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Circum-Tethys and circum-Pacific basins
Pergamon PII:
Journal of Southeast Asian Earth Sciences, Vol. 13, Nos 3-5, pp. 305-315, 1996 Copyright 0 1996 Published by Ekvier Science Ltd Printed in Great Britain. All rights reserved SO743%47(!%)00037-2 0743-9547/96 $15.00 + 0.00
Circum-Tethys and circum-Pacific basins Feng Fukai Institute of Petroleum Geology, Ministry of Geology and Mineral Resources, 20 Chengfu Road, Beijing 100083, China Abstract-Mesozoic circum-Tethys and circum-Pacific basins, which are genetically related to the Tethys and &cum-Pacific tectonic zones, are the regions with the most abundant petroleum and natural gas. These two tectonic systems intersect in China and most of the Mesozoic basins in China are closely associated with these two systems. Circum-Tethys basins developed against a background of crustal compression. They are the result of compressional thrusting and the resultant sinking of the crust. Warm-moderately hot foreland basins are the most representative. Hydrocarbon source rock evolved slowly. Threshold depth of oil-generation is great and petroliferous beds have relatively older ages and occur at greater depths. Circum-(west) Pacific basins were formed against a background of extension and thinning of the crust and frequent geothermal activities. Hot rift basins are the most representative. Hydrocarbon source rock evolved rapidly. Threshold depth of oil-generation is small and petroliferous beds have relatively younger ages and occur at shallower depths. Copyright 0 1996 Published by Elsevier Science Ltd
Introduction
Chinese data, based on his understanding Mesozoic and Cenozoic geology of China.
Tethys and circum-Pacific are two large Mesozoic tectonic zones. They intersect in China. There are numerous basins within and along these two tectonics zones. Most of the basins are around and genetically related to the two tectonic zones. In this paper these basins are called circum-Tethys and circum-Pacific basins. Basins in central and western China are related to the evolution of the Tethys, those in eastern China are associated with the circum-Pacific tectonic zone. It is reasonable to place the basins within these two ring-formed tectonic systems. Huang et al. (1977) divided China into three tectonics domains: the Old Asian Domain; the Tethys-Himalayan Domain; and the Circum-Pacific Domain. This division is based on the Paleozoic and Mesozoic geological features of China. The present paper will focus on Mesozoic and Cenozoic sedimentary basins which are related to the latter two tectonic domains. Due to the Tethys being a collisional orogen and the circum-Pacific still under the influence of sea-floor spreading and subduction, the tectonics, sedimentology, geothermal activity, geodynamic character and the distribution of oil, gas and other mineral resources associated with these basins have many differences.
Tectonic evolution of west Pacific and the circum-(west) Pacific basins The Pacific Ocean is a Mesozoic and Cenozoic ocean basin, which is still developing. The oldest oceanic crust has a Jurassic age (Bally 1980; Li et al. 1982). Continental margins surrounding the large oceanic basin possess the character of an active margin. There are two megasutures in the Tectonic Map of the World compiled by Bally (1980), the circumPacific zone and the Tethyan zone. These two zones, however, are not well displayed in China. This is due largely to insufficient data on China. In this paper, the author attempts to add in the
of the
Tectonics of Mesozoic continental margin and Mesozoic basins Mesozoic eastern Asian active margin. Mesozoic volcanic rocks, granites and fold belts are widely distributed in eastern China and eastern Asia, from Hainan through Guangdong, Fujian, Zhejiang, Shandong, Liaoning, Korea, Jilin, Heilongjiang to the easternmost part of Russia. They form a very complicated magmatic-tectonic zone (Fig. 1). Based on the following geophysical and drill-hole data on the East China Sea and south China Sea, this magmatic-tectonic zone has a tendency to extend into the marine regions: (1) 160 wells in the northern continental shelf of the south China Sea, extending to the Xisha Islands, have reached Mesozoic magmatic rocks (granites and volcanics, isotopic age 68-130 Ma); (2) Mesozoic granites and volcanics with isotopic ages of 89-123 Ma have been found in 10 localities (wells and outcrops on islands) in the southwestern part of the south China Sea (Wu 1990); (3) The gravity and magnetic fields of the fold belt between the continental shelf of the East China Sea and Diaoyu Island have similar characteristics as those of the terrestrial region of Fujian and Zhejiang, and are quite different from those of the Okinawa sea trough and Ryukyu island-arc east of the fold belt. Based on these, it is believed that the magmatic-tectonic zone of Fujian and Zhejiang extends eastwards to the Diaoyu Island. From the above data, the basement of the Tertiary basins in the East China Sea and south China Sea is a Mesozoic or older tectonic zone transformed by Mesozoic magmatic movement. Proterozoic substrata found in a few wells are probably old continental remnants cut apart by Mesozoic tectonics. This magmatic-tectonic zone with a width of about 1000 km extends about 5000 km through the terrestrial and marine regions of eastern Asia in a north-northeast
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direction. It is not concordant with the pre-Mesozoic tectonic lineaments. What is the relationship between the Mesozoic magmatic-tectonic zone and pre-Mesozoic tectonic zones? What are the tectonic characteristics of Mesozoic continental margin? Analysing the results of various researches the author arrives at the following conclusion. (1) Large scale magmatic activity, including eruptions and intrusive granites, indicate that strong heat flow events with a time range of 100 Ma took place at the continental margin of eastern Asia during the Mesozoic. Their intensities and spatial extent goes beyond the Cenozoic continental margin. The heat flow events are
the most prominent features of the Mesozoic. Both mantle-derived and crust-derived magmas indicate that mantle convection was very active. Uprising heat caused partial melting and rapid temperature rise in the crust. Against a background of high crustal temperatures, the older tectonic framework was severely altered. The pre-Mesozoic tectonic framework was replaced by a new framework. Because the magmatic-tectonic zone was developed on the basis of the old framework, it may keep some characteristics of the older tectonics. In the direction of the continental interior heat activities decrease gradually, magmatic activities and the degree of transformation becomes weaker, the boundary between
Fig. 1. Mesozoic and Cenozoic tectonics and basins of China and the adjacent regions. 1 Early Mesozoic erogenic belt. 2 Middle-Late Mesozoic erogenic belt. 3 Cenozoic collisional erogenic belt. 4 Cenozoic island-arc tectonics. 5 Boundary of tectonic regions. 6 Mesozoic volcanic arc of continent margin. 7 Mesozoic plutons. 8 Collisional suture. 9 Subduction zone. 10 Foreland thrust zone. 11 Important strike-slip fault zone (including interior fault depression). 12 Oceanic spreading ridge. 13 Strike-slip fault and other types of fault. 14 Foreland basin. 15 Intermontane basin and other types of basin. 16 Rift-fault depression. 17 Shear fracture. 18 Back-arc marginal sea.
Circum-Tethys
and circumPacific
the new and old tectonics becomes faint and transitional. Heat activities fluctuate alone this zone, the strongest occurring in Guangdong, Fujian, Zhejiang and Jiangxi. Mesozoic igneous rocks are of great importance. Paleozoic strata and older rocks are often cut apart, pushed and welded into a complex block by Mesozoic magmatic rocks. In the lower Yangtze region magmatic activities are relatively weaker. The structural styles of the Mesozoic tectonic movements comprise mainly of folding and basement detachment, bearing some signatures of an erogenic zone. (2) Most Mesozoic volcanics in eastern China are of a Late Jurassic to Early Cretaceous age. They belong mainly to the talc-alkaline series, and have a tendency to change into alkali-talc and alkaline series in the direction of the continental interior. It is usually thought that the former are eruptions at an active continental margin. (3) There are some relatively convincing theories for a subducting convergence zone in some parts of the Mesozoic tectonic zone: (a) there are Triassic and Jurassic deep-water marine deposits, flysch and ophiolite melange in the Sichote (SikhotekAlin area (Li et al. 1982); (b) the Hida double metamorphic zone (Early Mesozoic) and the Sambagawa double metamorphic zone I (Late Mesozoic) in Japan; (c) the Yushan high-pressure metamorphic zone (Late Mesozoic) in Taiwan; (d) Liu (1992) points out that there are at least two important Mesozoic sutures in eastern Asia: the Hainan-Hida suture zone (Early Mesozoic); and the Yuli-Sambagawa suture zone (Late Mesozoic). (4) From the above, this magmatic-tectonic zone represents a Mesozoic active margin in eastern Asia. Generally, it belongs to a collisional arc at continental margin, and possesses the characteristics of an island arc in some sections. Mesozoic basins. There are numerous Late Mesozoic basins in eastern China, especially in northeastern China, where about 200 Early Cretaceous coal-bearing fault depressions have been recognised (Li 1988). Below the large Late Cretaceous Songliao basin there are 40 Early Cretaceous fault depressions. In the eastern part of north China, Late Mesozoic fault depressions were also developed below Tertiary fault basins in the Bohai Gulf. Most of these basins are associated with volcanics. The volcanics erupted either before the formation of fault depression (e.g. the Great Hingan basin), or before the formation of depression and during the deposition process (e.g. the Songliao basin). Extensional basins in the south were formed slightly later, starting in the Late Cretaceous, e.g. the Subei basin, the Jianghan basin and small Cretaceous red basins in south China. These basins are not very similar to Cretaceous fault basins in the north. The formation of the basins in the south continued to the Tertiary. Many of them are developed in the Tertiary. The characteristics of Late Mesozoic basins can be summarised as follows: (1) The basins are formed under similar background, the extensional thinning of the crust. The basins are closely related to the Mesozoic magmatic-tectonic zone, most fault depressions are located within or adjacent to this zone (Figs 1 and 2). (2) Most of the basins have a “one-fold” structure, they are small listric fault depressions and rift valleys. Sedimentary features of the basins reveal that they are *BABI I,D-%Y
basins
Fig. 2. Distribution of circum-(west) Pacific basins. 1 Mesozoic and Cenozoic basins. 2 Oil and gas basin, 3 Cenozoic marginal sea basin. 4 Cenozoic subduction zone. 5 Mesozoic collisional suture. 6 Strike-slip fault.
