Relationship between decreased pressure gradient and reservoir filling degree of paleogene in Bonan Sag

Abstract

Overpressure plays an important role in oil and gas migration. Decreased pressure gradient, as a characterization of overpressure, refers to the variation range of pressure per unit distance and has great significance for studying the dynamic composition and mechanism of oil and gas migration in the process of hydrocarbon migration. To quantitatively identify the relation between overpressure and hydrocarbon accumulation, this study uses the Es3 Formation in Bonan Sag as an example. The pressure distribution characteristics, the plane decreased pressure gradient characteristics, and their relationship with reservoir filling degree are analyzed using the measured pressure and predicted pressure, combined with the calculation of decreased pressure gradient and reservoir filling degree. Results show that the pressure coefficient of the Es3 Formation in Bonan Sag reaches up to 1.85, the decreased pressure gradient on the plane exhibits heterogeneity, and the decreased pressure gradient of the deep sub-sag belt is the highest, reaching a maximum of 12.1 MPa/km, followed by steep slope and steep fault belts. The gentle slope belt has the lowest decreased pressure gradient of less than 4.4 MPa/km. The corresponding relationship between decreased pressure gradient and reservoir filling degree is different in the four structure belts. The deep sub-sag belt is bounded by the decreased pressure gradient of 9.7 MPa/km. When the decreased pressure gradient is less than 9.7 MPa/km, the reservoir filling degree increases with an increase in the decreased pressure gradient. When the decreased pressure gradient is greater than 9.7 MPa/km, the reservoir filling degree decreases with an increase in the decreased pressure gradient. The corresponding relationship in the steep fault belt is opposite that of the deep sub-sag belt. When the decreased pressure gradient is greater than 4 MPa/km, the reservoir filling degree decreases with a decrease in the decreased pressure gradient, whereas the relationship between the decreased pressure gradient and the reservoir filling degree is weak in the gentle slope belt.