四川盆地东部五峰组—龙马溪组黑色页岩有机质富集规律探讨

A study on the enrichment of organic materials in black shales of the Wufeng to Longmaxi Formations in eastern Sichuan Basin

关键词:  四川盆地东部  五峰组-龙马溪组  黑色页岩  斑脱岩  古生产力  氧化还原条件  有机质富集

Key words:  eastern Sichuan Basin  Wufeng-Longmaxi Formations  black shale  bentonite  enrichment of organic materials

摘要:

奥陶-志留交替时期是地球历史时期中的关键时期，该时期华南地区沉积的五峰组-龙马溪组黑色页岩是现阶段我国页岩气勘探的重点层位，多套斑脱岩存在于黑色页岩内，指示该时期研究区经历过多期连续的火山活动。本文选取了四川盆地东部华地1井和武隆黄莺乡五峰组-龙马溪组剖面，以黑色页岩和斑脱岩为研究对象，通过野外考察、元素地球化学等分析方法，系统地分析了有机质富集的主控因素（陆源输入、古生产力、古氧化还原条件），并在此基础上分析斑脱岩与页岩有机质富集间的关系。结果显示，研究区五峰组-龙马溪组黑色页岩TOC呈高-低-高模式，氧化还原指标（V/V+Ni、V/Cr、Ni/Co）表明水体经历了厌氧-富氧-厌氧的过程，生产力指标（Cu、Ba_bio、Si_bio）、陆源输入指标（Ti/Al、Zr/Al、Th/Al）表明五峰和龙马溪组具较高的生产力和陆源输入，观音桥组较两者均低，三者与有机碳的相关关系反映氧化还原条件对有机质富集作用最大，生产力次之，陆源输入最弱。从斑脱岩发育特征与有机碳、生产力和氧化还原条件的关系看，频繁的火山活动可能对五峰和龙马溪组有机质的保存产生双促作用，一是为海洋提供营养物质，提升其生物生产力；二是火山灰飘落至海水表面阻止了部分光照进入，海底缺氧环境加剧，造成生物大量死亡并落入海底，作为有机质埋藏。

Abstract:

Black shales of the Wufeng to Longmaxi Formations, the main target strata for exploration of shale-type gas in South China, were formed from the Late Ordovician to the Early Silurian, a crucial transitional period in the Earth's history. In this paper, based on geochemical data of back shales and bentonites from the Huadi No.1 well and the Huangying section in Wulong, the enrichment mechanism of organic materials of black shales of the Wufeng to Longmaxi Formations in the eastern Sichuan Basin is studied in detail. The results show that the TOC varies in a high-low-high mode from the Wufeng Formation to Longmaxi Formation, and the redox indices (V/V+Ni, V/Cr, Ni/Co) imply that the water experienced an anoxic-oxic-anoxic process. The productivity (Cu, Ba_bio, Si_bio) and terrigenous inputs indices indicate that black shales of the Wufeng and Longmaxi Formations are with higher productivity and terrigenous inputs, compared with those of Guanyinqiao Formation. The correlations between the TOC in the sediments and redox indexes, productivity and terrigenous inputs show that the enrichment process of organic materials in black shales is controlled by the redox indexes, the productivity and terrigenous inputs. The redox indexes played the most important role, the productivity was in the second place and terrigenous inputs were in the third. It is found that frequent volcanic activities maybe have dual effects on organic-rich shale:on one hand, volcanic ash provided a sufficient supply of nutrients, which triggered high marine productivity; on the other hand, volcanic ash prevented more sunshine coming into the ocean. The extremely anoxic environment led to more death of living things, and enhanced the amount of organic materials buried and preserved.