四川盆地五峰—龙马溪组黑色页岩有机岩石学特征研究

目前五峰—龙马溪组黑色页岩中显微组分缺乏统一的分类方案,命名也较为混乱,给页岩气勘探与评价造成了困难。为此,采用全岩光片和有机地球化学(TOC、δ¹³C_org)分析等方法,对四川盆地五峰—龙马溪组黑色页岩中显微组分进行有效识别与特征总结,探讨其可能成因,并判断其有机质类型。研究结果表明:五峰—龙马溪组页岩中显微组分主要由海相镜质组、腐泥组、动物有机碎屑组和次生组组成。其中,海相镜质组由无结构镜质体组成,呈浑圆状或长条状,具有强的光反射能力,但分布并不广泛;腐泥组主要由无结构腐泥体组成,为藻类遭受热降解过程而形成的无结构且无固定形态的显微组分,在富有机质页岩中广泛分布;动物有机碎屑组主要包括笔石表皮体、几丁虫和放射虫有机碎屑体;次生组由次生沥青体组成,广泛分布在页岩基质孔隙中,呈无固定形状。五峰—龙马溪组页岩中主要发育腐泥组和次生组,其次为动物有机碎屑组和海相镜质组,其有机质类型以Ⅰ-Ⅱ₁型干酪根为主,且腐泥组和次生组含量越高,有机质类型越好,其生烃潜力越大。

Study on Organic Petrology Characteristics of the Wufeng-Longmaxi Formation Black Shale,Sichuan Basin

At present, there is no unified classification and definition of organic macerals in the Wufeng-Longmaxi Formation black shale, which made it difficult for regional shale gas exploration and evaluation. In this study, organic macerals in the Wufeng-Longmaxi Formation black shale from the Sichuan Basin were identified and summarized via optical microscopic observation of polished blocks and organic geochemistry (TOC, δ¹³C_org), and their origins and organic matter types were determined. Our results show that organic macerals are mainly composed of the marine vitrinite, sapropelinite, organic zooclast and secondary groups. Among them, marine vitrinite group consists of round-/rod-shaped structureless vitrinite, and displays strong light reflection, but it is only locally distributed. The sapropelinite group is mainly composed of structureless sapropelinite, and is widely developed in organic-rich shale, which is the product of algal materials that underwent geological processes of thermal degradation. The organic zooclast group includes graptolite epidermis, chitinozoans and radiolarians. Meanwhile, the secondary group is composed of secondary bituminites, which is widely developed in matrix porosity of shales and is amorphous. The sapropelinite and secondary groups are the major types of organic macerals in the shales of Wufeng-Longmaxi Formation, whereas organic zooclast and marine vitrinite groups are less common. The organic matter type of studied shales is predominately type Ⅰ-Ⅱ₁ kerogen. Moreover, the relative contents of sapropelinite and secondary groups increase, and organic matter type tends to oil-prone, contributing to their greater potentials of hydrocarbon generation.