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Abstract: Ordovician karsts are well developed in the Yingmaili and Halahatang areas of the North Tarim Basin. The ancient karsts are an important factor for forming Ordovician hydrocarbon reservoirs in the central and western regions of the North Tarim Basin. They are distributed, like successive broad belts, along the south side of the North Tarim uplift and constitute a large complex ancient karst reservoir system together with the Lunnan bulge. The main controlling factors of the Ordovician karsts are uplift, interlamination parallel unconformity, fault systems, rock physical properties and high-energy sedimentary phases. The Ordovician karsts can be divided into uplift-controlled karst areas, the strata- and fault-controlled karst areas and limited karst areas while karsts of different origins can overlay each other. The ancient karsts are developed mainly in four periods: the Caledonian, Hercynian, Indo-China Yanshan, and Yanshan, and karsts of different periods overlay each other, too. The condition for developing large-scale ancient karst reservoirs exists in the area and the complex ancient karst systems can be rich in oil and gas. Along both sides of the Middle-Upper Ordovician clastic pinch-outs are the more favorable areas for large ancient karst reservoirs. [ABSTRACT FROM AUTHOR]

Abstract: The Kela-2 gas field, located in the Kuqa Depression of Tarim Basin, is the most productive gas field in China, based on per unit surface acreage. The gas trap is a subsurface anticline formed along an E–W trending thrust belt in front of the southern Tianshan Mountains. The gas pay zones are located in fine-grained sandstones of a fan delta front facies in the Lower Cretaceous Bashijiqik Formation (Klbs), and Eogene evaporitic rocks serve as regional cap rocks. The gases were derived primarily from over-mature Jurassic coal measures. Peak gas generation and final shaping of the Kela-2 structure occurred relatively late, in the late Himalayan orogeny (5–0Ma). Thrust faulting appears to have played a significant role in the hydrocarbon generation, migration, accumulation and preservation in this foreland basin. Detailed delineation of the Kela-2 gas field indicates that high-quality regional cap rocks are essential for the formation of the super-rich, abnormally pressured, giant gas fields, while buried anticlines under the overthrust zone are the favorable gas habitat. Late gas accumulation is considered to be decisive for the conservation of the giant gas fields. [Copyright &y& Elsevier]

Reports that the Lunnan area in the Tarim Basin has become an important onshore oil production base in China. Statement that the formation of the oil and gas pools in the low uplift of Lunnan has experienced a comparatively complex process of dynamics; Reference to the fact that, based on the hydrocarbon generation period of source rocks, the formation period of cap rocks and traps, the analysis of organic inclusion and the analysis of bitumen in the reservoir, this paper draws the conclusion that the low uplift area of Lunnan has experienced three pool formation periods: the Permian period, the Cretaceous-Early Tertiary period and the Late Tertiary-Quaternary period and two oil and gas reservoir adjustment periods: the Late Permian period and the Late Tertiary-Quaternary period; Indication of the comprehensive study that the large-scale Ordovician buried hill, formed in Early Hercynian, became the reservoir during the Permian period, because the Cambrian-Lower Ordovician oil was discharged laterally into the reservoir along the top of the Ordovician weathering crust from south to north; Claim that the reservoir experienced a complicated process—reconstruction in the end of Permian, adjustment in Cretaceous-Early Tertiary and re-discharging process in Late Tertiary-Quaternary, leading to the early original heavy oil reservoir of marine facies and the late original light oil reservoir and gas pool; Statement that carboniferous, Triassic and Jurassic oil and gas reservoirs result from upward adjustment and re-distribution of Ordovician oil and gas reservoirs; Reference to the fact that, of those results, the formation of Triassic-Jurassic oil and gas pools came under the influence of the northward-tilting structure; Vertical migration of the oil and gas sourcing from the different hydrocarbon source rock intervals into the base unconformity of Triassic system; Migration of the the oil and gas laterally from north to south and accumulated into th

