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Jiuquan basin is one of the piedmont basins of the Qilian Mountain range in northwestern China. The basin has an area of about 2,700 km2 (1,042 mi2) and trends WNW-ESE. Cenozoic to Mesozoic deposits, with a total thickness of 4,500 m (14,764 ft) overlie lower Paleozoic rocks. Some Carboniferous, Permian, and Triassic outcrops are exposed near the margin of the basin.
Jurassic and Cretaceous formations, characterized mainly by marsh-lake sedimentary facies, are the source beds within this area. The thickness changes of these formations are related to the effect of crustal movement during deposition. On the uplifted parts of this area, Jurassic and Cretaceous deposits are very thin or absent; otherwise, they generally developed to a thickness of about 2,500 m (8,200 ft).
The Tertiary formations have a thickness of about 2,000 m (6,560 ft) and consist chiefly of river or lake to alluvial sediments deposited under arid climatic conditions. In the lower part of these formations, the river-delta sand bodies are the regional reservoir beds.
Between the Mesozoic and Cenozoic systems, a large depositional interruption exists. Upper Cretaceous to Paleocene deposits are all absent.
There are three structural belts in this area. From south to north, they are the southern anticline belt, the center depression belt, and the northern monocline belt.
The first field (Laojunmiao oil field) was discovered in 1939. Since the founding of the People's Republic about 33 years ago, six oil fields (comprising 14 oil pools) have been sequentially discovered within this basin.
The discovery history of the oil fields can be divided into three stages. During the first stage (1939-59), shallow reservoirs in the Tertiary were explored, based primarily on oil seepage and surface structure drilling. Fields discovered during this period were the Laojunmiao, Yarxia, Beiyanghe, and Shiyougou. From 1960 to 1974, the second exploration phase drilled to the deeper formations. As a result, a buried basement hill was discovered under the shallow Yarxia reservoir, and a new Cretaceous reservoir was found in the direction of the source area. After 1975, exploration entered a new stage with the search for pre-Tertiary nontectonic-type reservoirs. With improved seismic apparatus and data processing, the study of sedimentary facies using seismic stratigraphy is being applied o exploration efforts. The improved seismic and data processing, in combination with advances in drilling techniques, have led to the discovery of new nontectonic-type oil fields in places that had been previously drilled. The number of nontectonic reservoirs and their reserves are growing.
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