库车坳陷克拉2气田异常流体压力演化史

根据克拉2气田现今实测地层压力分析了异常高压发育的特征,并通过流体包裹体测试求取包裹体形成时期的古压力。计算结果表明:下白垩统和古近系库姆格列木群砂岩储层在距今8～6Ma(康村组沉积期末)时,古压力系数大致为1.09～1.18,发育过剩压力为4.6～9.6MPa的异常高压。在此基础上,以古、今压力数据为约束条件,利用数值模拟技术定量恢复出下白垩统与侏罗系异常流体压力的演化历史。研究发现:下白垩统储层流体压力经历了沉积型异常流体压力形成发展(白垩纪-库车组沉积期末)和沉积型异常流体压力萎缩—构造挤压型异常流体压力孕育(库车组沉积期末至今)两个阶段。康村组沉积早期沉积型超压开始出现,库车组沉积期末达到顶峰,过剩压力达40～50MPa;此后,在沉积型超压萎缩—构造挤压型超压孕育过程的耦合下,过剩压力曾降至25～30MPa,更新世起超压略有升高。侏罗系流体压力演化较为简单,吉迪克组沉积期开始出现超压,至库车组沉积期末达到高峰,此后异常高压逐渐萎缩。在克拉2气田的主要成藏时期(距今3～1Ma),烃源岩较储层具有更高的异常压力,源—储压力差为10～30MPa。

EVOLUTION OF ABNORMAL FLUID PRESSURE IN THE KELA 2 GASFIELD, KUQA DEPRESSION

Large-scale gas accumulation was discovered in the Mesozoic of the Kela 2 Gasfield,Kuqa Depression of the Tarim Basin in Xinjiang.Many studies have been performed about the origin of abnormal overpressure.However,The previous studies were lack in paleopressure conditions and evolution of fluid pressure.In this paper,the data of present fluid pressure came from exploration wells.The paleopressure was calculated by measurement of fluid inclusions.It was found that overpressure was evident in Lower Cretaceous and Paleogene Kumugeliemu reservoir 8～6 Ma ago.Under the restriction of the present and ancient parameters,the evolution history of fluid pressure was reconstructed by numerical model.The research showed that the development of fluid pressure in Lower Cretaceous could be divided into two phases.From the Cretaceous to the end of sedimentary period of Kuqa Formation,the overpressure increased mainly due to depositional factors.High pressure occurred in sedimentary period of the Kangcun Formation,and the overpressure reached the culmination with the excess pressure of 40~50 MPa.During the second phase from the end of sedimentary period of Kuqa Formation to nowadays,overpressure due to depositional factors decreased gradually while overpressure due to extrusion stress increased.After the excess pressure firstly reduced to 25～30 MPa,it increased gradually.The evolvement of fluid pressure in the Jurassic was simpler than that in the Lower Cretaceous.Abnormal high pressure occurred in sedimentary period of the Jidike Formation.After it reached to the culmination by the end of sedimentary period of the Kuqa Formation,overpressure decreased gradually.On the basis of studies mentioned above,we contrasted the excess pressure value between source rocks and reservoirs in major accumulation period.It was discovered that the huge excess pressure value between source rocks and reservoirs was important reason which abundant gas accumulated on the background of high pressure.