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SGP-TR-204
Characteristics of Geothermal Reservoirs and Structural Geology for the Yangbajing
Geothermal Field: A Case Study for Formation of Geothermal Resources in South the
Nyaiqentanglha of Tibetan Plateau
Jianyun FENG, Zhiliang HE, Ying ZHANG, Zongquan HU and Pengwei LI
Mailing address, Petroleum Exploration and Production Research Institute of SINOPEC, Beijing 100083, China
E-mail address, [email protected]
Keywords: Yangbajing, Nyainqentanglha, Tibetan plateau, geothermal reservoirs, structure, geothermal channel
ABSTRACT
The Yangbajing geothermal field is situated at the central of the Dangxiong-Yangbajing fault basin, and is wedged between the
Nyainqentanglha metamorphic basement to the northwest and the Gangdese magmatic arc to the south. According to differences of the
burial depth and the fluid characters among the geothermal reservoirs in the geothermal field, the reservoirs are divided into two types
of shallow and deep-seated, both of which are belong to the same hydrothermal system except for different depth and rock composition.
The shallow-type reservoirs are formed by lateral-flow supply from deep-seated thermal fluid to the shallow Quaternary pores, buried
under the earth surface in a shallow depth, which are composed of Cenozoic alluvial-diluvial sandstone, conglomerate, tillite and
weathered granites. The deep-type reservoirs are distributed in north of the field, concentration and migration of the fluid are restricted
by regional faults and structural broken belt rigidly that saved most of the thermal fluid in the particular depth and area, the typical
thermal reservoir of structural-pores basement. Unlike the shallow-type reservoirs, the deep-seated ones consist of mylonitic granites,
granitic mylonites and broken granites that underwent both ductile shear and brittle shear deformation. These reservoirs and the
structures are universally developed in the whole basin south the Nyainqentanglha with the Yangbajing field as a typical case that
indicates a huge geothermal resource potential area in the basin. Under the background of rapid uplifting of the Nyainqentanglha, an
extensional environment formed in the Dangxiong-Yangbajing fault basin and connected the shallow normal faults with the deep-seated
ductile strike-slip faults, which brought the anatexis magma of metamorphic complex to the shallow crust that performed as a perfect
geothermal channel for the field. Thus the characteristics of geothermal reservoirs and structures enable the south foreland area of the
Nyainqentanglha to be the richest area of geothermal resources in Tibet.
1. INTRODUCTION
According to the theory of plate tectonics, there are three main plate boundaries, such as divergent, convergent, and transform,
corresponding to mid-ocean ridge, trench, and transform fault respectively. In the three boundaries, magmatism and accompanying
geothermal activities can be resulted usually from volcano eruption or magma intrusion that triggered by asthenosphere convection
except the transform fault. Thus the global geothermal belts are formed strictly in accordance with the relatively narrow plate
boundaries of divergent and convergent. For instance, the circum-Pacific geothermal belt is the boundary between the Pacific plate and
the Euro-Asia, the America and the other adjacent plates, while the Mediterranean-Himalayan global geothermal belt separates the
Euro-Asia plate and the India-Australia plate.
In consideration of the most advanced relics of interaction between two plates, mountain belts created by continent-continent collision
are perhaps the most dominant geologic features on the surface of the Earth. The youngest and arguably most spectacular of all is the
Himalayan-Tibetan orogen, which occupies the east-west trending, high-altitude Himalaya and Karakorum ranges in the south and the
vast Tibetan plateau to the north. This orogenic belt, largely created as a result of the Indo-Asian collision over the past 70±50 Ma, is
part of the greater Himalayan-Alpine system that was developed by the closure of the Tethys oceans between the two great land masses:
Laurasia in the north and Gondwana in the south(An, 2000). With the collision and compression continuing, the plateau has been
doubled in Crust gradually compared to the average thickness of the Crust, and has been uplifted to “roof of the world” under the
gravity equalization. Meanwhile, the thermal circumstance of the Crust is changed to constitute eastern part of the MediterraneanHimalayan global geothermal belt. In virtue of collision, the plateau is a patchwork of east-west trending blocks with a newer tendency
from north to south separating by sutures of different geotimes. For instance, the plateau is comprised of Himalayan Orogen(HS), YaluBrahmaputra River Suture Zone(YBRSZ), Lhasa-Gangdese Magmatic Belt(LGMB), Bangong Nujiang Suture Zone(BNSZ), Qiangtang
Terrane(QT) (Figure 1). As the YBSZ is regarded as the boundary of India and Asia plates with the strongest tectonic activity, along the
belt and its adjacent regions like the LGMB is the most favorable target for formation of geothermal resource. Actually, the study area
of this paper, the Yangbajing geothermal field, is situated at the LGMB in central of the Dangxiong-Yangbajing fault basin, which is
formed in an extensional environment characterized by a series of strike-slip and tensile dominated faults with northeast, northwest and
nearly south-north trending (Wang et al., 2011). This paper aims to introduce the characteristics of geothermal reservoir, geological
structure and potential geothermal resource of the Yangbajing geothermal field and its peripheral regions.
