鄂尔多斯盆地大牛地气田上古生界致密气轻烃地球化学特征

Geochemical characteristics of light hydrocarbons in Upper Paleozoic tight gas from Daniudi Gas Field, Ordos Basin

倪春华 吴小奇 王萍 王付斌 贾会冲 朱建辉 张毅 姜海健

NI Chunhua WU Xiaoqi WANG Ping WANG Fubin JIA Huichong ZHU Jianhui ZHANG Yi JIANG Haijian

1. 中国石化 石油勘探开发研究院 无锡石油地质研究所, 江苏 无锡 214126; 2. 中国石化油气成藏重点实验室, 江苏 无锡 214126; 3. 中国石化 华北油气分公司 油气勘探管理部, 郑州 450006; 4. 中国石化 华北油气分公司 勘探开发研究院, 郑州 450006

1. Wuxi Research Institute of Petroleum Geology, SINOPEC, Wuxi, Jiangsu 214126, China; 2. SINOPEC Key Laboratory of Hydrocarbon Accumulation, Wuxi, Jiangsu 214126, China; 3. Department of Oil and Gas Exploration Management, SINOPEC North China Company, Zhengzhou, Henan 450006, China; 4. Research Institute of Exploration and Production, SINOPEC North China Company, Zhengzhou, Henan 450006, China

大牛地气田是鄂尔多斯盆地典型致密砂岩大气田之一。为了更深入了解该区天然气的成因和来源，揭示天然气运移相态，对大牛地气田上古生界致密气开展了轻烃地球化学特征分析。研究表明，该区上古生界致密气C5-7轻烃组成具有异构烷烃优势分布，C6-7轻烃组成中芳烃含量整体偏低(＜10%)，甚至未检出芳烃，C7轻烃组成具有甲基环己烷优势分布特征，甲基环己烷相对含量均超过50%。上二叠统下石盒子组天然气K1值、K2值均与二叠系山西组和石炭系太原组天然气一致，而δ13C1值则与山西组天然气一致，与太原组天然气有明显不同。与山西组天然气相比，下石盒子组天然气整体具有偏低的苯/正己烷、苯/环己烷和甲苯/正庚烷比值，以及明显偏高的正庚烷/甲基环己烷比值。轻烃地球化学特征及烷烃气碳氢同位素组成综合表明，大牛地气田上古生界天然气为典型煤成气，其中山西组和太原组天然气均为原地自生自储，而下石盒子组天然气为下伏山西组烃源岩生成的天然气经历了游离相垂向运移聚集形成，太原组烃源岩不具有显著贡献。受天然气运移和水溶等作用影响，庚烷值、异庚烷值、苯/正己烷比值等轻烃指标直接用于判识大牛地气田致密气成熟度会存在偏差。

The Daniudi Gas Field is one of the typical tight sandstone gas fields in the Ordos Basin. In order to better understand the genetic types and source of natural gas and reveal the migration phase of natural gas, the geochemical analysis of light hydrocarbons in the Upper Paleozoic tight gas from the Daniudi Gas Field has been conducted in this study. The results indicate that, the C5-7 light hydrocarbons in the Upper Paleozoic tight gas are dominated by iso-alkanes, and the C6-7 light hydrocarbons display low contents of aromatics (<10%), which are even undetectable. The C7 light hydrocarbons are dominated by methylcyclohexane (MCH) with the relative MCH contents exceeding 50%. The K1 and K2 values of natural gas from the Lower Shihezi Formation (P1x) are consistent with those from the Shanxi (P1s) and Taiyuan (C3t) formations, respectively. The δ13C1 values of the P1x gas are consistent with those of the P1s gas and significantly different from those of the C3t gas. Compared with the P1s gas, the P1x gas mainly displays lower benzene/n-hexane, benzene/cyclohexane and toluene/n-heptane ratios, as well as significantly higher n-heptane/methylcyclohexane ratios. Geochemical characteristics of light hydrocarbons and carbon and hydrogen isotopic compositions of alkanes indicate that, the Upper Paleozoic natural gas from the Daniudi Gas Field is typical coal-derived gas. The P1s and C3t gases were in-situ self-generated and self-accumulated, whereas the P1x gas was mainly derived from the underlying P1s source rocks and accumulated via vertical migration in free phase, with insignificant contributions of the C3t source rocks. Due to the effect of migration and water solution of natural gas, the deviation exists in the identification of thermal maturity for tight gas from the Daniudi Gas Field using the light hydrocarbon indexes such as heptane and isoheptane values and benzene/n-hexane ratios.

国家自然科学基金项目(42172149,U2244209)和中国石化科技部攻关项目(P23230，P22132，P21077-1)联合资助。

https://doi.org/10.11781/sysydz202402366