originally extensional structures, though some have experienced later tectonic inversion and compression of different degrees. A few basins have a “two-fold” structure-faulting in the lower part and sinking in the upper part. These two different basin structures represent different patterns of sinking and dynamic regime. (3) Most Late Mesozoic fault depressions have a high geothermal gradient, they belong to hot basins (see below). (4) Fault depressions of “one-fold” structure slip along major synsedimentary fault and have a listric structure-steep in the upper part and gentle in the lower part. Sinking is related to strike-slip faults, the depth of the faults does not surpass the extent of the upper crust. They seem not to be related to mantle movement. Stages of depression are not clear or missing. This kind of basins are called “shear fault depressions” in this paper, e.g. the Great Hingan fault depression. (5) Fault basins of “two-fold” structure are developed in two stages. Extension of the crust results in block faulting and sinking, this is the rifting stage. Gradual cooling of the crust results in slow geothermal sinking, this is the post-rifting stage (McKenzie 1978; Royden
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1987). The formation of these kinds of basin is connected with the active mantle upwelling. They are called rift basin in this paper (e.g. the Songliao basin), corresponding to the thermal excited basins of Reading (1986). Structure of Cenozoic active continental margin and Cenozoic basins The Cenozoic continental margin developed on the Mesozoic active continental margin. The system of island arc-back arc marginal sea basins (briefly arc-basin system) is a typical feature of the circum(west) Pacific tectonics. Oceanic-continental subduction results in rifting of the Mesozoic continental margin arc. Continental stretching occurs at first; then comes the sea-floor spreading; marginal sea and island arc are formed at the end. The Japanese island arc, for example, was a part of a continental margin arc in the Mesozoic. It rifted off from the continent in the Cenozoic. The Japanese (marginal) sea was formed only in the Miocene. The Ryukyu island arc and its back-arc basin-the Okinawa trough was formed quite late. It has been spreading since the Pliocene. Due to the short spreading time, oceanic crust had not appeared yet. It was still in the period of continental crust thinning. The formation of the south China Sea is relatively complicated. It was a part of the eastern Asian continent in the Mesozoic. The first group of rift valleys occurred in the Early Tertiary, oceanic crust appeared in the Oligocene and developed to the Middle Miocene (32-17 Ma) (He Liansheng 1990). Finally sea-floor spreading stopped and the central ocean basin was formed. The former Early Tertiary rift valleys was split into the north and south continental shelf basins and small ocean basins. Because of the rotation and northward movement of the Philippine island arc, it collided with the Asian continent at Taiwan in Late Miocene-Pliocene, and became the eastern boundary of the south China Sea. The south China sea is a marginal sea basin, but not necessarily a back-arc basin. The three marginal seas mentioned above have a short history, only 30-50 Ma, i.e. the spreading began in the Oligocene, about 30 Ma later than the continental shelf rift basins nearby, such as the East China Sea continental shelf basins, the Pearl River mouth and Qiongdongnan basins on the north continental shelf of the south China Sea, the Zengmu, Nansha and North Balawang basins on the south continental shelf of the south China Sea. They began to form in Early Tertiary (Late Cretaceous has been suggested in some literature, but this date is not supported by reliable data). What is the relationship between these continental shelf basins and the marginal sea basin? To which basin system do they belong? According to the Wilsons cycle, there should be a stage of continental extension and rifting before sea floor spreading, i.e. the continental shelf basins mentioned above represent the rifting phase prior to sea-floor spreading. Therefore, it is reasonable to assume that the continental shelf basins near a marginal sea basin belong to one system. Continental shelf basins are called “periphery basins of marginal sea”. These periphery basins can be further divided into different types according to their evolution history. Although Tertiary basins developed in the interior of the continent, such as the Bohai Gulf, Subei-south Yellow Sea, north Yellow Sea and Jianghan basins,
show similarity to the above-mentioned continental shelf basins in basin structure, development history and lithology, these continental interior rift basins are difficult to link with the island arc-subduction system 1000 km away. The characteristics of Tertiary basalt within these basins indicate that they belong to continental rift basins, the basalt being quite different from the Mesozoic volcanics at the continental margin.
Tectonics of the Tethys and the circum-Tethys basins Huang and Chen (1987) concluded: “The Tethys was a wide sea area lying south of the super Eurasian continent. Most part of it is oceanic and transitional. It started to form in Late Carboniferous and Early Permian. Due to plate subduction and collision, rifting and sea floor spreading, the extent of the sea area changed greatly with time. It became more and more narrow and closed in the Tertiary, but there remained a part which continued to develop to today”. Figure 3 outlines the tectonics of the Tethys and the distribution of periphery basins and interior basins. The Tethys proper has already closed and formed grand mountain chains, which is represented in Europe by the Alps and in Asia by the Himalayas. The eastern part of the Tethys has not yes closed, it is still developing. Between the Tethys tectonic zone and the periphery basins are foreland thrust fault zones, more or less the A-type subduction zone of Bally (1980). The still open Sumatra-Jawa island arc is a B-type subduction boundary. Within the tectonic zone (China) four collision sutures of the Indosinian, Early Yanshanian, Late Yanshanian and Himalayan periods have been found. They represent the multi-phase collision of Eurasia and Gondwana and the paleo-, meso- and neo-Tethys. Besides some intermontane basins within the tectonic zone and some remnant blocks which have not been strongly deformed, most basins are distributed at the periphery of the erogenic mountain belts, forming a relatively complete circle of basins. They are called in this paper “circum-Tethys basins”. “Foreland basins” are located near the foreland thrust zone. They are most closely related to the evolution of the tectonic zone, and are normally connected with the craton basin in the direction to the interior of the continent. Figure 3 shows 33 periphery basins, 23 of them (70%) can be considered as foreland basins, the rest are strike-slip basins and other unclassified basins. Therefore, the foreland basins are the most representative of circum-Tethys basins. The intermontane basins (excluding the area not closed), however, are representatives of basins within the tectonic zone. In the eastern part, which is still developing, the basins behind the subduction zone and island arc are back-arc basins or marginal sea basins. Before the closure of the Tethys, the Gondwana continent was to the south and the Eurasian continent was to the north. During the permo-Triassic periods, “China” was on the northern side of the Tethyan ocean, the Songpan-Ganzi sea belonged to the northern Tethys and was a sedimentary miogeosynclinal basin with a sialic crust (Huang and Chen 1987). From the Permian to Late Triassic, “southwestern China” and the upper Yangtze region were close to the Songpan-Ganzi sea
Circum-Tethys
and circum-Pacific basins
trough of northern Tethys. They were continental shelf shallow sea basins dominated by carbonate platform deposits. The sea trough to their west was dominated by flysch and deep-sea deposits. Two marine areas were connected, both contained northern Tethyan faunas and there was no old continent between them. Before the closure of the Qinling sea trough (an aulacogen of the northern Tethys in the interior of the continent), the Ordos basin also experienced short transgressions and had similar Early Triassic faunas as those of the upper Yangtze sea basin. The Tarim Basin might have been connected with the Kekeshaer sea trough in the Permian and changed into a near-sea continental basin in the Triassic. The Qaidam was a peninsula extending into the Songpan-Ganzi sea and the western Qinling sea trough in the Triassic. It was surrounded by the sea on three sides. The above indicate that these regions were connected with the Tethys and that they are periphery basins of the Tethys. As a result of the northward movement of the blocks derived from the southern continent and the closing of the Tethys from south to the north, a series of collision mountain chains were formed at different times. The first erogenic belt formed is the Songpan-Ganzi folding belt (Indosinian). The next are the Tanggula fold belt (early Yanshanian) and the Lhasa-Tengchong fold belt (late Yanshanian). The last is the Himalayan mountain chain (Himalayan). As the Tethys disappeared in China the grand Qinghai-Tibet High Plateau was formed. Four collision erogenic events influenced the periphery basins mentioned above to varying degrees. They even affected basins far away, such as the Junggar, Hexizoulang and Ordos. The Indosinian, Yanshanian and Himalayan tectonic phases also produced unconformities and disconformities in these areas. The Indosinian and Himalayan are the most prominent phases which strongly affected the periphery
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basins and the pre-Mesozoic erogenic belts. The effect of the Indosinian phase on the periphery basins is that the former continental shelf basins, offshore basins (normally called first cyclothem) changed into “foreland basins” which had genetic relations with the foreland thrust zone and are dominated by terrestrial sediments (normally called second cyclothem). The relationship between the periphery basins and the Tethys is explained here, using the Sichuan basin as a example. (1) From the Permian to early-Late Triassic the Sichuan basin was located between the Yangtze craton and the northern Tethys sea, with the Songpan-Ganzi sea to its west and the Qinling aulacogen (a branch of the Tethys extending landward) to its north. The Qinling sea closed first in the Middle Triassic, the SongpanGangzi sea closed in the Late Triassic (Feng and Wu 1980). (2) The Longmenshan and Qinling-Dabashan foreland zones thrust from west to east and from north to south, forming complicated thrust tectonic belts, e.g. klippes, imbricate thrust sheets. From late-Late Triassic to Quaternary, shallow-water terrestrial coarse elastics were deposited. The most representative sediments are molasse deposits with a thickness of 3000 m, including 7-8 conglomerate beds. These reflect the multi-phase movement, thrusting and faulting of the erogenic belt. (3) Large foreland basins (with a width of more than 400 km) display structures of foreland basin such as: (a) thrust fault; (b) foredeep depression; (c) frontal rise; (d) second-order depression. They corresponding respectively to the second-order structures: (i) the Longmenshan; (ii) the western Sichuan depression; (iii) the central Sichuan rise; (iv) the eastern Sichuan paleosyncline (Indosinian). Middle and small-sized foreland basins possess only the structures (a) and (b). The structure of the cover is often related to the deep part of the crust. Based on geodynamical analysis, foreland basins are the
Fig. 3. Tethys and circum-Tethys basins. 1 Tethys tectonic zone. 2 Boundary of the Tethys. 3 Collisional suture. 4 Subduction boundary. fault. 6 Circum-Tethys basins. 7 Oil and gas basins. 8 Inner Tethys basin.
