Sedimentary mechanism and distribution pattern of beach-bar sandbodies mainly dominated by contemporaneous faults in Banqiao Sag

Key words:  beach-bar  contemporaneous fault  distribution regularity  2nd member of Shahejie Formation  Banqiao Sag

Abstract:

The sedimentary characteristics of beach-bar sandbodies were compared among different fault-blocks of Es₂ in Banqiao Sag using the seismic,logging and core data, and the formation mechanism and distribution pattern of beach-bar were studied. The results indicate that the reservoirs of Es₂ in Banqiao Sag have single beach-bar sandbodies whose average thickness is higher than 10 m,and the cumulative thickness nearly reaches 100 m. In addition to siltstone and fine sandstone, there are also gravel deposits. The contemporaneous faults result in the differences in lithology, mineral, grain size and sedimentary tectonics between fault blocks,which fundamentally control the deposition mechanism and distribution regularity of beach-bar sandbodies. There are two kinds of distributary models of the beach-bar sediments where the contemporaneous fault is the main controlling factor:the autochthonous and continuous type of beach-bar deposition in down-throw side of the fault,and the staggered and intermittent type of beach-bar deposition in up-throw side of the fault. The former has thick single sandbody and well developed interlayers,generally presents a positive cycle,and horizontally adjoins the footwall of the contemporaneous fault. The latter shows more inverse cycle characteristics,has single sandbodies staggering with each other vertically,and has a banding distribution which runs parallel to the edge of the lake basin horizontally.