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Characteristics of Lower Palaeozoic Marine Carbonate Rocks in Dongying Sag, East Bohai Bay Basin


Pre-Cenozoic carbonate rocks are very important reservoirs in Bohai Bay Basin. Currently, the degree of prospecting on this kind of reservoirs is higher in the west of the basin, but relatively lower in the east. Therefore, it is very important to learn the reservoir space types and vertical heterogeneity of the Pre-Cenozoic carbonate rocks in the east, which is meaningful for the exploration on this kind of reservoirs. Our studies focus on lower Palaeozoic marine carbonate rocks in Dongying sag,East Bohai Bay Basin, which have an unconformable contact with upper formations. Applying different research methods, such as core observation, thin section examination,imaging logging,drilling data,combined with previous studies on the regional tectonic evolution,we studied the characteristics of lower Palaeozoic marine carbonate rocks in Dongying sag. It shows that primary pores in the reservoirs have almost disappeared through intense and complicated diagenesis caused by repeated uplift/denudation and burial. Secondary pores or caves and fissures predominate in the reservoirs. The secondary pores and caves, which can be subdivided into inter-gravel dissolution pores, vein dissolution pores, intercrystalline dissolution pores and matrix dissolution pores, resulted from karstification and burial dissolutions (organic acids dissolution and hydrothermal dissolution). The fissures were caused mainly by tectonic movements and partly by the collapse of the caves. When it comes to the vertical distribution of reservoir space, there are some differences between pores/caves and fissures. The distribution of fissures is relatively homogeneous, they exist in every depth intervals of the reservoir with a uniform density. Whereas the distribution of the pores or caves can be divided into two sections. The top section extends for about 100m below the upper unconformity. This section contains all kinds of secondary pores or caves. Pore sizes are obviously larger and caves with or without infillings are common in this section. Karstification and burial dissolutions are both well developed in this part, so we can call it “cooperation zone”. Unconformity related karstification created large cave systems firstly, then subsequent burial dissolutions developed and enlarged the pores or caves formed previously. Under the “cooperation zone”, karstification is poorly developed or absent, and burial dissolutions predominate. Pore density is relatively lower and caves are almost absent.