泥岩化学压实作用的超压响应与孔隙压力预测

Overpressure responses for chemical compaction of mudstones and the pore pressure prediction

为探索泥岩化学压实作用的增压机制及其超压响应特征,结合国内外相关研究,对泥岩化学压实作用与超压发育关系、化学压实阶段超压响应特征以及孔隙压力预测方法进行了系统总结,分析了东营凹陷泥岩化学压实作用在孔隙压力预测中的意义.认为泥岩化学压实作用以黏土矿物转化为主导机制,通过改变泥岩的微观结构及岩石物理属性,化学压实作用影响了超压的发育及其测井响应,建立不同压实阶段正常压实趋势线是有效判识超压成因机制、准确预测孔隙压力的基础.研究结果表明:1)泥岩压实过程可以划分为机械压实阶段(<70℃)、过渡阶段(70~100℃)以及化学压实阶段(>100℃),黏土矿物转化(尤其是蒙脱石向伊利石转化)控制了泥岩的化学压实行为;2)泥岩化学压实的增压效应主要包括3个方面:形成垂直于最大有效应力方向排列的黏土颗粒组构,促进泥岩压实、导致自生胶结物沉淀降低泥岩渗透率,增强体系封闭性、使得泥岩发生骨架弱化作用,引起有效应力转移;3)在有效应力-孔隙度/密度和声波时差/电阻率-密度交会图中,化学压实导致的超压表现出非弹性卸载特征,即随着有效应力减小,呈现出孔隙度降低,而声波时差和密度同时增大的测井响应;4)适用于化学压实阶段的孔隙压力预测方法包括伊利石压实曲线法、纵横波速度比法、偏移-修正法、Dutta压实模型法以及两步测井预测法等;5)东营凹陷泥岩黏土矿物快速转化深度与超压顶深(2 000~2 800 m)大致相同,超压发育及测井响应与化学压实密切相关,基于化学压实正常趋势线预测的深部(>3 000 m)孔隙压力与实测压力更接近.

To explore the pressuring mechanism of mudstone chemical compaction and its influence on overpressure responses, the effects of chemical compaction on the formation of overpressure, the response characteristics of overpressures in chemical compaction stage, and the pressure prediction methods were systematically summarized on basis of the related domestic and foreign researches. The implications of mudstone chemical compaction in Dongying depression for pressure prediction were also investigated. It indicates that the transformation of clay minerals is the dominant mechanism of mudstone chemical compaction. The microstructure and the petrophysical properties of mudstone are significantly changed by the transformation of clay minerals and associated clay diagenesis reactions, which in turn affects the development of overpressure and its logging responses. The normal compaction trends of the different compaction stages are the basis for overpressure mechanisms identification and pore pressure prediction.The results showed that:(1) The compaction of mudstones may go through three stages, i.e. the mechanical compaction stage(<70 ℃), transitional stage(70-100 ℃), and chemical compaction stage(>100 ℃). The transformation of clay minerals controls the behavior of chemical compaction, especially the transformation from smectite to illite.(2) The pressuring mechanism of chemical compaction mainly includes three effects. The mudstone compaction processes are promoted by the aligned fabric of clay grains due to clay minerals growing normal to the maximum effective stress. The effectiveness of the closed system is enhanced by the permeability reducing relate to the authigenic cementation significantly. The load transfer happened in mudstone through framework weakening can also lead overpressure.(3) The plots of effective stress-porosity/density and acoustic time difference/resistivity-density can be used to distinguish the chemical compaction. The overpressure caused by chemical compaction shows characteristics of inelastic unloading, that is, as the effective stress decreases, the porosity decreases, while the acoustic time difference and density increase simultaneously with the decreases of effective stress.(4) Pore pressure prediction methods suitable for pressure predicting of diagenetically altered mudstones include illite compaction curve method, vP/vS method, budge-fudge method, Dutta compaction model method, and two-step logging analysis method, etc.(5) The depth of the rapid transformation of clay minerals has a good consistency with the top of overpressure zone(2 000-2 800 m) in Dongying depression, which indicats that the overpressure and its logging responses may be related to the chemical compaction of mudstones. The measured pressure in intervals deeper than 3 000 m is closer to the predicted pressure based on the normal compaction trend of chemical compaction.