准噶尔盆地吉木萨尔凹陷二叠系超压成因及分布预测

Mechanism of Generation and Prediction of Distribution of Overpressure within the Permian Strata in the Jimusar Depression,Junggar Basin

准噶尔盆地吉木萨尔凹陷二叠系普遍发育异常高压,被认为是影响钻井工程高效安全和油气成藏动力条件评价的关键因素。但其成因机制目前缺乏研究,目的层埋藏较深,现有超压计算方法精度不高,超压分布特征也不够明确。本文基于钻井、测井和测试资料对研究区目的层现今实测地层压力进行分析,结合泥岩压实特征、源岩有机地球化学特征以及现今构造背景探讨了超压成因机制,同时利用岩石静力平衡原理改进了压力预测模型,以成因为导向,形成适用于研究区的压力预测方法,并对目的层进行了压力分布预测。研究表明:(1)吉木萨尔凹陷二叠系局部发育高幅超压,其中芦草沟组压力系数最高可达1.8;(2)研究区二叠系超压形成机制主要包括沉积成因的非均衡压实和特殊成因的生烃增压,可能存在水热增压;(3)现有压力预测模型假设过强且精度较低,本文基于岩石静力平衡原理构建的颗粒应力法计算模型,不仅消除特察模型缺陷,且预测精度明显高于传统方法;(4)异常高压主要发育在凹陷西部深洼区及中西部斜坡带,以J36井区为最高,该分布规律揭示局部高幅超压受到生烃凹陷控制。上述成果及认识对吉木萨尔凹陷二叠系钻探和油气勘探评价有比较重要的指导意义。

Abnormal pore pressure,developed in Permian strata throughout the Jimusar Depression,eastern Junggar Basin,is recognized as the key factor affecting the safety and efficiency of the drilling process and evaluation of the dynamic conditions of hydrocarbon accumulation.However,the causes of overpressure have still not been studied systematically,the accuracy of traditional overpressure calculation methods remains insufficient due to its deep burial,and the distribution law requires improvement.In this study,the current measured pore pressure features of the target layer were analyzed based on drilling,logging,and formation testing data,in combination with the compaction characteristics of the mudstone,organic geochemical characteristics,and present tectonic setting.Then,the mechanisms of overpressure generation were evaluated.Furthermore,based on the static equilibrium principle of formation,the pore pressure prediction model was improved theoretically.In addition,a new method suitable for the study area was developed considering the overpressure generation mechanism,and the pressure distribution in the study area was predicted.The study produced several interesting results:(1)High-amplitude overpressure is generally developed in the Permian of the Jimusar Depression,and the pressure coefficient of the Lucaogou Formation is as high as 1.8.(2)The overpressure of the Permian in the study area is caused mainly by disequilibrium compaction and hydrocarbon-generating supercharging,but a hydrothermal mechanism may also exist.(3)Traditional pressure prediction models have extremely strict assumptions and low accuracy.Therefore,based on the principle of the static equilibrium equation,grain stress parameters were introduced to eliminate the defects of the Terzaghi model,and a new pressure calculation model with significantly improved accuracy was proposed.(4)The overpressure is mainly developed in the western deep sag and the central-western slope zone,the high-pressure coefficient is mainly located in the J36 well area,which is controlled by the hydrocarbon-generating depression.The aforementioned results and implications have important guiding significance for drilling in the Permian and evaluating hydrocarbon exploration in the Jimusar Depression.