Spatial coupling relationships of gas hydrate formation in the Tibetan Plateau

Qiang Zhou , WanLun Li , WeiTao Chen , YongJiang Wang

1. China Aero Geophysical Survey & Remote Sensing Center for Land and Resources, Beijing 100083, China

2. China Geological Library, Beijing 100083, China

3. China University of Geosciences, Wuhan, Hubei 430074, China

1 Introduction

The Tibetan Plateau, considered as the world’s roof,attracts people from all over the word because of its unique climate, exquisite scenery and mystic qualities of its inhabitants. The Plateau has also attracted attention due to its extremely subtle geology, and rich endowment of ores and energy resources which are posed for excavation and development.

Due to construction and opening of the Qingzang Railway, the Plateau will experience a new vigorous period of growth and development. The need for energy will be a huge challenge for this development. Natural gas hydrate, a new source of next generation energy, has been found in several oceans and a pilot project in a high latitude permafrost area of Canada (Zhu, 2006), showing great application potential. Although permafrost regions in the Tibetan Plateau lie at middle latitudes, some specifications can be correlated with gas hydrates in high latitude permafrost regions (Zhang and Xu, 2001).Meanwhile, there is a potential for the existences of gas hydrates in the Plateau (Xu and Cheng, 1999). Chenet al.(2005) adopted a predictive method based on stable thermodynamic field, according to both thickness and geothermal gradients of the Plateau’s frozen zone, and predicted about 1.2×1011–2.4×1014m3endowment of gas hydrates in the Tibetan Plateau. Recently, Luet al. (2010)acquired experimental samples of gas hydrates through drilling at the Muli perennial frozen region on the south edge of Qilian Mountains. These activities have created enthusiasm for the exploration of gas hydrates in this region. However, with global climate change, the Tibetan permafrost may begin to thaw leading to unstable conditions in relation to preservation of gas hydrates,thus inevitably releasing a great deal of gas hydrates.The Tibetan Plateau, as a potential gigantic "carbon reserve", will certainly affect global climate as soon as the ore of gas hydrate is explosively released. Therefore,whether for the sake of energy or environmental protection, it is necessary to ascertain the distribution of gas hydrates on the Tibetan Plateau.

This paper has performed a comprehensive analysis of basic geology and petroleum geology, uplifted of the Plateau and formation of frozen earth, as well as glacial migration, and discussed the coupling relationship between petroleum geology of the Tibetan Plateau and gas hydrates both spatially and temporally. We conclude that the Tibetan Plateau has great potential for the development of gas hydrates.

2 Space-time correlation between formation of gas hydrate and petroleum resource

2.1 Petroleum resource on Tibetan Plateau

The Tibetan Plateau, lying at the eastern segment of the Tethys petroliferous tectonic region, has the highest production of oil and gas in the world as well as the most abundant reserves, and has developed a series of relatively large, both in scale and sedimentary thickness,Mesozoic and Cenozoic petroliferous basins, including both groups of terrestrial and oceanic basins (Zhaoet al.,2000a). The period and distribution of hydrocarbon source rocks in the Tibetan petroliferous basins are similar to those of central and eastern regions of the middle segment of the Tethys. The major source rocks should be Jurassic to Cretaceous marine sediments related with evolution of the Tethys (Wanget al., 2006). For instance,main source rocks in the Qiangtang Basin are upper Triassic to upper Jurassic, while those in the Cuoqing Basin are lower Cretaceous. In addition, there are also hydrocarbon source rocks in the Biru, Changdu, and Kekexili basins. According to a preliminary estimate, total amount of hydrocarbon generated in source beds of upper Paleozoic, Mesozoic and Cenozoic in the aforementioned basins is about 38,128.5×108t. The amount of hydrocarbon generated by Mesozoic source beds in the Qiangtang Basin alone reaches to 9,930.92×108t, accounting for 26% of the total in the Tibetan Plateau.Within three major Mesozoic hydrocarbon source beds,the proportions of hydrocarbon amounts generated by upper Triassic, Jurassic and lower Cretaceous are nearly equivalent. Mesozoic source beds in the Tibetan Plateau mainly produce gas, and the amount of oil including gas condensate only account for 29.36% of the total quantity of hydrocarbon generation, and the amount of oil generated in the Qiangtang Basin account for 49% of the total quantity of oil generation. When secondary hydrocarbon generation is considered, Mesozoic marine source beds in the Qiangtang Basin are estimated to generate secondary hydrocarbons, in the amount of 924.28×108t,accounting for 9.31% of the total hydrocarbons generated during the Mesozoic in this basin (Zhaoet al., 2000b).In 1993, the China National Star Petroleum Corporation(CNSPC) drilled the first petroleum discovery well at Lunpola in northern Tibet. In 1999, the pilot production was successfully completed, thus ending the speculation that Tibet lacked natural oil reserves (http://www.cngascn.com/up_files/news/84306500.pdf).

