ABSTRACT

This paper investigates the temperature and pressure distribution characteristics and their evolution in the Upper Ordovician strata in the Tazhong area, Tarim Basin, northwestern China. Based on hundreds of temperature data from boreholes, the present-day temperature ranges from 100°C to 130°C, increasing from the southeast to the northwest and having a “cool” belt in the central fault horst. Pore pressures in the Upper Ordovician reservoirs range from 35 to 62 MPa (5076 to 8992 psi). Overpressures developed in the southwestern region and along the eastern segment of the central fault horst to the northern slope. The Lower Ordovician reservoirs in the east reached 140°C during the Late Ordovician, but those in the west heated continuously and reached the maximum temperature (>130°C) during present day. Paleopressures reconstructed from fluid inclusion data indicate that overpressures caused by hydrocarbon generation were formed in the areas away from fault belts but were absent in the area around the no. 1 fault belt. The temperature evolution indicates that the middle−lower Cambrian source rocks reached gas generation stage (vitrinite reflectance [Ro] > 2.0%) at the end of the Ordovician, whereas the Middle−Upper Ordovician source rocks are still in the high-maturity stage (Ro < 1.3%) at the present day. Deep oil and gas migration is driven by overpressure, and commercial reservoirs are predominantly distributed in the normally pressured zones along the no. 1 fault belt. The formation of gas condensate is controlled by specific temperature and pressure characteristics.

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