川南地区五峰组–龙马溪组页岩有机质的组构差异及对内部孔隙的控制

Control of Pore Characteristics by Structure Differences in Organic Matter within the Wufeng-Longmaxi Shales, Southern Sichuan Basin

摘要: 有机孔是富有机质海相页岩储层中重要储气空间，控制了页岩气的富集与运移。为探明页岩有机孔的非均质性成因，基于扫描电镜与激光拉曼联用等技术，探讨川南地区五峰组–龙马溪组页岩有机质内部化学结构差异对有机孔发育的控制作用。结果表明：① 根据有机质赋存状态及其孔隙发育特征，将有机孔分为三大类：无定形孔隙、结构型孔隙和生物碎屑孔隙；② 不同类型有机孔发育具有非均质性，生物碎屑孔隙面孔率约为4%，结构型孔隙面孔率约为13%，无定形孔隙面孔率约为27%，无定形孔隙发育较好，有利于页岩气的富集；③ 不同原始成烃生物形成的有机质的激光拉曼光谱特征具有明显差异，无定形孔隙RBS平均为240.6 cm⁻¹，小于结构型孔隙的平均RBS 249.8 cm⁻¹；无定形孔隙的平均D/G (0.8)大于结构型孔隙的平均D/G (0.6)；无定形孔隙的成烃生物是以浮游藻类为主的多细胞藻类，结构型孔隙的成烃生物是以底栖藻类为主的单细胞藻类；④ 成烃生物的差异导致有机质的芳构化程度和有序度不同，有机质芳构化程度高对有机质孔隙的发育起抑制作用，结合激光拉曼光谱的G-FWHM，D-FWHM，RBS和D/G等参数判断笔石等动物碎屑、底栖藻等单细胞藻类和浮游藻类等多细胞藻类成烃后的有机质芳构化程度依次降低，有机孔的发育程度依次增高。

Abstract: Organic pores serve as the primary gas storage spaces in organic-rich marine shale reservoirs, controlling the enrichment and migration of shale gas. To investigate the genesis of heterogeneity in shale organic pores, this study employs scanning electron microscopy (SEM) combined with laser Raman spectroscopy (LRS), to explore how differences in chemical structures within organic matter from the Wufeng Formation to Longmaxi Formation shales in the southern Sichuan Basin control the development of organic pores. Results indicate: (1) Based on occurrence states of organic matter and pore development characteristics, organic pores are classified into three categories: amorphous pores, structural pores, and bioclastic pores; (2) The development of different pore types exhibits significant heterogeneity-bioclastic pores show a facial porosity of approximately 4%, structural pores about 13%, while amorphous pores reach 27%. Amorphous pores demonstrate superior development, which favors shale gas enrichment; (3) The laser Raman spectral characteristics of organic matter derived from different source organisms exhibit significant differences. Amorphous pores exhibit lower average Raman band shift (RBS) values (240.6 cm⁻¹) compared to structural pores (249.8 cm⁻¹). The average D/G ratio for amorphous pores (0.8) exceeds that of structural pores (0.6). Hydrocarbon-generating organisms for amorphous pores are primarily multicellular algae dominated by planktonic species, whereas structural pores originate mainly from unicellular benthic algae; (4) Differences in hydrocarbon-generating organisms result in variations in aromatization degree and orderliness of organic matter. Higher aromatization degrees inhibit pore development. Through comprehensive analysis of Raman parameters including G-FWHM, D-FWHM, RBS, and D/G ratios, we determine that graptolites and other animal debris, benthic unicellular algae, and planktonic multicellular algae exhibit progressively decreasing aromatization degrees after hydrocarbon generation, corresponding to increasingly developed organic pores.