In any closed fluid system there exists a relationship between pressure and temperature. For liquid system, a change in density with a change in temperature impacts on the fluid pressure if the fluid expansion is contained in a rigid system boundary. Geothermal energy in the earth interior has been exploited for different purposes such as heating and power generation. But no influence of geothermal energy on hydrocarbon reservoir fluid in situ in the earth interior has ever been thought of. This pressure contribution to reservoir pressure due to geothermal energy is what this paper seeks to quantify by examining the thermal conductivity of the reservoir fluids- formation system and the reservoir architecture. Because of the complex representation of the different components of the reservoir, the relationship between the pressure and temperature in the reservoir has a complex polynomial relation which depends on changing reservoir parameters.