HYDROCARBON ACCUMULATION OF FOUR ASSOCIATION TYPES OF FORELAND BASINS IN CENTRAL AND WESTERN CHINA:PART I.GEOLOGICAL CONDITIONS
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Abstract:
Foreland basins are important areas of exploration for petroleum in China.According to the basin construction and evolution,the foreland basins in central and western China can be divided into four types of assemblages including the presenile,the reconstructed,the regenerated and the superimposed.This paper discusses the geological conditions of accumulation,such as the assemblages of source rock,reservoir and seal,in different foreland basins based on the examples of northwestern Junggar foreland basin(the presenile),western Sichuan foreland basin(the reconstructed),northern Chaidamu foreland thrust(the regenerated) and southern Junggar foreland basin(the superimposed).The thermal evolutions of main source rocks are stated in the four types of foreland basins.The foreland development in the late period of Himalayan movement has an obvious control on the thermal evolution of source rocks in the Chaidamu foreland thrust(the regenerated) and southern Junggar foreland basin(the superimposed).Keywords:
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This paper reviews the previous and current research on foreland basins, including the concept, modes of tectonics-sedimentation, types, characteristics, and hydrocarbon bearing features. At the same time, the features and hydrocarbon potential of the rejuvenated foreland basins around intracontinental orogens in Mid-West China are discussed. Conclusions are as follows: ① The rejuvenated foreland basins, juxtaposing with peripheral foreland basin and retro-arc foreland basins, should be considered as a main type of foreland basins; ② The foreland basins, on the basis of their geodynamic environments, can be subdivided into three fundamental types: the peripheral foreland basins, retro-arc foreland basins and rejuvenated foreland basins. The former two types are the representative foreland basins, and the latter one is a rather unique type around introcontinental orogens in the Mid-West of China. ③ Due to different regional background and evolutional processes of three types of foreland basins, individual styles of oil and gas system are developed in these foreland basins.
Tectonic subsidence
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The thrust belt-related foreland basins in central and western China are characterized by non-marine source rocks.The source rocks in peripheral and retro-arc foreland basins were formed in the period when the basins were developing,such as the Permian source rocks found in the northwestern and southern Junger basin,the southwestern Tarim basin,the Tuha basin and the Triassic source rocks found in the western Sichuan and western Ordos basins.The source rocks in reactivated foreland basins were formed in the Triassic-Jurassic,the Cretaceous,and the Paleogene,before the thrust-related foreland basins were formed.Based on their sedimentary environments,all of the source rocks in the foreland basins may be classified into five types: passive continental margin marine facies,relic marine-lacustrine facies,limnetic facies,intracontinental fresh-water lacustrine facies,and intracontinental semi-saline to saline lacustrine facies.The organic matter from the limnetic coal measure is of type III and gas bearing.The organic matter from the other four types of source rocks is of type I and II and oil-bearing.Oil and gas resources in the foreland basins in the central and western China are ca.89.17×10~8 t and ca.101 464×10~8 m~3 respectively,showing that gas is enriched in Chinese foreland basins compared with foreland basins elsewhere.
Paleogene
Tectonic subsidence
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Foreland basin is the richest in oil and gas distribution of the world's different petroliferous basins.It has more favorable petroleum geology conditions and greater exploration potential.In this paper,the authors compare and analyze the coupling relationship between basins and adjacent mountains of foreland basins and the controlling factor of oil and gas,then acquire the initial understanding of the oil and gas reservoir characteristics foreland basins.That the structure of foreign typical foreland basin is a single,generally superimposed over a passive continental margin or back-arc basins.While the basement structure of Chinese central and western foreland basins is of much complexity,multi-stage evolution and more cause types.The distribution of oil and gas is related to the development characteristics of the fold-thrust belt.