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Figure 1: Sketch geologic maps for East Asia, Tibet and its adjacent areas. The Yangbajing geothermal field is shown as a black
rectangle on the subjacent magnified map.
2. GEOLOGICAL CHARACTERISTICS
The Yangbajing geothermal field is occupied in center of the Tibetan Plateau between the Dangxiong County and Nimu County in the
LGMB as a faulted basin of Yadong-Kangma-Yangbajing-Naqu regional active tectonic zone, southeast of Nyainqentanglha Mountain,
separates the Angang basin to the south and the Ningzhong basin to the north in NE-SW trending. There are no outcrops in the basin but
Quaternary systems of loose deposits except connection area of range and basin. The loose deposits are mainly of alluvial and proluvial
scattered from piedmont to central basin. In southeast and northeast margin of the Yangbajing basin near the mountains, a rock
association of low-grade metamorphosed strata developed locally in the area, including black slate, siltstone, quartzite, schist, limestone,
which is considered to be formed in Triassic to Late Cretaceous with biostratigraphical evidences of Cucullale sp. and Aslarto sp. in the
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limestone. Though the bedrock of Nyainqentanglha Mountain is also be concluded to be formed in Mesozoic for its metamorphic extent
and residual sedimentary structures within the rock, which is called Nyainqentanglha Rock Group composed of biotite hornblende
gneiss with a granitic migmatization, its real time of formation is still debatable for pre-Cambrian or Mesozoic that we treat them as an
unknown-age rock association in this paper. Besides, a suite of Upper Cretaceous intermediate-acid volcaniclastic rocks were
discovered overlying on the Mesozoic strata with uncomformity contact in northeast of the basin and Laduogang areas. Above the
Upper Cretaceous volcaniclastic rocks, the Miocene red clastic rock, such as sandstone and glutenite developed at Ladura area in
northeast of the basin with a little uncomformity contact relationship. In northeast of the basin and south margin of Nyainqentanglha
Mountain, a suite of fault-controlled extrusive rocks overlying on the Miocene clastic rock in uncomformity, be composed of andesite,
andesite rhyolite, quartz andesite, volcanic breccia, and tuffaceous rock(Figure 2).
Figure 2: Sketch Geological map for the Yangbajing geothermal field.
The normal and strike-slip faults of detachment are distributed in both sides of the basin adjacent to the mountains. And formation of
hydrothermal system in the geothermal field has direct relationship with secondary faults of the fault system south the Nyainqentanglha,
in trending of northeast, northwest and north northeast in chronological order respectively (Figure 3). The faults develop in the west side
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are more completely and actively than the east side, and are mostly dipped to the central basin with a little steep dip angle of about 60°
(Zhang, 2010). More interesting is that, the south-north trending faults are mostly normal, while the northeast ones are sinitral strikeslip, most probably resulted from inhomogeneity of oriented stresses with different direction and intensity.
Figure 3: Main faults developed in the Yangbajing geothermal field.
The Yangbajing Basin is estimated to be formed during period from late Mesozoic to early Cenozoic, and has received more than 300m
to 400m continental clastic material deposition in thickness since Miocene. As the Cenozoic tectono-magmatic event burst into a climax
in Neogene, the piedmont margin inherited faults reacted again accompanying by magmatism along the fault planes, and the initial
geothermal activity in the area commenced from then on. After that, the main faults and basin graben continued to develop induced by
rapid uplift of the Himalayan Mountains in Late Pleistocene, and northwest trending faults formed in Middle Pleistocene tillite cutting
the previous formed northeast trending faults. In intersection and unsubstantial part of the two groups of faults, deep-seated thermal
fluid can ascend to surface or subsurface of the earth resorting to the tectonic fracture channel. In view of alteration strata of Middle
Pleistocene to Holocene, all the hydrothermal alteration developed in Middle Pleistocene tillite except the overlying Upper Pleistocene.