5 Strike-slip
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result of a combine effect of horizontal compression of the thrust zone and the loading of the nappe, these two effects cause the flexure and sinking of the crust (Feng 1993). The Yarlung-Zangbo collisional event (Tertiary) not only caused the large-scale rise of the Qinghai-Tibet High Plateau, but also strongly affected the western and northern parts of China, as well as the areas further away. The pre-Mesozoic erogenic belts, such as the Tianshan, Kunlunshan and Qilianshan rose again and overthrust the areas nearby, forming Cenozoic thrust zones. Pliocene-Early Quaternary molasse deposits of large thickness (several hundred to several thousand meters) were deposited at the margins, such as the deposits of the two frontal depressions south
and north of Tarim, the Xiyu conglomerate of the southern frontal depression of the Junggar basin, the Yumen Conglomerate in Hexizoulang, the Yaan-Dayi conglomerate of the western frontal depression of the Sichuan basin and the Siwalik Conglomerate of the southern slope of the Himalayas (Huang 1979). These molasse deposits are wholly or partly tectonically deformed, either folded or faulted, forming the late structural trap (e.g. the Sichuan, Tarim and Qaidam basins). The Junggar, Turpan-Hami and Hexizoulang basins do not seem to be related to the Tethys before the Cenozoic. After the Yarlung-Zangbo collisional event, the pre-Mesozoic erogenic belts rose once again.
Fig. 4. Crustal division of China and the adjacent regions. 11 Strongly compressed and thickened continental crust. 12 Normal and thickened continental crust. 111 Extended and thinned continental crust. II2 Transitional continental crust. III Oceanic crust. 1. Depth of the Moho.
Circum-Tethys
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and circum-Pacific basins
Analysis of the differencesbetween the eastern and western tectonic systems Figure 4 shows the macro-distribution of the Moho in China and adjacent regions. Both the configuration of the macro-distribution of crustal thickness and vertical structure have a two-fold character. The gravity gradient zone (reflecting the change of crustal thickness) running from Hingan via Taihang to Wuling is a boundary (about 40 km). The crust east and west of this zone have great differences. The western part is the thickened continental crust area (area A) with a thickness of 40-70 km. It has experienced several collisional erogenic events in the Mesozoic and Cenozoic, the crust has been compressed and shortened. The eastern part is the extended and thinned continental crust and transitional crustal areas with a thickness of 10-40 km. Here the Mesozoic and Cenozoic crustal extension and thinning took place. Area (,: S-70 km, corresponding to the Tethys proper. The upper part of the crust experienced brittle deformation, faulting and folding, overthrusting and supposition. There is a low speed zone of 5 km thickness in the depth of 20 km, it may be the ductile shear surface (or detachment surface). The lower part of the crust, roughly half of the crust, can be explained as a ductile-plastic deforming part (according to the Shaoyang-Heishui transaction). Area (*: 40-50 km, including pre-Mesozoic erogenic belts and a series of large Mesozoic, Cenozoic basins. A prominent character of this area is that it extends around the Area (,, forming a half circle. It corresponds to the region where the circum-Tethys basins occur. Crust thickness does not change greatly within a basin, it is generally 4245 km. Though it experienced compressional deformation, the thickness of the crust was not much affected. It can represent the normal thickness of a continental crust, and thus be used as a standard for judging thickening and thinning of the crust in other areas. The crust thickness of old erogenic belts outside the basins has increased slightly. The crust structure within the basins: comprising a sedimentary cover, a metamorphic basement and the lower crust, each has a thickness of about one-third of the crust (see the Shaoyang-Heishui transaction). Area ((I: Includes the continental region of the eastern part and parts of the offshore continental shelf, with a tendency to thin eastward (reduced from 40 km to 25 km). The northern section: locally Moho doming (about 30 km) in the eastern part of north China, the Bohai Gulf and the Songliao basin, the maximum thinning is about 10 km. In the lower Yangtze region and in south China on the southern section there is no clear thinning of the crust. The degree of crustal thinning often controls the scale of Mesozoic and Cenozoic basins. Moho anomalies generally correspond to large Mesozoic and Cenozoic extensional rift basins. Small faulted basins are always related to the thinning, there is usually no Moho anomaly. Structures of the crust and mantle are generally related to the active degree of Cenozoic tectonics and degrees of faulting. When the low-velocity layer of the upper mantle is shallow and there is a low-velocity layer in the crust, recent tectonic activities are strong (earthquakes, volcanic eruptions), e.g. the eastern part of
north China and the Bohai Gulf. The former Mesozoic active regions changed into tectonically stable regions in the Cenozoic, e.g. middle and lower Yangtze regions and south China. There is no low-velocity layer in the upper mantle (Liu 1992). The change in the crustal thickness is related to mantle activity. The upwelling of the asthenosphere raises the temperature of the lithosphere and crust, causing uneven extension and thinning of the crust and an increase in tectonic activities. If there is no superposition of Cenozoic mantle activity, the Mesozoic tectonically active regions become tectonically stable regions. crustal region, including Area ((2: Transitional Cenozoic island arc-marginal sea. Crust thickness varies between 25-10 km. Based on SV velocity layer analysis, the low-velocity layer of the upper mantle is very shallow (45-80 km), the uprising current is especially active (Liu 1992). This is the primary driving force for the strong Cenozoic tectonic activitie-arthquake, thermal activity, volcanic eruption, continental extension and sea-floor spreading. Very active vertical convection of the asthenosphere, i.e. the uprising current is the primary reason for the formation of the arc-basin system in west Pacific.
Geothermalfield and geothermal subdivisionof the basins of the two systems Geothermal distribution in the crust is not even, some places are relatively hotter and belong to the hot crust; some places are relatively colder and belong to the cold crust. Hot and cold are relative. Average continental geothermal parameter is used in this paper as a standard to judge the geothermal property of a region. When the geothermal parameter of a region is greater than the average, it is called a hot region (basin); when it is smaller than the average, it is called a cool region (basin); when it is close to the average, it is called a moderately hot region (basin). Regions with very high anomaly are very hot regions (basins). Heat flow is a very important parameter to describe the thermal condition and process in the interior of the earth. The average continental heat flow is 60 f 5 m WJm2. Because there are only limited measured heat flow values, we use geothermal gradient as a supplementary parameter for geothermal analysis. These two parameters are used for geothermal division of basins (regions; Table 1; Fig. 5). There is enormous heat energy in the interior of the earth, which is transmitted continuously to the earth’s surface in the form of rock conduction, magma, Table 1. Geothermal division of basins Basin region Hot basin Moderately
Geothermal gradient Heat flow d(“C/km) Q (mW/m*) 30-60 22-33
62.7-100 N-62.7
< 22 > 60
c 50 > loo
hot basin Cool basin Very hot basin
Average continental crust heat flow 60 + 5 mW/m*
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Fig. 5. Geothermal division of Chinese basins. 1 Very hot region (basin). 2 Hot basin. 3 Moderately hot basin. 4 Cool basin. 5 High temperature tin and tungsten ore deposits zone. 6 Low temperature mercury and antimony ore deposits zone. 7 The Tethys. 8 Boundary of the two systems. 9 Subduction zone. 10 Mesozoic magmatic zone. volcanoes and hot springs . Heat sources are generally divided into mantle source, magma source and crust source sustained by the decay of radioactive elements. The geothermal field of a basin or a region is not fixed, but changes with the thermal-tectonic events. A thermal-tectonic event breaks the old equilibrium and the geothermal parameter increases rapidly. This marks the start of a thermal cycle. As the heat dissipates, the crust becomes cooler and reaches a new equilibrium. A thermal equilibrium to a new equilibrium constitutes a thermal cycle. According to the active pattern of thermal evolution of the oceanic lithosphere, heat flow is an exponential function of time. A thermal cycle lasts 100-150 Ma. The highest cooling rate is in the first 50 Ma after the thermal event, then it becomes lower and lower. We applied the oceanic thermal evolution pattern to the rift basins in eastern China and found that the two parts have similar cooling tendency. The author thinks that tectonic events, such as continental extension, sea-floor spreading, subduction, collision, sinking of basins are all movement patterns in the geological framework. Since they are movement, they must have a driving force and need the relevant energy. Turcotte and Schubert (1982) believed that the basic mechanism of plate tectonics must supply the energy for earthquakes, volcanoes and erogenic movement, and the only energy resource that is big enough is in the interior of the earth. The author basically agrees with this opinion, but also takes second-order forces into consideration. For example, tectonic deformation caused by the rotation of the earth
and the load of geological bodies can influence the sinking of basins. Figure 5 is a thermal division of major Chinese basins based on the thermal parameters given in Table 1. It should be pointed out that this division is based on modern thermal conditions, the paleothermal conditions being not considered. We think that paleogeotherm is a time variable. If we use it as a standard, we cannot get a definite result. Therefore it is not used as the basis of basin division. We can see in Fig. 5 that the general geothermal distribution tendency is warmer in the east and cooler in the west. It conforms basically with the modern crustal thickness and the Mesozoic, Cenozoic tectonic framework. Extensional basins related to &cum-Pacific tectonics are generally hot basins, only a small number of a basins including a few compressional basins are moderately hot basins. The hottest places within a basin are deep faults and depressions. Circum-Tethys basins and foreland basins are generally cool-moderately hot basins. The coldest place is the foredeep depression of foreland basins. The thermal distribution pattern is quite different from that of the circum-Pacific basins. These two systems have clear differences. Basins, other than the Mesozoic and Cenozoic basins, cannot be systematically classified because of the lack of sufficient thermal parameters. Only the tendencies of the geothermal distribution can be found. It has been mentioned before that heat flow is an exponential function of time, and a geothermal cycle lasts 100-150 Ma. Tectonically active regions since the
Circum-Tethys
and circumpacific
Tertiary are hot or very hot regions, e.g. the Cenozoic Yarlung-Zangbo suture and the Okinawa sea trough are the hottest. Pre-Mesozoic and Early Mesozoic tectonic regions (more than 150 Ma) have reached the thermal equilibrium, if they are not superimposed by Late Mesozoic-Cenozoic geothermal events. Therefore, they are moderately hot regions, e.g. south China, the Yangtze region (excluding the coastal region and Late Mesozoic-Tertiary extensional basins). The regions with the Yanshanian granites was hot in the Mesozoic. They have since cooled down and are moderately hot regions. Regions with the Yanshanian granites in south China metallogenic provinces. Mesozoic are important geothermal field can be determined based on the distribution of the different ore belts. Tungsten and tin ore deposits are high temperature ore deposits. They are mainly distributed in southern Jiangxi, southeastern Hunan and Hainan Island. Huang (1945) assigned them to out-Pacific high temperature ore province. Low temperature ore deposits, such as mercury and antimony ore deposits are distributed in the western margin of the Jiangnan Old Continent, southern Guizhou, southeastem Yunnan and the Nanpanjiang synclinorium. No high temperature tungsten and tin ore deposits occur in these regions. Huang called them inner-Pacific and partly Tethys low temperature province; the circum-Tethys (basin) zone in this paper, and are similar to the Sichuan basin which is moderately hot-cool. This implies that the circum-Tethys zone, including the Sichuan basin, is not only cool at present, but also was cool during the Yanshanian in Mesozoic. This conforms with the values of paleogeothermal gradient 24.4” C/km and heat flow 58 m W/m2 at the end of the Cretaceous. The features of geothermal field distribution indicate that circum-Tethys and circum-Pacific basins are two types of basins. They are not only different in basin structure, crustal thickness and geodynamical character, but also in the geothermal gradients.
