Discussion on the Characteristics and Origin of Low-Permeability Reservoir of Coal-Bearing Strata in L3 Structure, Dongying Formation, Bodong Lower Uplift

The elastic reservoir of Dongying formation in Bodong Lower Uplift is characterized by low-permeability, which is a main factor influencing the exploration results. By using various analysis methods, the reservoir characteristics and mechanisms are studied in L3 structure, Dongying formation, Bodong Lower Uplift. The study results show that in the reservoirs,the dissolution is strong, and the reservoirs are low in the content of plagioclase and the carbonate content, high in the content of kaolinite in clay minerals, with obvious quarts overgrowth in the reservoir of Dongying formation in L3 structure. The reservoir is basically medium porosity and low to very low permeability, which is characterized by small pore apertures, bad connectivity, narrow pore throat, and middle or high class of discharge pressure. Trace element analyzing results indicate that the sedimentary environment was freshwater when Dongying formation was deposited. It is confirmed by logging and core data that multilayer carbonaceous mudstone and coal exist in the formation. The dissolution of unstable plagioclase was formed in low temperature environment by humic acid in early stage. The dissolution pores was complicated and diminished by later compaction and diagenesis, which is the main reason for low permeability. The low content of early-stage carbonate cements result in low compressive strength is another important influencing factor.