Thus, geothermal activities of the Yangbajing basin began in Pliocene of Neogene, and the geothermal field completely formed in
period from late Middle Pleistocene to early Late Pleistocene.
3. GEOTHERMAL RESERVOIR CHARACTERISTICS
The basement rock of the geothermal field is predominated by Cenozoic granites with Quarternary loose gravel caprock on the surface.
Underwent multiple tectonic and magmatism in the strong stressed region, the rocks were broken and fractured to form structural broken
belt with different type and strength extent, providing a perfect channel and geothermal reservoir for ascending of thermal fluids. The
geothermal reservoir in the area can be divided into two types of shallow geothermal reservoir and deep-seated geothermal reservoir
according to their depth and petrology (Duo, 2003).
3.1 Shallow Geothermal Reservoir
The shallow geothermal reservoirs distribute in range from 4210 meters to 3930 meters downward above sea level with an average
thickness of 240 meters, of which mainly are Middle Pleistocene gravel strata and underlying weathered broken granites with porosity
and permeability developed very well. The thermal fluids are occurred in fractures of basement rocks and pores of gravle caprocks,
belonging to geothermal reservoir of basement fracture and Quarternary porous types which maximum temperature can reach to 150℃170℃. And the wellhead temperatures and pressures from the reservoir are commonly 125℃-140℃ and 0.12MPa-0.32MPa separately.
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In the whole, the shallow geothermal reservoirs are occurred in the field with relatively low temperature, low pressure and low fluid
flow, but distribute in an extensive area, especially in regions for faults intersection or uplifting of basement rocks.
3.2 Deep-seated Hyperthermia Geothermal Reservoir
The deep-seated hyperthermia geothermal reservoirs in the geothermal field are occurred below altitude of 3630 meters underneath the
ground, and are composed of mylonitic granites, granitic mylonites, and broken granites underwent a diplex deformation both of ductile
and brittle shearing. Accumulation and migration of deep-seated hyperthermia fluids are controlled strictly by northeast faults in the
area in fault structural fracture zone and basement fractures at certain depth, belonging to structural basement fracture type of
geothermal reservoir. According to borehole data, the deep-seated hyperthermia geothermal reservoir can be divided into two parts. The
first part is discovered in altitude range of 3630 meters to 3120 meters underneath the ground with a thickness of 350 meters and
maximum temperature higher than 250℃, of which the wellhead temperatures and pressures from the reservoir are commonly 200℃
and 0.5MPa-1.5MPa separately. And the second part distributes in the range from 3030 meters to 2420 meters above sea level with a
thickness of 610 meters and most temperatures over 300℃, even 330℃ as maximum. Thus the deep-seated geothermal reservoirs are
more favorable than the shallow ones for electric power generation.
To reserve the geothermal fluids preventing from vaporizing or draining away, there must be caprocks and fluid-resisting layer to
sandwich the geothermal reservoirs in proper structures. For instance, the caprocks for shallow geothermal reservoirs are kaolinized
gravel deposits, granites and volcaniclastic rocks with strong alteration in the altitude over 4150 meters, and the sedimentary thickness
of which is about 210 meters in average. While the caprocks for deep-seated geothermal reservoirs are relatively intact and hard granites
in altitude from 3970 meters to 3470 meters with an average thickness of 400 meters. And the fluid-resisting layers for deep-seated
geothermal reservoirs are mylonitic granites and granitic cataclasite in altitude from 3530 meters to 2960 meters with thickness from
160 meters to 500 meters.
