The near-critical initial state of the formation fluid with a unique content of C5+ components in a tight reservoir with high temperature and abnormally high reservoir pressure (AHRP) causes significant difficulties in collecting samples of the formation fluid and assessing their adequacy to the actual reservoir mixture both in composition and identified phase state (gas condensate or oil) and phase behavior. The paper presents the result of the work of the project multidisciplinary team, which includes specialists from the PJSC «NOVATEK» group of companies and the leading Russian Institute of Oil and Gas Problems (OGRI RAS), of studying the near-critical gas condensate system in a HPHT tightreservoir. The effect of well completion and sampling conditions on the composition, phase state and phase behavior of fluid samples is shown. The influence of formation water and residual liquid hydrocarbons (HC), capillary effects, adsorption, and the specifics of the PVT-model tuning on the dew point pressure, the condensate lossdynamics, and the variation of composition and phase state of the fluid with depth is assessed. The results made it possible to clarify the understanding of the reservoir system,develop a program for further research, and improve the history matching of the flow model. The performed modeling of various factors in a porous medium for the conditions of the studied reservoir showed that the presence of formation water and capillary effects do not noticeably affect the phase behavior of the fluid, but the possible presence of scattered hydrocarbons and the adsorption / desorption of hydrocarbon components can lead to considerable changes in the dew point pressure and the condensate loss dynamics. The significant influence of the PVT model tuning on the calculated distribution of the initial fluid composition over the reservoir volume is demonstrated, including the position of the gas-oil contact and its type (classical gas-liquid contact or supercritical transition to the liquid state).