A study on the relationship between graptolites and shale gas enrichment in the Wufeng–Longmaxi Formations of the Middle–Upper Yangtze region in China

Previous studies on graptolites have mainly focused on stratigraphic division and sedimentary environment analysis. On the basis of previous studies of Ordovician and Silurian graptolites in the Middle–Upper Yangtze region, this paper makes full use of core analysis, drilling geological, experimental analysis, and logging data from wells and combines graptolite types in different sedimentary environments to identify 4 graptolite zones, WF1–WF4, and 9 graptolite zones, LM1–LM9, in the Silurian Longmaxi Formation in the Middle–Upper Yangtze region, among which WF2–LM4 present high total organic carbon (TOC), organic pore development, and shale gas enrichment and are favorable development zones. This study reveals that enrichment in graptolites has positive correlations with TOC, organic porosity, and total hydrocarbon content. The evolution of graptolites has contributed to the enrichment in organic matter and the formation of organic pores in shales and affects shale gas–bearing properties to a certain extent, which is an important indicator for shale gas reservoir evaluation.