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Ren-qiu oil field is in a buried block of Sinian (upper Proterozoic) rocks located in the Ji-zhong depression of the western Bohai Bay basin in eastern China. The main reservoir consists of Sinian dolomite rocks. It is a fault block with a large growth fault on the west side which trends north-northeast with throws of up to 1 km (0.6 mi) or more. The source rocks for the oil are Paleogene age and overlie the Sinian dolomite rocks.
The structural fractures are the main factor forming the reservoir of the buried-block oil field. Three structural lines, trending northeast, north-northeast, and northwest, form the regional netted fracture system. The structural fractures are best developed along the north-northeast fault zones and at the intersections of other structural lines. Since the regional stress field was changed during the late Mesozoic Era, the mechanical properties of the north-northeast fault zones were changed from compressive shear to extensive shear. Therefore, the expansion of the structural fractures provided a good pathway for the activity of karst water.
The north-northeast growth fault controlled the structural development of the buried block. The block was raised and eroded before the Tertiary sediments were deposited, so that the Sinian dolomite rocks were exposed to the surface and underwent weathering and leaching for a long period. In the Eocene Epoch, the Ji-zhong depression subsided, but the deposition, faulting, and related uplift of the block happened synchronously as the block was gradually submerged. At the same time, several horizontal and vertical karst zones were formed by the karst water along the netted structural fractures. The Eocene oil source rocks lapped onto the block and so the buried block, with many developed karst fractures, was surrounded by a great thickness of source rocks.
As the growth fault developed, the height of the block was increased from 400 m (1,300 ft) before the Oligocene to 1,300 m (4,250 ft) after. As the petroleum was generated, it migrated immediately into the karst fractures of the buried block along the growth fault. The karst-fractured block reservoir has an 800-m (2,600-ft) high oil-bearing closure and good connections developed between the karst fractures. This is the high-yield Ren-qiu buried block oil field.
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