白云凹陷天然气生成与大中型气田形成关系

Discussion on Natural Gas Generation and Giant-Medium Size Gas Field Formation in Baiyun Sag

通过对白云凹陷源、热、生烃机理、油气成因、资源规模及大中型气田分布特征等进行研究,认为白云凹陷存在文昌组和恩平组2套有效气源岩,其中恩平组浅湖相—沼泽相烃源岩是目前已发现油气的主力烃源岩。白云凹陷烃源灶处于变热流密度地质背景下,烃源岩熟化率早低晚高,主生气期相对较晚,介于23.8～0Ma之间。烃源岩热模拟生气实验表明,文昌组湖相烃源岩以生油为主,高成熟阶段生成的天然气主要为原油裂解气(约80%);恩平组浅湖相—沼泽相烃源岩油气兼生,以生气为主,主要为干酪根裂解气(约80%)。白云凹陷已发现的油气主要来自恩平组浅湖相—沼泽相烃源岩,天然气以来源于恩平组烃源岩干酪根裂解气为主,以原油裂解气和文昌组烃源岩晚期干酪根裂解气为辅。白云凹陷总生烃量为985.442 5×108t,以生气为主(81.5%),生油为辅(18.5%),主生气强度介于(500～3 500)×104t/km2之间。研究认为研究区高生气强度的白云凹陷烃源灶为大中型气田的形成提供了物质基础,近源晚期成藏提高了天然气田聚集的效率,运聚单元的资源规模控制着大中型气田的宏观分布。

Based on the study of source rock,thermal history,hydrocarbon generation,hydrocarbon origin,resource amount and distribution of giant-medium size gas fields in the Baiyun sag,we concluded that the Baiyun sag has developed two sets of effective gas-source rocks of Wenchang and Enping Formations,in which the Enping Formation source rock has proved to be the main contributor for the discovered oil and gas in the Baiyun sag.Hydrocarbon generation kitchens in the Baiyun sag is within the variable heat flow,experienced the lower maturation ratio at earlier stage and the higher ratio at later stage.Thus,the main gas generation period is relatively late,ranging from 23.8Ma to present.Based on results of thermal simulation experiment of source rocks,the lacustrine source rock in the Wenchang Formation mainly prone to generate oil;the shallow lake-marsh facies source rock in Enping formation is apt to generate gas（about 80 percent of kerogen decomposing gas）.The discovered oil and gas in the Baiyun sag mainly is sourced from the shallow lake-marsh facies source rock in Enping Formation.The discovered gas comes greatly from the kerogen decomposition of Enping Formation source rock and little from the contribution of Wenchang Formation source rock at later stage.The total hydrocarbon amount is 985.442 5×108t in the Baiyun sag,with gas and oil accounting for about 81.5% and 18.5%,respectively.Gas generation intensity is in range of（500-3500）×104t/km2.Therefore,the strong intensity of gas generation in the Baiyun sag is a requirement for the giant-medium size gas field formation,the near source and late accumulation raises the accumulation efficiency of natural gas,the resource amounts in the migration and accumulation units constrain the distribution of giant-medium size gas fields.