These petroleum resources provide enough source material to form gas hydrates, a prerequisite of existing gas hydrates.

2.2 Spatial and temporal correlation between petroleum resource and gas hydrate

Numerous residual basins are preserved in the Tibetan Plateau (Zhaoet al., 2000b), which are tectonic residual basins formed by original sedimentary basins through late reworking following rapid and total uplift of the Plateau. These residual basins, after the Plateau’s uplift of 4,000–5,000 m, guarantee the preservation of petroleum resources, an important factor controlling formation of gas hydrates (Fu, 2005).

The Qiangtang Basin formed during the late Triassic to late Jurassic, and the initial shape of which may have been preserved in the mid Cretaceous. This residual basin has been subjected to two obvious reworking during the late Yanshanian and Himalayan periods. Hydrocarbon generation is closely related with the evolution of the basin. In this region, the double occurrence of hydrocarbon generation, migration and accumulation of large quantities of oil and gas, and eventually trapped as ores might be limited to the end of the Oligocene. Also,the preservation period of petroleum might be after the Miocene (Wanget al., 2001). All four major source beds,namely T3x, J2b, J2x and J3s, have undergone hydrocarbon generation at least two or more times, among which primary generation occurred in the late Jurassic to early Cretaceous, almost simultaneous with regional structural deformation of the late Yanshanian period.Hydrocarbon gas is mainly stored in anticline traps of Jurassic to Cretaceous age. Secondary hydrocarbon generation mainly occurred in the early Tertiary, with local or regional well-preserved petroleum resource generated during this period (Zhaoet al., 2000b). In the Qiangtang Basin, numerous thrust-detached structures developed in the late Miocene, forming "duplicate structures with passive roof" providing space for accumulation of oil and gas. On the other hand, the Neotectonic movement since the Tertiary formed stable lozenge terranes, which mainly show differential block uplift, are relatively stable within the blocks, and have little effect on the preservation of oil and gas. These Himalayan fault blocks or anticlines might also form effective accumulation areas for oil and gas (Wanget al., 2001).

The Tibetan Plateau from the Oligocene to Pliocene underwent strong rifting and volcanism, thus its tectonics, magmatism and uplift of the Plateau are closely coupled. During the uplift process of the Tibetan Plateau,formed due to tectonic evolution, corresponding structural traps became available for oil and gas to accumulate and become ores. Also, due to strong tectonic movement, the scale of uplift and incision was relatively large, destroying the preservation of oil and gas. Zhaoet al. (2000b) suggested that secondary petroleum migration in the Qiangtang Basin served as both accumulation and destruction, dominated by accumulation,with early generated oil and gas possibly being destroyed. During the Early Yanshanian and Late Himalayan, regional magmatism was relatively strong, with local temperature increase that obviously promoted thermal evolution of organic material, and destroyed early generated petroleum reservoirs. During the pulse uplift of the Plateau, both structural and thermal dynamic forces driving hydrocarbon migration destroyed early petroleum reservoirs to some extent, but at the same time generating new reservoirs. During the Qingzang movement of the Pliocene to Pleistocene, strong heat sources promoted organic materials within basin hydrocarbon source rocks into a highly mature stage, however plentiful light hydrocarbons are difficult to preserve due to structural destruction.