Distributions of oil and gas resources in the two types of basins The distribution of oil and gas resources is controlled by numerous geological factors. Since the circum-Tethys basins are different from the circum-Pacific basins in their tectonic background, geothermal condition, basin type and structure, these two types of basins must have different distribution, generation and storage patterns of oil and gas resources. Diferences
in stratigraphy and sedimentary environment
The most important factor which influences the abundance of oil and gas is the material condition which is the basis for the formation of oil and gas. Most of the circum-Tethys basins are composed of foreland sequences (first cyclothem) of Paleozoic cratons and foreland sequences which are related to erogenic belts (second cyclothem). The first cyclothem are basically marine sequences, which are mainly carbonate platform sediments. Deeper water deposits of basinal facies are developed in some places. These type of basins have a long evolving history and relatively complete strata (Sinian-Tertiary). Most oil and gas are from the first
basins
313
cyclothem. The second cyclothem was developed after the formation of erogenic mountain chains. It has sufficient material supply, the speed of deposition is often greater than the speed of sinking, an overcompensated, shallow-water environment is therefore often sustained. Coarse elastic sediments are well developed, very thick molasse sequence is a typical character of foreland basins. Thick-bedded coarse, red sediments are prominent. Deep-water environment is very difficult to develop. Even though it can occasionally be formed, it is a local phenomenon far away from the erogenic belt. The circum-Pacific zone is formed by continent-ocean collision; there is a great difference between it and the Tethyan collisional mountain chain. The Alps, the Himalayas, the Qinghai-Tibet High Plateau and the Cenozoic reactive mountain chains, such as the Tianshan, Kunlun, Qinling and Qilian, are all high giant mountain systems. There are big altitude differences between them and the adjacent basins. Coarse elastic materials are deposited in the basins at a very high rate, forming an overcompensated, shallow water environment. The circum-Pacific mountains are not comparable in scale and height with the mountain chains in the west. The altitude differences between them and the basins nearby are small, sediment supply is lower. Undercompensated deep lake can be easily formed when the sinking rate is greater than the rate of sediment supply in a faulted lake or an aulacogen lake. In the east, basins with well developed deep-lake sediments are good sources of oil and gas. These sediments are the most important source rocks of terrestrial basins. Widely distributed Cretaceous and Early Tertiary coal measures are gas sources and storage beds for this type of basin. Fault basins began to form mostly in Early Cretaceous but some only started in the Tertiary. As far as we know, oil and gas are developed mainly in younger sequences. If oil and gas exist in older sequences in the deeper part of a fault depression, they are mostly from the overlying sequences. Older source rocks have rarely been found. Geothermal energy is essential for the change from a sedimentary basin to an oil or gas basin. A hot basin supplies suthcient thermal energy to the organic materials. High geothermal gradient (30-50’ C/km) ensured that the kerogenite can rapidly and fully split and change into oil and gas. The second result of a high geothermal gradient is the small threshold depth of oil formation. The threshold depth of the Cretaceous System is 1100-1300 m (Songiiao and Erlian basins). The Lower Tertiary sediments are heated over a relatively shorter period and the threshold depth of oil formation is correspondingly deeper, 2500 & m (Bohai Gulf, Subei, East China Sea, South China Sea). Circum-Tethys basins are cool-moderately hot basins, geothermal gradient 25-15” C/km, heat flow 55-40 m W/m2. Compared with the hot basins the thermal parameters are reduced by between a third and a half. At lower temperature, the organic materials evolve slowly, but this can be compensated by time. The threshold depth of oil formation increases by three times, 3000-5000 m. The upper part of the sequence (Tertiary and part of Mesozoic) is often immature. Mature sequences are mainly the Paleozoic and part of the Mesozoic sequences. The source rocks and storage beds of the basins in the east are younger terrestrial sequences; oil and gas in the basins to the west are mainly from Paleozoic marine sequences and some Mesozoic
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terrestrial sediments. It is easy to see that the two types of basins have quite different distributions of oil and gas. Type of oil and gas accumulation
The trap structures of the compressional basins in the middle and west are anticlines and large-scale domes. The trap structures of the extensional basins in the east are mainly complicated block traps and anticlines. In the middle and west, carbonates are the major oil and gas storage beds and elastic sediments play a minor role. In the east, elastic rocks are the major storage beds and carbonate storage beds are rare. The depth and age of oil and gas beds naturally influence the porosity and permeability. Although the change in physical properties of a storage bed is determined by numerous factors, a general tendency is that the basins in the east are dominated by intermediatehigh porosity beds, low porosity beds are not important; basins in the middle and west are dominated by intermediate-low porosity beds, non-conventional gas in tight formations constitutes a large portion (e.g. Sichuan, Ordos and Junggar basins). A few basins, e.g. the Tarim, have better storage beds. Because of the differences in the lithology of oil and gas beds and the storage conditions of the two types of basins, there are differences in the pattern of oil and gas accumulation. Macrodistribution of oil and gas resources
Figure 6 shows the macrodistribution of oil and gas basins in China. The classification of basins are based on the oil and gas proportion in the resource, the basin type,
geotherm, age of source rocks, abundance and maturity of organic materials and the amount of oil and gas discovered. A comprehensive survey of the distribution of oil and gas resources in the world revealed that some contain mainly oil, some mainly gas, and some contain both, It also shows the diversity and unevenness in the distribution of oil and gas. Such a situation is determined by many factors, which include not only the properties of oil- and gas-forming materials, but also the age of source rocks, depth, geothermal field and the heating time. The basin classification, based on their resource proportion, includes well explored production basins and basins still under exploration. For the purpose of calculation and analysis, the latter are more important. Oil basin. Oil resource constitutes more than 75% of the total resources, with gas less than 25%. Most Late Mesozoic and Tertiary terrestrial basins in the east are oil basins. The characteristics of oil basins are: (1) hot rift and fault basins being the majority and cool and moderately hot basins being the minority; (2) oil-bearing sequences are young, mostly Tertiary, some Cretaceous and only a few are Triassic and Permian. Gas basins. Gas constitute more than 70% of the total resources, with oil less than 30%. Basins with well developed and evolved Paleozoic carbonate rocks, such as the Sichuan and Ordos basins have the largest gas reserves. Basins dominated by well developed coal-bearing formations are the East China Sea, west Taiwan, southwest Taiwan and southeast Hainan basins. Oil-gas basins. These basins contain both oil and gas in nearly equal proportions. The upper and middle parts
Fig. 6. Types of Chinese oil and gas basins. 1 Oil basin. 2 Gas basin. 3 Oil-gas basin. 4 Cool basin. 5 Moderately hot basin. 6 Hot basin. 7 Foreland basin. 8 Intermontane basin. 9 Rift valley. 10 Shear fracture.
Circum-Tethys
315
and circum-Pacific basins
contain oil, while the deeper parts contain gas, e.g. the Tarim, Junggar and Turpan-Hami basins. Generally speaking, circum-Tethys basins (Chinese part) contain more gas than oil; circum-Pacific basins (Chinese part) contain more oil than gas.
REFERENCES
Huang Jiqing (1979) On the formation of PlioceneQuatemary molasse in the Tethys-Himalayan tectonic domain and its relation with the Indian plate motion. Geological Papers for International Exchange-Cont. 26th Int. Geol. Cong. Vol. 1, pp. l-2. Geological Publishing House, Beijing. Huang Jiqing and Chen Bingwai (1987) The evolution of the Tethys in China and adjacent regions. Geological Publishing House., Beijing. Huang Jiqing, Ren Jishun, Jiang Chunfa Zhang Zhimeng and Xu Zhiqin (1977) An outline of tectonics of China. Acta Geologica Sinica (2).
Bally A. W. (1980) Basins and subsidence-A summary. Dynamics of plate interiors. Geodynamics Series, pp. 520. Feng Fukai (1993) Basins-thermodynamics-oil and gas. In Analysis of Chinese oil and gas basins. pp. 92-106. Petroleum Industrial Publishing House, Beijing.Feng Fukai and Wu Chengye 1980, The role of the Indosinian movement in the development of the Sichuan oil and gas basin. Geological Papers for International ExchangeCont. 26th Int. Geol. Con. Vol. 1, pp. 269-219. Geological Publishing House, Beijing. He Liansheng (1990) Formation and evolution of south China Sea and the relation to oil and gas resources. In Orogenic belts, Basins, Circum-Pacific tectonics. Geological Publishing House, Beijing. Huang Jiqing (1945) On major tectonic forms ofchina. (2nd edn., 1994) Geological Publishing House, Beijing.
Li Chunyu (1982) Tectonic map of Asia. Cartographic Publishing House, Beijing. Li Sitian (chief editor) (1988) Fault basin analysis and coal accumulation. Geological Publishing House, Beijing. Liu Guangding (chief editor) (1992) Geological and geophysicalfeatures of Chinese seas and adjacent regions. Science Press, Beijing. McKenzie D. (1978) Some thoughts on evolution of sedimentary basin. Earth Planet. Sci. Lett. 40, 25-32. H. D. (1986) Sedimentation environments and jacies, 2nd. Edn.
Reading
and tectonics,
Sedimentary
Royden L. (1987) Thermal modelling of sedimentary basins. Turcotte D. L. and Schubert G. (1982) Geodynamics-application of continuous medium physics to geological problems (Chinese Translation) Seismological Publishing House, 1986.
