Abstract:

Analysis of the causes of overpressure in the Kuqa depression reveals that there are two types of overpressure:the depositional overpressure and the tectonic compressive overpressure.Constrained by such direct evidences as measured pressure,forming pressure of fluid inclusions,and palaeo-pressure during the epoch with the maximal burial depth,the depositional overpressure in different geologic periods are reconstructed in accordance with the mathematical model of fluid pressure evolution.On the basis of mechanism of structural compaction,the amplitude of tectonic overpressure in the Himalaya period is evaluated.The evolution history of fluid pressure in target strata is obtained and the development of gas pressure potential is restored by combining these two kinds of overpressures.The impact of hydrodynamic force upon gas migration and pooling is also studied.The results show that the target was under depositional overpressure before the deposition of Kuche Formation,and since then went through the decline of depositional overpressure and rising of tectonic overpressure.Three modes of gas migration including continuous charging,early charging with later standstill and early charging with early standstill are recognized based on analyses of the pressure relationship between the Jurassic source rock and the lower Cretaceous reservoir rock.Gas pressure potential gradients in different geologic periods show that the lateral migration and accumulation was most active after the deposition of Kuche Formation.The major cause of the late accumulation of gas was the intensifications of gas migration and accumulation by tectonic movement since the Quaternery period.

Key words: overpressure, excess pressure, gas potential gradient, hydrodynamic force, hydrocarbon migration, hydrocarbon accumulation characteristic, Kuqa Depression