Hetianhe is a big carbonate gas field which is found and demonstrated in the period of "Chinese National Ninth 5-Year Plan". The proved reserve of Hetianhe gas field is over 600x10&sup8; m³. Its main producing layers are Carboniferous bioclastic limestone and Ordovician carbonate composed of buried hill. The former is stratified gas pool with water around its side, and the latter is massive gas pool with water in its bottom. The gases in the gas pools belong to dry gases with normal temperature and pressure systems. Based on the correlation of gas and source rock, the gases are mainly generated from Cambrian source rocks. According to the researches on source rock and structure evolution, and the observations on the thin section to reservoir bitumen and the studies on homogenization temperature of fluid inclusions, the gas pool has been identified and divided into three formation periods. The first is Late Caledonian when the oil generated from the Cambrian source rocks and migrated along faults, as a form of liquid facies into Ordovician carbonate reservoir and accumulated there. After that, the crust uplifted, the oil reservoir had been destroyed. The second is Late Hercynian when condensate gases generated from the Cambrian source rocks and migrated into Ordovician reservoir, as a form of liquid facies. Since the fractures had reached P strata, so the trap might have a real poor preservation condition, and the large-scale gas pool formation had not happened. The third gas reservoir formation period occurred in Himalaya. The fractures on both sides of Hetianhe gas field developed violently under the forces of compression, and thus the present fault horst formed. The dry gases generated from Cambrian source rocks and migrated upwards as the form of gas facies into Ordovician and Carboniferous reservoirs, and the large gas pool as discovered at present was formed finally.

Reports that, based on the theory of plate tectonics, combining with the isotopic dating of ophiolite, igneous and volcanics, geochemical tests rare earth element analyze and seismic interpretation, this paper studies the pre-Carboniferous tectonics and sedimentary formation of the south edge of the Tarim Basin and proves that there exists the Kunlun Ocean under tensional tectonics during the Sinian and Cambrian in the south edge of the Tarim Plate. Statement that, due to the collision orogenesis, there formed the peripheral foreland basin in the south edge of Tarim; Reference to the fact that Upper Silurian and Devonian molasses sedimentary system superposed on the Sinian and Middle Silurian passive margin flysch sedimentary system and formed the bivariate structure of the foreland basin; Identification, based on the field geology and seismic interpretation, that the formation of the Silurian and Devonian have the character of half deposit which shows thick in the south area and thin in the north, and the pre-Carboniferous thrust compression tectonics remained in the foreland thrust belt, which further demonstrates that there existed the Silurian and Devonian peripheral foreland basin on the south edge of the Tarim Basin.

Reports that, analyzing the characteristics of the Tethyan tectonic field, the authors think that the Tethyan tectonic field underwent three evolutional stages: closing of Paleo-Tethys and rifting of Neo-Tethys from early Permian to late Triassic, subduetion of Neo-Tethys and collision between the Indian plate and the Eurasia plate from Jurassic to early of low Tertiary, and collision between the Arab plate and the Eurasia plate and the A-type subduction of Indian plate from late of low Tertiary to the present. Suggestion that, combining the evolution of the Tethyan orogenic belt with the characteristics of the Northern Margin basin, the sedimentary and tectonic characteristics and types of the Northern Margin basin are controlled by the formation and evolution of the Tethyan orogenic belt and the ingression of Tethys; Statement that the evolution of Northern Margin basin can be divided into three development stages: back-arc foreland basin from late Permian to Triassic, the back-arc fault subsidence and depression from Jurassic to the early of low Tertiary, and the reactive foreland basin from the late of low Tertiary to the present; Conclusion that the Northern Margin basin in the Tethyan tectonic field is an important region for natural gas accumulation, and the Tarim Basin is a part of this region.

Reports that using the modern tectonic geology theories and methods such as the plate tectonic analysis, the paleo-structure analysis, the structural-lithofacies analysis, and the fault related fold and petroleum system, and combining with the seismic data, well drilling data and the circumferential field geology, study on the structural characteristics and petroleum prospect in the Tarim Basin has been carried out. Results showing that the Tarim Basin is a large superimposition and combination basin with continental crustal basement, composed of a Paleozoic craton and Meso-Cenozoic foreland basins; Characteristics of the basin, including that the kernel part of the basin is the marine facies Paleozoic craton, superimposed 4 continental facies foreland basins; Statement that though the scale of the paleozoic craton of the Tarim Basin is relatively small, the structure is steady; Reference to the petroleum prospect of the Paleozoic craton: multiphase pool-generation and accumulation controlled by ancient uplift; Report that the Meso-Cenozoic foreland basins in the Tarim Basin, which are distributed on the cratonic circumference and are a long-term subsidence, turned into rejuvenated foreland basins after the Meso-Cenozoic period; Petroleum prospects, including coal-bed generating hydrocarbon, abundant natural gas, pool-generation in later and recent periods, the oil and gas distribution controlled by the foreland thrust belt; Statement that the structural characteristics of Tarim provide it with a superimposition and combination petroleum system of multiple resources, multiple reservoirs and multiphase pool-generation; Report that the oil and gas exploration prospect covers two large fields: the Paleozoic craton and the Meso-Cenozoic foreland thrust belt.