Journal of Southeast Asian Earth Sciences, Vol. 13, Nos 3-5, pp. 305-315, 1996 Copyright 0 1996 Published by Ekvier Science Ltd Printed in Great Britain. All rights reserved SO743%47(!%)00037-2 0743-9547/96 $15.00 + 0.00
Circum-Tethys and circum-Pacific basins Feng Fukai Institute of Petroleum Geology, Ministry of Geology and Mineral Resources, 20 Chengfu Road, Beijing 100083, China Abstract-Mesozoic circum-Tethys and circum-Pacific basins, which are genetically related to the Tethys and &cum-Pacific tectonic zones, are the regions with the most abundant petroleum and natural gas. These two tectonic systems intersect in China and most of the Mesozoic basins in China are closely associated with these two systems. Circum-Tethys basins developed against a background of crustal compression. They are the result of compressional thrusting and the resultant sinking of the crust. Warm-moderately hot foreland basins are the most representative. Hydrocarbon source rock evolved slowly. Threshold depth of oil-generation is great and petroliferous beds have relatively older ages and occur at greater depths. Circum-(west) Pacific basins were formed against a background of extension and thinning of the crust and frequent geothermal activities. Hot rift basins are the most representative. Hydrocarbon source rock evolved rapidly. Threshold depth of oil-generation is small and petroliferous beds have relatively younger ages and occur at shallower depths. Copyright 0 1996 Published by Elsevier Science Ltd
Introduction
Chinese data, based on his understanding Mesozoic and Cenozoic geology of China.
Tethys and circum-Pacific are two large Mesozoic tectonic zones. They intersect in China. There are numerous basins within and along these two tectonics zones. Most of the basins are around and genetically related to the two tectonic zones. In this paper these basins are called circum-Tethys and circum-Pacific basins. Basins in central and western China are related to the evolution of the Tethys, those in eastern China are associated with the circum-Pacific tectonic zone. It is reasonable to place the basins within these two ring-formed tectonic systems. Huang et al. (1977) divided China into three tectonics domains: the Old Asian Domain; the Tethys-Himalayan Domain; and the Circum-Pacific Domain. This division is based on the Paleozoic and Mesozoic geological features of China. The present paper will focus on Mesozoic and Cenozoic sedimentary basins which are related to the latter two tectonic domains. Due to the Tethys being a collisional orogen and the circum-Pacific still under the influence of sea-floor spreading and subduction, the tectonics, sedimentology, geothermal activity, geodynamic character and the distribution of oil, gas and other mineral resources associated with these basins have many differences.
Tectonic evolution of west Pacific and the circum-(west) Pacific basins The Pacific Ocean is a Mesozoic and Cenozoic ocean basin, which is still developing. The oldest oceanic crust has a Jurassic age (Bally 1980; Li et al. 1982). Continental margins surrounding the large oceanic basin possess the character of an active margin. There are two megasutures in the Tectonic Map of the World compiled by Bally (1980), the circumPacific zone and the Tethyan zone. These two zones, however, are not well displayed in China. This is due largely to insufficient data on China. In this paper, the author attempts to add in the
of the
Tectonics of Mesozoic continental margin and Mesozoic basins Mesozoic eastern Asian active margin. Mesozoic volcanic rocks, granites and fold belts are widely distributed in eastern China and eastern Asia, from Hainan through Guangdong, Fujian, Zhejiang, Shandong, Liaoning, Korea, Jilin, Heilongjiang to the easternmost part of Russia. They form a very complicated magmatic-tectonic zone (Fig. 1). Based on the following geophysical and drill-hole data on the East China Sea and south China Sea, this magmatic-tectonic zone has a tendency to extend into the marine regions: (1) 160 wells in the northern continental shelf of the south China Sea, extending to the Xisha Islands, have reached Mesozoic magmatic rocks (granites and volcanics, isotopic age 68-130 Ma); (2) Mesozoic granites and volcanics with isotopic ages of 89-123 Ma have been found in 10 localities (wells and outcrops on islands) in the southwestern part of the south China Sea (Wu 1990); (3) The gravity and magnetic fields of the fold belt between the continental shelf of the East China Sea and Diaoyu Island have similar characteristics as those of the terrestrial region of Fujian and Zhejiang, and are quite different from those of the Okinawa sea trough and Ryukyu island-arc east of the fold belt. Based on these, it is believed that the magmatic-tectonic zone of Fujian and Zhejiang extends eastwards to the Diaoyu Island. From the above data, the basement of the Tertiary basins in the East China Sea and south China Sea is a Mesozoic or older tectonic zone transformed by Mesozoic magmatic movement. Proterozoic substrata found in a few wells are probably old continental remnants cut apart by Mesozoic tectonics. This magmatic-tectonic zone with a width of about 1000 km extends about 5000 km through the terrestrial and marine regions of eastern Asia in a north-northeast
305
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direction. It is not concordant with the pre-Mesozoic tectonic lineaments. What is the relationship between the Mesozoic magmatic-tectonic zone and pre-Mesozoic tectonic zones? What are the tectonic characteristics of Mesozoic continental margin? Analysing the results of various researches the author arrives at the following conclusion. (1) Large scale magmatic activity, including eruptions and intrusive granites, indicate that strong heat flow events with a time range of 100 Ma took place at the continental margin of eastern Asia during the Mesozoic. Their intensities and spatial extent goes beyond the Cenozoic continental margin. The heat flow events are
the most prominent features of the Mesozoic. Both mantle-derived and crust-derived magmas indicate that mantle convection was very active. Uprising heat caused partial melting and rapid temperature rise in the crust. Against a background of high crustal temperatures, the older tectonic framework was severely altered. The pre-Mesozoic tectonic framework was replaced by a new framework. Because the magmatic-tectonic zone was developed on the basis of the old framework, it may keep some characteristics of the older tectonics. In the direction of the continental interior heat activities decrease gradually, magmatic activities and the degree of transformation becomes weaker, the boundary between
Fig. 1. Mesozoic and Cenozoic tectonics and basins of China and the adjacent regions. 1 Early Mesozoic erogenic belt. 2 Middle-Late Mesozoic erogenic belt. 3 Cenozoic collisional erogenic belt. 4 Cenozoic island-arc tectonics. 5 Boundary of tectonic regions. 6 Mesozoic volcanic arc of continent margin. 7 Mesozoic plutons. 8 Collisional suture. 9 Subduction zone. 10 Foreland thrust zone. 11 Important strike-slip fault zone (including interior fault depression). 12 Oceanic spreading ridge. 13 Strike-slip fault and other types of fault. 14 Foreland basin. 15 Intermontane basin and other types of basin. 16 Rift-fault depression. 17 Shear fracture. 18 Back-arc marginal sea.
Circum-Tethys
and circumPacific
the new and old tectonics becomes faint and transitional. Heat activities fluctuate alone this zone, the strongest occurring in Guangdong, Fujian, Zhejiang and Jiangxi. Mesozoic igneous rocks are of great importance. Paleozoic strata and older rocks are often cut apart, pushed and welded into a complex block by Mesozoic magmatic rocks. In the lower Yangtze region magmatic activities are relatively weaker. The structural styles of the Mesozoic tectonic movements comprise mainly of folding and basement detachment, bearing some signatures of an erogenic zone. (2) Most Mesozoic volcanics in eastern China are of a Late Jurassic to Early Cretaceous age. They belong mainly to the talc-alkaline series, and have a tendency to change into alkali-talc and alkaline series in the direction of the continental interior. It is usually thought that the former are eruptions at an active continental margin. (3) There are some relatively convincing theories for a subducting convergence zone in some parts of the Mesozoic tectonic zone: (a) there are Triassic and Jurassic deep-water marine deposits, flysch and ophiolite melange in the Sichote (SikhotekAlin area (Li et al. 1982); (b) the Hida double metamorphic zone (Early Mesozoic) and the Sambagawa double metamorphic zone I (Late Mesozoic) in Japan; (c) the Yushan high-pressure metamorphic zone (Late Mesozoic) in Taiwan; (d) Liu (1992) points out that there are at least two important Mesozoic sutures in eastern Asia: the Hainan-Hida suture zone (Early Mesozoic); and the Yuli-Sambagawa suture zone (Late Mesozoic). (4) From the above, this magmatic-tectonic zone represents a Mesozoic active margin in eastern Asia. Generally, it belongs to a collisional arc at continental margin, and possesses the characteristics of an island arc in some sections. Mesozoic basins. There are numerous Late Mesozoic basins in eastern China, especially in northeastern China, where about 200 Early Cretaceous coal-bearing fault depressions have been recognised (Li 1988). Below the large Late Cretaceous Songliao basin there are 40 Early Cretaceous fault depressions. In the eastern part of north China, Late Mesozoic fault depressions were also developed below Tertiary fault basins in the Bohai Gulf. Most of these basins are associated with volcanics. The volcanics erupted either before the formation of fault depression (e.g. the Great Hingan basin), or before the formation of depression and during the deposition process (e.g. the Songliao basin). Extensional basins in the south were formed slightly later, starting in the Late Cretaceous, e.g. the Subei basin, the Jianghan basin and small Cretaceous red basins in south China. These basins are not very similar to Cretaceous fault basins in the north. The formation of the basins in the south continued to the Tertiary. Many of them are developed in the Tertiary. The characteristics of Late Mesozoic basins can be summarised as follows: (1) The basins are formed under similar background, the extensional thinning of the crust. The basins are closely related to the Mesozoic magmatic-tectonic zone, most fault depressions are located within or adjacent to this zone (Figs 1 and 2). (2) Most of the basins have a “one-fold” structure, they are small listric fault depressions and rift valleys. Sedimentary features of the basins reveal that they are *BABI I,D-%Y
basins
Fig. 2. Distribution of circum-(west) Pacific basins. 1 Mesozoic and Cenozoic basins. 2 Oil and gas basin, 3 Cenozoic marginal sea basin. 4 Cenozoic subduction zone. 5 Mesozoic collisional suture. 6 Strike-slip fault.
