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The genetic mechanisms of the secondary pore development zones in the lower part of the fourth member of the Shahejie Formation (Es4x) were studied based on core observations, petrographic analysis, fluid inclusion analysis, and petrophysical measurements along with knowledge of the tectonic evolution history, organic matter thermal evolution, and hydrocarbon accumulation history. Two secondary pore development zones exist in Es4x, the depths of which range from 4200 to 4500 m and from 4700 to 4900 m, respectively. The reservoirs in these zones mainly consist of conglomerate in the middle fan braided channels of nearshore subaqueous fans, and the secondary pores in these reservoirs primarily originated from the dissolution of feldspars and carbonate cements. The reservoirs experienced “alkaline–acidic–alkaline–acidic–weak acidic”, “normal pressure–overpressure–normal pressure”, and “formation temperature increasing–decreasing–increasing” diagenetic environments. The diagenetic evolution sequences were “compaction/gypsum cementation/halite cementation/pyrite cementation/siderite cementation–feldspar dissolution/quartz overgrowth–carbonate cementation/quartz dissolution/feldspar overgrowth–carbonate dissolution/feldspar dissolution/quartz overgrowth–pyrite cementation and asphalt filling”. Many secondary pores (fewer than the number of primary pores) were formed by feldspar dissolution during early acidic geochemical systems with organic acid when the burial depth of the reservoirs was relatively shallow. Subsequently, the pore spaces were slightly changed because of protection from early hydrocarbon charging and fluid overpressure during deep burial. Finally, the present secondary pore development zones were formed when many primary pores were filled by asphalt and pyrite from oil cracking in deeply buried paleo-reservoirs.

Genetic mechanisms of secondary pore development zones of Es 4 x in the north zone of the Minfeng Sag in the Dongying Depression, East China