Together with other data, we consider that the hydrothermal system of Yangbajing geothermal field is supplied by ice and snow melt
water and precipitation water from Nyainqentanglha Mountain downward along fault structural fracture zone to deep-seated aquifer, and
is formed through the circulating and heating in deeply water supplies within some specific depth and scope. After heated by magma or
deep-seated convection and injected with renewable water, the hyperthermia fluids with high enthalpy begin to ascend along the
regional deep fractures and faults to form deep-seated geothermal reservoir of basement fracture type at altitudes of 2430m-3030m and
3120m-3630m. The remaining thermal fluids keep on ascending triggered by vertical active faults and geothermal convection, and
gradually begin to vaporize and escape non-condensable gases and water vapor that form acid alteration caprocks for shallow
geothermal reservoir due to decompression when encountering mixing with cold water from northwest. As the ascending thermal fluids
are blocked by the caprocks, the fluids migrate laterally along the NE-SW trending faults to collect in shallow geothermal reservoirs.
Therefore, the shallow geothermal reservoirs have close relationships with the deep-seated reservoirs, and are productions of heating
and supplying from the latter.
4. TEMPERATURE FIELD
Temperature is a crucial parameter for the geothermal field when evaluating quality of the geothermal resource. The hyperthermia fluids
migrate and collect in permeable reservoir, and tend to gather in proper area to form a geothermal field or hot spring under the favorable
conditions of geology and hydrogeology (Zhao, 2010). In the whole process, temperature field reflects storage and movement status of
hyperthermia fluids in the geothermal field.
Both the north and the south areas of the Yangbajing geothermal field have hyperthermia centers showing on the planar temperature
contour maps of different burial depth, forming a hyperthermia belt in north-south trending in the whole. Measured maximum
temperature for the north area is 172℃ and the south area is 160℃, which have a clear gradation of cooling outwards from the centers
on the planar temperature contour maps. In the burial depth of 50 meters, the fluids over 150℃ exist only in the south area; in 100
meters, the fluids over 150℃ exist in both the south and the north areas and the former bigger than the latter in area; in 150 meters, the
fluids over 150℃ also exist in both the south and the north areas and nearly with the same area; while in 200 meters, the fluids over 150
℃ exist in both the south and the north areas and the latter bigger than the former in area (Figure 4).
In north area of the field, as there are no caprocks, the upper mixing fluids of hot and cold waters (with the temperature lower than 85℃
) reach to dynamic equilibrium with the lower hyperthermia fluids, covering a coat on the underneath hot fluids and restraining flashing
of steam. The natural steam is hardly to generate through convection equilibrium when cold water supply from mountains is limited.
Thus the temperature field in the north area has a temperature gradient of 1.0℃/m, reflecting the mixing fluids in the upper part. The
shallow geothermal reservoirs are heated by hot water conduction.
In south area of the field, the upper temperature gradient is about 0.8℃/m-1.5℃/m, indicating a warming clay caprock of conduction;
the middle hyperthermia gradient is low or zero, implying a convection of hot water in Quarternary geothermal reservoirs; while the
lower negative temperature gradient represents a cooling down status in basement rocks. The hyperthermia fluids reserve in strata with
high permeability and cool down outwards rapidly.
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Figure 4: Isothermal maps of the Yangbajing geothermal field for different burial depths from 50 meters to 200 meters.
5. PRESSURE FIELD
Pressure is another pivotal parameter for the geothermal field when evaluating quality of the geothermal resource as it may affects
efficiency of the electric power generation. All the geothermal fluids migrate and gather in reservoirs directed by pressure decreasing
from internal to external and shared a co-pressure surface with foreign cold fluids in margin of the geothermal field. As more and more
geothermal resources extracted, pressure of geothermal fluids will decrease sharply and co-pressure surface will gradually shrink
inwards accordingly, which attracts more and more cold fluids intruding into the field outside the area.
Comparing the pressure distribution map with the temperature field distribution map, the two parameters coincide with each other very
well and high pressure areas are also high temperature areas, implying the predominant role of high enthalpy (Figure 5). Temperature in
the north area is higher than the south area, but the pressure is reversed, which is conducted by burial condition of geothermal reservoirs
and seepage field. As deficiency of caprocks in the north area, cold fluids can intrude into the shallow geothermal reservoir easily and
join in the shallow water-circle and convection reserving a low temperature water-cap of several decameters thick on the underneath
reservoir. The low temperature water-cap prevents the hyperthermia fluids from vaporizing freely and vaporized itself to steam-cap
when thinning to some extent and maintaining an equilibrium status between steam and pressure for the hyperthermia fluids. Therefore,
lower pressure of the north area in the field results from pressure loss and heat dissipating during process of convection, conduction and
natural vaporization of near surface heat exchange in an open condition.
