Pore structure analysis and classification of pyroclastic reservoirs in the Dehui fault depression based on experimental and well-logging data

The classification of pyroclastic reservoirs based on their pore structure has attracted the attention of many researchers in recent years. In this study, through thin section observations and scanning electron microscopy (SEM), mercury injection capillary pressure (MICP), nuclear magnetic resonance (NMR) and other relevant analyses, the reservoir in the third member of the Huoshiling Formation in the Dehui fault depression is classified into three separate types. The results show that a large number of devitrification corrosion pores are developed in the type-I reservoir, while the type-II reservoir contains matrix and dissolution pores. Moreover, the type-III reservoir only contained matrix-pores. Considering the complexity of the pore structure of the pyroclastic reservoir rocks, we established a relationship between the pseudo-capillary pressure curves and the NMR logging for the micro- and macro-pores in each reservoir type. Through the processing and analysis of key wells, it was found that the threshold pressure and median pressure obtained via NMR logging correspond well with the experimental core data. This enabled us to classify the reservoir type in several other wells and delineate the prolific regions in the study area that contain type-I and type-II reservoirs. The pseudo-capillary pressure curve can be used to continuously evaluate the pyroclastic reservoir pore structure longitudinally, which has very important practical significance in production applications.