川西坳陷中三叠统雷口坡组沥青地球化学特征及气源示踪

Geochemical characteristics of bitumen and tracing of gas source in the Middle Triassic Leikoupo Formation， Western Sichuan Depression

吴小奇 陈迎宾 翟常博 周凌方 周小进 杨俊 王彦青 宋晓波

Xiaoqi Wu Yingbin Chen Changbo Zhai Lingfang Zhou Xiaojin Zhou Jun Yang Yanqing Wang Xiaobo Song

中三叠统雷口坡组是四川盆地西部天然气勘探的热点层系之一，目前对其储层沥青发育和地球化学特征的研究与认识较为薄弱。基于储层沥青含量、反射率、碳同位素和生物标志化合物等开展川西雷口坡组沥青的成因和来源研究，并基于沥青地球化学特征分析来开展气源示踪，丰富和完善了该区气源对比的研究方法和途径。研究表明，雷口坡组储层中沥青零星分布于缝合线和裂缝中，δC值介于-28.6 ‰ ~ -24.6 ‰。钻井岩心样品中沥青的等效镜质体反射率平均为3.14 %，为典型的热裂解成因。野外露头沥青样品等效镜质体反射率较低，香水镇黄连桥和石元乡马鞍塘剖面沥青经历了生物降解，而天池乡卸军门剖面沥青为原油脱沥青质作用形成。雷口坡组储层沥青与自身碳酸盐岩具有较好的亲缘性，而根据沥青含量计算所得川西大气田雷口坡组裂解生气强度和圈闭总裂解生气量与现今气藏规模不匹配。川西大气田主体并非来自古油藏的原位裂解，而是由高演化阶段的原油裂解气直接充注形成。天然气主体来自下伏上二叠统烃源岩，雷口坡组自身碳酸盐岩贡献较为有限。相关认识为揭示川西雷口坡组成藏机理、明确勘探方向提供了科学依据。

The Middle Triassic Leikoupo Formation （T） is one of the exploration targets for natural gas in western Sichuan Basin. However， the development and geochemical characteristics of bitumen in the formation is barely studied and understood. Analyses are therefore carried out on the content， reflectance， carbon isotopes， and biomarkers of bitumen samples from the formation to determine the genesis and source， and a gas source tracing is also performed based on geochemical characteristics of the bitumen， thus shedding some light on finding new ways of gas-source correlation in the area. The results indicate that the bitumen distributes sporadically along sutures and fissures， with δC values ranging from -28.6 ‰ to -24.6 ‰. The bitumen from core samples is of typical thermogenic origin as suggested by an average equivalent vitrinite reflectance of 3.14 %. The bitumen from outcrops has low equivalent vitrinite reflectance values， suggesting diversified origins： samples from the Huanglianqiao outcrop， Xiangshui Town， and the Ma’antang outcrop， Shiyuan Town， are the result of biodegradation， whereas those from the Xiejunmen outcrop， Tianchi Town， are derived from de-asphalting of crude oil. It also indicates a close affinity between the bitumen and carbonate rocks in the Leikoupo Formation. However， the oil cracking intensity in the formation of the area and the total gas amount generated through cracking in the whole trap calculated according to bitumen contents do not match the scale of current large gas pools. It is suggested that the pools are sourced from direct charging of gas from cracking of oil in a high maturity stage rather than the in-situ cracking of paleo-oil， and that most of the gas is from the underlying Upper Permian source rocks， rather than from the carbonates in the Leikoupo Formation. These understandings may help revealing the accumulation mechanisms and specifying exploration direction in the Leikoupo Formation of western Sichuan Basin.

10.11743/ogg20220213