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Figure 5: Temperature and Pressure fields for the Yangbajing geothermal field.
6. GEOTHERMAL FIELD MODEL
The Yangbajing geothermal field occurred in strike-slip and detachment fault system south the Nyainqentanglha Mountain, of which the
fault structural fracture zone in metamorphic complex configuring hyperthermia fluids ascending area of the geothermal field. Thus the
strike-slip and detachment fault system becomes the basic and crucial geologic condition for formation of the field. The regional south
margin fault of the Nyainqentanglha Mountain acted since Jurassic in sinistral strike-slip movement that formed metamorphic complex
and ductile sheared zone. After that, the anatexis magma ascended and intruded into the metamorphic complex to act as a shallow
magmatic heat source in a burial depth of about 5 kilometers. In the circumstance, some certain areas began to appear heat abnormity.
Tibetan Plateau has been uplifted in the whole since late period of Pleistocene, and consequently induces a rapid lifting of the mountains
in both side of the Yangbajing basin and enables deposit of Middle Pleistocene-Holocene. At the same time, a series of secondary
inherited faults in NE, NW, and NNE trending developed in margin of the basin and cut the basin into a rhombic shape. Supplied by
precipitation, the hyperthermia fluids gathered to form a hrdrothermal system above the magmatic heat source at appropriate depth and
reservoir in strike-slip and detachment fault system, particularly in shallow brittle deformed extensional faults, protogenic joints and
secondary fractures. A groundwater circulation system was formed by gathering and migration of the hyperthermia fluids in center of
the fault system, and brought the fluids into surface or subsurface through shallow fluids upflow along structural channels (Figure 6).
Figure 6: Ideal model for formation of the Yangbajing geothermal field.
7. CONCLUSIONS
Based on detailed analysis of tectonic setting, geology, structural geology, reservoir, temperature and pressure, the following
conclusions can be drawn from the results described above.
The Yangbajing geothermal field occurred in Yangbajing Cenozoic basin in Lhasa-Gangdese Magmatic Belt between YaluBrahmaputra River Suture Zone and Bangong Nujiang Suture Zone, controlled by strike-slip and detachment fault system south the
Nyainqentanglha Mountain.
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Formation of hydrothermal system in the geothermal field has direct relationship with secondary faults of the fault system south the
Nyainqentanglha acting as a perfect geothermal channel, in trending of northeast, northwest and north northeast in chronological order
respectively. The hydrothermal activities display strongly in intersection areas of the several groups of faults, and have joined in
geothermal circulation since about late period of Middle Pleistocene – early period of Late Pleistocene.
There are two types of geothermal reservoir in the Yangbajing geothermal field, such as shallow basement fracture and Quarternary
porous reservoir, and deep-seated hyperthermia structural basement fracture reservoir.
ACKNOWLEDGMENTS
The authors would like to thank Xiancai Hu for making time available to give some constructive suggestions and precious data. And we
would also like to acknowledge all those who have worked in this field over the last 30 years and it is to them we thank for our
understanding of the basic geology to progress.
REFERENCES
An, Y.: Himalaya and Tibet: Mountain Roots to Mountain Tops, Journa of Asian Earth Sciences, Book Review, 18, (2000), 507-508.
Duo, J.: The Basic Characteristics of the Yangbajing Geothermal Field - A Typical High Temperature Geothermal System, Engineering
Science, 5(1), (2003), 42-47.(in Chines with English abstract)
Wang G. C., Cao K., Zhang K. X., Wang A., Liu C., Meng Y. N. and Xu Y. D.: Spatio-temporal framework of tectonic uplift stages of
the Tibetan Plateau in Cenozoic. Science China of Earth Sciences, 54, (2011), 29–44, doi: 10.1007/s11430-010-4110-0 (in Chines
with English abstract)
Zhang Z. Y.: Temperature inversion of remote sensing data zone and Investigation of geothermal anomaly in Yangbajing Region of
Tibet. Thesis for Master Degree of Chengdu University of Technology, (2010).(in Chines with English abstract)
Zhao A. C., Zhao Y. S., Guo J. J. and Zhang N.: Study of Geothermal Extraction Scheme of Hot Dry Rock in Tibetam Yangbajing
Region. Chinese Journal of Rock Mechanics and Engineering, 29(2), (2010), 4089-4095. (in Chines with English abstract)
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