities of its deep structure. For instance, increased values of the heat flow are conditioned by the impact of such tectonic structures like faults and mud volcanoes. One of the stations in the west of the SCB recorded abnormally high value (480 mW/m2) of the heat flow (Fig. 2). The available geophysical data do not explain this anomaly as impact of the intrusive body [Lebedev and Tomara, 1981]. That is due to the evacuation of fluids along the active dislocations (faults or vents of mud volcanoes).

2. Complex geothermal and seismoacoustic profile through the SCB (position of the profile is shown in Fig. 1)

I - temperature of the upper layer of sea bottom sediments, ^oC; II - heat flow, mW/m²; III - seismoacoustic profile of the sedimentary series as deep as the top of the Middle Pliocene: a - zone of faults near the west coast; b,c - mud volcanos. 1-14 - numbers of stations.

It should be mentioned that measurement of the heat flow is of a point character and main geologic factors affecting density of the heat flow are of a local character. All these require special investigations in the areas covering local areas of active faults and mud volcanoes and remote calm areas. Distribution of mud volcanoes and heat flows in the SCB (Fig. 3) proves the above mentioned. The mud volcanoes are spread in the areas of contoured isolines of the heat flow 40 m Bt/m2. Anomaly of the heat flow of 600 mW/m² in the South Caspian (Fig. 2) in the authors vision [Tomara, 1979] corresponds to a dislocation discovered by seismoacoustic profiling and geothermal measurements.

3. Distribution of mud volcanos and contours of density of heat flows in the SCB

The western part of the SCB has a complex picture of the heat field corresponding to its tectonic structure, which is complicated by faults and underwater mud volcanoes. The heat flow usually varies 20 to 70 mW/m² there. At the same time there were recorded acute anomalies, which on order exceeding normal heat flow. The eastern part of the SCB is characterized by quiet thermal regime.

The MCB on the whole is characterized by calm geothermal regime with average heat flow of 50 mW/m². In the Derbend depression there was recorded a local anomaly in three points (210 and 134 mW/m²). North of this anomaly there was investigated a profile where density of the heat flow varies from the west eastwards from 54 to 92 mW/m². South of the anomaly there was investigated a profile where the average value of density of the heat flow is 52 mW/m² [Tomara, 1979].

There were expressed many opinions on these anomalies. The most probable of them is the underwater discharge of thermal waters due to the pinching out of high-thermal water-bearing horizons in the given area of the sea floor. At the same time other processes should be taken into account as well. They are lithogenic heat-generation which in terms of the Derbend depression may increase the observed heat flow by 42 mW/m², mud volcanism, effects of transformation of the organic matter of the sediment, effects taking place in the boundary of water-sediment (which have not been studied well enough yet), etc.

To create a complete picture of fields of heat flows let's consider vicinities of the Caspian Sea. The Central-Mangyshlak rift zone characterized by the heat flow density 58 mW/m². To the west that, the East-Manych rift zone is situated: thickness of the earth crust is about 32 km, temperature in the Mohorovicic surface is 650^oC, and the heat flow density is 55-60 mW/m². The Tersk-Caspian zone: thickness of the earth crust is about 32-35 km. Temperature in the base of the crust is about 600^oC, the heat flow density is 35-50 mW/m2. The South-Emben paleorift is characterized by heat flow of 58 mW/m2. Thickness of the earth crust is 10-16 km [Murzagaliyev, 1998].

Discussion. Thus, the thermal field in the water area of the Caspian Sea is rather complex. It is conditioned by complex deep structure of the region and geodynamic processes. Thermal field of the Caspian Sea water area is associated with the existence of different contradiction opinions (fixism and mobilism) which have been in geologic science up to now. For this reason some data were not published or were not used in the mapping. This, fig. 2 demonstrates a point of determination of density of the heat flow where its value is 480 mW/m² [Lebedev and Tomara, 1981]. In the previous papers [Tomara, 1979] the author thoroughly describe measurements and proved that this anomaly is not a mistake but a result which was justified there times. Moreover, density of the heat flow was 600 mW/m² in this area (14,5 ?cal/cm² sec). May be author tried to change this anomaly a little in their further publications (up to 480 mW/m²). Later on this abnormal values (209 mW/m² in the Middle Caspian) were not used anywhere among ordinary values 30-80 mW/m².