Basement
Tectonic subsidence
Hydrocarbon exploration
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The foreland basins of the Mesozoic and Cenozoic in mid_western area of China are located in the eastern end of Northern Basin groups of Tethys tectonics.The forming of foreland basin was controlled by the revolution of Tethys Tectonics.Two epochs of foreland basins were developed in mid_western China during the Early Mesozoic and the Late Cenozoic respectively.Foreland basins may be divided into three types and four kinds of responding foreland basins (or thrust belts).All of those foreland basins were controlled by the evolution of Thetys tectonics.Two epochs of foreland basins both underwent the tension tectonics.The tectonics environments of the foreland basins formed the specific geological characteristics for natural gas.Effective resource rock was grown in the tension tectonics settings.Foreland thrust belts were developed in the compression tectonics settings.The foreland was well match with the trapping time and space in the large rowed and belted structural traps.The early foreland basin and thrust belt were altered and superposed by the structures of the late foreland basin.The traps in the main body of the early foreland basin almost were seriously damaged,and the traps in the thrust frontier,fore_deep and fore_bulge slopes and uplift of the early foreland basin could be well preserved.For the late foreland basin,the large structural traps in the thrust belt and fore bulge have some good conditions for pooling gas and will be the key exploration targets recently.
Tectonic subsidence
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Located in the area between orogenic belt and craton, intracontinental foreland basin in Western China was formed by the intracontinental fold thrust movementresulted from the long distance effect of plate collision. Without marine source rocks and marine molasses that are always developed along the passive continental margin, the intracontinental foreland basin is different from the typical foreland basin in foreign countries to some extent. With abundant hydrocarbon resources, the intracontinental foreland basin has attracted increasing attention of petroleum geologists in recent years. Main differences between intracontinental foreland basin and typical foreland basin have been analyzed and main sedimentary characteristics of the intracontinental foreland basin have been proposed. During the fold faulting episode, the superposition of coarse grained sediments could form apron shaped sedimentary body. While during the inactive period of the fold thrust movement, the well developed coal bed and mudstone could form source rocks of high quality. However, during the whole process, the migration of depression center and the excursion between depression center and deposition center are common. According to the superposition style of deposits, the intracontinental foreland basin has been subdivided into step forward, superposition and step backward types and sedimentary sequence model for each type has been established. Finally, the hydrocarbon generation, accumulation and preservation condition of each type of basin have been discussed, which will be helpful to the hydrocarbon exploration according to their respective sequence model and to the improvement of exploration efficiency.
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There are abundant oil and gas resources in the foreland basins in central and western China.Many outstanding successes of petroleum exploration have been obtained recently in that region.However,the exploration activity is rare because of the complex geology in the foreland basins.A classical foreland basin overseas is a linear depression located between the active orogen and stable craton.Nevertheless,in central and western China,there are mainly rejuvenated foreland basins resulting from convergence within the craton,related to the reactivation of old Tethys tectonic domains during the new Tethys stage.Therefore,there are many differences in comparison with the classical foreland basin abroad.The difference,due to the superimposition of foreland structures of two periods,is responsible for differences in basin geometry,in plate flexure and subsidence,in sedimentation,and in structural deformation.According to these differences,we have classified the foreland basins in central and western China into four types,i.e.,alteration foreland basin,palingenetic foreland basin,superposition foreland basin and caduceus foreland basin.They have different structural characteristics that determine the different reservoir features and the variable exploration potential for these three types of foreland basin.
