GEOLOGICAL FEATURES OF UNCONFORMITY STRUCTURES AND FORMATION CHARACTERISTICS OF STRATIGRAPHIC RESERVOIRS IN JIYANG DEPRESSION,BOHAI BAY BASIN, CHINA (Invited Presentation)

Based on the core observation and micro research of minerals and elements, the unconformity structure in Jiyang Depression can be divided into three parts of weathered clay layer, semi-weathered rocks and un-weathered rocks. When it comes to weathered clay layer, it can be seen that the lamination disappeared and the feldspar and calcite were altered thoroughly. In addition, the clay minerals and elements, such as silicon and aluminium, are rich, but calcium and magnesium has been leached out. The porosity and permeability of weathered clay became worse because of the effect of post-diagenesis. However, in semi-weathered rocks, some calcite and feldspar still remains. But the element enrichment and leaching degree is lower than that in weathered clay layer. The dissolved pores and fractures are well developed, and the porosity and permeability conditions of semi-weathered rocks are better than that of weathered clay layer. The unconformity-forming process can be divided into three periods including weathered stage, denudation stage and diagenetic stage. Weathered stage was the basic period when unconformity formed. Denudation stage was critical period for unconformity preservation. And Diagenetic period determined the mineral compositions and poroperm characteristics of unconformity.

The differences in mineralogical and porosity characteristics of different structural layers have contributed to the differences in the contributions to the formation of stratigraphic reservoirs. Weathered clay layer has low porosity and permeability, and can be used as high quality caprock. In contrast, semi-weathered rocks usually have good porosity and permeability, and can act as the main channel of hydrocarbon migration and favorable locations for accumulation. However, as unconformity structural layers in continental basins are mainly composed of frequently alternating interbeds of sand and mud, the horizontal continuity either for permeable layers of the unconformity or of for its top non-permeable layers was poor. Considering the physical properties of mudstone were basically unchanged after weathering process, it can be conclude that although both horizontal and vertical migrations along unconformity might occur, it was difficult for the unconformity to act as a pathway for long distance of hydrocarbon migration.