准噶尔盆地中部超压封闭层的岩石学特征与封闭机理

本文依据准噶尔盆地中部地区钻井地层压力实测数据,勾画出该区超压顶界面的空间分布形态以及超压封闭层的厚度;重点阐述了2口钻井超压封闭层的岩性构成、成岩作用机理和岩石学与岩石物性特征;通过对比研究,探讨了封闭层的封闭机理。研究表明该区超压顶界面为一个向南倾斜的曲面,横向上可穿过不同的地层层面。超压封闭层由一组砂泥互层的碎屑岩组成,厚度约166～250m,砂岩含量大于50%。砂岩封闭层已达到晚成岩 A_2亚期阶段,自生高岭石、石英次生加大和碳酸盐胶结作用发育,主要与中下侏罗统煤系烃源岩成熟演化过程中排出的大量有机酸溶解铝硅酸岩及其自生矿物有关。封闭层砂岩为致密储层,渗透率<1.0md、孔隙度<10%,由于其内发生了天然气充注,气毛细管封闭成为封闭层的主要封闭机理。

Petrologic feature and sealing mechanism for the pressure seal in the center of Junggar basin

A huge abnormally overpressured compartment had been developed in the hinterland of Junggar basin, northwest China. But little attention was paid to the pressure seal for the overpressure compartment, including its constitute, spatial geometry, petrological signatures, and the overpressure - sealing mechanism. Based on a variety of data on formation pressure and petrology from more than 20 drilling wells, it is indicated that the overpressure top was encountered in laterally varied depth ranging from 3827m in the north to 5500m in the southwest, roughly shaping a southwest tilted tortured - surface, cutting across lower Jurassic and lower Cretaceous in the north and south, respectively. The pressure seal thickness is about 166 -250m among wells. In the area interested, the seal was occurred mainly within coal - bearing Jurassic, lithologieally being composed of alternating sandstones and shales, with sandstone more than 50%. Strong diagenetieally cementation was happened within sandstone intervals of the seal, indicative of abundant authigenie kaolinite, quartz, calcite and siderite. The kaolinite and quartz overgrowth was stemmed from eorrasion of aluminosilieate by rich organic acids generated within coal - bearing lower and middle Jurassic. Due to pore throat - plugging by the cements, the sandstone of the seal was tightened with a sharply decreased permeability less than 1.0rod. Moreover, the sandstone was charged with much gas expelled from coal measure nearby. Consequently, gas capillary sealing was considered as major mechanism for the sealing to the overpressure compartment.