Abstract: Overpressure is considered prevalent in the northwestern Qaidam Basin. Understanding the spatial distribution and genetic mechanism of overpressure is key to the evaluation of hydrocarbon accumulation conditions and resource potential of the region. Here, we analyzed the plane and section distribution characteristics of the areas abnormal high pressure based on the measured pressure and pressure calculated by the equivalent depth method. Combining the time-difference curve of acoustic wave of mudstone, deposition rate, lithologic characteristics, hydrocarbon generation potential of organic matter and tectonic compression, we quantitatively evaluated the genetic mechanism of overpressure. The results showed that the initial depth of overpressure in the area was about 1500 m, developed mainly in the Lulehe to upper Ganchaigou Formations of the Miocene and with abnormally high pressure increasing with depth. In general, undercompaction, organic matter generation and tectonic compression were important factors for overpressure formation and evolution. The results showed that, in the northwestern Qiandam Basin, undercompaction was the most important controlling factor, with a contribution rate of up to 60%; the contribution from tectonic compression came in second at about 20%30%; and the contribution from organic matter generation was the least. Moreover, the salinity characteristics of the northwestern Qaidam Basin indicated that the formation of the overpressure was also affected by both pore fluid salinity and the salt-bearing plastic layer deposited in the salt-brackish water environment.

Key words: northwestern Qaidam Basin, overpressure, formation mechanism, undercompaction, quantitative evaluation