During the multi-pulse and rapid uplift of the Tibetan Plateau, especially from the Holocene to the end of the Pleistocene, the whole Plateau experienced the formation of ever-frozen earth at about 0.71 Ma B.P..Free and difficult-to-preserve natural gas, can under conditions of low temperature and high pressure, be preserved in a cryospheric setting. Therefore, petroleum resources and gas hydrates in the Tibetan Plateau has certain spatial and temporal correlations with each other. A preliminarily conclusion is that petroleum resources might reside below gas hydrates, and gas hydrates is dominated by a mixture of both light and heavy hydrocarbons.

3 Coupling relationship between formation of frozen earth and preservation of gas hydrate

The Tibetan Plateau has been subjected to motile-uplift and planation multiple times during the Cenozoic, especially geological processes after the Plateau entered a frozen period. These tectonic movements are significant for both formation of frozen earth and preservation of gas hydrates. The Kun(lun)-Huang(he)movement period occurred in 1.1–0.6 Ma B.P. (late Early Pleistocene to early Middle Pleistocene), and is an important stage of uplift during the formation of the Tibetan Plateau (Figure 1). After this movement, the Tibetan Plateau uplifted to an altitude above 3,000 m. This uplift, coupled with an orbit transition called the "Middle Pleistocene Evolution" and global cooling, caused the Plateau’s mountains to enter into the cryosphere, forming the largest Quaternary glacier in the Tibetan Plateau.This glacieris equivalent to the maximum glacial period within the Qingzang glacier series at the Marine Isotope Stage (MIS) 20–16 with a total area exceeding 5.0×105km2(about one quarter of the Plateau area). Glacial area at the maximum period incorporated the central and eastern mountainous districts, namely Tanggula, Amne-Machin, Golok and Daocheng Haizi mountains, which is 18 times that of the modern glacier. Except for summer,stable snow cover on the Plateau, along with the glacier’s large area, increasing surface reflectance and strengthening of the cold high pressure above the Plateau in winter, has allowed further cooling of the Plateau (Shiet al., 1998).

Figure 1 Stepwise uplifting of the Tibetan Plateau since 3.4 Ma B.P. (Shi et al., 1998)

The Last Glacier Maximum (LGM) occurred at the end of the Late Pleistocene. The rapid uplift of the Plateau led to surface change from a warm, semi-arid prairie to a cold, arid periglacial environment, forming a large area of permafrost. During the Holocene, this large scale ice age ended, the Plateau experienced continual uplift to an altitude above 4,000 m, and the region remained a periglacier environment due to its special altitude. During the early Holocene warming period (10–8 ka B.P.), after the cold and dry climate of the last glacial period, the climate of the Plateau gradually became warm and humid. The Middle Holocene relative warming period (8–3 ka B.P.) is a climatic optimum during a post glacier stage of the Plateau. The Late Holocene cool period since 3 ka B.P., with a colder climate due to continuous uplift of the Plateau, is the coldest stage during this period, and mountainous glaciers on the Plateau generally advanced. Climatic instability of the Megathermal in the Holocene is obvious,with rapid cooling in both warm and warming periods(Shiet al., 1998).

In general, annual average temperature below -2 °C is suitable for discontinuous permafrost formation, while annual average temperature below -8 °C is suitable for continuous permafrost formation. For the Tibetan Plateau in LGM, namely the MIS 16 stage (about 0.8 Ma B.P.), the glacier line of balance ranges between altitudes of about 3,500–4,700 m, summer temperature at the balance line is about -2 to -4 °C, and annual average temperature lies between -4 to -12 °C. Supposing at that time the average altitude of the Tibetan Plateau was 3,500 m, surface annual average temperature would be below 0 °C. When annual average temperature drops below -2.5 °C, Plateau permafrost is widely formed,especially in the western portion of the Plateau with relatively less rainfall, higher relief and colder temperatures,indicating that the Tibetan Plateau fully entered the cryosphere during LGM (Shiet al., 1998).

