玛湖凹陷南部二叠系风城组致密储层特征及致密油赋存状态OA

In the exploration of tight-oil reservoirs of the Permian Fengcheng Formation in the Mahu Sag,frequent simultaneous production of oil and water has occurred,and the microscopic occurrence states of the tight reservoirs remain unclear,restricting deeper oil and gas exploration.A detailed investigation of pore distribution,oil saturation,and fluid occurrence in tight reservoirs is key to efficient exploration and development.Using cast thin sections,argon-ion scanning electron microscopy(Ar+SEM),high-pressure mercury intrusion,nuclear magnetic resonance(NMR),confocal laser scanning microscopy(CLSM),X-ray diffraction(XRD)and other experimental methods,a petrological characterization of the tight reservoirs of the Fengcheng Formation in the southern Mahu Sag was carried out to identify reservoir space types and pore-size distributions and to examine the oil-water occurrence states of the main lithologies.The results show:1)the Permian Fengcheng Formation tight reservoirs in the Mahu Sag are dominated by siliceous and carbonate minerals;the main lithologies are conglomerate,conglomeratic sandstone,dolomitic sandstone,and mudstone.2)The tight reservoirs primarily develop micron-scale and micro-nano-scale pores.Conglomerates are dominated by micron-scale intergranular pores and intergranular dissolution pores,with pore sizes ranging from 2 to 100 μm.Conglomeratic sandstones are also dominated by micron-scale pores,with pore sizes of 1 to 20 μm.Dolomitic sandstones are dominated by micro-nano-scale dissolution pores,with pore sizes of 0.01 to 5 μm.Among them,the conglomerate reservoirs exhibit slightly better petrophysical properties than the other two reservoir types,providing relatively favorable storage conditions.3)The fluid occurrence characteristics differ among the conglomerate,conglomeratic sandstone,and dolomitic sandstone reservoirs:in conglomerate reservoirs,large pores mainly contain heavy oil while small pores are dominated by bound water,so heavy oil constitutes a large overall proportion;in conglomeratic sandstone reservoirs,large pores are mainly filled with heavy oil while small pores host a coexistence of bound water and film-like light oil,forming a"large pores oil-bearing,small pores water-bearing"pattern;in dolomitic sandstone reservoirs,heavy and light oils form an oil-film coating structure in the large pores,whereas small pores are filled with heavy oil.These findings have theoretical significance for understanding tight-oil distribution within complex pore networks and provide geological evidence for further exploration and development of Fengcheng Formation tight oil in the Mahu Sag.