Evolution of the temperature-pressure system and far-source hydrocarbon accumulation in Junggar Basin

The controls of the formation and evolution of the temperature-pressure system on a far-source hydrocarbon accumulation in the Junggar Basin were studied systematically based on the analyses of temperature and pressure fields and guided by the theory of system thermodynamics. The uplifted area of the basin is featured by high temperature, high temperature gradient and high heat flow, while the depressed area is opposite. Overpressure developed in the depressed area and decreases in irregular rings to the edge of the basin. Strong overpressure developed in the south of the basin and the central depressed area, medium overpressure developed in the east of the basin, and weak overpre-ssure developed on the northwestern margin of the basin. Vertically there are three sets of relatively closed temperature-pressure systems in the basin: the Permian-Upper Triassic system, the Lower Jurassic system and the Cretaceous-Paleogene system. The sedimentation rate of the basin was relatively fast during the Hercynian period, and the energy accumulation of temperature and pressure took place from the Permian to the Late Triassic. Tectonic movements were intense in the Yanshanian period, and fracture activities cut through cap rocks frequently at the edge of the uplifts. The temperature-pressure field adjusted, leading to oil and gas migration and accumulation through cap rocks. It was the main period for the formation of far-source reservoirs. The basin was tilting, descending in the south and uplifting in the north, in the Himalayan period, and fault activity was weak. The temperature-pressure system was relatively stable and helped oil and gas preservation. The far source lithologic oil and gas reservoirs are mainly distributed in the low-energy field of temperature-pressure in the horizontal, such as the northwest of the basin, Mobei-Mosuowan area, Luliang uplift and the eastern uplift. These areas are favorable for exploration of far-source oil and gas reservoirs in the Junggar Basin.