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Various types of geofluids exist in deep and ultra-deep layers in petroliferous basins. The geofluids are much more active under high-temperature and high-pressure (HTHP) conditions, but their properties are unclear. We simulated the mixing of different fluids in CH4/C3H8/C6H14/C8H18-water systems and C6H14/C8H18-CO2-H2O systems at temperatures of 25°C to 425°C and pressures of 5 MPa to 105 MPa, using an in-situ micron quartz capillary tube thermal simulation system and molecular dynamics numerical simulation software. The mixing processes, patterns, and mechanisms of various fluids were analyzed at microscale under increasing temperature and pressure conditions. The results show that the miscibility of fluids in the different alkane-H2O and alkane-CO2-H2O systems is not instantaneous, but the miscibility degree between different fluid phases increases as the temperature and pressure rise during the experiments. The physical thermal experiments (PTEs) show that the mixing process can be divided into three stages: initial miscibility, segmented dynamic miscibility, and complete miscibility. The molecular dynamics numerical simulations (MDNSs) indicate that the mixing process of fluids in the alkane-H2O and alkane-CO2-H2O systems can be divided into seven and eight stages, respectively. The carbon number affects the miscibility of alkanes and water, and the temperature and pressure required to reach the same miscibility stage with water increase with the carbon number (C3H8, C6H14, CH4, C8H18). CO2 has a critical bridge role in the miscibility of alkanes and water, and its presence significantly reduces the temperatures required to reach the initial, dynamic, and complete miscibility of alkanes and water. The results are of great significance for analyzing and understanding the miscibility of geofluids in deep and ultra-deep HTHP systems. [ABSTRACT FROM AUTHOR]