Influence of graptolite debris on shale gas generation and accumulation

As one of the standard fossils of the Ordovician and Silurian, graptolites are widely distributed in the Upper Ordovician Wufeng Formation to the Lower Silurian Longmaxi Formation in the Lower Paleozoic of South China. This stratum shows a good potential for shale gas exploration; however, there is a disagreement about the contribution of graptolite debris to the generation and storage of shale gas. Acid treatment was carried out on shale samples from the Wufeng-Longmaxi formations in well Weiye 1 in the Weiyuan and Changning areas of the Sichuan Basin. Graptolite debris was collected and analyzed with Microscopic Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy and Transmission Electron Microscopy. The composition and pore development characteristics of the organic matter in graptolites were examined, and the contribution of graptolite bodies to shale reservoir capacity was discussed. The graptolite bodies were mainly composed of organic matter of collagen, which shows a poor gas potential. However, the inner fetal tube of the graptolite body is hollow, and strawberry-like pyrite is developed in the body cavity, which is favorable for the storage of shale gas. The graptolite bodies are superposed on each other to form a multi-layer structure, and the vertical and horizontal stripes and the nano-mineral partitions in the graptolite skin form gaps between the stacked graptolite skins, thereby improving shale gas storage. Therefore, it is believed that the large number of graptolites in the Wufeng-Longmaxi formation shale is one of the important reasons that affect the shale gas enrichment in this segment.