汞含量和同位素在海相天然气成因鉴别中的应用——以塔里木盆地满西地区为例

Marine natural gas resources are abundant in the western part of Manjar Depression, Tarim Basin. In recent years, great breakthroughs have been made in Shunnan, Shunbei and Gucheng areas, showing good prospects for natural gas exploration. Because of the deep burial depth of marine reservoirs and well-developed faults, natural gas in the western part of Manjar Depression is not only including oil cracking gas but also primary cracking gas. Meanwhile, gas in the western part of Manjar Depression was subjected to secondary alteration such as TSR and hydrothermal fluid. Because of the complex geological conditions, there are many genetic types of natural gas in this area, and the commonly used geochemical indicators are difficult to effectively identify the genetic types and sources of natural gas. In this project, mercury content and isotopes in natural gas are used as technical means, combined with conventional oil and gas geochemical indicators, to explore and analyze the main genetic types of deep marine natural gas under complex geological background, and to explore the response characteristics of mercury content and isotopes in marine natural gas after TSR and hydrothermal fluid alteration, and to establish identification indicators of oil cracking gas, primary cracking gas and TSR or hydrothermal alteration. The new identification indicators of mercury content and isotope in natural gas will fill in the blank of research on mercury isotopes in natural gas.

塔里木盆地满加尔坳陷西部地区海相天然气资源丰富。近年来，在顺南、顺北及古城地区取得重大突破，展现出良好的天然气勘探前景。由于该区海相气藏埋藏深度大、断裂发育，天然气既有原油裂解气也干酪根裂解气，同时，遭受TSR和热液流体等次生作用改造。复杂的地质条件使得该地区天然气成因类型多，常用的地球化学指标难以有效识别天然气成因类型及其来源。本项目以天然气汞含量和同位素作为技术手段，结合常规油气地球化学指标，探讨分析复杂地质背景下深层海相天然气的主要成因类型，并探索经历TSR和热液流体改造海相天然气中汞含量和同位素的响应特征，探索建立原油裂解气、干酪根裂解气，及其遭受TSR或热液改造后天然气中汞含量和同位素新鉴别指标，填补天然气中汞的同位素研究空白。

近年来，塔里木盆地满西地区（主要指顺北-顺南地区）发现了一批超深层海相油气藏，然而对于该区天然气成因还存在争论，是否经历次生改造也尚不明确。由于满西地区断裂体系和热液流体异常发育，深层海相油气可能伴生多种次生作用，使得油气源对比难度大。本项目以满西地区海相天然气为主要研究对象，结合区域构造、潜在烃源岩分布和邻区天然气地化特征，明确该地区天然气成因类型，探讨次生改造作用对天然气地化特征的影响。主要取得以下2个方面认识：.（1）顺北-顺南地区海相天然气为腐泥型有机质形成的油型气，顺北地区天然气为原油伴生气，顺南地区为原油裂解气。顺北地区海相天然气为原油伴生气，既有干酪根裂解气，又有原油伴生气；顺南地区发育典型原油裂解气。顺北4号断裂带与顺北8号断裂带是原油裂解气与干酪根裂解气的过渡带，8号带以东主要天然气主要位原油裂解气，4号带以西天然气主要为干酪根讲解气。根据CO2含量和碳同位素组成，顺北地区天然气中CO2表现为无机成因，而顺南地区具有有机成因CO2特征。研究区稀有气体同位素组成表现为壳源成因，无明显幔源组分的贡献。.（2）顺北-顺南地区海相天然气均遭受了不同程度的次生改造作用。次生作用不仅促进了油气相态的改变，也可能形成了烷烃气同位素的异常。通过原油热裂解和TSR模拟实验表明顺北地区海相天然气中H2S主要为含硫化合物热裂解所形成。结合顺南地区油气地化特征认为该区发生了一定程度的TSR作用。顺北和顺南地区油气均遭受了不同程度的热液流体改造。热液流体促进原油裂解，形成的饱和烃在色谱图上具有UCM特征。满西地区油气受到的次生作用整体上有从顺北7号、5号、1号断裂带至顺南地区逐渐增强的趋势，从而使顺北7号断裂带向顺南地区原油裂解程度逐渐增强。.本项目研究对对深化塔里木盆地台盆区深层海相天然气成因与资源潜力具有重要意义。