海相沉积盆地膏盐层热效应的定量研究-以塔里木盆地为例

Salt rocks are usually distributed in the marine sedimentary basins in the world, and more and more exploration practices show that the formation of giant hydrocarbon reservoirs are generally associated with salt rocks. The salt rocks in the Tarim basin are characterized by coexistence of multiple suits, obvious change in thickness, large occurring area and complex deformation style and history. Although salt related study is always the key issue in the field of petroleum geology, and much advancement have been achieved, nearly all focus on the excellent plasticity derived salt related structure and serves as seal of cap rocks to oil and gas fields. Thermal properties measurements show that the salt rock is high in thermal conductivity and very low in heat generation, this contrast in thermal properties between surrounding rocks will undoubtedly result in geothermal anomalies and influence the hydrocarbon generation; however, this geothermal effect of salt rock is not paid much attention in previous studies. In this project, we will quantitatively investigate on the geothermal effects of salt rocks from three aspects as follows: (1) combined with thermal properties measurements and numerical modeling, the amplitude, scope and controlling factors of the influence of salt on present-day geothermal regime are will explored, to provide new interpretations for heat flow anomaly in salt bearing basins; (2) Based on borehole thermal history inversion and sedimentation and subsidence history reconstruction, we will investigate on the influence of salt rocks on sub-salt organic matter maturity, to reveal the temporal-spatial characteristics of its geothermal effects and its manifestation manner; (3) Taking the Tarim basin as case study, and consider the kinematic-thermal couple modeling and knowledge of research above, we will study the comprehensive influence of multiple salt related structures on sub-salt organic matter maturity in this basin. According to the mentioned topics addressed, this research not only can reveal the temporal-spatial evolution characteristics and action manner of geothermal effects of the salt rock, but also can provide scientific evidence for potential evaluation of deep sub-salt hydrocarbon resource and as reference for corresponding oil and gas exploration.

大量勘探实践表明，世界上许多油气藏的形成都与膏盐层有关。塔里木盆地发育多套厚度变化大、面积分布广、变形复杂的膏盐层。前人相关研究多集中在盐岩的可塑性而致的盐构造和致密性而作为优质盖层这两方面。实际上，膏盐岩具有高的热导率和低的生热率，这一独特的热物性势必会引起地热异常，进而影响烃源岩热演化。膏盐层的热物性导致的热效应关注不够，本项课题中，我们将定量研究塔里木盆地膏盐层的热效应：（1）结合实测物性数据和理论模型，探讨膏盐层对现今热状态影响的幅度、空间范围以及主控因素，进而为盆地热流异常解释提供依据；（2）基于沉积埋藏史和热史反演，定量分析膏盐层对其下烃源岩成熟度的影响，揭示膏盐层对热演化影响的时空特征和作用方式；（3）结合上述认识和热-运动学耦合模型，探讨塔里木盆地多期盐构造的热效应。本研究不仅可揭示膏盐层热效应的体现方式和时空特征，而且能为盐下油气资源评价和勘探提供理论依据和决策参考。

海相沉积盆地广为发育膏盐层，勘探表明盆内油气藏多与膏盐层有关。塔里木盆地是我国最大的海相沉积盆地，该区以往的盐构造研究主要集中于其作为良好盖层的封堵性能和作为构造圈闭的塑性变形和盐相关构造。实测表明，盐岩热导率远大于普通沉积岩，生热率却很低，盐岩强烈的热物性差异必然会对盆地地温场及烃源岩热演化产生显著影响，但这一问题尚未引起重视。本项目经过4年的工作，结合野外地质考察、室内物性测试、数值模拟实验和综合研究等多种手段，定量探讨了盐构造的热效应及其对烃源岩热演化的影响。获得如下主要成果：（1）基于岩石热物性测试和钻孔温度测量，获得了塔里木盆地现今地热背景（平均地温梯度为22℃/km、地表热流为43mW/m2），盆地表现为低温冷盆的前寒武克拉通热状态特征。稳定的大地构造环境和岩石物性横向差异造成了盆地现今地热分布格局，盐构造的聚热效应使得库车前陆盆地表现出高热流异常；（2）基于理论模型和有限元二维数值模拟实验，定量揭示了盐构造热效应的方式、幅度、范围和主控因素。发现盐层会造成其上地层显著增温（3~13%）和盐下地层降温（11~35%），进而分别加速盐上烃源岩和抑制盐下烃源岩的热演化过程。盐层的热导率、几何形态、厚度和埋深是引起地热异常幅度的主控因素。异常范围与盐体尺寸有关，横向上可达盐体宽度2倍、垂向上为盐体厚度2~3倍；（3）热史模拟表明，库车前陆盆地古近系盐构造使得下伏侏罗系烃源岩温度显著降低，进而抑制其热演化过程， 成熟度Ro值相应地降低了18%，使得原本应处于过成熟阶段的烃源岩现今仍处于生气阶段，有利于盐下深层油气的生成与保存。截止目前，本项目已正式发表学术论文6篇，其中国际SCI论文2篇、EI论文3篇；参写学术专著1部；参加EGU、AGU、IUGG等国内外重要学术会议5次；培养研究生2名。上述成果首次为盐下深层油气资源赋存提供了确凿的地热学证据，对我国海相沉积盆地深层油气资源潜力评价与勘探具有重要参考意义。