祁连山冻土区天然气水合物气源研究

With the discovery of gas hydrate in recent years, the permafrost regions of Qilian Mountain have become hotspots in the research of South Qilian Basin. Many scholars have already reached a consensus on the genetic type of the source of gas hydrate by different researches, but the source beds of gas hydrate, however, is still a matter of opinion. Study on the source of gas hydrate could not only provide favorable basis for a profound understanding of the distribution of gas hydrates but also important information for a thorough exploration of occurrence regularities of petroleum and a detailed evaluation of the resource potential of oil and gas in South Qilian Basin. Based on the previous researches and, by choosing source rocks of hydrocarbon from strata of Carboniferous, Permian, Triassic and Jurassic in the permafrost regions of Qilian Mountain as our objects, this study firstly analyzes the hydrocarbon generating potential, organic matter type and thermal evolution degree of source rocks. Then it compares with other existing gas hydrates on organic matter type and thermal evolution degree so as to make sure of the similarities and differences of the organic matter type and thermal evolution degree of source rocks, alkane gas and gas hydrate, and preliminarily determines the genetic relationship between the alkane gas of gas hydrate and hydrocarbon source rocks. At the same time, by analyzing the components, carbon isotopes of the released gas from hydrocarbon source rocks and the alkane gas from gas hydrate, comparing the similarities and differences of analyses of the two gases above, and coupled with the hydrocarbon generating potentials of source rocks from different horizons in the study area, this study furthermore clarifies the genetic relationships of hydrocarbon source rocks and gas hydrate, which also help establish the source beds of gas hydrate.

祁连山冻土区因天然气水合物的发现而成为南祁连盆地研究的热点地区，众多学者从不同的角度对该区天然气水合物气源母质类型取得一致认识，但对天然气水合物气源层位认识尚存争议。天然气水合物气源研究不仅能够为认识该区天然气水合物分布规律供有利依据，还可以为探索南祁连盆地油气赋存规律和评价油气资源潜力提供重要信息。本项目在已有研究基础之上，选择祁连山冻土区内石炭系、二叠系、三叠系和侏罗系烃源岩为研究对象，分析烃源岩的有机质丰度、母质类型和热演化程度，并与已有天然气水合物母质类型和热演化程度进行对比，明确烃源岩、解析气与天然气水合物母质类型、热演化程度的异同，初步确定天然气水合物烷烃气与烃源岩的亲缘关系。同时，以烃源岩解析气、天然气水合物烷烃气的组分、碳同位素测试分析为依据，对比二者异同，并结合研究区不同层位烃源岩的生烃潜力，进一步明确天然气水合物与烃源岩的亲缘关系，厘定天然气水合物气源层位。

木里坳陷是南祁连盆地天然气和天然气水合物最有前景的地区。项目选择研究区内三叠系和侏罗系为研究对象，通过野外地质调查、岩心观测确定了上三叠统-侏罗系烃源岩的分布层位，在此基础上分析了190个来自于木里坳陷侏罗系窑街组的天然气和天然气水合物的分解气的分子组成和碳氢同位素组成，并将天然气进行了详细的分类，详细研究了天然气的成因类型、热成熟度、次生蚀变。此外，利用黄金管在封闭体系中热解了下侏罗统窑街组的九个烃源岩样品，并对热解气体甲烷、乙烷和丙烷的稳定碳同位素组成进行了分析，与天然气进行了比较。结果表明：（1）天然气主要由烷烃气体和少量的CO2、N2组成，大部分烷烃气来自于热成因，含有少量生物成因气。热成因烷烃气的干酪根类型主要油型气，含有少量煤成气。大部分油型气主要来自于腐泥型干酪根在成熟阶段的产物，以原油裂解生气为主，煤成气为腐质型干酪根在高成熟阶段干酪根直接降解生气；（2）天然气水合物烷烃气发生了次生蚀变作用，主要包括生物降解作用、扩散运移。生物降解作用导致甲烷及其同系物的碳同位素系列发生反转（δ13C1＜δ13C2＜δ13C3＞δ13C4）;扩散运移作用导致δ13C1-δ13C2值增大伴随着甲烷含量降低以及甲烷及其同系物的氢同位素系列发生反转；（3）侏罗系烃源岩的热模拟气体是单源气体，大部分天然气的乙烷和丙烷与热解气体具有相同的来源。中低温泥页岩热模拟气（300-400℃）的乙烷和丙烷与研究区的腐泥型烷烃气接近；高温阶段腐质型干酪根的热模拟气（400-500℃）的乙烷和丙烷与研究区的腐质型烷烃气相近；（4）油型气体来源于下侏罗统上部的泥岩和油页岩。煤型气体来源于下侏罗统的泥岩、炭质泥岩和煤层。