A Preliminary Quantitative Stratigraphic Correlation of Graptolite-Bearing Wells in the Wufeng–longmaxi Succession of South China Using Graphic Correlation

The graptolite-bearing black shales of the Wufeng Formation and the lower part of the Longmaxi Formation in South China constitute a key stratigraphic interval for both high-resolution biostratigraphy and shale gas exploration. However, stratigraphic correlation among wells remains challenging because graptolite range records from individual cores are commonly incomplete. In contrast to outcrop sections, where relatively continuous sampling is possible, the limited amount of drill core material in a single well may cause the lowest observed occurrence of a taxon to lie above the true base of its stratigraphic range. This problem reduces the completeness of biostratigraphic event records and hinders the establishment of a robust high-resolution regional correlation framework. To address this issue, we compiled published and existing graptolite range data from approximately 20 wells and outcrop sections in the Wufeng–Longmaxi succession of South China, including about 35 genera and more than 70 species. First and last occurrence events were used as the principal biostratigraphic markers. These event sequences were analyzed using graphic correlation to achieve quantitative stratigraphic comparison among sections and to project biostratigraphic events from different wells and outcrops onto a unified reference coordinate system. On this basis, we present a preliminary Composite Standard Reference Section (CSRS) for the region. Preliminary results indicate that graphic correlation can integrate incomplete graptolite range data from different wells and improve the consistency of event ordering among sections, thereby providing a more unified framework for high-resolution biostratigraphic correlation. Some well intervals show marked deviations from the general correlation trend, which may reflect stratigraphic incompleteness, condensed intervals, or fault-related repetition or omission. These observations suggest that graphic correlation has considerable potential for refining subsurface stratigraphic correlation in the Wufeng–Longmaxi succession. This study represents a preliminary attempt to introduce quantitative stratigraphic methods into graptolite-based well correlation of the Wufeng–Longmaxi succession in South China. The framework developed here provides a methodological basis for further refinement of the regional CSRS, improved high-resolution stratigraphic correlation, and more accurate identification of favorable shale gas intervals.