THE CONTROLLING FACTORS OF JURASSIC HIGH QUALITY RESERVOIRS IN QINAN FAULT STEP BELT OFF BOHAI SEA

The Jurassic reservoir in the Qinan fault step belt is heterogeneous due to complex internal textures. It is one of the major problems faced in oil exploration. Based on the study of rock types, reservoir physical properties, reservoir space types and pore structures, it is concluded that the Jurassic of the area is a kind of proximal sediments rich in pyroclastic materials intercalating coal seams. The reservoir is characterized by medium porosity and low permeability. High quality reservoirs are sometimes found in the low permeability sequence. Pore structure analysis shows that permeability may change greatly though the porosity remains similar. Secondary pores caused by multi-cyclic dissolution of feldspar and volcanic materials dominate sometimes. The quality of reservoirs depends upon the dissolution by acidic fluids. The coal strata are believed the source of organic acids, which promoted the dissolution and the reduction of carbonate minerals. The pyroclastic materials have double affect on the reservoir, on the one hand, they are apt to be corroded and dissolved to increase the reservoir porosity and block the process of quartz overgrowth, on the other hand, it may reduces the compaction of sediments which are favorable for reservoir properties.