Tectonic subsidence
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Being impacted by India-Eurasia continental collision and its agelong convergence,the Cenozoic tectonics of the Middle and the Western China are obviously characterized by the Circum-Tibet Plateau Basin and Range System and the foreland basins including the foreland thrust belts in the basin-mountain joint areas. Based on a great number of geological and geophysical data,this study first brought up the concept and the distribution features of the Circum-Tibet Plateau Basin and Range System, and then discussed its deep structure characteristics and origin-dynamics mechanism. Through deep investigation into the structure features of this system,this study thought those foreland basins came into being as superposed basins from the regenerated foreland basins in the Himalayan tectonic movements and circum and post-arc foreland basins in the late Hercynican-Indosinian orogeny, and their prototype basins included circum foreland basins in the Hercynian-Indosinian orogeny (such as the northern Tarim basin, the West Sichuan basin,and the western Ordos basin,etc.), post-arc foreland basins in the Hercynian-Indosinian orogeny (such as the southern Tarim basin,etc.)and the regenerated foreland basins in the Himalayan orogeny (such as the Kuqa basin,the Southwestern Tarim basin,the Huainan basin,etc.). These three prototype basins in addition to the thrusting and uplifting movements in the Himalayan orogeny are classified into four types of superposed foreland basins:the superimposed,the neogenic,the converted,and the premature decayed. Within the Circum-Tibet Plateau Basin and Range System there developed several foreland thrust belts with complex tectonic deformation and the main structures include fault-correlative folds combination,salt-correlative folds,strike slip-thrust combination,and so on. In the end,through analysis on the geological conditions for oil and gas in the Circum-Tibet Plateau Basin and Range System, this paper considers that there are such favorable conditions for abundant oil and gas resources that this area will surely be the important target for oil and gas exploration in China.
Orogeny
Tectonic subsidence
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The foreland basin is a major area for basin analysis and petroleum exploration due to its important structural location and abundant oil/gas resources. Structural, sedimentary and drilling data are integrated to comprehensively study the evolution and sediment filling and their significance of hydrocarbon accumulation in western Sichuan foreland basin, an important area of gas exploration in the Sichuan Basin. The evolution of western Sichuan foreland basin initiated at Carnian stage of Indosinian epoch and ended in Himalayan epoch. The development of the foreland basin in different epochs is quite different in respects of migration of depocenter, variation of tectonic stress field, and change of tectonic framework. These differences determine the generation, migration, accumulation and preservation of gas in western Sichuan foreland basin. The occurrence of gas pools in the study area is mainly controlled by the down-warped zones and uplifted zones in the process of foreland basin formation. Mianzhu-Yanting and Qionglai-Xinjin uplifted zones formed in Yanshan epoch are favorable for hydrocarbon accumulation, thus are the main plays for petroleum exploration in western Sichuan foreland basin.
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Abstract A comprehensive study has been carried out to subdivide and correlate the Upper Carboniferous and Permian sedimentary successions in the Junggar basin based on outcrops and drilling and geophysical data. The study results, combined with geological analyses of the basin's periphery and the basement, as well as studies of the sedimentary rocks within the basin, the unconformities, tectonic geometry, kinematics and geodynamics, lead to the conclusion that the Junggar basin was characterized by the development of foreland basin systems during the Late Carboniferous and Permian. During that period, three foreland basin systems were developed: (1) the northwest foreland basin system, which trended nearly north‐south from Mahu to the Chepaizi Palaeo‐mountain during its early stage of development and thus it was also referred to as the west foreland basin system; (2) the Karamaili foreland basin system in the east and (3) the Northern Tianshan foreland basin system in the south. These systems are different in size, development stage and time of formation. The first two are developed earlier than the third, but they are smaller in size. All the structures in the Junggar basin have resulted from the integration and superposition of structural elements in the above three systems. In general, the development of the Junggar basin can be divided into four stages. Stage I was marked by the creation and evolution of the marginal western foreland and the peripheral Karamaili foreland basin systems during the Late Carboniferous‐early Early Permian (C 3 P 1 ja ). Stage II was characterized by the development of complicated foreland basin systems during the middle‐late Early Permian (P 1 jb ‐P 1 f ) when the three foreland basin systems took their shapes. Stage III was the integration stage of peripheral foreland basin systems during the Middle Permian (P 2 ) in which steady and slow tectonic subsidence prevailed. Stage IV coincided with the shrinking of foreland basin development during the Late Permian (P 3 ).
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geodynamics
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