The temperature of LGM is about 7 °C below today’s temperature, so the area of permafrost should be relatively enlarged. The area of modern permafrost in the Tibetan Plateau is about 1.6×106km2, its peripheral boundary equivalent to annual average temperature of -2 to -3 °C. Inferred from modern annual average isothermals, the area where modern temperature is 3–4 °C,should have developed permafrost during LGM (lower than -3 °C). Thus, permafrost distribution during LGM is roughly divided into: (1) north, incorporating most of the Qaidam Basin, reaching to Qinghai Lake and the Gonghe Basin; (2) south, the upstream valley of YarlungZangbo River; and (3) east, a significantly expanded area (Figure 2). Thus, the area of permafrost in the Tibetan Plateau was up to about 2.2×106km2, almost 40%larger than today (Shi and Zheng, 1997).

Figure 2 The extent of permafrost during the present and LGM in the Tibetan Plateau (Shi et al., 1997)

The rapid decrease of water supply for lakes and ponds in the Tibetan Plateau at the end of the Late Pleistocene, together with increasing evaporation, led to increase salt content and salinity of lake water, causing an enrichment of carbonates (Zhanget al., 1993).The study on Qaidam Basin gypsum beds indicated that there were lacustrine deposits from fresh water between 35–3 ka B.P., but it has gradually become salt lake and deposited gypsum since 23 ka B.P.. Chenet al. (1990)suggested that most evaporite deposits in Qarham salt lake of the Qaidam Basin began at about 24 ka B.P.,and potash salt deposits appeared at about 16–19 ka B.P.and formed a dry salt lake due to an extremely dry climate. The deposited loess, both at the east bank of Golmud Reservoir and Nachitai terrace (18.931–15.377 ka B.P.) are also the products of that period. All of these agree well with climatic change in the Tibetan Plateau. Meanwhile, it possibly indicates, due to extreme low temperatures, that permafrost formation might consume a large amount of fresh water because of gas hydrate generation. The salinization of lake water is closely related to gas hydrate formation since they have temporal correlation.

According to literature about the history of hydrocarbon generation (Zhaoet al., 2000b), the major period generating hydrocarbons in main petroleum generating basins, such as the Qiangtang Basin, is late Jurassic to early Cretaceous, while regional or local secondary hydrocarbon generation mainly occurred in the early Tertiary. Before the rapid uplift of the Tibetan Plateau, both structural and thermodynamic forces combined to drive hydrocarbon migration, leading to alternating destruction and accumulation of petroleum reservoirs. Since about 3.4 Ma B.P., frequent seismic and fault activities following rapid uplift of the Plateau provided available channels of migration for hydrocarbon gas. Glacial and periglacial processes are widespread during the Quaternary,with pressure from ice cover promoting gas hydrate stability within underlying deposits. This forms a stabile zone of gas hydrates under conditions of suitable temperature and pressure, which are eventually preserved under frozen cover through rapid decrease of Tibetan Plateau temperatures.

4 Coupling relationships between glacial formation,evolution and gas hydrate accumulation and pool forming

The formation process of stabilized zone and reservoir of gas hydrates in a terrestrial permafrost region is related to overlying past and/or present glaciers(Jianget al., 2002). An ice layer of 3–4 m thick will lead to a geological static load increase of 2,700–3,600 N/cm2in the underlying stratum, forcing fluids, such as water, oil and gas, out of fine dispersed rocks with weak permeability into strata with good reservoir properties. The strong forces produced by flowing water during the process of glacier movement will washout and destroy reservoirs of oil and gas, causing hydrocarbon advancement in the direction of glacier movement. Thus, it changes the distribution of hydrocarbon again. Glacier movement force soil and gas from below ice cover to the periglacial zone. Therefore, under silimar conditions, the periglacial zone of past or modern glacial shields should have relatively more petroleum resources (Jianget al., 2002).

