四川盆地及周缘志留系龙马溪组一段深层页岩储层特征及其成因

深层页岩储层是页岩气勘探开发的重要接替领域。针对四川盆地及周缘龙马溪组一段深层页岩,系统开展了X-衍射全岩(XRD)、总有机碳(TOC)、大薄片及氩离子抛光大片成像研究。结果表明,相对于浅层页岩,深层页岩储层具有高硅质和低TOC含量、低粉砂纹层含量、高孔隙度和更有效的孔隙网络特征。由浅层至深层,页岩硅质含量由30%增至62%。相应地,碳酸盐含量由32%降至14.3%,粘土矿物含量由33%降至7.8%,TOC含量由7.1%降至4.25%。深层页岩储层发育条带状粉砂纹理,与浅层页岩相比,其粉砂纹层含量减少、单层厚度减薄。黑色页岩发育有机孔、无机孔和微裂缝,由浅层至深层,页岩总面孔率由1.6%增至10.8%,有机孔和无机孔均明显增加,且微裂缝占比由1%增至12%。深层页岩储层有机孔、无机孔和微裂缝相互连通,形成有效的孔隙网络。深层高硅质含量、高孔隙度和更有效的孔隙网络与生物成因硅有关,低TOC含量与远离物源有关,低粉砂纹层含量与水深较大有关。生物成因硅在成岩过程中可形成大量有机孔、无机孔和微裂缝,且其可有效保存孔隙。有机质的生成受营养物质供给影响,远离物源区营养供给较少。粉砂纹层主要由碳酸盐...

Characterization and genesis of deep shale reservoirs in the first Member of the Silurian Longmaxi Formation in southern Sichuan Basin and its periphery

Deep reservoirs are among the future major targets for shale gas exploration and development in China. A study on the first Member of deep Longmaxi Formation (Long 1 Member) shales in the Sichuan Basin and its periphery was carried out by means of whole rock component X-ray diffraction (XRD), TOC determination, large thin section and argon ion (Ar+) milling SEM imaging. The results show that, compared with shallow shales, the deep shale reservoirs are characterized by relatively higher silica content, lower TOC, lower silty lamina fraction, higher porosity, and more effective pore networks. From shallow to deep, the silica content increases from 30% to 62%. Accordingly, the contents of carbonate minerals, clay minerals, and TOC decrease from 32%, 33%, and 7.1% to 14.3%, 7.8%, and 4.25% respectively. The stripe-shaped siltstone-bearing lamina developed in deep shale reservoir is significantly lower in content and thickness compared with shallow shales. Organic/inorganic-matter pores micro-fractures are widespread within the black shales. From shallow to deep, the thin section porosity increases from 1.6% to 10.8% along with significantly increasing organic and inorganic pores, as well as micro-fractures (with a proportion increase from 1% to 12%). In deep shales, the organic/inorganic-matter pores and micro-fractures are interconnected to form an effective pore network. In addition, the higher silica content and porosity, as well as effective pore network are possibly associated with the biogenic silica; the relatively lower TOC may be due to the long distance from provenance; and the low silty lamina fraction is related to deep water environment. Biogenic silica can produce and preserve a large amount of organic/inorganic-matter pores and well as micro-fractures during diagenesis. The supply of nutrient substance determines the generation of organic matter, and the distance to provenance is in negative correlation to nutrient supply. Silty lamina is mainly composed of carbonate minerals, which, however, are hard to form in deep reservoirs.