originally extensional structures, though some have experienced later tectonic inversion and compression of different degrees. A few basins have a “two-fold” structure-faulting in the lower part and sinking in the upper part. These two different basin structures represent different patterns of sinking and dynamic regime. (3) Most Late Mesozoic fault depressions have a high geothermal gradient, they belong to hot basins (see below). (4) Fault depressions of “one-fold” structure slip along major synsedimentary fault and have a listric structure-steep in the upper part and gentle in the lower part. Sinking is related to strike-slip faults, the depth of the faults does not surpass the extent of the upper crust. They seem not to be related to mantle movement. Stages of depression are not clear or missing. This kind of basins are called “shear fault depressions” in this paper, e.g. the Great Hingan fault depression. (5) Fault basins of “two-fold” structure are developed in two stages. Extension of the crust results in block faulting and sinking, this is the rifting stage. Gradual cooling of the crust results in slow geothermal sinking, this is the post-rifting stage (McKenzie 1978; Royden
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1987). The formation of these kinds of basin is connected with the active mantle upwelling. They are called rift basin in this paper (e.g. the Songliao basin), corresponding to the thermal excited basins of Reading (1986). Structure of Cenozoic active continental margin and Cenozoic basins The Cenozoic continental margin developed on the Mesozoic active continental margin. The system of island arc-back arc marginal sea basins (briefly arc-basin system) is a typical feature of the circum(west) Pacific tectonics. Oceanic-continental subduction results in rifting of the Mesozoic continental margin arc. Continental stretching occurs at first; then comes the sea-floor spreading; marginal sea and island arc are formed at the end. The Japanese island arc, for example, was a part of a continental margin arc in the Mesozoic. It rifted off from the continent in the Cenozoic. The Japanese (marginal) sea was formed only in the Miocene. The Ryukyu island arc and its back-arc basin-the Okinawa trough was formed quite late. It has been spreading since the Pliocene. Due to the short spreading time, oceanic crust had not appeared yet. It was still in the period of continental crust thinning. The formation of the south China Sea is relatively complicated. It was a part of the eastern Asian continent in the Mesozoic. The first group of rift valleys occurred in the Early Tertiary, oceanic crust appeared in the Oligocene and developed to the Middle Miocene (32-17 Ma) (He Liansheng 1990). Finally sea-floor spreading stopped and the central ocean basin was formed. The former Early Tertiary rift valleys was split into the north and south continental shelf basins and small ocean basins. Because of the rotation and northward movement of the Philippine island arc, it collided with the Asian continent at Taiwan in Late Miocene-Pliocene, and became the eastern boundary of the south China Sea. The south China sea is a marginal sea basin, but not necessarily a back-arc basin. The three marginal seas mentioned above have a short history, only 30-50 Ma, i.e. the spreading began in the Oligocene, about 30 Ma later than the continental shelf rift basins nearby, such as the East China Sea continental shelf basins, the Pearl River mouth and Qiongdongnan basins on the north continental shelf of the south China Sea, the Zengmu, Nansha and North Balawang basins on the south continental shelf of the south China Sea. They began to form in Early Tertiary (Late Cretaceous has been suggested in some literature, but this date is not supported by reliable data). What is the relationship between these continental shelf basins and the marginal sea basin? To which basin system do they belong? According to the Wilsons cycle, there should be a stage of continental extension and rifting before sea floor spreading, i.e. the continental shelf basins mentioned above represent the rifting phase prior to sea-floor spreading. Therefore, it is reasonable to assume that the continental shelf basins near a marginal sea basin belong to one system. Continental shelf basins are called “periphery basins of marginal sea”. These periphery basins can be further divided into different types according to their evolution history. Although Tertiary basins developed in the interior of the continent, such as the Bohai Gulf, Subei-south Yellow Sea, north Yellow Sea and Jianghan basins,
show similarity to the above-mentioned continental shelf basins in basin structure, development history and lithology, these continental interior rift basins are difficult to link with the island arc-subduction system 1000 km away. The characteristics of Tertiary basalt within these basins indicate that they belong to continental rift basins, the basalt being quite different from the Mesozoic volcanics at the continental margin.
Tectonics of the Tethys and the circum-Tethys basins Huang and Chen (1987) concluded: “The Tethys was a wide sea area lying south of the super Eurasian continent. Most part of it is oceanic and transitional. It started to form in Late Carboniferous and Early Permian. Due to plate subduction and collision, rifting and sea floor spreading, the extent of the sea area changed greatly with time. It became more and more narrow and closed in the Tertiary, but there remained a part which continued to develop to today”. Figure 3 outlines the tectonics of the Tethys and the distribution of periphery basins and interior basins. The Tethys proper has already closed and formed grand mountain chains, which is represented in Europe by the Alps and in Asia by the Himalayas. The eastern part of the Tethys has not yes closed, it is still developing. Between the Tethys tectonic zone and the periphery basins are foreland thrust fault zones, more or less the A-type subduction zone of Bally (1980). The still open Sumatra-Jawa island arc is a B-type subduction boundary. Within the tectonic zone (China) four collision sutures of the Indosinian, Early Yanshanian, Late Yanshanian and Himalayan periods have been found. They represent the multi-phase collision of Eurasia and Gondwana and the paleo-, meso- and neo-Tethys. Besides some intermontane basins within the tectonic zone and some remnant blocks which have not been strongly deformed, most basins are distributed at the periphery of the erogenic mountain belts, forming a relatively complete circle of basins. They are called in this paper “circum-Tethys basins”. “Foreland basins” are located near the foreland thrust zone. They are most closely related to the evolution of the tectonic zone, and are normally connected with the craton basin in the direction to the interior of the continent. Figure 3 shows 33 periphery basins, 23 of them (70%) can be considered as foreland basins, the rest are strike-slip basins and other unclassified basins. Therefore, the foreland basins are the most representative of circum-Tethys basins. The intermontane basins (excluding the area not closed), however, are representatives of basins within the tectonic zone. In the eastern part, which is still developing, the basins behind the subduction zone and island arc are back-arc basins or marginal sea basins. Before the closure of the Tethys, the Gondwana continent was to the south and the Eurasian continent was to the north. During the permo-Triassic periods, “China” was on the northern side of the Tethyan ocean, the Songpan-Ganzi sea belonged to the northern Tethys and was a sedimentary miogeosynclinal basin with a sialic crust (Huang and Chen 1987). From the Permian to Late Triassic, “southwestern China” and the upper Yangtze region were close to the Songpan-Ganzi sea
Circum-Tethys
and circum-Pacific basins
trough of northern Tethys. They were continental shelf shallow sea basins dominated by carbonate platform deposits. The sea trough to their west was dominated by flysch and deep-sea deposits. Two marine areas were connected, both contained northern Tethyan faunas and there was no old continent between them. Before the closure of the Qinling sea trough (an aulacogen of the northern Tethys in the interior of the continent), the Ordos basin also experienced short transgressions and had similar Early Triassic faunas as those of the upper Yangtze sea basin. The Tarim Basin might have been connected with the Kekeshaer sea trough in the Permian and changed into a near-sea continental basin in the Triassic. The Qaidam was a peninsula extending into the Songpan-Ganzi sea and the western Qinling sea trough in the Triassic. It was surrounded by the sea on three sides. The above indicate that these regions were connected with the Tethys and that they are periphery basins of the Tethys. As a result of the northward movement of the blocks derived from the southern continent and the closing of the Tethys from south to the north, a series of collision mountain chains were formed at different times. The first erogenic belt formed is the Songpan-Ganzi folding belt (Indosinian). The next are the Tanggula fold belt (early Yanshanian) and the Lhasa-Tengchong fold belt (late Yanshanian). The last is the Himalayan mountain chain (Himalayan). As the Tethys disappeared in China the grand Qinghai-Tibet High Plateau was formed. Four collision erogenic events influenced the periphery basins mentioned above to varying degrees. They even affected basins far away, such as the Junggar, Hexizoulang and Ordos. The Indosinian, Yanshanian and Himalayan tectonic phases also produced unconformities and disconformities in these areas. The Indosinian and Himalayan are the most prominent phases which strongly affected the periphery
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basins and the pre-Mesozoic erogenic belts. The effect of the Indosinian phase on the periphery basins is that the former continental shelf basins, offshore basins (normally called first cyclothem) changed into “foreland basins” which had genetic relations with the foreland thrust zone and are dominated by terrestrial sediments (normally called second cyclothem). The relationship between the periphery basins and the Tethys is explained here, using the Sichuan basin as a example. (1) From the Permian to early-Late Triassic the Sichuan basin was located between the Yangtze craton and the northern Tethys sea, with the Songpan-Ganzi sea to its west and the Qinling aulacogen (a branch of the Tethys extending landward) to its north. The Qinling sea closed first in the Middle Triassic, the SongpanGangzi sea closed in the Late Triassic (Feng and Wu 1980). (2) The Longmenshan and Qinling-Dabashan foreland zones thrust from west to east and from north to south, forming complicated thrust tectonic belts, e.g. klippes, imbricate thrust sheets. From late-Late Triassic to Quaternary, shallow-water terrestrial coarse elastics were deposited. The most representative sediments are molasse deposits with a thickness of 3000 m, including 7-8 conglomerate beds. These reflect the multi-phase movement, thrusting and faulting of the erogenic belt. (3) Large foreland basins (with a width of more than 400 km) display structures of foreland basin such as: (a) thrust fault; (b) foredeep depression; (c) frontal rise; (d) second-order depression. They corresponding respectively to the second-order structures: (i) the Longmenshan; (ii) the western Sichuan depression; (iii) the central Sichuan rise; (iv) the eastern Sichuan paleosyncline (Indosinian). Middle and small-sized foreland basins possess only the structures (a) and (b). The structure of the cover is often related to the deep part of the crust. Based on geodynamical analysis, foreland basins are the
Fig. 3. Tethys and circum-Tethys basins. 1 Tethys tectonic zone. 2 Boundary of the Tethys. 3 Collisional suture. 4 Subduction boundary. fault. 6 Circum-Tethys basins. 7 Oil and gas basins. 8 Inner Tethys basin.
