页岩气保存条件的碳同位素约束:以上扬子板块五峰-龙马溪组页岩为例  

Shale gas preservation conditions and their carbon isotope constraints:A case study of the Wufeng-Longmaxi Shale in the Upper Yangtze Block

作　　者：徐璐 刘睿 张康斌 唐余锋 陈增裕 XU Lu;LIU Rui;ZHANG Kangbin;TANG Yufeng;CHEN Zengyu(School of Geoscience and Technology,Southwest Petroleum University,Chengdu 610500,China;Sichuan Provincial Key Laboratory of Natural Gas Geology,Chengdu 610500,China)

机构地区：[1]西南石油大学地球科学与技术学院,四川成都610500 [2]天然气地质四川省重点实验室,四川成都610500

出　　处：《沉积与特提斯地质》2024年第3期630-640,共11页Sedimentary Geology and Tethyan Geology

基　　金：国家自然科学基金项目(42072184,41702157);中国石油-西南石油大学创新联合体项目(2020CX010302)。

摘　　要：天然气碳同位素对研究油气保存条件有着重要的指示意义。本文以上扬子长宁、涪陵、正安地区五峰—龙马溪组页岩为例,通过页岩气组分分析、单体碳同位素分析,探讨造成不同地区碳同位素倒转程度(δ^(13)C_(1)-δ^(13)C_(2))差异的原因,重建五峰—龙马溪组页岩气保存条件的时空动态演化过程,并揭示复杂构造变形区保存条件对页岩气富集的制约。结果表明:(1)高—过成熟阶段干酪根裂解气与原油二次裂解气混合是导致五峰—龙马溪组页岩烃类气体碳同位素发生倒转的主要原因。(2)原油二次裂解气的贡献程度不同导致不同地区烷烃碳同位素倒转程度(δ^(13)C_(1)-δ^(13)C_(2))存在一定差异;整体上,自盆外正安地区到盆内涪陵、长宁地区,原油二次裂解气的占比依次增大,碳同位素倒转程度增大,页岩含气量也依次增加,指示页岩系统的封闭性逐渐变好。(3)以δ^(13)C_(1)为约束的页岩系统保存条件定量评价模型显示,五峰—龙马溪组页岩气系统的开放程度(θ)在正安地区约66%~82%,涪陵地区约70%~77%,长宁地区约65%~70%。(4)中新生代上扬子地区发生差异构造变形,使得四川盆地外五峰—龙马溪组页岩系统的封闭性遭到更严重的破坏,促使油气大量运移和排出,保存条件相对较差。在构造变形比较强烈的地区,良好的保存条件对页岩储层的形成以及页岩气的富集至关重要。The carbon isotope composition of natural gas provides significant insights into the preservation conditions of oil and gas.This study focuses on the Wufeng-Longmaxi Formation in the Changning,Fuling,and Zheng'an areas,analyzing shale gas components and the carbon isotopes of monomer hydrocarbons to understand the variation in alkane carbon isotope reversal(δ^(13)C_(1)-δ^(13)C_(2))in different areas.We reconstruct the spatial and temporal evolution of preservation conditions in the Wufeng-Longmaxi Shale of the Yangtze Block and explore how preservation conditions affect shale gas enrichment.The results show that:(1)Carbon isotope reversal of hydrocarbons is commonly observed in the Wufeng-Longmaxi Shale,primarily caused by the mixing of secondary cracking gas with primary cracking gas during the high or post-maturation stage.(2)The mixing ratio of these two types of cracking gases is the major factor controlling the degree of carbon isotope reversal.As the proportion of secondary cracking gas increases,the extent of carbon isotope reversal and shale gas content gradually increase from the periphery towards the interior of the Sichuan Basin,indicating relatively good sealing conditions within the basin.(3)A quantitative evaluation model based onδ^(13)C_(1)shows that the system openness(θ)of the Wufeng-Longmaxi shale ranges from 66%to 82%in the Zheng'an area,70%to 77%in the Fuling area,and 65%to 70%in the Changning area.(4)Differential structural deformation of the Meso-Cenozoic strata in the Upper Yangtze Region has severely damaged the sealing conditions outside the Sichuan Basin,facilitating hydrocarbon migration and expulsion,and the preservation conditions are relatively poor.Therefore,good preservation conditions are crucial for shale gas enrichment,particularly in regions that have undergone extensive deformation.

关 键 词：页岩气 保存条件 碳同位素倒转 含气性 五峰—龙马溪组页岩 

分 类 号：P618.13[天文地球—矿床学]