Stratigraphic division and correlation of Wufeng-Longmaxi black shales in and around the Sichuan Basin

Abstract Stratigraphic division and correlation is crucial for the identification of shale gas sweet spots and drilling design. In this study, stratigraphic division and correlation was carried out for the Wufeng-Longmaxi formations in southern China from the prospective of lithostratigraphy, sea level changes, and biostratigraphy using data from seismic investigation, wells, and outcrops. The Wufeng Formation was divided into two, member Wu 1 and member Wu2, and the Longmaxi Formation was also divided into two members, i.e. Long 1 and Long 2. The Long 1 Member was further divided into two sub-members, i.e. Long 11 and Long 12; Long 11 was subdivided into 4 layers (Long 111, Long 112, Long 113, and Long 114). It was found that there are three eustatic cycles in the Wufeng-Longmaxi formations. Cycle I corresponds to the Wufeng Formation, with the maximum flooding surface at the top of the Wu 1 Member. Cycle II corresponds to the Long 1 Member, with the maximum flooding surface at the top of Long 113. Cycle III corresponds to the Long 2 Member. Furthermore, 4 graptolite biozones (WF1 to WF4) were identified in the Wufeng Formation and 9 graptolite biozones (LM1 to LM9) in the Longmaxi Formation. WF1-2 and WF3-4 correspond to the Wu 1 and Wu 2 Members, respectively, and LM1, LM2-4, LM5, LM6, and LM7-9 correspond to Long 111, Long 112, Long 113, Long 114, and Long 12 and Long 2, respectively. High-quality shales mainly occur in the Wufeng Formation and Long 11. The major intervals that should be investigated with regards to shale gas production include LM1–LM5 (10 m thick) in the Weiyuan Block and WF1–LM5 (20–35 m thick) in the Changning Block. Long 111 is believed to be an optimal target for drilling due to its high TOC content, siliceous content, porosity, microfracture density, and horizontal/vertical permeability ratio.