5 Strike-slip
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result of a combine effect of horizontal compression of the thrust zone and the loading of the nappe, these two effects cause the flexure and sinking of the crust (Feng 1993). The Yarlung-Zangbo collisional event (Tertiary) not only caused the large-scale rise of the Qinghai-Tibet High Plateau, but also strongly affected the western and northern parts of China, as well as the areas further away. The pre-Mesozoic erogenic belts, such as the Tianshan, Kunlunshan and Qilianshan rose again and overthrust the areas nearby, forming Cenozoic thrust zones. Pliocene-Early Quaternary molasse deposits of large thickness (several hundred to several thousand meters) were deposited at the margins, such as the deposits of the two frontal depressions south
and north of Tarim, the Xiyu conglomerate of the southern frontal depression of the Junggar basin, the Yumen Conglomerate in Hexizoulang, the Yaan-Dayi conglomerate of the western frontal depression of the Sichuan basin and the Siwalik Conglomerate of the southern slope of the Himalayas (Huang 1979). These molasse deposits are wholly or partly tectonically deformed, either folded or faulted, forming the late structural trap (e.g. the Sichuan, Tarim and Qaidam basins). The Junggar, Turpan-Hami and Hexizoulang basins do not seem to be related to the Tethys before the Cenozoic. After the Yarlung-Zangbo collisional event, the pre-Mesozoic erogenic belts rose once again.
Fig. 4. Crustal division of China and the adjacent regions. 11 Strongly compressed and thickened continental crust. 12 Normal and thickened continental crust. 111 Extended and thinned continental crust. II2 Transitional continental crust. III Oceanic crust. 1. Depth of the Moho.
Circum-Tethys
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and circum-Pacific basins
Analysis of the differencesbetween the eastern and western tectonic systems Figure 4 shows the macro-distribution of the Moho in China and adjacent regions. Both the configuration of the macro-distribution of crustal thickness and vertical structure have a two-fold character. The gravity gradient zone (reflecting the change of crustal thickness) running from Hingan via Taihang to Wuling is a boundary (about 40 km). The crust east and west of this zone have great differences. The western part is the thickened continental crust area (area A) with a thickness of 40-70 km. It has experienced several collisional erogenic events in the Mesozoic and Cenozoic, the crust has been compressed and shortened. The eastern part is the extended and thinned continental crust and transitional crustal areas with a thickness of 10-40 km. Here the Mesozoic and Cenozoic crustal extension and thinning took place. Area (,: S-70 km, corresponding to the Tethys proper. The upper part of the crust experienced brittle deformation, faulting and folding, overthrusting and supposition. There is a low speed zone of 5 km thickness in the depth of 20 km, it may be the ductile shear surface (or detachment surface). The lower part of the crust, roughly half of the crust, can be explained as a ductile-plastic deforming part (according to the Shaoyang-Heishui transaction). Area (*: 40-50 km, including pre-Mesozoic erogenic belts and a series of large Mesozoic, Cenozoic basins. A prominent character of this area is that it extends around the Area (,, forming a half circle. It corresponds to the region where the circum-Tethys basins occur. Crust thickness does not change greatly within a basin, it is generally 4245 km. Though it experienced compressional deformation, the thickness of the crust was not much affected. It can represent the normal thickness of a continental crust, and thus be used as a standard for judging thickening and thinning of the crust in other areas. The crust thickness of old erogenic belts outside the basins has increased slightly. The crust structure within the basins: comprising a sedimentary cover, a metamorphic basement and the lower crust, each has a thickness of about one-third of the crust (see the Shaoyang-Heishui transaction). Area ((I: Includes the continental region of the eastern part and parts of the offshore continental shelf, with a tendency to thin eastward (reduced from 40 km to 25 km). The northern section: locally Moho doming (about 30 km) in the eastern part of north China, the Bohai Gulf and the Songliao basin, the maximum thinning is about 10 km. In the lower Yangtze region and in south China on the southern section there is no clear thinning of the crust. The degree of crustal thinning often controls the scale of Mesozoic and Cenozoic basins. Moho anomalies generally correspond to large Mesozoic and Cenozoic extensional rift basins. Small faulted basins are always related to the thinning, there is usually no Moho anomaly. Structures of the crust and mantle are generally related to the active degree of Cenozoic tectonics and degrees of faulting. When the low-velocity layer of the upper mantle is shallow and there is a low-velocity layer in the crust, recent tectonic activities are strong (earthquakes, volcanic eruptions), e.g. the eastern part of
north China and the Bohai Gulf. The former Mesozoic active regions changed into tectonically stable regions in the Cenozoic, e.g. middle and lower Yangtze regions and south China. There is no low-velocity layer in the upper mantle (Liu 1992). The change in the crustal thickness is related to mantle activity. The upwelling of the asthenosphere raises the temperature of the lithosphere and crust, causing uneven extension and thinning of the crust and an increase in tectonic activities. If there is no superposition of Cenozoic mantle activity, the Mesozoic tectonically active regions become tectonically stable regions. crustal region, including Area ((2: Transitional Cenozoic island arc-marginal sea. Crust thickness varies between 25-10 km. Based on SV velocity layer analysis, the low-velocity layer of the upper mantle is very shallow (45-80 km), the uprising current is especially active (Liu 1992). This is the primary driving force for the strong Cenozoic tectonic activitie-arthquake, thermal activity, volcanic eruption, continental extension and sea-floor spreading. Very active vertical convection of the asthenosphere, i.e. the uprising current is the primary reason for the formation of the arc-basin system in west Pacific.
Geothermalfield and geothermal subdivisionof the basins of the two systems Geothermal distribution in the crust is not even, some places are relatively hotter and belong to the hot crust; some places are relatively colder and belong to the cold crust. Hot and cold are relative. Average continental geothermal parameter is used in this paper as a standard to judge the geothermal property of a region. When the geothermal parameter of a region is greater than the average, it is called a hot region (basin); when it is smaller than the average, it is called a cool region (basin); when it is close to the average, it is called a moderately hot region (basin). Regions with very high anomaly are very hot regions (basins). Heat flow is a very important parameter to describe the thermal condition and process in the interior of the earth. The average continental heat flow is 60 f 5 m WJm2. Because there are only limited measured heat flow values, we use geothermal gradient as a supplementary parameter for geothermal analysis. These two parameters are used for geothermal division of basins (regions; Table 1; Fig. 5). There is enormous heat energy in the interior of the earth, which is transmitted continuously to the earth’s surface in the form of rock conduction, magma, Table 1. Geothermal division of basins Basin region Hot basin Moderately
Geothermal gradient Heat flow d(“C/km) Q (mW/m*) 30-60 22-33
62.7-100 N-62.7
< 22 > 60
c 50 > loo
hot basin Cool basin Very hot basin
Average continental crust heat flow 60 + 5 mW/m*
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Fig. 5. Geothermal division of Chinese basins. 1 Very hot region (basin). 2 Hot basin. 3 Moderately hot basin. 4 Cool basin. 5 High temperature tin and tungsten ore deposits zone. 6 Low temperature mercury and antimony ore deposits zone. 7 The Tethys. 8 Boundary of the two systems. 9 Subduction zone. 10 Mesozoic magmatic zone. volcanoes and hot springs . Heat sources are generally divided into mantle source, magma source and crust source sustained by the decay of radioactive elements. The geothermal field of a basin or a region is not fixed, but changes with the thermal-tectonic events. A thermal-tectonic event breaks the old equilibrium and the geothermal parameter increases rapidly. This marks the start of a thermal cycle. As the heat dissipates, the crust becomes cooler and reaches a new equilibrium. A thermal equilibrium to a new equilibrium constitutes a thermal cycle. According to the active pattern of thermal evolution of the oceanic lithosphere, heat flow is an exponential function of time. A thermal cycle lasts 100-150 Ma. The highest cooling rate is in the first 50 Ma after the thermal event, then it becomes lower and lower. We applied the oceanic thermal evolution pattern to the rift basins in eastern China and found that the two parts have similar cooling tendency. The author thinks that tectonic events, such as continental extension, sea-floor spreading, subduction, collision, sinking of basins are all movement patterns in the geological framework. Since they are movement, they must have a driving force and need the relevant energy. Turcotte and Schubert (1982) believed that the basic mechanism of plate tectonics must supply the energy for earthquakes, volcanoes and erogenic movement, and the only energy resource that is big enough is in the interior of the earth. The author basically agrees with this opinion, but also takes second-order forces into consideration. For example, tectonic deformation caused by the rotation of the earth
and the load of geological bodies can influence the sinking of basins. Figure 5 is a thermal division of major Chinese basins based on the thermal parameters given in Table 1. It should be pointed out that this division is based on modern thermal conditions, the paleothermal conditions being not considered. We think that paleogeotherm is a time variable. If we use it as a standard, we cannot get a definite result. Therefore it is not used as the basis of basin division. We can see in Fig. 5 that the general geothermal distribution tendency is warmer in the east and cooler in the west. It conforms basically with the modern crustal thickness and the Mesozoic, Cenozoic tectonic framework. Extensional basins related to &cum-Pacific tectonics are generally hot basins, only a small number of a basins including a few compressional basins are moderately hot basins. The hottest places within a basin are deep faults and depressions. Circum-Tethys basins and foreland basins are generally cool-moderately hot basins. The coldest place is the foredeep depression of foreland basins. The thermal distribution pattern is quite different from that of the circum-Pacific basins. These two systems have clear differences. Basins, other than the Mesozoic and Cenozoic basins, cannot be systematically classified because of the lack of sufficient thermal parameters. Only the tendencies of the geothermal distribution can be found. It has been mentioned before that heat flow is an exponential function of time, and a geothermal cycle lasts 100-150 Ma. Tectonically active regions since the
Circum-Tethys
and circumpacific
Tertiary are hot or very hot regions, e.g. the Cenozoic Yarlung-Zangbo suture and the Okinawa sea trough are the hottest. Pre-Mesozoic and Early Mesozoic tectonic regions (more than 150 Ma) have reached the thermal equilibrium, if they are not superimposed by Late Mesozoic-Cenozoic geothermal events. Therefore, they are moderately hot regions, e.g. south China, the Yangtze region (excluding the coastal region and Late Mesozoic-Tertiary extensional basins). The regions with the Yanshanian granites was hot in the Mesozoic. They have since cooled down and are moderately hot regions. Regions with the Yanshanian granites in south China metallogenic provinces. Mesozoic are important geothermal field can be determined based on the distribution of the different ore belts. Tungsten and tin ore deposits are high temperature ore deposits. They are mainly distributed in southern Jiangxi, southeastern Hunan and Hainan Island. Huang (1945) assigned them to out-Pacific high temperature ore province. Low temperature ore deposits, such as mercury and antimony ore deposits are distributed in the western margin of the Jiangnan Old Continent, southern Guizhou, southeastem Yunnan and the Nanpanjiang synclinorium. No high temperature tungsten and tin ore deposits occur in these regions. Huang called them inner-Pacific and partly Tethys low temperature province; the circum-Tethys (basin) zone in this paper, and are similar to the Sichuan basin which is moderately hot-cool. This implies that the circum-Tethys zone, including the Sichuan basin, is not only cool at present, but also was cool during the Yanshanian in Mesozoic. This conforms with the values of paleogeothermal gradient 24.4” C/km and heat flow 58 m W/m2 at the end of the Cretaceous. The features of geothermal field distribution indicate that circum-Tethys and circum-Pacific basins are two types of basins. They are not only different in basin structure, crustal thickness and geodynamical character, but also in the geothermal gradients.
