Discussion on the contribution of graptolite to organic enrichment and gas shale reservoir: A case study of the Wufeng–Longmaxi shales in South China

Abstract

The graptolitic shale of the Wufeng–Longmaxi Formations is widely deposited across the Ordovician and Silurian transition in South China, which is the target of shale gas exploration and development within China. The contribution of graptolites to organic enrichment and reservoir of gas shale is discussed below based on the statistics of nearly 1000 shale samples from the Wufeng Formation and the bottom part of the Longmaxi Formation in the southern and northern margins of the Yangtze plate. The assessment involves graptolites abundance, the total organic carbon (TOC) content analyses, and the different scales of scanning electron microscopy analyses of related samples. The TOC content of the Wufeng–Longmaxi graptolitic shales (including graptolites and non-graptolites, i.e., the host shale) is mainly controlled by that of its host shale, while less affected by the graptolites abundance, indicating that the graptolites barely influence the organic enrichment. Graptolites consist of a large number of organic matter with reticular biological tissue structure; they account for 20%–50% of the graptolitic area. The aforementioned also developed honeycomb-shaped pores with pore sizes ranging 110 nm-1.7 μm (an average of about 500 nm), which are higher than those of the organic pores in the host shale (108–770 nm, average 330 nm), proving that graptolites have an important contribution to shale gas storage space. Since there are a large number of graptolites within the shales from the Wufeng Formation and the bottom part of the Longmaxi Formation, the laminated and stacked local pattern of their distribution provides abundant storage space for shale gas. Moreover, the feature also serves as the predominant channel for shale gas flow. Therefore, the widely developed graptolites should be considered as one of the essential factors controlling enrichment and high productivity of shale gas in the Wufeng–Longmaxi Formations.