Significant Role of Structural Fractures in Renqiu Buried-Hill Oil Field in Eastern China

Renqiu oil field is in a buried hill of Sinian (late Precambrian) rocks located in the Jizhong depression of the western Bohai Bay basin in eastern China. The main reservoir consists of Sinian dolomite. The buried hill is a fault block bounded on the west side by a large growth fault that trends north-northeast and has throws of up to 1 km (3,300 ft). The source rocks for the oil are Tertiary sediments that overlie the Sinian dolomite. Structural fractures are the main factor forming the reservoir of the buried-hill oil field. Three dominant structural trends—northeast, north-northeast, and northwest—form the regional fracture system. The fractures are best developed along the north-northeast fault zones and at the intersections of other structural trends. Because the regional stress field altered during the late Mesozoic, the mechanical properties of north-northeast fault zones were changed from compressional shear to extensional shear. Consequently, the enlargement of the structural fractures provided good channels for the circulation of meteoric water. The north-northeast growth fault controlled the structural development of the fault block. The block was raised and eroded before the Tertiary sediments were deposited, so the Sinian dolomite was exposed and underwent a long period of weathering and leaching. During the Eocene, the Jizhong depression subsided, but deposition, faulting, and relative uplift of the block occurred simultaneously as the block was gradually submerged. At the same time, several horizontal and vertical karst zones were formed along the fracture system by the circulating water. The Eocene source rocks lapped onto the block; therefore, the buried hill, with many developed karst fractures, was surrounded by a great thickness of source rocks. As the growth fault developed, the height of the fault block increased from 400 m (1,300 ft) before the Oligocene to 1,300 m (4,300 ft) after. As petroleum was generated, it migrated immediately along the growth fault into the karst-fracture system of the buried hill. The karst-fracture block reservoir has 800 m (2,625 ft) of oil-bearing closure and has good interconnected porosity between the leached fracture zones. Renqiu buried-hill oil field offers a high yield.