The Study On Gas Accumulation Dynamics Of Upper Paleozoic In The Southwestern Area Of Ordos Basin

On the basis of analyzing the static data such as drilling, well logging, experimental analysis as well as the dynamic data from gas testing and pilot production of the exploratory well, this article guided by the modern theory of hydrocarbon accumulation mainly studied the source of gas accumulation dynamics, the state of fluid pressure in the ancient time, the genesis of the overpressure and the specific values of hydrocarbon generation pressurization. And further to link the dynamics with gas accumulation, then summarizes the gas pool forming dynamics mode. The main achievements and understanding are as follows:Through the testing of the fluid inclusion and the recovery of paleo-pressure by calculating, it show that Shihezi formation and Shanxi formation of the upper Paleozoic in Ordos basin have a widespread overpressure in ancient times, mainly for the weak overpressure-overpressure system.The formers considered that the main mechanism for generating the overpressure is disequilibrium compaction, while as for this problem we hold different views. We suppose that in this area the main reason for generating the overpressure is not disequilibrium but gas generation.By establishing the numerical model and calculating the numerical size of the overpressure, it showed that the overpressure mainly occurs after the gas accumulation entering a period that a large amount of hydrocarbon generated, and the accumulated overpressure caused by gas generation has a positive correlation with the test gas production, which indirectly showed that the main causes of the overpressure is gas generation, and this kind of overpressure as the main force of the primary migration played an important role of the gas accumulation.Considering the upper research results, we contend that there are two main types of the gas pool forming dynamics model in the study area. One is before the end of early Cretaceous, a large amount of gas were formed by source rocks, the great overpressure is the main dynamics resource of gas migration, because of the lack of condition for mass lateral migration in the study area, the gas precede an episodic migration driven by overpressure, its characteristics are vertical, close and rapid accumulation, mainly reflected in the gas pool of Shan 1 member. The other one is after the late early cretaceous, due to the strata uplift in this time, the temperature, pressure, stress drop and hydrocarbon generation pressurization without energy supply can no longer be the power of gas migration, the late gas concentration largely under the action of diffusion migration, mainly reflected in the gas pool of He 8 member.