Now we possess new data demonstrating possibility of rather higher values of the heat flow anomalies. Near mud volcano Nakon Mosby in the Barents Sea there were recorded heat flows up to 1045 mW/m² [Eldholm et al., 1999].

Geothermal investigations in the crater of mud volcanoes in Azerbaijan demonstrated that thermal gradients (heat flows as well) in mud volcanoes may several times higher than heat flows normal for the earth crust [Mukhtarov, Adigezalov, 1997].

This data demonstrate disintegrated character of the crust and high fluid-dynamic activity in the boundaries of the dissected areas. In these boundaries processes of heat-transportation are shown by equations of heat-mass transportation in porous media. As a result in local areas abnormally high heat flows may be formed.

Its main peculiarity is that it allows generation of hydrocarbons in deep (8-15 km) horizons of the Caspian region [Mukhtarov, Adigezalov, 1999; Mukhtarov et al., 2003].

References

1.Aliyev S.A., Ashirov T., Sudakov N.P. New data on the heat flow through the Caspian Sea floor // Izv.AN Turkm. SSR, ser. phystech. and geol. sciences, 1979, №2, p.124-126 (in Russian).

2.Aliyev S.A. Geothermal fields of depression zones in the SCB and their relation with oil-gas potential // Authors thesis … doc.geol.-min. sciences. Baku: GIANAS. - p.30

3.Ashirov T. Geothermal field of Turkmenia. - M.:Nauka, 1984. - p.160

4.Eldholm O., Sundvor E., Vogt P.R., Hjelstuen B.O., Grane K., Nilsen A.K., Gladczenko T.P., 1999, SW Barents Sea continental margin heat flow and Hakon Mosby Mud Volcano, Geo-Marine Letters, 19; 29-37.

5.Kashkai M.A., Aliyev S.A. Heat flow in the Kura depression / In book: Deep heat flow in the European part of the USSR. - Kiev: Nauk. Dumka, 1974, p.95-108.

6.Catalogue of data on heat flow in the territory of the USSR. M.: 1973. - p.64.

7.Lebedev L.I., Tomara G.A. Some peculiarities of distribution of heat flow in the South Caspian. /In book: Geothermometers and paleotemperature gradients.-M.:Nauka, 1981, p.156-161.

8.Levin L.E. Thermal regime and oil-gas potential of sedimentary basins of the Black Sea. Caspian region. // Exploration and protection of the earth interior, 2001, №2, p.9-13.

9.Levin L.E., Viskovski Yu.A. The SCB: evolution and thermal regime of oil-gas systems. /In book: III Azerbaijan International Geophysical Conference. Abstracts. Baku: 2000. p.239.

10.Lubimova E.A., Nikitina V.N., Tomara G.A. Thermal fields of the inner and marginal seas in the USSR. - M.: Nauka, 1976, p.224.

11.Mamedov A.V. The Caspian Sea in Mesozoic and Cenozoic. // Izv. AN Azerbaijana, series of Earth Sciences, 1998, №1, p.3-11.

12.Murzagaliyev D.M. Geodynamics of the Caspian region and its reflection in geophysical fields. // Geology of oil and gas, 1998, №2 p.10-15.

13.Mukhtarov A.Sh., Adigezalov N.Z. Thermal regime of mud volcanos in the East Azerbaijan. Proceedings of Geology Institute, Baku, 1997, issue 26, p. 221-228.

14.Mukhtarov A.Sh., Adigezalov N.Z. Thermal evolution of the Lower Kura depression and terms of hydrocarbons maturity (example of Kurowdagh field). // Izv. AN Azerbaijana, series of the Earth Sciences, 1999, №1, p.14-20.

15.Mukhtarov A.Sh., Tagiyev M.F., Imamverdiyev R.A. Models of oil-gas generation and prediction of the phase state of hydrocarbons in the Baku Archipelago. // Izv. AN Azerbaijana, series of Earth Science, 2003, №2, p. 17-25.

16.Tomara G.A. Heat flow of deepwater depressions in the Caspian Sea. / In book: Experimental and theoretical studies of heat flows. M.: Nauka, 1979, p.99-112.

App

Drawing

Drawing

Drawing

Размещено на Allbest.ru