酒泉盆地营尔凹陷下白垩统异常高压成因及模拟

Origin and modeling of overpressure in Lower Cretaceous of Ying’er sag, Jiuquan Basin, northwest China

酒泉盆地营尔凹陷下白垩统普遍发育异常高压,综合利用试油、测井、黏土矿物转化等信息分析地层压实特征和异常高压的成因,在此基础上,利用流体流动压实模型对压力进行模拟。研究结果表明:营尔凹陷纵向上可划分出常压带(<2.5 km)、浅层超压带(2.5-3.2 km)、压力过渡带(3.2-3.6 km)和深层超压带(>3.6 km)共4个压力带,平面上以长沙岭为界具有南强北弱的超压特征;超压带普遍具有欠压实特征,晚期的快速沉降所导致的不均衡压实作用是形成超压的最主要原因,黏土矿物转化对超压带的分布具有重要的影响,受有机质低丰度(w(TOC)<1.5%)的限制,生烃作用对超压的直接贡献较小;通过对比机械压实和化学压实不同模型的模拟结果发现,机械压实和化学压实的联合模型模拟的孔隙压力和孔隙演化史更加合理,说明不均衡压实和化学成岩的共同作用是形成营尔凹陷现今超压特征的主要原因。

Ying＇er sag of Jiuquan Basin develops overpressure in Lower Cretaceous. The compaction and overpressure were analyzed by using pressure direct measurements, wireline log and transformation of clay minerals data. To test the analyzed origins of overpressure, pressure modeling by the fluid-flow method was performed. The results show that there are four pressure zones vertically, i.e. hydrostatic regime （〈2.5 kin）, shallow overpressure regime （2.5-3.2 kin）, pressure transition regime （3.2-3.6 kin） and deep overpressure regime （〉3.6 km） and the overpressure decreases from south to north taking Changshaling as the dividing line. The primary cause of overpressure is disequilibrium compaction resulted from late rapid subsidence since Miocene. The observation that the rapid conversion of smectite to illite coincides with the overpressure regimes is interpreted to the direct contribution of transformation of clay minerals to overpressure. Limited by the abundance of organic matter （w（TOC）〈l.5%） of overpressure zones, the hydrocarbon generation contributes little to overpressure. Different mechanical and chemical fluid-flow pressure models wereperformed and compared. The modeled pore pressure and porosity evolution by mechanical compaction combined with chemical compaction are consistent with the observation results and more reasonable, which suggests that disequilibrium compaction and chemical compaction are the main causes of overpressure in Ying＇er sag.