Since the Middle Pleistocene, there were three Pleistocene glaciations in the Tanggula Mountains, namely Kunlun Ice Age, Penultimate Ice Age, and Last Ice Age as well as two Late Holocene glaciations, namely Neoglaciation and Little Ice Age (Jiao and Shen, 2003; Duanet al., 2005). At the peak of the Little Ice Age in Tanggula Mountains, the Late Holocene glacier was 0.5–2.0 km longer, and 8%–12% larger than at present,and the snowline was 20–65 m lower than today’s snowline. The maximum, middle and minimum of extending distances, increased areas and lowering snowlines corresponds to the eastern, middle and western segments of this glacier. The extending extent of the Pleistocene glacier during the Neoglaciation is 3.0–5.0 km away from the end of the present glacier. During LGM, glacial line at Tanggula Puerto descended to an altitude of 5,040–5,060 m. The glacier at that time was 3.0–10.0 m longer than present, with an area 1.5–4.5 times larger than today, and descending snowline to 140–250 m (Jiao and Shen, 2003). During the last glacial period in the Middle Pleistocene, the glacier was transitional between monsoon continental and oceanic, with a snowline of 5,238 m in altitude, mainly formed down-incised troughs in valleys. The Kunlun Ice Age, which is the earliest and largest ice age during the Quaternary glacial period, is possibly a monsoon marine ice cap, which formed a high glacial drift platforms and deep secluded valleys (Deng and Zhang, 1992). The glacial extent at Tanggula Mountains during maximum ice age, according to statistics from the central and western segments, is 24,500 km2,12 times larger than present (Shiet al., 1995). The total area of the Tanggula Quaternary glacier (Figure 3) ranges between 36,000 and 40,000 km2, 16–18 times larger than present (Jiao and Shen, 2003).

Surface indications of petroleum have been found at periglacial positions of the ice cap during maximum glacial period (Figure 3), such as at Anduo, Yanshiping, and Angdaercuo (Zhaoet al., 2000b; Wanget al., 2001).These petroleum indicators occur mainly in fractures or in crystal caves, consistent with Jurassic reservoir pores.Chromatograph of petroleum saturated hydrocarbon at the Anduo 114 highway maintenance station indicates that alkane is dominant, with carbons on the major peak includingnC22andnC24. Naphthenic hydrocarbon content is also relatively high, while most of alkane and isoparaffin before C20are lost. Compared with J3s limestone in this region, the maturity of crude oil is relatively higher,and deviate from evolutionary curve of normal hydrocarbon source rocks, which indicate oil seepage has been subjected to secondary migration for some distance. In the process of migration, light hydrocarbons in the oil seepage is almost lost, but was not subjected to relatively strong biodegradation (Zhaoet al., 2000b). The late migration of oil seepage might be affected by glacial movement which is helpful for the generation of gas hydrates.

Figure 3 A comparison of present and LGM glacier extent in the Tibetan Plateau (Shi et al.,1998)

5 Conclusions

Based on extensive literature, we preliminarily conclude that Mesozoic marine hydrocarbon source rocks,widely distributed in the Tibetan Plateau, provided significant source material for formation of gas hydrates in permafrost. Carbonate rocks and mud shale, the major source of hydrocarbons, is an important reservoir stratum for gas hydrates. According to research on the history of hydrocarbon generation, the main period of hydrocarbon generation in the Qiangtang Basin of the Tibetan Plateau, is late Jurassic to early Cretaceous. Secondary generation of hydrocarbons, regionally or locally,mainly occurred in the Early Tertiary. Before rapid uplift of the Tibetan Plateau, petroleum reservoirs had been alternatively destroyed and accumulated due to combined structural and thermodynamic forces driving hydrocarbon migration. Since about 3.4 Ma B.P., the Plateau has been subjected to widespread glaciation and periglacial processes, following strong uplift of the Plateau, again producing hydrocarbon migration. Pressure of ice cover forces out free hydrocarbon gas from within underlying sediments incorporated with water, forming gas hydrates, which are then preserved and covered by frozen layers produced by sharp falling temperatures in the Tibetan Plateau.

Therefore, we conclude that gas hydrate has good spatio-temporal coupling relationships among conditions of accumulation and preservation based on compressive analysis of petroleum generation, evolution, uplift of Plateau, formation of frozen earth, glacial advancement and regression.

We would like to thank for anonymous reviewers helping improving this manuscript. Thanks are also given to the authors whose literatures are incited here, especially those not listed below. This paper is supported by Research Project No. 200420140001 of China Geological Survey.

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