Distributions of oil and gas resources in the two types of basins The distribution of oil and gas resources is controlled by numerous geological factors. Since the circum-Tethys basins are different from the circum-Pacific basins in their tectonic background, geothermal condition, basin type and structure, these two types of basins must have different distribution, generation and storage patterns of oil and gas resources. Diferences
in stratigraphy and sedimentary environment
The most important factor which influences the abundance of oil and gas is the material condition which is the basis for the formation of oil and gas. Most of the circum-Tethys basins are composed of foreland sequences (first cyclothem) of Paleozoic cratons and foreland sequences which are related to erogenic belts (second cyclothem). The first cyclothem are basically marine sequences, which are mainly carbonate platform sediments. Deeper water deposits of basinal facies are developed in some places. These type of basins have a long evolving history and relatively complete strata (Sinian-Tertiary). Most oil and gas are from the first
basins
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cyclothem. The second cyclothem was developed after the formation of erogenic mountain chains. It has sufficient material supply, the speed of deposition is often greater than the speed of sinking, an overcompensated, shallow-water environment is therefore often sustained. Coarse elastic sediments are well developed, very thick molasse sequence is a typical character of foreland basins. Thick-bedded coarse, red sediments are prominent. Deep-water environment is very difficult to develop. Even though it can occasionally be formed, it is a local phenomenon far away from the erogenic belt. The circum-Pacific zone is formed by continent-ocean collision; there is a great difference between it and the Tethyan collisional mountain chain. The Alps, the Himalayas, the Qinghai-Tibet High Plateau and the Cenozoic reactive mountain chains, such as the Tianshan, Kunlun, Qinling and Qilian, are all high giant mountain systems. There are big altitude differences between them and the adjacent basins. Coarse elastic materials are deposited in the basins at a very high rate, forming an overcompensated, shallow water environment. The circum-Pacific mountains are not comparable in scale and height with the mountain chains in the west. The altitude differences between them and the basins nearby are small, sediment supply is lower. Undercompensated deep lake can be easily formed when the sinking rate is greater than the rate of sediment supply in a faulted lake or an aulacogen lake. In the east, basins with well developed deep-lake sediments are good sources of oil and gas. These sediments are the most important source rocks of terrestrial basins. Widely distributed Cretaceous and Early Tertiary coal measures are gas sources and storage beds for this type of basin. Fault basins began to form mostly in Early Cretaceous but some only started in the Tertiary. As far as we know, oil and gas are developed mainly in younger sequences. If oil and gas exist in older sequences in the deeper part of a fault depression, they are mostly from the overlying sequences. Older source rocks have rarely been found. Geothermal energy is essential for the change from a sedimentary basin to an oil or gas basin. A hot basin supplies suthcient thermal energy to the organic materials. High geothermal gradient (30-50’ C/km) ensured that the kerogenite can rapidly and fully split and change into oil and gas. The second result of a high geothermal gradient is the small threshold depth of oil formation. The threshold depth of the Cretaceous System is 1100-1300 m (Songiiao and Erlian basins). The Lower Tertiary sediments are heated over a relatively shorter period and the threshold depth of oil formation is correspondingly deeper, 2500 & m (Bohai Gulf, Subei, East China Sea, South China Sea). Circum-Tethys basins are cool-moderately hot basins, geothermal gradient 25-15” C/km, heat flow 55-40 m W/m2. Compared with the hot basins the thermal parameters are reduced by between a third and a half. At lower temperature, the organic materials evolve slowly, but this can be compensated by time. The threshold depth of oil formation increases by three times, 3000-5000 m. The upper part of the sequence (Tertiary and part of Mesozoic) is often immature. Mature sequences are mainly the Paleozoic and part of the Mesozoic sequences. The source rocks and storage beds of the basins in the east are younger terrestrial sequences; oil and gas in the basins to the west are mainly from Paleozoic marine sequences and some Mesozoic
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terrestrial sediments. It is easy to see that the two types of basins have quite different distributions of oil and gas. Type of oil and gas accumulation
The trap structures of the compressional basins in the middle and west are anticlines and large-scale domes. The trap structures of the extensional basins in the east are mainly complicated block traps and anticlines. In the middle and west, carbonates are the major oil and gas storage beds and elastic sediments play a minor role. In the east, elastic rocks are the major storage beds and carbonate storage beds are rare. The depth and age of oil and gas beds naturally influence the porosity and permeability. Although the change in physical properties of a storage bed is determined by numerous factors, a general tendency is that the basins in the east are dominated by intermediatehigh porosity beds, low porosity beds are not important; basins in the middle and west are dominated by intermediate-low porosity beds, non-conventional gas in tight formations constitutes a large portion (e.g. Sichuan, Ordos and Junggar basins). A few basins, e.g. the Tarim, have better storage beds. Because of the differences in the lithology of oil and gas beds and the storage conditions of the two types of basins, there are differences in the pattern of oil and gas accumulation. Macrodistribution of oil and gas resources
Figure 6 shows the macrodistribution of oil and gas basins in China. The classification of basins are based on the oil and gas proportion in the resource, the basin type,
geotherm, age of source rocks, abundance and maturity of organic materials and the amount of oil and gas discovered. A comprehensive survey of the distribution of oil and gas resources in the world revealed that some contain mainly oil, some mainly gas, and some contain both, It also shows the diversity and unevenness in the distribution of oil and gas. Such a situation is determined by many factors, which include not only the properties of oil- and gas-forming materials, but also the age of source rocks, depth, geothermal field and the heating time. The basin classification, based on their resource proportion, includes well explored production basins and basins still under exploration. For the purpose of calculation and analysis, the latter are more important. Oil basin. Oil resource constitutes more than 75% of the total resources, with gas less than 25%. Most Late Mesozoic and Tertiary terrestrial basins in the east are oil basins. The characteristics of oil basins are: (1) hot rift and fault basins being the majority and cool and moderately hot basins being the minority; (2) oil-bearing sequences are young, mostly Tertiary, some Cretaceous and only a few are Triassic and Permian. Gas basins. Gas constitute more than 70% of the total resources, with oil less than 30%. Basins with well developed and evolved Paleozoic carbonate rocks, such as the Sichuan and Ordos basins have the largest gas reserves. Basins dominated by well developed coal-bearing formations are the East China Sea, west Taiwan, southwest Taiwan and southeast Hainan basins. Oil-gas basins. These basins contain both oil and gas in nearly equal proportions. The upper and middle parts
Fig. 6. Types of Chinese oil and gas basins. 1 Oil basin. 2 Gas basin. 3 Oil-gas basin. 4 Cool basin. 5 Moderately hot basin. 6 Hot basin. 7 Foreland basin. 8 Intermontane basin. 9 Rift valley. 10 Shear fracture.
Circum-Tethys
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and circum-Pacific basins
contain oil, while the deeper parts contain gas, e.g. the Tarim, Junggar and Turpan-Hami basins. Generally speaking, circum-Tethys basins (Chinese part) contain more gas than oil; circum-Pacific basins (Chinese part) contain more oil than gas.
REFERENCES
Huang Jiqing (1979) On the formation of PlioceneQuatemary molasse in the Tethys-Himalayan tectonic domain and its relation with the Indian plate motion. Geological Papers for International Exchange-Cont. 26th Int. Geol. Cong. Vol. 1, pp. l-2. Geological Publishing House, Beijing. Huang Jiqing and Chen Bingwai (1987) The evolution of the Tethys in China and adjacent regions. Geological Publishing House., Beijing. Huang Jiqing, Ren Jishun, Jiang Chunfa Zhang Zhimeng and Xu Zhiqin (1977) An outline of tectonics of China. Acta Geologica Sinica (2).
Bally A. W. (1980) Basins and subsidence-A summary. Dynamics of plate interiors. Geodynamics Series, pp. 520. Feng Fukai (1993) Basins-thermodynamics-oil and gas. In Analysis of Chinese oil and gas basins. pp. 92-106. Petroleum Industrial Publishing House, Beijing.Feng Fukai and Wu Chengye 1980, The role of the Indosinian movement in the development of the Sichuan oil and gas basin. Geological Papers for International ExchangeCont. 26th Int. Geol. Con. Vol. 1, pp. 269-219. Geological Publishing House, Beijing. He Liansheng (1990) Formation and evolution of south China Sea and the relation to oil and gas resources. In Orogenic belts, Basins, Circum-Pacific tectonics. Geological Publishing House, Beijing. Huang Jiqing (1945) On major tectonic forms ofchina. (2nd edn., 1994) Geological Publishing House, Beijing.
Li Chunyu (1982) Tectonic map of Asia. Cartographic Publishing House, Beijing. Li Sitian (chief editor) (1988) Fault basin analysis and coal accumulation. Geological Publishing House, Beijing. Liu Guangding (chief editor) (1992) Geological and geophysicalfeatures of Chinese seas and adjacent regions. Science Press, Beijing. McKenzie D. (1978) Some thoughts on evolution of sedimentary basin. Earth Planet. Sci. Lett. 40, 25-32. H. D. (1986) Sedimentation environments and jacies, 2nd. Edn.
Reading
and tectonics,
Sedimentary
Royden L. (1987) Thermal modelling of sedimentary basins. Turcotte D. L. and Schubert G. (1982) Geodynamics-application of continuous medium physics to geological problems (Chinese Translation) Seismological Publishing House, 1986.